BACKGROUND OF THE INVENTION
Both of the above mentioned pat
ents
are directed to frequency synthesizer tuning systems for use with
television receivers to enable operation of the receivers with
minimal viewer fine tuning adjustments. By the utilization of the
frequency synthesizer tuning systems of these patents, the fine
tuning adjustment which is necessary with conventional types of
television receiver tuning systems has been substantially
eliminated. The system employed in the '953 patent permits utilization
of a frequency synthesizer tuning system which correctly tunes to a
desired television station or channel even if the transmitted
signals from that station are not precisely maintained at the
proper frequencies. The '535 patent is directed to a signal seek
tuning system adaptation of the frequency synthesizer tuning system
of the '953 patent which still permits implementation of all of
the desired wide-band pull in range of the frequency synthesizer
system of the '953 patent.
The systems of the foregoing
patents operate effectively to correct automatically for frequency
offsets in a frequency synthesizer tuning system without affecting
the operation of the conventional frequency synthesizer used in the
system. The systems of these patents are in widespread use
commercially and permit direct selection, with automatic fine
tuning adjustment, of any desired VHF channel which the viewer
wishes to observe. In addition, the signal seek adaptation disclosed
in the '535 patent couples all of the advantages of the frequency
synthesizer tuning system of the '953 patent with the desirability
of providing bidirectional signal seek operation.
While the
systems disclosed in the foregoing patents operate in a highly
satisfactory manner to accomplish the desired results of accurate
tuning without the necessity of fine tuning adjustments, the
circuitry for accomplishing the desired results is somewhat
complex. It is desirable to reduce the circuit complexity and the
number of signal detectors for accomplishing these results without
compromising the accuracy of operation of the system.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide an improved tuning system for a television receiver.
It
is an additional object of this invention to provide an improved
frequency synthesizer tuning system for a television receiver.
It
is another object of this invention to provide an improved
frequency synthesizer tuning system for a television receiver which
includes a provision for adjusting the synthesizer loop for
frequency offsets in the received signal with a minimum number of
signal detectors.
It
is a further object of this invention to tune the local RF
oscillator of a television receiver to the correct frequency for a
selected channel with a frequency synthesizer tuning system, and
automatically to change the reference frequency of the synthesizer
system, or adjust the count of a programmable divider that produces a
signal that divides the frequency of the local oscillator of the
tuner, if the AFT signal produced by the AFT frequency discriminator
of the receiver is outside a predetermined range corresponding to
correct tuning.
It is still another object of this invention
to provide an improved frequency synthesizer tuning system for a
television receiver which operates to adjust the synthesizer loop for
frequency offsets in the received signal over a relatively wide
pull in range in response to the output of the receiver frequency
discriminator by changing the division ratio of a programmable
frequency divider in the reference oscillator leg or local oscillator
leg of the synthesizer loop at a first relatively high rate from
an initial nominal value to a pre-established maximum in one
direction, and then resetting the division ratio to a second nominal
value once the maximum is reached and continuing to incrementally
change the division ratio in the same direction from the second
nominal value until a properly tuned condition is indicated by the
output of the receiver AFT frequency discriminator, followed by
control at a lower rate of operation to maintain tuning during
transmitting station drifts.
In accordance with a preferred
embodiment of this invention, the frequency synthesizer tuning
system for a television receiver includes a stable reference
oscillator and a voltage controlled local oscillator in the tuner. A
programmable frequency divider is connected between the output of
the reference oscillator and one input to a phase comparator, the
other input of which is supplied by the output of the local
oscillator. The output of the phase comparator then comprises a
control signal which is supplied to the local oscillator to control
the frequency of its operation.
A
counter circuit is connected to the programmable frequency divider
for initially setting the divider to a predetermined division
ratio upon selection of a desired channel by the viewer. The
counter then operates to change the programmable fraction of the
division ratio at a first relatively high rate in a direction
controlled by the output from the receiver picture carrier
discriminator in the absence of a predetermined signal output
derived from the discriminator. A control means causes the counter
circuit to count in this direction until it is determined that a
station is tuned or a predetermined maximum count is attained if no
station is correctly tuned, thereupon resetting the counter circuit
to a count which is a predetermined amount less than the maximum
predetermined count. Counting is continued in the same predetermined
direction from the new lesser count to continuously change the
programmable fraction of the frequency divider in accordance with
the state of operation of the counter.
The
high rate operation of the counter is terminated by the control
means in response to a predetermined signal from the output of the
discriminator, indicating that a station is correctly tuned, or after
a fixed time-out interval; so that the system automatically
adjusts for frequency offsets of the received signal which
otherwise would cause the station to be mistuned if a conventional
frequency synthesizer tuning system were used. After termination of
the high rate operation of the counter, it is switched to a lower
rate operation for maintaining tuning during transmitting station
drifts.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a television receiver employing a preferred embodiment of the invention;
FIG. 2 is a detailed block diagram of a portion of the circuit of the preferred embodiment shown in FIG. 1;
FIG. 3 is a detailed circuit diagram of a portion of a circuit shown in FIG. 1;
FIG. 4 is a flow chart of the control sequence of operation of the circuit shown in FIG. 1 and 2; and
FIG.
5 shows a waveform and time/frequency chart, respectively, useful
in explaining the operation of the circuit shown in FIGS. 1, 2 and
3.
DETAILED DESCRIPTION
Referring now to the drawings,
the same reference numbers are used throughout the several figures
to designate the same or similar components.
FIG. 1 is a block diagram of a television receiver, which may be a black and white or color television receiver. Mo
st
of the circuitry of this receiver is conventional, and for that
reason it has not been shown in FIG. 1. Added to the conventional
television receiver circuitry of FIG. 1, however, is a frequency
synthesizer tuning system, in accordance with a preferred embodiment
of the invention, which is capable of automatically changing the
reference frequency when a frequency offset exists in the received
signal for a particular channel.
Transmitted composite
television signals, either received over the air or distributed by
means of a master antenna TV distribution system, are received by an
antenna 10 or on antenna input terminals to the receiver. As is
well known, these composite signals include picture and sound
carrier components and synchronizing signal components, with the
composite signal applied to an RF and tuner stage 11 of the
receiver. The stage 11 includes the conventional RF amplifiers and
tuner sections of the receiver, including a VHF oscillator section
and a UHF oscillator section. Preferably, the UHF and VHF
oscillators are voltage controlled oscillators, the freuency of
operation of which are varied in response to a tuning voltage
applied to them to effect the desired tuning of the receiver.
The
output of the RF and tuner stages 11 is applied to the remainder
of the television receiver 14, which includes the IF amplifier
stages for supplying conventional picture (video) and sound IF
signals to the video and sound processing stages of the receiver 14.
The circuitry of the receiver 14 may be of any conventional type
used to separate, amplify and otherwise process the signals for
application to a cathode ray tube 16 and to a loudspeaker 17 which
reproduce the picture and sound components, respectively, of the
received signal.
The receiver 14 also includes a conventional
AFT or automatic fine tuning discriminator circuit and
additionally may include a synch separator circ
uit
for producing an output in response to the presence of vertical
synchronizatin pulses, a picture carrier detection circuit, and an
automatic gain control (AGC) amplifier. Outputs representative of
these sensor components are shown as being coupled over a group of
lead 20 to sensory circuitry 22, which in turn couples outputs
representative of the operation of these various sensor circuits to
a microprocessor unit 23 for controlling the operation of the
microprocessor unit.
The microprocessor unit 23 is utilized
in the system of FIG. 1 for controlling the operation of a
frequency synthesizer tuning system capable of automatic offset
correction. When the viewer desires to select a new channel, he
enters the desired channel number into a channel selection keyboard
25. There are a number of different keyboards which may be employed
to accomplish this function, and the particular design is not
important to this invention. The channel selector keyboard 25 also
may include switches or keys for initiating a signal seek function
in either the "up" or "down" direction.
Information
represented by the selection of channel numbers on the keyboard 25 is
supplied to the microprocessor unit 23 which provides output
signals over a corresponding set of leads 27 to the tuners (local
oscillators) 11 to effect the appropriate band switching control for
the tuners 11 in accordance with the particular channel which has
been selected. In addition, the keyboard 25, operating through the
microprocessor unit 23, provides output signals which operate a
channel number display 29 to provide an appropriate display of the
selected channel number to the viewer.
The
microprocessor M3870 unit 23 also processes the signals which are
used to operate the channel number display 29 through a
multiplexing circuit operation to decode the selected channel
number into a parallel encoded signal. This signal is applied to
corresponding inputs of the count-down counter or programmable
frequency divider 31 to cause the division number of the divider 31
to relate to the divided down frequency of the tuner local
oscillators connected to the input of the divider 31 through a
prescaler divider circuit 32 to the frequency of the reference
oscillator 34. Thus, the division number or division ratio of the
local oscillator frequency obtained from the output of the
programmable divider 31 is appropriately related to the frequency of
the reference crystal oscillator 34.
The
output of the oscillator 34 also is applied through a countdown
circuit or programmable frequency divider 35. Conventional frequency
synthesizer techniques are employed; and the microprocessor unit 23
automatically compensates, through appropriate code converter
circuitry, for the non-uniform channel spacing of the television
signals. It has been found most convenient to cause the programmable
frequency divider 31 to divide by numbers corresponding directly to
the oscillator frequency of the selected channel, for example, 101,
107, 113 . . . up to 931.
In accordance with the time
division multiplex operation of the microprocessor 23, the count of
the programmable frequency divider 35 initially is adjusted to a
fixed count by the application of appropriate output signals from
the microprocessor unit 23 to a point selected to be at or near the
mid-point of the operating range of the programmable frequency
divider 35. Thus, the output of the divider 35 is a stable
reference frequency (because the input is from the reference
crystal oscillator 34) which is used to establish initially and to
maintain tuning of the receiver to the selected channel.
The
output of the programmable divider 35 is applied to one of two
inputs of a phase comparator circuit 37. The other input to the
phase comparator circuit 37 is supplied from the selected one of
the VHF or UHF oscillators in the tuner stages 11 through the
programmable frequency divider 31. The phase comparator circuit 37
operates in a conventional manner to supply a DC tuning control
signal through a phase locked loop filter circuit 39 and over a
lead 40 to the oscillators in the tuner system 11 to change and
maintain their operating frequency.
With the exception of the
use of the microprocessor unit 23, the operation of the system
which has been described thus far is that of a relatively
conventional frequency synthesizer system incorporated into a
television receiver. This system is similar to the system of the
'953 patent. As in the system of that patent, the system shown in
FIG. 1, when the transmitted station or station received on a
master antenna distribution system provides the station or channel
signals at the proper frequency, operates as a relatively
conventional frequency synthesizer system. If, however, there is a
frequency offset in the received signal to cause the carrier of the
received signal to be displaced from the frequency which it should
have to some other frequency, it is possible that the system would
give the appearance of mistuning to the received station. The
microprocessor 23, operating in conjunction with the sensory
circuitry 22, is employed in conjunction with the countdown or
programmable frequency divider circuit 35 to eliminate this
disadvantage and still retain the advantages of frequency
synthesizer tuning.
Reference now should be made to FIG. 2 which shows details of t
he
interface between the keyboard 25, the microprocessor unit 23, and
the circuitry used in the frequency synthesizer portions of the
system. A commercially available microprocessor which has been used
for the microprocessor 23, and which forms the basis for the
diagramatic representation of the microprocessor in FIG. 2, is the
Matsushita Electronics Corporation MN1402 four-bit single-chip
microcomputer. This microcomputer has two, four-bit parallel input
ports labeled "A" and "B". In addition, three output ports, a five-bit
output port "C" and two four-bit output ports "D" and "E" are
provided. The internal configuration of the microcomputer 23 includes
an arithmetic logic unit (ALU), a read only memory (ROM) for
storing instructions and constants, and a random access memory
(RAM) used for data memory, arranged into four files, each file
containing 16 four-bit words. These words are selected by X and Y
registers and this memory is used, for example, for timers,
counters, etc., and also is used to hold intermediate results. To
facilitate an understanding of the operation of the system, a
portion of this memory is shown in FIG. 2 as a clock 81 and a
reversible counter 82 connected between the "B" input port and the
"D" output port. The microcomputer 23 is programmed to permit it to
operate in conjunction with the remainder of the circuits shown in
FIG. 2. The programming techniques are standard, and the
microcomputer 23 itself is a standard commercially available
circuit component.
There are several system parameters that
must be selected in the operation of the system shown in FIG. 2.
The selection of the nominal frequency of the two signals that feed
the phase comparator circuit 37 is an example. Channel selection is
provided by changing the frequency division ratio of the selector
counter 31 which divides the local oscillator signal after this
signal is passed through a prescaler circuit 32 and a divide-by-two
divider circuit 41. The nominal frequency from the programmable
frequency divider 31 (selector counter) is selected so that the
local oscillator (tuner) 11 can be set exactly on frequency for all
channels.
Since
the frequency divider 31 is able to divide only by integer
numbers, one distinct frequency possibility in the range of one KHz
is obtained, another in the range of two KHz, etc. A choice must
be made as to which of these values is optimum. Each value yields
the nominal frequency of all of the 82 channels by simply
multiplying by an appropriate integer for each channel. To simplify
the phase locked loop filtering problem by the filter 39, it is
desirable that the frequencies of the signals supplied to the phase
comparator 37 are as high as possible. This permits rapid
acquisition of a new channel along with a very clean DC control
signal to adjust the local oscillator. A trade-off for this,
however, must be made to permit fine tunning adjustment of the local
oscillator automatically to correctly tune in stations which are
off their assigned frequency, or to manually provide this feature,
if desired. The two-speed operation of the system in accordance
with the present invention allows a better trade-off to be made by
allowing rapid acquisition and then a slower speed for precise
tuning.
A compromise solution which is utilized in the circuit
of FIG. 2 is to cause the frequency division chain from the local
oscillator 11 in the tuner to the phase comparator 37 to be composed
of the fixed divide-by-256 prescaler 32, and a fixed divide-by-4
division, which is accomplished by the divider 41 at the input of
the counter 31 and a second divider 42 at the output of the counter
31. The variable frequency divider counter 31 then is loaded by
means of three latch circuits 44, 45 and 46 at an appropriate time
by the time division multiplex operation of the microcomputer 23
and a number that programs the programmable frequency divider
counter 31 to divide by the numerical value of the frequency of the
local oscillator in MHz for the channel selected. For example, if
the receiver is to be tuned to channel 2, which has a nominal local
oscillator frequency of 101 MHz, the programmable frequency
divider 31 is set to divide by 101. If the receiver is to be tuned
to channel 83, which has a nominal local oscillator frequency of
931 MHz, the programmable frequency divider 31 is set to divide by
931. In both cases, the variable divider 31 produces a 1 MHz
signal. However, because of the fixed divide-by-256 and the two
fixed divide-by-two dividers in series with the programmable divider
31, an output frequency of 976.5625 Hz is supplied from the output
of the divider 42 to the upper input of the phase comparator 37.
The
division ratio of the selector counter 31 is established by
appropriate output signals from the latch circuits 44, 45 and 46, as
mentioned above. The initial operation for changing, or maintaining,
the division ratio of the divider 31 is established by an entry of
the two digits of the selected channel number in the keyboard 25.
The microcomputer 23 operates as a time division multiplex system
for continuously monitoring the input ports and the output ports to
control the operation of the remainder of the system. The selection
of the two digits of the desired channel number is affected by a
time division multiplex iscanning of the outputs of the D output
port of microcomputer 23 and providing that information at the A
input port.
From
here the information is translated again to the D output ports to
the appropriate drivers of the channel number display circuit 29 and
to the latches 44, 45 and 46, and to a pair of similar four bit
latches 49 and 50 which control the divider ratio of the counter 35.
Although
the D output ports of the microcomputer 23 are connected in common
to all of these various portions of the circuit, the selection of
which of the latches are enabled to respond to the particular
output signals appearing on the D output ports at any given time is
effected through the C and E output ports of the microcomputer 23
in a time division multiplex fashion. A decoder circuit 52,
connected to the lowermost three outputs of the E output port of
the microcomputer 23, is used to apply unique decoding signals at
different times in the tim
e
division multiplex sequence of operation of the microcomputer 23
to the five latch circuits 44, 45, 46, 49 and 50, respectively. At
any given time in the sequence, only one of these latch circuits is
enabled for operation. A latch load signal is applied from the
upper output (EO3) at each cycle of operation of the signals
appearing on the E output port to set the latch circuit which is
enabled by the output of the decoding circuit 52 with the data
appearing on the other inputs to the latch circuit. This data
simultaneously appears on the four outputs of the D output port of
the microcomputer 23.
Thus, in rapid sequence, the latch
circuits 44, 45 and 46 are set to store the division number
corresponding to the selected channel entered onto the keyboard 25,
and the latch circuits 49 and 50 are each operated to set the
programmable divider reference counter 35 to a center or nominal
count, which is always the same upon the selection of a new channel
on the keyboard 25. Similarly, the two right-hand outputs of the C
output port (CO6 and CO5) enter the two digits of the selected
channel number in the drivers of the display circuit 29 at the
proper time in the binary encoded sequence when these digits appear
on the four-bit binary encoded representation of the D output
port. This results in a visual display of the channel number
selected.
In addition to the selection of a channel number
directly by the keyboard 25, the keyboard also may include an
additional switch 56, which is scanned in the time division multiplex
sequence to determine if the receiver is placed in a "seek" mode
of operation (when the signal seek capability is incorporated into
such a receiver). Operating in conjunction with the signal seek
switch 56 are a pair of "up" and "down" seek direction input
switches shown with a graphic representation of the seek directions
on the keyboard 25. A further provision is provided by two keys
labeled "U" and "D", which are used for "manual" fine tuning of the
receiver in the "up" or "down" directions depending upon which of
the two keys U or D has been operated. The keyboard 25 includes one
additional switch 58 which may be used to disable the automatic fine
tuning (AFT) portion of the circuit by rendering the microcomputer
insensitive to the signal output from the AFT circuit, in a manner
described more fully subsequently.
As is apparent from the
foregoing, the microcomputer 23 provides the intelligence, decision
making, and control for the system operation. It is a complete self
contained computer. The decisions or signal inputs upon which the
microcomputer 23 bases its operation include, in addition to the
inputs from the keyboard 25, inputs on sensory inputs into the B
input port and into the SNS1 and SNS0 inputs as shown in FIG. 2.
These input signals are used to provide an indication to the
microcomputer 23 of the presence or absence of a received signal;
and if the presence of such a signal is indicated, the inputs
provide a further indication of the accuracy of the tuning of the
receiver to that signal. If the system is being operated solely in a
manual mode of operation (AFT switch 58 open), the microcomputer
23 disregards all of this sensory information and tunes to the
frequency allocation of the channel selected in the manner described
above. The system will stay tuned to this condition, operating as a
conventional frequency synthesizer, whether or not a station is
present in the received signal.
When
the system is placed in its automatic mode of operation (similar
to the mode of operation of the above mentioned '953 patent), the
counter 82, integrally formed as part of the microcomputer 23,
continuously adds or subtracts one number at a time from the nominal
value or programmable division fraction entered into the programmable
frequency divider 35 at the outset of each new channel number
selection when frequency offset (mistuning) is present. The counter
82 is driven at a relatively high counting rate by clock pulses from
the clock 81 during this initial or forced search mode of
operation. Thus, automatic offset correction is provided for any
channel which is off its assigned frequency. The offset correction
automatically adjusts the frequency of the local oscillator by
changing the division ratio of the signal from the reference
oscillator 35 applied to the lower input of the phase comparator 37.
By doing this, the output of the phase comparator 37 applied to
the local oscillator 11 varies to cause the oscillator to be tuned
in the proper direction to compensate for the transmitting station
mistuning.
When the system is operating in its automatic mode
of operation, the microcomputer 23 responds to the sensor
information applied to it on its B input ports and on the S1 input
port shown in FIG. 2. These inputs are obtained from the various
outputs of the operational amplifiers shown connected to the
corresponding input ports in the detailed circuit of FIG. 3.
Depending upon whether the receiver is provided with a signal seek
feature or not, one or more of the sensory inputs of the circuit of
FIG. 3 are used. The s
ystem
shown in the drawings has a capability of correcting for frequency
offsets larger than 1.5 MHz on channels 2 and 7 and approximately 2
MHz on channels 6 and 13. The remainder of the channels have a
range between these two values.
If the receiver is not tuned
properly, the micromputer 23 executes the localized search of the
tuning range mentioned above. Since there is a necessary settling
down time for the tuning of a television receiver immediately
following selection of a new channel, a time interval of 250
milliseconds has been selected to prevent any localized search or
offset frequency correction until the expiration of this "settling
down" time period. If, at the end of this 250 millisecond time
interval, a properly tuned station is present, this is indicated by
the sensory outputs from the television receiver and no localized
search is effected to change the division ratio or programmable
divider count in the reference counter 35 for a system that also
has signal seek.
A system with no signal seek capability is
described later that requires less sensory input but which uses a
time period where a forced search is required directly after the
settling time interval.
Upon
termination of the 250 millisecond settling down period, the
microcomputer 23 is rendered responsive to the sensory input signals on
its sensory input signal ports. In the simplest form, only the
output of the frequency discriminator 60 (FIG. 3) applied to three
comparators 61, 62 and 63 is used to provide the necessary tuning
information to the microcomputer 23. The outputs of these comparators
are applied to the B12 and B11 inputs of the microcomputer.
The
comparator 61 simply is a conventional comparator for determining
whether or not the output of the frequency discriminator is
positive or negative, as indicated in the upper waveform of FIG. 5.
The comparators 62 and 63 are each adjusted with appropriate
reference input levels to provide a narrow window centered about
the center tuning frequency (fc) of the receiver. If the tuning of
the receiver, as indicated by the output of the frequency
discriminator 60, is outside this window on either side of the
central axis shown in FIG. 5, one output condition is indicated on
the input terminal B11 of the microcomputer. Only when the tuning
frequency is within the tuning window, indicative of a properly
tuned receiver, is the appropriate input applied to the
microcomputer input terminal B11. This input overrides any other
input that may be present on the input terminal B12 and is
indicative of a properly tuned receiver. The input from the
frequency discriminator 60, as applied to the microcomputer on its
input port B12, is used to determine the direction of operation of
the counter 82 of the microcomputer for the localized search count
signals applied to the latch circuits 49 and 50 to change the count
of the reference programmable divider counter 35 on a step-by-step
basis.
The lower graph of FIG. 5 plots the relative frequency
of the local oscillator 11 to the received signal frequency with
respect to time. The various arrows are used to indicate the manner
of operation of the counter 82 in the microcomputer 23 in
conjunction with the reference counter 35 for adjusting for any
mistuning conditions which may exist after the initial station
selection has been effected in the manner described above.
If
the receiver is properly tuned, the outputs from the comparators 62
and 63 of FIG. 3 which are combined together and applied to the
input port B11 of the microcomputer 23, provide an indication that
the tuning is within the properly tuned center frequency window. As
a consequence, no further operation of the microcomputer to change
any of the outputs applied to the latch circuits 49 and 50 for the
duration of this condition is effected. On the other hand, if the
receiver is mistuned on either side of the proper tuning frequency,
the various operating characteristics shown in FIG. 5 are effected.
Assume
initially that the receiver is capable of making tuning
adjustments over a range of fc plus Δf to fc minus Δf, as indicated
in the top waveform of FIG. 5. Three specific examples of
mistuning will then be considered. Initially, assume that the local
oscillator is mistuned relative to the received signal to a
frequency f1 as shown in the lower graph of FIG. 5. In this
condition, the outout of the frequency discriminator 60 is positive
since this signal frequency lies to the lefthand side of the
center or properly tuned region of operation of the discriminator.
Under this condition of the operation, the input signal applied to
the sensor port B12 of the microcomputer 23 is such that the
microcomputer counter 82 is caused to advance in a positive
direction to change the programmable division ratio or count of the
reference counter 35 in a manner to force the output of the phase
comparator 37 to adjust the frequency of the local oscillator until
the proper tuning indicated at point B in the lower graph of FIG. 5
is reached. The time interval for accomplishing this result is
measured from the upper end of the arrow representative of the
frequency f1 to the point B.
Now assume that the receiver
mistuning is to a frequency f2 which as shown in FIG. 5 as located
on the righthand-side of the center axis fc. In this condition, the
discriminator output is negative. This is reflected in the output
of the comparator 61 applied to the input port B12 of the
microcomputer 23. The polarity of this sign
al
is identified by the microcomputer 23 to cause the counter 82 in
it to operate in the reverse direction. As this count is applied on
a step-by-step basis through the latch circuits 49 and 50 to the
reference counter 35, the division ratio or count of the reference
counter (divider) 35 is changed. As a result, the reference
oscillator signal applied to the phase comparator 37 causes the
phase comparator 37 output to drive the local oscillator frequency
in a direction opposite to that considered in the first example.
This is shown by the vector interconnecting the top of the arrow
representative of f2 to point A on the time/frequency graph of FIG.
5.
As discussed in the general discussion above, whenever the
tuning frequency reaches the narrow window on either side of fc, the
outputs of the comparators 62 and 63 provide the necessary
indication on the sensory input port terminal B11 to cause
termination of the operation of the counter 82 in the microcomputer
23. Then the reference counter 35 remains set to the count attained
just prior to the appearance of this input signal on the input port
B11 of the microcomputer 23.
A third mistuning condition can
exist, and ordinarily this condition results in an ambiguity which
cannot be corrected simply by responding to the signal polarity at
the output of the frequency discriminator. This is indicated by
the mistuned condition where the difference between the local
oscillator frequency f3 and the transmitter frequency is such that
the signal f3 lies in the range to the right of the negative
portion of the discriminator output shown in the upper waveform of
FIG. 5. In this condition, the associated sound causes the
discriminator output to be positive; so that the television
receiver normally would attempt to tune toward the next adjacent
channel and away from the properly tuned center frequency of the
channel which is desired. The output of the discriminator 60 in
this situation is the same as it was in th
e
first example considered for frequency f1; so that the counter 82
of the microprocessor 23 operates to change the count in the
reference counter 35 in a manner to cause the local oscillator
frequency to go higher toward a frequency f3 +Δf, as shown in FIG.
5.
A predetermined number of counts of the counter 82 in the
microcomputer 23 are necessary for the microcomputer to count
through the frequency range Δf, and this range is selected to be
within the pull in or operating range of the system. Once this count
has been attained, the microcomputer counter 82 immediately is
reset back to a count which corresponds to a frequency 2 Δf lower
than the frequency attained by the maximum count. This is indicated
in FIG. 5 by the frequency f3-Δf. Because the microcomputer counter
82 is limited to counting a number of counts equal to Δf, this new
frequency now is on the lefthand side of the center line fc, shown
in both waveforms of FIG. 5. This places the local oscillator
frequency at a point such that the frequency discriminator output is
the positive output shown on the lefthand-side of the upper
waveform of FIG. 5. Counting continues in the same direction as
previously. This time, however, it is in a proper direction to bring
about correct tuning; and when the center frequency is reached,
the output of the comparators 62 and 63 cause the microcomputer 23
to stop its count. The proper tuning point attained is indicated at
point C on the graph of the lower part of FIG. 5.
Because
the counter 82 of the microcomputer is limited to a maximum count
equivalent to Δf above its initial count and thereupon is reset to a
new count equivalent to 2 Δf lower than the maximum count, it is
not necessary to utilize any other sensory inputs in order to
properly tune the receiver over a wide pull in range (as much as
plus or minus 2 MHz). Only the output of the conventional frequency
discriminator 60 is used to provide the necessary sensory inputs.
The
counter 82 of the microcomputer 23 is operated by the clock 81
during the foregoing sequence of operation, immediately following the
selection of a new channel by the operation of the keyboard 25, at
a fast or high speed operation. Typically, the counter steps are
10 milliseconds per step; so that there are no initial visual
effects which can be noticed by an observer of the television
screen of the receiver being tuned. The maximum forced search
period is approximately 900 milliseconds in duration. At the end of
this time interval, a timer in the microcomputer 23 causes a
signal to be applied through the outputs of the E output port to
the decoder circuit 52 indicative of the completion of this time
interval. The decoder 52 then applies a pulse on an output lead
connected to the B13 input of the B input port of the microcomputer
23. This pulse is sensed by the microcomputer 23 and is applied to
the clock 81 to change the clock rate to a much slower rate,
approximately one-third (1/3) or one-fourth (1/4) the rate used
previously during the forced search mode of operation. This then
permits the system to accomodate station drifts which normally
occur at a very slow rate during the transmission and reception of a
television signal. As a consequence, it is possible to use more
filtering in the filter 39 on the tuning line (FIG. 1) and employ a
smaller frequency window for the channel verification sensed by
the circuitry shown in FIG. 3.
The
result is a more precise tuning from the receiver than is
otherwise possible if only a high speed operation of the clock 81
is utilized.
When the channel once again is changed by
operation of the keys in the keyboard 25 or operation of the
channel selection circuitry from a remote control unit, this new
channel input is sensed by the microcomputer 23 from the signals
applied to the A input port and the clock 81 is reset to its fast
time or the forced search mode of operation; and the process
resumes.
Instead of employing an additional decoding function
in the decoder 52, a separate decoder also could be connected to
the outputs of the D output ports to feed back the signal to the
B13 input terminal of the B input port of the microcomputer 23. The
operation of the system to change the rate or frequency of the
pulses applied by the clock 81 to the counter 82 otherwise is the
same as described above.
Although applicant has found that it
is preferable to correct for mistuning or frequency offsets by
adjusting the count or division ratio of the counter 35, such
offset adjustments also could be effected by adjusting the count in
the counter 31 in the local oscillator signal line. The operation in
such a case is the same as described above for adjusting the count
in the counter 35.
If the receiver is to be used with an
automatic signal seek mode of operation, however, additional sensory
inputs are necessary. These inputs operate in conjunction with the
output of the frequency discriminator 60. The operation of the
microcomputer 23 in controlling the count of the reference
programmable frequency counter divider 35 is the same as described
above. The additional sensory inputs simply are used in conjunction
with the outputs of the comparators 62 and 63 to signal the
microcomputer 23 to assure that tuning is to a picture channel
rather than an adjacent sound channel. This is accomplished by
utilizing the output of the synchronizing signal separator 65 which
is applied to a comparator 67 to produce an output signal to the
SNS1 sensory input of the microcomputer 23 only when vertical
synchronizing signal components are present.
In addition, the
output of a picture carrier detector 69 is applied to the input of a
comparator 70 to produce an output to the B10 sensory input of the
microcomputer 23. If the picture carrier detector 69 is producing
an output indicative of the presence of a carrier, but no output is
being obtained from the vertical synch separator 65 at the same
time, the system is mistuned to a sound carrier and the
microcomputer 23 is permitted to continue its localized search until a
properly tuned station is found. Only when there is coincidence of
signals from the picture carrier detector 69, the synch signal
separator 65, and the automatic frequency discriminator window as
determined by the comparators 62 and 63, is the microcomputer
operation terminated to indicate that a properly tuned channel is
present.
Further insurance of tuning the receiver only to a
strong signal also can be provided by the addition of an AGC
amplifier 72. This is connected to a comparator 74 coupled to the
B10 input port along with the output of the picture carrier
detector comparator 70. When the AGC amplifier 72 is used as a
sensory input, the microcomputer operation, when the system is used
in a signal seek mode, is only terminated to indicate reception of
a valid signal when that signal is strong enough to produce the
desired output from the comparator 74. The signal level which is
acceptable is set by a potentiometer 75.
It should be noted
that when the system is operated in a signal seek mode, the sensory
inputs must indicate the reception of a properly tuned signal
within a pre-established time period. If no signal is sensed by the
various sensory input circuits operating in conjunction with one
another as described above, the microcomputer 23 automatically steps
to the next channel number and repeats the sequence of operation
described above. This is when it is placed in its signal seek mode of
operation. If signal seek is not employed, the additional sensory
circuits 65, 69 and 72 are not necessary, and the inputs to the
microcomputer which are provided from these sensory circuits are not
utilized. The sensory signal input which is used both for a
receiver without a signal seek capability of operation and for a
receiver which has a signal seek mode of operation in it, is the
output of the frequency discriminator 60 operating in conjunction
with the comparators 61, 62 and 63 as described above.
As
indicated above, the wideband method of tuning precisely to an
incoming signal that is at the wrong frequency described here only
needs the frequency discriminator sensory information. The method
that uses the additional sensors described above is needed to make
this system operate compatibly with signal seek but it is not
restricted to seek operation.
For
a system that does not use signal seek operation, only the
frequency discriminator sensory input is required for proper
operation. The discriminator 60 is used for both fine tuning
direction information and to produce a frequency window to indicate
the presence of a correctly tuned station (channel verification).
Initially, after a channel change, there is a 250 millisecond
settling time, the same as the operation described above with
compatible seek. After that, however, comes a period of time where a
forced localized search is produced by the microcomputer 23. The
forced search is needed to insure that the system will correctly
tune to stations that initially may be tuned to the undesired zero
voltage crossover in the right half of the upper curve of FIG. 5.
Such signals may be within the frequency window of the discriminator
60; and if a search is not forced, this system will not correctly
tune. The compatible seek system described previously correctly
tunes the local oscillator without a forced search, because the
picture carrier detector and vertical detector do not give an output
for this situation and the system automatically goes into its search
mode of operation. However, the non-seek system does not have a
picture carrier sensor input and must be forced to search for an
initial period of time sufficient to allow the system to tune up to
its maximum frequency and then reset (loop) back to a frequency of 2
Δf lower. Then it is tuned to the positive left half portion of
the discriminator curve (FIG. 5) and the frequency window created
by the discriminator 60 is sufficient to insure proper tuning. If
the discriminator output produced by the desired incoming signal
created an initial situation that produces the correct tuning
direction information, i.e., in the left half of the curve of FIG.
5, or in the right half portion that gives the correct direction
and
frequency
window information, the forced search would not be needed.
However, the forced search will produce a correct tuning situation
anyway. In these cases, the tuning either is correct to begin with
or correct tuning is reached quickly. Then, even though the forced
search is active, it simply alternates up and down through the
correct tuning point because each time the receiver is tuned a little
high in frequency, it produces a negative output from the
discriminator 60; and the tuning direction signal causes the system
to tune down in frequency.
Then,
a positive discriminator output is produced, and the system tunes
up in frequency. This continues until the forced search is removed
by time-out of the microcomputer 23 (a fraction of a second). At
such time, the receiver is correctly tuned by the frequency window of
the discriminator to be very near fc. The system cannot tune to
the undesired discriminator crossover shown in the right half
portion of FIG. 5 because the polarity of the tuning direction
signal always causes it to tune away from that point.
The
fast time or forced search operation of the system can be
terminated in a different way other than the preestablished
time-out period described above in conjunction with the operation
of the circuit shown in FIG. 2. Generally, it is desirable to build
into the system (or program into the system by means of software)
such a maximum time-out period to effect the operation which has
been described above to terminate the search and cause the clock 81
thereafter to operate in a low speed mode of operation. Termination
also can be accomplished by sensing the number of changes in the
direction sensor input applied to the B12 terminal of the B input
port to cause the search to be terminated when this direction
changes three times (or more). By doing this, any flicker that
might be observed on the screen of the television receiver is
minimized, since the forced search still takes place at the high
rate of application of clock pulses from the clock 81 to the
counter 82 in the same manner described above.
Termination
of the search, however, also may be effected by means of a search
terminate counter 78 (FIG. 3), which is advanced by pulses applied
to it each time the output of the comparator 61 changes its sign
(indicative of a change in direction for the counter 82) as applied
to it through the B12 input port, as described earlier. After three
of these changes, or some other number if desired, an output pulse
is obtained from the search terminate counter 78 and is applied to
the SNS0 input of the microcomputer 23. This causes the operation
of the clock 81 to be switched to its low speed mode of operation
to terminate the fast or "forced search" mode of operation. The
next time a new channel number is entered on the keyboard 25, a
reset pulse is applied to the search terminate counter 78 to reset
it to its original or zero count, thereby readying it for another
sequence of operation. It is apparent that the search terminate
counter 78 may not always be operated to terminate the count, since
the time-out interval which is sensed by the decode circuit 52 and
applied to the B13 input port of the microcomputer 23 may occur
before there are three changes of direction of the search. In any
event, the next time a new channel number is entered into the
keyboard 25, the search terminate counter 78 is reset; so that it
is irrelevant whether this counter reaches a full count or not to
effect the termination of the forced search operation of the
system.
FIG. 4 shows the control sequence of the system which
is stored in the ROM (Read Only Memory) of the microcomputer 23.
The microcomputer 23 operates by always running through the flow
sequence, via loops L1, L2 and L3. Loop L1 corresponds to a new
channel selection by two digit number entry. Loop L2 corresponds to
channel number increment or decrement by an up or down key
operation, respectively, or by seek operation. Loop L3 corresponds
to fine tuning, either manual or automatic. To obtain exact timing for
system control, the microcomputer 23 receives a standard timing
pulse from the output of the reference counter 35 divided in a
divide-by-five counter 80 and applied to the A13 input port of the
microcomputer 23. The control functions which are programmed into
the microcomputer 23, as indicated in the flow chart of FIG. 4, are
outlined in the following paragraphs.
Channel Number
Correction: An invalid two digit channel number entry (0, 1, 84,
99) is corrected. When the operation of the receiver is in the
signal seek mode, the next channel up from 83 is channel 2, and the
next lower channel from channel 2 is 83.
PLL
Control I: For a given channel number, a corresponding binary code
for the PLL selector counter 31 is derived as described
previously. For UHF channels, the local oscillator frequency
separation between two adjacent channels is 6 MHz and the code for
PLL is generated by the microcomputer 23 through means of a simple
calculation. This code then is transferred from the microcomputer
23 to the latches 44, 45 and 46 as described previously.
PLL
Control II: This routine of the microcomputer 23 is used to
transfer the fine tuning data to the latches 49 and 50 which
control the count of the reference counter 35 in the PLL circuit.
Channel
Number Display: The channel number is transferred from the
microcomputer 23 to the driver latches of the display driver
circuit 29.
Key Input Detection: The keyboard is arranged as
the matrix circuit shown in FIG. 2. ROM programming for scanning
and acknowledging a keyboard entry only after successive
indications provides protection against false entry due to contact
bounce. The four data output lines of the D output port of the
microcomputer 23 are used to transfer data to the phase lock loop
section of the circuit and to the display circuit 29, as well as for
scanning the keyboard matrix circuit.
Time Count: The
microcomputer 23 receives a basic timing pulse of approximately 200 Hz
from the output of the divider 80 and performs various controls for
each timing pulse. By way of example, sensing for the vertical
synch input (when the system is used with a signal seek capability)
on the input port SNS1 takes place every 2.5 milliseconds.
Automatic seek timing is selected to be 133 milliseconds for UHF
channels. All of these timing pulses are derived from the basic
synchronization timing pulse applied to the microcomputer on the
A13 input port from the output of the divider 80. Various other
timing values used in the microcomputer to properly time multiplex
sequence the operation are derived from this basic timing pulse.
Sensor
Input Detection: As described previously, the output of the
comparators shown in FIG. 3 reflect the status of the tuning of the
television receiver. If no signal seek mode of operation is used,
only the frequency discriminator or AFT discriminator 60 is
necessary. When a system is being used in a signal seek mode, a
proper television signal receipt is indicated by the presence of a
vertical synch signal at the output of the synch signal separator
65 and corresponding outputs are applied to the input leads B10 and
B11 (high level input signals) indicative of tuning to the
"correct tuned" frequency discriminator window and reception of a
picture carrier. As stated previously, the signal present on the B12
input lead is used to determine the direction of tuning when the
receiver is operated in its automatic mode.
Mode Detection: The
status of the seek and automatic/manual (A/M) switches are
detected. If the A/M switch (not shown) is in its automatic
position, automatic seek and offset correction are active. If only
the seek switch is on, only seek is performed. If the A/M switch is
in manual, manual fine tuning (MFT) is active.
Automatic
Mode: If the TV receiver is not properly tuned for VHF channels in
automatic, the local oscillator frequency is shifted automatically
toward proper tuning. The fine tuning data is generated in the
microcomputer 23 and is transferred to the latches 49 and 50 for the
reference counter 35 in the PLL circuit.
Manual Fine Tuning
(MFT) Control: The local oscillator frequency is shifted by pushing
the fine tuning up (U) or down (D) pushbutton or switch. This MFT
control can be applied to VHF channels as well as to UHF channels.
Channel
Up/Down: When a channel up (upward pointing arrow) or down
(downward pointing arrow) key closure in the keyboard 25 is
detected, or upon a direct access to an unused channel, this routine
is activated and the system will advance to the next channel in
the selected direction.
The foregoing embodiment of the
invention which has been described above and which is illustrated in
the drawings is to be considered illustrative of the inventi
on,
which is not limited to the specific embodiment selected for this
purpose. For example, hard-wired logic could be used to achieve the
various circuit operations which are accomplished by the
microcomputer 23 in conjunction with the other portions of the
system. The relative ease of programming and debugging the
microcomputer 23, however, make it much simpler to implement the
system operation with the microcomputer than with hard-wired logic.
With respect to the sensor circuit inputs to the system, an added
degree of operating assurance can be provided by the addition of a
sound carrier sensor in addition to the picture carrier sensor shown
in FIG. 3. If this feature is desired, the output of the
comparator for the sound carrier is combined with the outputs of
the comparators 70 and 74 at the input terminal B10 of the B input
port of the microcomputer 23. Because of the manner of the circut
operation which has been described previously, however, the addition
of a sound carrier detector to the system is not considered
necessary, even for a system operating in the signal seek mode of
operation. This is in contrast to conventional television receivers
having a signal seek operation, in which detection of the sound
carrier generally is a necessity to insure that mistuning of the
receiver to an adjacent sound carrier does not take place.
