Note: Descriptions are shown in the official language in which they were submitted.
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CHARACTERIZED FAST TUNING
CONTROL FOR A TELEVISION SYSTEM
The present invention relates to television
systems and more parti~ularly to fast tuning subsystems
for switching channels in a television system. Still
more particularly it relates to such subsystems in which
a selected substitute television signal in a substitute
channel can be substituted indistinguishably for one or
more normal television signals in respective normal
channels, as for market research purposes.
Back~round of the Invention
Marketing research techniques have been
daveloped in which a substitute television signal in a
substitute channel, containing a commercial the
effectiveness of which is to be assessed, is substituted
for a normal television signal in a normal channel in
homes of selected test viewers so that the effectiveness
of the commercial can bP evaluatPd. This allows the
promoter of a service or product to assess the reaction
of a small, demographically controlled panel ~f test
viewers before the wide airing of a commercial which may
prove ineffective.
One example of such a television signal
substitution system is disclosed in United States Patent
No. 4,404,589. As there disclosed, substitute
television program signals are transmitted in at least
one substitute channal along with signal substitution
control signals. A control box or terminal at each test
viewer receiver responds to the signal substitution
: 30 control signals by selectively switching to a substitute
television program from a normal program. The signal
substitution control signals include a numbçr of
; different terminal command si~nals and a number ~f
di~ferent event command signalsO Each of the terminal
command signals incllldes a respective test viewer
address ignal for identifying a respective test viewer
receiver and a number o~ event identification signals .
: identi~ying respective signal substitution events in
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which this terminal is to participate. Each of the
event command signals includes a respectiYe event
address signal corresponding to a respective event, an
appropriate substitution control command, a substitute
channel identification signal, and one or more normal
channel identification signals for identifying the
normal channels ~rom which the receiver is to ~e
switched. The current event command signals correspond-
ing to each allowable event address are stored in the
terminal for later correlation with the terminal's
participation event list and wi~h the viewer'~ selected
channel signal. When the viewer selected channel
corresponds to a normal channel identification signal
associated with a current event command whose event
address signal corresponds to an event in which the
respective terminal is to participate, the substitute
channel is substituted for the channel selected by the
viewer for a period determined by the event command
signals. Subsequent responses to the events, such as
purchases, of the respective vi~wers are then
individually tabulated and analyzed against the
responses of viewers receiving the normal signals.
When a viewer changes channels on a modern
television receiver, the channel change is carried out
in, for example, about a quarter of a second. The
change is accompanied by momentary disruption of the
picture and a sound pop or a period of sound muting.
When a market research company causes a channel
substitution, it is desirable that the substitution be
carried out so quickly and uno~trusively a~ to be
imperceptible to the normal test viewer. If the
substitution were distinguishable, it ~ould, at least
subconsciously, influence the response o~ the test
viewer to the commercial. That is, were the viewer to
know or suspect he was receiving a test commercial, he
might react in a manner in which he believes he is
expected to re~ct, rather than acting normally, skewing
the test results from normal response. Therefore, it is
1 315901
desirable that the tuning be accomplished extremely
rapidly so as to be indistinguishable. More
specifically, the transition time between channels
should be kept well within about 60 microseconds to
prevent an a~dible pop due to loss of the television
signal intercarrier frPquency modulated with the sound
subcarrier. The normal and substitute channel tuning
should bP very accurately matched to ensure no shift in
picture quality, particularly that o~ the chroma
signal. The transition should be timed to occur during
the vertical blanking interval between picture fields so
that the change is not seen by the viewer.
Switching channels may require a large
frequency change in the tuner. For example, if ~he
normal channel is a low YHF channel ~wherein Channel 2
has a video carrier frequency of 55.25 MHz) and the
substitute channel is a high UHF channel (wherein
Channel 70 has a video carrier frequency of 807.25 MHz),
the tuner might have to slew through more than 700 MHz.
The vertical blanking interval of standard NTSC video,
during which the substitution is to be effected, takes
1.3 milliseconds. The factor that is most critical in
making the substitution indistinguishable is the sound.
The audio stage of the television receiver is not tuned
to the sound carrier, but is tuned to the 4~5 MHz
intercarrier beat frequency generated between the video
carrier and the sound carrier in each VHF and UHF
channel. When the tuner of the receiver tunes between
channels, the intercarrier beat frequency disappears
because both the video and sound carriers are no longer
simultaneously present in the IF pass band. When the
audio stage of the television receiver has no signal
applied, its internal limiter amplifier will amplify
noise up to an audible ampli~ude level. This causes the
pop heard during viewer controlled channel rhanging.
This presents no problem when the viewer changes
channels, for it to be expected. However, if an audible
pop were produced during siynal substitution, it wsuld
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alert the viewPr to the fact of substitution.
In order to avoid the effect of noise during
signal substitution, the channel change must be
sufficiently fast that the human ear cannot distinguish
it. The total energy of a noise burst is the integral
of power over time, but the human ear is essentially
logarithmic in perception and can hear extremely low
energy noise pulses. To make the noise attendant a
channel change unobtrusive, the change should be
accomplished in less than about 60 microsecond~. Not
only is relatively fa~t tuning required, but al~o the
tuning must be relatively accurate to recover the 4.5
MHz intercarrier beat. Due to the close proximity of
the sound carrier o~ an adjacent channel to the video
carrier o~ a substitute channel, a maximum error of
about + 500 KHz is requir~d ~or both the video and sound
subcarriers of the substitute channel to be within the
pass band.
Previous signal substitution systems have
employed a cable television distribution system with a
control box for channel switching located at each test
viewer's home. ~hese systems have employed a fast
electronic tuner having a voltage controlled oscillator
whose output frequency determined the channel to which
the tuner was tuned. A voltage divider network estab-
lished predicted tuning voltages n~cessary to cause the
local oscillator to translate each individual channel's
frequency to that o~ at least one channel of the tele-
vision receiver. The tuner was made to select ~
particular channel very quickly by jamming the appro-
priate control voltage into the local o~cillator causing
it to slew rapidly to the n~w ~requency. This is ~nown
as jam tuning. Thus, by directing an electronic switch
in the local ~scillator control circuit to change ~rom a
normal channel voltage to the su~stitute channel volt-
age, a rapid substitution could be made, This prior art
tuner controller system was predictive in nature in that
the channel tuning control voltages corresponding to the
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desired input channels were determined by testing prior
to or during installation of the control box at the home
0f the test viewer. A problem encountered was that with
time the correct tuning voltages tended to drift.
Drifting resulted in frequency errors which
caused loss of picture definition, and color hue or
saturation changes. The automatic fine tuning circuitry
in the tele~ision set of the test viewer might correct
the error, but it would correct the error in a visible
manner due to its slow operating speed. With time) the
driftin~ became so extreme as to require that the
control boxes be removed from test view~r homes for
recalibration.
Brief Summarv of the Invention
The present invention provides an improved fast
tuning subsystem combining accurate jam tuning with
feedback to take care of long term drift. A phase--
locked loop feedback system samples the output of the
local oscillator in the tuner to determine if a frequen-
cy error be present. The frequencies of the ~hannels
are generally very accurately maintained, but signifi-
cant frequency error could arise from the tuner in the
control box. If such an error were present, the phase
detector would provide an error signal for combination
with the predicted voltaqe signal and application of the
resultant voltage signal to the voltage controlled local
oscillator o~ the tuner, thereby causing the tuner to
provide the desired frequency output. In this control-
ler system, the predicted or characterization voltages
for each frequency is stored in a look-up table in a
memory associated with a ~iCrOprOceSSQr. Each of the
predicted voltag~ signals is pr~determined for the
respective channels for a particular tuner to be s~bstan-
tially equal at the time of characterization to the volt-
age applied to the voltage controlled oscillator that
provides a null tuning error signal~ In the predictive,
feed forward portion of the control operation, the micro-
processor, upon selection of a substitute channel,
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applies a signal representative of that characterization
voltage to a precision digital to analog converter which
applies its analog output to the voltage controlled
oscillator. With time, due to aging of the components,
the voltage values for the various ~requencies stored in
the look-up tab~e become somewhat err~neous. This
results in the voltages applied during the feed ~orward
moda becoming incorrect for the various channels, and
the relatively slow operating phase-locked loop adding a
corrective offset to assure accurate tuning to the
desired frequency. The response time of the feedback
mode is made substantially slower than that of the
predictive mode so as to decouple ~he one control from
the other.
Accsrding to one aspect o~ the invention a fast
tuning subsystem is used in switching from a current
channel to a selected channel of a television system.
The tuning su~system includes a tuner for selecting a
channel to be received, the tuner including a voltage
controlled oscillator having a control input for
determining, in response to voltage applied to its
control input, the channel ~requency the tuner
selectively receives. Predictive means supplies a
selected predicted voltage signal to the control input
of said oscillator, the predictive means including an
associated memory in which is stored siynals
corresponding to respective voltage signals predicted
for applying ~o the oscillator to tune the tuner to
receive the channel frequencies of the respective
channels. The oscillator is responsive to a selected
predicted voltage signal to slew the channel frequency
at a rapid ~lew rate to the ~requency corresponding to
said selected predicted voltage signal. An error
detector generates a tuning error signal indicative of
the frequency error o~ the tuner off the frequency for
the selected channel. A feedback circuit combines the
tuning error signal with the selected predicted voltag~
signal to provide a control signal at the control input
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for adjusting the channel frequency to reduce khe tuning
error. The operation of the predictive means is
substantially decoupled from the operation of the
feedback circuit to prevent transients in the slew mode
of the predictive means from ~ubstantially perturbing
the feedback mod~.
~ t is an aspect of the invention that the
channel frequency be slewed su~ficiently fast that the
total spurious intercarrier sound energy detected during
the slew mode be sufficiently small as to be inaudible
to a viewer, and where transients occasioned by
operation o the predictive means do not perturb the
operation of the feedback circuit so as to cause
transient visible de~radation of picture as observed by
a viewer.
In another aspect the response time of the
feedback circuit is made sufficiently slower than the
response time of the predictive means as to result in
substantial decoupling. Preferably, the response time
of the feedback circuit is at least four orders of
magnitude slower than the response time of the
predictive means. In another aspect the error detector
means includes means for limiting the error signal in
magnitude during the interchannel slewing, preferably a
phase-error detector limiting the error signal to +
radians during interchannel slewing.
According to another aspect, the error detector
compares the frequency of the voltage controlled
; oscillator with the reference to generate the tuning
error signal. According to another aspect, such an
~ error detector comprises a reference oscillator for
i generating a referance signal at a frequency that is l/N
ti~es the frequency of the voltage controlled oscillator
at the frequency at which the tuner selectively receives
respecti~e channels, where N is a different respective
integer for each channelO The error detector further
includes divide by N means for dividing the output
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frequency of the voltage controlled oscillator by ~ and
means for comparing the divided by N signal with the
reference signal to produce the tuning error signal~
Preferably, the means for decoupling includes a loop
filter in circuit with the feedback means.
Another important aspect is the characterizing
of the predictive control signal. This must be done
accurately and rapidly in the course of manufacture or
the cost of characterization of a large number of
channels could well become excessive. According to the
present invention, the control signals for each tuner
are characterized by, for each channel, applying a
respectiYe preliminary control voltage signal estimate
to the respective control input, utilizing the resulting
tuning error signal from the error detecting means to
determin~ the control voltage signal at the control
input so as to reduce the tuning error signal to less
than a predetermined magnitude, and storing in the
memory means a signal corresponding to the determined
control voltage as a signal corresponding to a
respective predicted control voltage signal.
In another aspect the error detector and the
feedback means comprise ~ phase-locked loop generating a
tuning error signal indicative of phase error, the
derivative of the tuning error ~i~nal is utilized to
determine successive incremental updates to the
preliminary control voltage signal so as to reduce the
tuning error ~ignal to less than a predete~mined
magnitude, and a signal corresponding to the then
characteriz~d preli~inary control voltage signal is
~tored in memory as corresponding to the determined
control voltage signal.
In anoth2r aspect the determination of the
characterized con~rol voltage signal is made for each
channel by applying a respective preliminary control
volta~e signal to the control input, utilizing the
resulting tuning error signal to modify the preliminary
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control voltage signal by a predetermined amount to
produce a modified preliminary control voltage signal,
applying the modified preliminary control voltage signal
to the control input, and repeating the steps until th~
resulting tuning error signal is less than the
predetermined magnitude.
In another aspect each predetermined amount is
in acc~rdance with a binary search process wherein
successive predetermined amounts decrease by half. In
another aspect each predetermined amount is
substanti~lly the same in accordance with a linear
search process. In 5till another aspect each
pr~determined amount is in accordance with a binary
search process for a nu-mber of steps wherein successive
predetormin d amounts decrease by half and is thereafter
in accordance with a linear ~earch process wher~in each
said predetermined amount is substantially the same~
Other aspects, objects and advantages of the
invention will become apparent from the following
detailed description, particularly when taken in
conjunction with the accompanying drawings.
Brief Description of the Drawinas
FIG. 1 is a diagram, partly schematic and
partly bl~ck in nature, illustrating a prior art fast
tuning subsystem having predictive tuning voltage
control;
FIG. 2 is a block diagram illustrating a f~st
tuning subsystem according to the present invention
having not only predictive tuning voltage control but
al o a slowly operating phase-lo~ked 1OGP for improYing
the accuracy of the tuned freguency over time; and
FIG. 3 is a block diagram illustrating the
characterizing of the subsystem in the course of its
production.
Corresponding reference characters indicate
corresponding components throughout the several figures.
Detailed Description of the Preferred Embodiments
A control box is used in a television system
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for switching between substitute channels and normal
channels. The control box is located, for example, at
the home of each test viewer and provides a signal input
on one of the channels selectable by the televisisn
receiver ~f the test viewer. Each box is under the
c~ntrol of a remote superYiSory controller which
selects, for example, commercials in a substitute
channel for insertion into a selected norMal channel for
the purpose of testing the effectiveness of each
commercial using an appr~priately selected panel of
viewers. An example of a television ~ystem with
multi-event signal substitution is shown and discussed
in the above-mentioned United States Patent No.
4,~04,589.
As mentioned above, it is desirable that a test
viewer not know when a substitute commercial is inserted
in place of a commercial in a normal television
channel. The jud~ment of the test viewer could be
influenced, if only subconsciously, if the substitution
were visually or aurally distinguisha~le. one way the
test viewer could distinguish a substitution is
interference with or disruption of the picture or
sound. Such interference typically occurs when a
television viewer changes normal channels because the
electronic tuner of a conventional television set
usually takes a fraction o~ a second to tune to the
chosen channel. During this time the 4.5 M~z
intercarrier sound signal disappears resulting in a
popping noise because the limiter amplifier in the audio
FM receiver of the television set amplifies random
thermal noise and intervening spurious signals up to the
full amplitude levelO One aspect of the present
invention is the ~bility to switch between channals so
: swiftly that the act of signal substitution is
~ 35 indistinguishable by the test viewer.
: A previously used predictive tuning system is
shown in FIG. 1. A control box 100 as previously
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employ~d in fast tuning a cable system is th~re shown.
A voltage divider 102 is formed by a number of
potentiometers 104 each of whi~h is factory calibrated
for the estimated tuning voltag~ of a respective one of
the normal and substitute channels (shown as normal
channels 2-13 and substitutP channels A and B,
respectively). A switch SW3 permits the t~st ~iewer to
select a normal channel. ~n electronic switch SW4 is
under the control o~ a signal from a substitute conkrol
circuit 106 as provided by way of a data receiver 107
which extracts the control signals from signals r~ceived
over a cable 108 from a testing facility. The switch
SW4 switches among the viewer selected normal ~hannel
and the two substitute channels to apply the respective
estimated control voltage to a tuner or frequency
converter 110. The tuner 110 includes a local
oscillator (L.0) 112 and a mixer 114O The local
oscillat~r 112 is a voltage controlled oscillator
oscillating at a frequency determined ~y the control
voltage applied to its control input terminal ~rom the
switch SW4. The output of the local oscillator 112 is
mixed in the mixer 114 with the signal on the cable 1~8
to produce beat frequencies. A beat frequency for the
channel to be detected is at an intermediat frequency
which is later converted by a second converter 115 to
the frequency to which the viewer's television set 116
is tuned, such as that of a channel not used in the
local broadcast area, usually channel 3 or channel 4.
Upon operation of the switch SW4 during a vertical
blanking interval, the predetermined control voltage for
tuning to the desired channel is jammed into the tuner
110. Among the shortcomings ~f ~his system are that
potentiometers are difficult to adjust precisely during
factory calibration, and the system sffers no pr~vision
for compensation for drift due to the aging of
compone~ts. With time, the freguency errors o~ this
predicti~e system became so intolerable that the various
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channels could not be properly tuned, necessitating the
return of the control boxes 100 to the factory for
recalibration.
A fast tuning control subsystem according to
the present invention is illustrated in FIG. 2. The
subsystem includes a control box 120 ~or receiving
signals over a signal transmission medium, which
includes a cable 122 in this embodiment. The signals
are received by a fast tuner or frequency csnverter 124
and a data receiver 126. The t.uner 124 separates the
television signals; whereas the data receiver dete~ts
and separates the control signals received ~rom the
supervisory testing facility. The ~ast tuner 124 is
under the control of a microprocessor 128 and has two
phases or modes of operation, a jam phase and a feedback
phase.
The first phase is the ja~ phase where a
predicted control voltage for the desired channel is
applied to the local oscillator (LØ) 130 of the tuner
124. The local oscillator 130 is a voltage controlled
oscillator oscillating at a frequency determined by the
control voltage applied to its control input terminal.
The output of the local oscillator 130 is mixed in a
mixer 132 with the signals received on the cable 122 to
produce a beat frequency that for the selected channel
corresponds to a fixed intermediate frequency. The
signals at the intermediate frequency are then converted
by a second converter 133 to a frequency to which the
viewer's television set 134 is tuned. Signals
corresponding to respective control voltages for tuning
the various channel~, both substitute and normal, are
contained in a channel table 138 in an addressable read
only memory associated with the microprocessor 128. The
various voltages are predetermined as part of a
characterization process performed as a last step of the
manufacture of the control box 120, as will be explained
further below in connection with FIG. 3. The
corresponding signals are tored in the memory table 138
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at respective addresses where they are addressed in
accordance with an appropriat channel sel~ction
algorithm of a channel ~elector 136 of the
microprocessor 128 under the control of a substitution
controller 137 of the microprocessor, acting under
instructions from the data receiver 126. The
microprocessor 128 also receives a signal indicating the
channel selection made by the television viewer, as from
a selector switch, not shown. Upon channel selection,
the microprocessor 128 calls up the corresponding
control signal from the memory table 138 and applies the
corresponding predetermined, estimated or predicted
voltage signal (in digital form) to a precision digital
to analog converter (DAC) 140 haviny 12-14 bit
accuracy. The DAC 140 converts the signal to analog
form and applies it to the local o~cillator 130 by way
of a summing circuit 142. The memory table 138 used to
store the predicted ~ignal is an erasable programmable
read only memory such as an EPROM or an EEPROM. These
programmable devices contain a channel table 138 custom
programmed to contain signals corresponding to accurate
estimates of the frequency control ~oltage required for
each individual frequPncy converter module 124.
The second phase of tuning is the feedback mode
employing an error detection system to determine the
difference between the desired frequenry output of the
local oscillator 130 and actual frequency output. It
can usually be safely assumed that the input signal has
a reasonably accurate and stable frequency and that
significant freguency error results only from the local
oscillator 130 in the tuner 124. Thus any frequency
error of the tuner output can be accurately assessed by
measuring the frequency output of the local oscillator
130. The output of the local oscillator serv~s as an
input to a ph~ae-locked loop (PLL) 144 includiny a
divide by N divider 146 controlled by the microprocesaor
128 ~or selecting an integer N by which the tuner output
frequency i~ divided to reduce it to a frequency that is
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supposed to match that of a crystal reference oscillator
148. ~y comparing the phase of the reference oscillator
148 and the phase of the output of the divider 146 using
a phase detector 150, an error signal i5 established ~or
feeding back through a loop filter 152 to the su~mer 142
~or c~mbining with the output of the DAC 140 to bring
the local oscillator precisely on the frequency for
tuninq in the desired channel.
This second or feedback mode of operation is
substantially decoupled ~rom the ~irst or jam mode of
operation in order that th2 two modes not interfere with
one anoth~r. It would be counterproductive for the
phase-locked loop to be trying to synchronize the
oscillator frequency with the reference while the
predicted control voltage is slewing $he frequency.
More specifically, the act of slewing or jamming
provides a random unbalance of the phase-locked loop 144
which ~he phase-locked loop seeks ~o reduc~. By making
the response time of the phase-locked loop 144
sufficiently long relative to the response time o~ the
predictive jamming mode, the two modes may be
substantially decoupled. Specifically, the response
time must be sufficiently long so that the rate of
correction of the random phasP unbalance be sufficiently
slow so that the resulting transient local oscillator
frequency of~set be bounded to prevent significant
transient visible picture degradation. The response
time of the jamming mode i~ made sufficiently rapid as
to slew the tuner to the new channel so fast that the
total intercarrier sound noise energy detected during
the slew mode is so small as to be inaudible to the
~iewer. The transients generated as the tunex slews
through intervening channel frequencies do not perturb
the operation of the feedback means so much as to cause
transient visible degrada~ion of the picture observed by
a viewer immediately after the jam mode. A response
time for the jam mode of less than 60 microseconds has
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been found appropriate and is readily achieved by
appropriate choice of a common local oscillator 130 and
a common DAC 1~0. A response time for the phase-locked
loop 144 four orders o~ magnitude ~lower than the
response time of the predictive slewing circuit has
proven effective in providing adequate decoupling. In
the specific embodiment actually used, a settling time
of the order of three seconds for the phase-locked loop
144 provided effective decoupling. Such settling time
for the phase-locked loop 144 is provided by the filter
constants of the loop filter 152. The error det~ctor
150 includes means for limiting the error ~ignal during
interchannel slewing to furth~r limit the error that
must be corrected. This is an inherent property of a
pha~e-locked loop, which limits the error signal to less
than + ~.
Characterization
Characterization is a process performed at the
end of the production cycle of each control box 120, or
during the recalibration of each control ~ox 120, in
which the table of predicted tuning voltages for each
normal and substitute channel, called the channel table,
is determined and entered in the memory 138. It is
these tuning voltages which are jammed into the local
oscillator 130 of the tuner 124 to cause the tuner to
slew quickly to th~ chosen channelO These voltages are
predetermined with high accuracy because the tuner must
arrive promptly within a narrow range of the desired
channel in order to prevent a correction in the feedback
mode of an initia~ frequency error suf~icient to cause
visible picture degradation. Within 1~0 kHz has proven
ade~uate. In order to realize the desired accuracy,
which may approach one part in 10,000, a digital to
analog converter having 12 to 14 bit accuracy is
provided.
A problem with this factory characterization is
that it is a very expensive operation because each
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individual box 120 must be individually characterized
for each of up to about 70 channels. Up to 1000
decisions may be necessary to characterize each box
12~. The phase-locked loop 144 settles to equilibrium
relatively slowly, taXing at least several ~econds.
This is much too ~low to be able to use the equilibrium
condition in making these charac-terization decisions in
a low cost manner. As will be explained in greater
detail below, these decisions are each more efficiently
made in a fraction of a second using a process based
upon the derivative of the output of the phase detector
150. The process depends upon the ~act that frequency
is the instantaneous derivative of phase.
There are numerous considerations in obtaining
the desired accuracy. A first consideration is that all
critical components of a control box 120 mu~t be used in
the characterization of that box. That is, at least th2
digital to analog converter 140, the ~requency co~verter
124 and the phase-locked loop 144 of the control box 120
are actually used in its characterization. This is
because the required accuracy is so high that minute
differences which could occur due to the use of
different yroupings of components cannot be tolerated.
A second accuracy consideration is burn in and
stabilization. To achieve the required acruracy, the
control box 120 is brought up to its final operating
temperature and allowed to operate at that temperature
until the components reach thermal equilibrium. A third
area of concPrn where extreme accuracy is required
relates to th linearity and hysteresis of the
components, ~uch as resi~tors and capacitors.
In order to achieve high speed and ~t the same
time remove th~ effect~ of hysteresis, the
¢haracterization search for the estimated tuning voltage
for each channel is divided into two parts~ ~he first
part is a high speed binary sear~h. A crude estimate of
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the channel tuning voltage is made initially, but
typically an eight bit binary search is carried out to
get a more accurate estimate of the tuning voltage for
the channel. Binary or dichotomizing searches are well
known to those o~ skill in the art and need not be
further discussed herein. Such a search involves
stepping the test signal in the proper direction by
amounts successively decreasing by half to narrow the
error. To the extent that components in the circuit
exhibit hysteresis sr any degree of thermal m~mory, the
result of the binary search is not accurate e~en when
carried to small steps.
The second part of the characterization ~earch
involves backing off in the test value some small extent
and then proceeding to sweep up through the estimated
value (resulting from the binary search) using a linear
and constant velocity sweep. Throuyh the u~e of a
linear and constant velocity sweep, all hysteresis
effects are aligned in a common direction for all the
channels characterized. Any residual hysteresis error
is reduced to a constant error term, like the error due
to characterizing the box in a temperature environment
slightly different than ambient temperature of a test
viewer's home, and can readily be removed by the PLL
control loop 144.
To implement the characterization process, a
characterization test jig 154 is used. The test jig 154
is connected between the output of the phase detector
150 and the microprocessor 128. The test jig includes a
differentiator 156 and a polarity discriminat~r 158 for
detecting the sense of tuning error, an EP~OM programmer
160~and a characterization control computer 162 ~or car-
rying out the test seguence. An incxamental weighting
algorithm is provided by weighting operator 164 included
in the microproc~ssor 128 itsel~. A ~witch SW~ switches
the input of the 190p ~ilter 152 to ground from its
connection to the phase detector 150 to assure that the
phase-locked loop 144 does not simultaneously try to
correct incremental tuning voltage steps introduced
during the characterization process. The phase detector
thus sanses only the error of the control voltage
applied to the summer 142 by the DAC 140.
The characterization control computer 162 puts
the test jig 154 through its program sequence. The
incremental weighting algorithm responds to the sense of
the error as determined by the polarity discriminator
162. It responds by putting out an incremental signal
corresponding to an increment or decrement in the
control signal to the local oscillator 130. This signal
is summed by a summer 166 with a tentative predicted
voltage signal from the channel table 138. The initial
tentative predicted voltage signals are stored in a test
read only m~mory (ROM) which is plugged into the
microprocessor 128 for test purposes to provide initial
tentative predicted voltage signals for each channel.
As each incremental signal is added to a tentative
predicted voltage signal, the sum is stored in a random
access memory as a current tentative predicted voltage
signal. When the test program is completed for all
channels, the EPROM programmer is enabled to put the
final predicted voltage signals in an EPROM. The EPROM
is then physically inserted in the microprocessor 128 in
lieu of the test ROM.
Regarding the necessary speed of the decision
making during both the binary saarch mode and the linear
; sweep mode of the characterization process for each
channel, there may be as many as 1000 decisions
necessary to fully characterize each tuner. For the
. sake of economy, each decision must be made within a
smal~ fraction of a second. This speed cannot be
obtained by allowing khe phasP locked loop 144 to settle
to accurate values~because the time constant of the
phase-locked loop itsel~ is many seconds. On the other
hand, frequency is the instantaneous derivative of
,, .
~ 13~901
--19--
phase. In a search process, all that is required for
each decision is a determination of the sense o~ any
error; that may be determined by observing whether a
currently observed phase error is increasing or
S decreasing. This is determined from the slcpe of the
instantaneous derivative of pha~e by the differentiator
156 which determines the derivative and the polarity
discriminator 158 which determines sen~e. Put more
basically, the output of the differentiation is
proportional to the rate of change of the phase. If the
slope is downward, it means the current value i~- above
the proper value. On the other hand/ if the slope is
upward, the present value is below the proper value. In
this way simple binary decisions can be made rapidly.
In the binary search mode, the incremental
weighting algorithm 164 puts out incremental signals
wherein each signal after the first is half the value of
the preoeding incremental signal and vf sense determined
by the signal from the polarity discriminator 158. The
binary search is programmed for eight jumps or tests.
After the binary search the linear search continues by
fixed increments until the sense changes. That
determines the final predicted voltage signal. Thus,
the resulting table of tuning voltages is a merger of
the invariant field and the accumulated increments to
create a channel table, customized to each control box
120. This table is burned into a nonvolatil~ read only
memory, EPROM, and assembled into the memory 138 of each
microprocessor assembly 128. As critical components
age, ths control boxes 120 may be returned to the
factory occasionally ~or recalibration.
Although a preferred embodiment of the
inYention has bee~ disclosed in some detail, various
modifications may be made therein within the ~cope of
the invention. For example, although in the preferred
embodiment the system has been de~igned for operation
over cable, the invention may also be used with
over the-air signals.
