Fine tuning indicator for TV with synchronous detector

A fine tuning indicator for a television receiver uses a synchronous detector for generating a control voltage indicative of the tuning accuracy. The control voltage is sensed by an indicating circuit which produces a visual indication of the tuned or mistuned condition of the receiver.

BACKGROUND OF THE INVENTION 
This invention relates generally to television receivers and particularly 
to fine tuning indicators for use with television receivers. 
Television receivers generally provide a fine tuning control to permit a 
viewer to accurately tune the receiver to a selected television channel. 
However, most viewers are not able to determine precisely when the 
receiver is accurately tuned. Consequently, the television receiver may be 
inaccurately tuned and, as a result, not produce an optimum picture. 
In the highly competitive television market, performance, reliability and 
cost are all of major concern. In this respect, adequate performing fine 
tuning indicators are generally cumbersome and expensive. Even though most 
"full feature" receivers include a form of automatic frequency control 
(AFC) for maintaining the tuner locked to the received signal, it is 
desirable to optimize receiver tuning prior to AFC activation. 
Consequently there is a need for fine tuning indicators to help the 
average viewer tune his receiver, at least initially before releasing the 
tuning system to AFC. 
In video detection systems, the simple envelope detector has long been the 
most popular and have proven quite satisfactory. Recently however there 
has been a trend toward synchronous detectors, which offer enhanced 
performance. 
The present invention offers attractive advantages with a receiver using a 
synchronous detector in that it uses the control voltage generated by the 
synchronous detector locking loop to control a fine tuning indicator, thus 
providing a reliable tuning indicator at a low cost. 
OBJECTS OF THE INVENTION 
Accordingly, it is a general object of the invention to provide an improved 
television receiver tuning indicator. 
It is a more specific object of the invention to provide an accurate fine 
tuning indicator for a synchronous detector type television receiver. 
SUMMARY OF THE INVENTION 
The fine tuning indicator described herein utilizes the existing television 
synchronous detector for generating a control voltage of predetermined 
value when the receiver is properly tuned and for varying the value of the 
control voltage in accordance with the degree of mistuning of the 
receiver. The control voltage is sensed by an indicating circuit which 
produces a visual indication of the tuned or mistuned condition of the 
receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
It is conventional to convert the received high frequency television signal 
to a lower frequency called the intermediate frequency (IF) signal. The IF 
frequency is generally 45.75 megahertz. After suitable amplification, the 
IF signal is demodulated to obtain the video and synchronizing components. 
Most television receivers employ peak detectors for demodulating the IF 
signal. More advanced receivers incorporate a synchronous detector because 
it produces a demodulated signal of lower distortion. Such a synchronous 
detector includes a conventional APC (automatic phase control) loop which 
generates an error signal or control voltage for locking the frequency of 
the detector oscillator to the frequency of the IF signal. The detector 
oscillator and the IF signal are then fed to a multiplier whose output is 
a low distortion demodulated video signal. There are also so-called 
quasi-synchronous detectors which do not incorporate an APC loop. It 
should be remembered that for this invention only conventional synchronous 
detectors are of interest, that is, those which have a phase lock loop 
circuit. 
It has been found that the control voltage developed by the synchronous 
detector can be advantageously used as an indicator of receiver tuning 
accuracy and in television receivers employing such synchronous detectors 
they may be used for driving a fine tuning indicator. Hence, the 
synchronous detector may be made to do double duty, namely, demodulate the 
IF signal and provide a control voltage for use as an indication of the 
accuracy of receiver tuning. 
Referring now to the FIGURE, there is shown an exemplary embodiment of a 
fine turning indicator which employs a single synchronous detector 10 both 
for video demodulation and for providing a control voltage indicative of 
the accuracy of tuning of the receiver. An indicating circuit 12 responds 
to the synchronous detector control voltage and produces a visual 
indication of the accuracy of receiver tuning. 
Referring more specifically to synchronous detector 10, it includes a 
voltage controlled detector oscillator 14, a first multiplier 16, an 
amplifier 18 (optional), a resistor 20, a second multiplier 22 and a 
series connection of a resistor 34 and a capacitor 36. Resistor 20, 
resistor 34 and capacitor 36 form a low pass filter network. Lead 24 
supplies a conventional IF signal from an IF amplifier 26 and a limiter 28 
to the input of the synchronous detector. The receiver circuitry preceding 
IF amplifier 26 is a conventional tuner and is, therefore, not shown. 
The operation of synchronous detectors is well known. Suffice it to say 
that the various elements of detector 10 cooperate to lock the frequency 
of detector oscillator 14 to the frequency of the IF signal on lead 24. To 
control the oscillator frequency a control voltage is generated on a lead 
30 which is at the junction of resistors 20 and 34. This control voltage 
is DC when lock has been achieved and, in the illustrated embodiment, has 
a predetermined magnitude when the IF signal is at its nominal frequency 
of 45.75 megahertz. Only when the receiver is properly tuned will the 
frequency of the IF signal be equal to 45.75 megahertz. Detuning of the 
receiver results in the frequency of the IF signal being either above or 
below nominal, depending on the direction of mistuning. 
When the frequency of the IF signal deviates from nominal, the value of the 
control voltage on lead 30 deviates in a substantially linear manner. For 
example, when the IF frequency is above 45.75 megahertz, the control 
voltage is lower than the predetermined value. Similarly, when the IF 
frequency is low, the control voltage is higher than the predetermined 
value. Thus the control voltage on lead 30 is representative of the 
accuracy of receiver tuning. 
To demodulate the IF signal, an output of oscillator 14 and the signal from 
IF amplifier 26 are fed to multiplier 22. At the output 32 of multiplier 
22, demodulated video appears and is treated by conventional television 
circuitry (not shown) for creating a television picture. 
The filtered control voltage at lead 30 is fed, via a resistor 38, to a 
pair of comparators 40 and 42. In general, comparators 40 and 42 compare 
the value of the control voltage with a pair of reference voltages which 
are, respectively, above and below the nominal or predetermined value. if 
the control voltage sensed by comparators 40 and 42 is outside the range 
of the reference voltages, one of a pair of visual indicators, in the form 
of light-emitting diodes 44 and 46, is energized to present a visual 
indication of the mistuning of the receiver. 
More specifically, the comparison of the control voltage to the reference 
voltages is effected by coupling the control voltage to the negative input 
48 of comparator 40 and to the positive input 50 of comparator 42, and by 
coupling one reference voltage to the positive input 52 of comparator 40 
and the other reference voltage to the negative input 54 of comparator 42. 
Comparators 40 and 42 may include respective feedback resistors 41 and 43. 
The voltages appearing at inputs 52 and 54 are developed by a voltage 
divider comprising fixed resistors 56 and 58 and a pair of variable 
resistors 60 and 62. The series combination of resistors 56, 58, 60 and 62 
is coupled, as shown, between ground and a source of positive potention 
+V. A wiper arm 64, associated with variable resistor 60, picks off the 
desired reference voltage from resistor 60 and applies it to positive 
input terminal 52 of comparator 50 via a resistor 66. Wiper arm 68, 
associated with resistor 62, picks off the other reference voltage and 
applies it to negative input terminal 54 of comparator 42 via a resistor 
70. Preferably, wiper arms 64 and 68 are adjusted such that the reference 
voltages appearing at input leads 52 and 54 of the comparators are above 
and below, respectively, the nominal or predetermined value of the control 
voltage. Thus a voltage range is formed within which the control voltage 
may vary without developing an indication of mistuning. For example, if 
the predetermined or nominal value of the control voltage is 5.8 volts, 
the reference voltage appearing at input 52 of comparator 40 may be 6.8 
volts and the reference voltage appearing at input 54 of comparator 42 may 
be 4.8 volts. Accordingly, a tuning range is provided within which the 
control voltage may vary without generating an indication of mistuning. 
However, should the receiver be detuned such that the control voltage 
exceeds the level of voltage on input 52 of comparator 40, comparator 40 
will develop an output for actuating light-emitting diode 46, thereby 
giving a visual indication of both the fact and direction of receiver 
mistuning. Should the receiver be mistuned in the opposite direction, the 
value of the control voltage at input 50 of comparator 42 will be less 
than the value of the reference voltage on input 54 and comparators 42 
will develop an output for activating light-emitting diode 44. 
It will be appreciated that when the receiver is properly tuned (within a 
predetermined tuning range) the value of the control voltage will be 
within the range of the reference voltages at inputs 52 and 54 of the 
comparators and neither of light-emitting diodes 44 or 46 will be 
activated. Thus, comparators 40 and 42 generate a first or non-actuating 
signal when the control voltage has a value between the reference voltages 
and a second or actuating signal when the control voltage is outside the 
reference voltages. 
In some instances, synchronous detector 10 may require calibration in order 
to insure that the control voltage appearing at lead 30 is of a known or 
predetermined value when the receiver is properly tuned. Such calibration 
may be effected by applying a signal of precisely 45.75 megahertz to the 
synchronous detector at lead 24. With the synchronous detector now 
receiving a signal whose frequency is equal to the frequency of the IF 
signal when the receiver is properly tuned, oscillator 14 may be adjusted 
to develop at lead 30 a control voltage whose value is precisely equal to 
its predetermined or nominal value. In the example used, oscillator 14 may 
be adjusted to develop a control voltage at lead 30 of 5.8 volts. 
The method of adjustment of oscillator 14 will vary according to its 
specific construction. However, where oscillator 14 includes an adjustable 
coil, the coil itself may be adjusted to achieve the desired level of 
control voltage on lead 30. 
Because the value of control voltage on lead 30 generally varies in a known 
manner according to the deviation in frequency of the IF signal, the 
extent of its variation may be measured or calculated such that the 
reference voltages appearing at inputs 52 and 54 of comparators 40 and 42 
may be set to correspond to known frequency deviations. For example, if 
the IF tuning limit for the receiver is .+-.150 kilohertz, the reference 
voltages appearing at inputs 52 and 54 of comparators 40 and 42 may be set 
to the values which the control voltage assumes with these IF frequency 
deviations. Within the .+-.150 kilohertz limits, neither of the 
light-emitting diodes will be activated. Should the IF frequency deviate 
by more than 150 kilohertz from nominal, one of diodes 44 or 46 will be 
activated to present to a viewer a visual indication of the mistuned 
condition. The viewer need only tune the receiver until neither of the 
light-emitting diodes is activated, at which point the viewer will know 
that the receiver is properly tuned (within a predetermined acceptable 
tuning range). 
Although the embodiment described above has been illustrated as presenting 
a visual indication of mistuning by the activation of either of a pair of 
light-emitting diodes, many other visual methods of presenting an 
indication of mistuning are possible. For example, the outputs of the 
comparators may be applied to a logic gate whose output is coupled to a 
single light emissive element for activating that element only when the 
receiver is properly tuned. When the receiver is mistuned, the light 
emissive element will be off. 
Other variations may include a tuning meter which may be calibrated to 
indicate proper tuning when the value of control voltage at lead 30 is at 
nominal and to indicate mistuning when the control voltage falls outside a 
predetermined range. 
The major advantage of the fine tuning indicator described is that the 
synchronous detector can supply the control voltage for a visual tuning 
indicator in addition to demodulating the IF signal. Accordingly, the cost 
of implementing the fine tuning indicator is reduced. 
Although the fine tuning indicator described has been illustrated in terms 
of a preferred embodiment, it will be obvious to those skilled in the art 
that many alterations and modifications may be made without departing from 
the spirit and scope of the invention. For example, many alternate visual 
indicators may be used to present an indication of receiver tuning in 
response to the control voltage developed by the synchronous detector. 
Tuning meters, one or more light-emitting diodes or lamps may be used and 
the reference voltages, if any, may be generated by any of many well-known 
circuits. Accordingly, all such modifications and alterations are deemed 
to be within the scope of the invention as defined by the appended claims.