Patent GB-1168425-A (GB)
Title: Fine Tuning Indicator
IPC: H04N5/50
Filed: 1968-03-12
Abstract: 1,168,425. Television tuning indicators; gating circuits; frequency discriminators. WESTING- HOUSE ELECTRIC CORP. 12 March, 1968 [11 April, 1967], No.11963/68. Headings H3A, H3Q and H3T. A television tuning indicator comprises means for generating a signal having a predetermined time relationship to a scanning line, means for generating a second signal whose time relationship to the scanning line depends on the degree of mistuning of the receiver, and gating means to apply these signals to the picture tube and so display a fixed vertical line and a second vertical line whose displacement from the first line indicates the amount and direction of mistuning; the lines will coincide when the receiver is correctly tuned. Fig. 1 shows a colour television receiver including within block 10 the tuning indicator means of the invention, this being made operative when required by a switch 76 which supplies voltage to the indicator circuitry. A signal from the I.F. stages is applied to a discriminator 82 giving an output depending on the I.F. which varies with the mistuning of the receiver. To make this output independent of the amplitude of the received signal it is combined in an amplifier 86 with a signal representing the peak value of the video signal, obtained from the first video amplifier via a peak detector 126. The output is passed through a peak detector 90 to a gate 98, controlled by a bi-stable circuit 102. The vertical flyback pulses are differentiated at 106 and change over the bi-stable 102 so that the gate 98 passes, on alternate frames, a constant voltage from a reference source 96 and the control signal varying according to mistuning. A monostable circuit 110 is triggered by horizontal flyback pulses and reverts to its stable state at some time during the scanning line, dependent on the voltage it receives from the gate 92. The reversion is differentiated at 114 and triggers a monostable circuit 116 to provide gating pulses at 32 to blank out the video signals for a short period during a scanning line. While the gate 98 is transmitting the constant voltage from 96 this blanking will occur at a fixed point on the scanning line, and a vertical black bar will appear in a fixed position. During the next frame the gate 98 will pass the control voltage so that a second black bar will appear horizontally displaced from the first by an amount depending on the mistuning of the receiver. At the correct tuning point the two lines will coincide. Circuitry of units 82, 122, 86 and 88 (Fig. 4). The discriminator 82 comprises a tuned circuit L1, C2 resonant at a frequency 1MHz above the correct I.F. so that its response will be higher or lower depending on the direction of mistuning of the receiver. The output is rectified by diode D1, smoothed and applied to the buffer amplifier Q1. The latter also receives an input from the video peak detector comprising transistor Q3 and capacitor C5 which gives an output representing the peak value of the video waveform. The two inputs to the amplifier Q1 are of opposite polarity so that changes due to varying amplitude of the received signal cancel out and the output of the amplifier Q1 depends only on the frequency of the signal from the I.F. stage. This output is rectified by a peak detector comprising transistor Q2 and capacitor C4, and applied as one input to the gate 92 (Fig. 6). Circuitry of units 106, 102, 92, 96, 110, 114, 116, 32 (Fig. 6). The gate 92 comprises a pair of diodes D2, D3 to which are applied respectively the mistuning signal from Fig. 4 and a reference voltage set by resistor R10. A bi-stable circuit 102 comprises transistors Q4, Q 5  which are changed over by each frame flyback pulse applied through a differentiating capacitor C8 and diodes D4, D5. When transistor Q5 is non-conducting both diodes D2 and D3 are conducting, and the output on lead 98 is the control voltage from Fig. 4 since the impedance of R10 and R11 is greater than the output impedance of the circuit of Fig. 4. When transistor Q5 conducts and Q4 is turned off, the collector potential of the latter reverse biases both diodes D2 and D3, and the output on lead 98 is the reference voltage Vr. This output voltage is fed to a comparator 110 which is a monostable circuit comprising transistors Q6 and Q7. Q6 is normally off but is turned on by a horizontal flyback pulse at 115 and the circuit enters its quasi-stable state whose duration depends on the time constant of C10, R25 and the voltage on line 98. The instant when the monostable circuit reverts to its former state thus depends on the voltage from gate 92. In one frame period this will be the fixed value Vr and in the next frame period it will have a value depending on the mistuning of the receiver. The step output is differentiated by C11, R40 and the pulse lengthened by a further monostable circuit Q8, Q9. The output of this controls a gate 32 to connect either its own voltage level or the normal video signal to the second video amplifier stage and the picture tube, depending on which of the diodes D6, D7 is conductive.