Stereo tv decoder

  
The composite input signal is preamplified by transistor Ql and is then coupled to the high-pass filter composed of C3, C4, R6, and R7. The filtered audio is then passed to IC1, an MC1310P `Coilless Stereo Demodulator.` That IC is normally used to demodulate broadcast-band FM signals, but by changing the frequency of its on-board VCO (Voltage Controlled Oscillator) slightly (from 19 kHz to 15.734 kHz), we can use that IC to detect stereo-TV signals. A block diagram of the MC1310P is shown in Fig. 22-5. Notice that the components connected to pin 14 control the VCO's frequency, hence the pilot-detect and carrier frequencies.
Stereo tv decoder - schematic

For use in an FM receiver, the VCO would run at four times the 19 kHz pilot frequency (76 kHz), but for our application, it will run at four times the 15.734 kHz pilot frequency of stereo TV, or 62.936 kHz. The MC1310P divides the master VCO signal by two in order to supply the 31.468 kHz carrier that is used to detect the L - R audio signal. The L - R signal undergoes normal FM detection, and at that point we've got two audio signals: L + R and L - R. The decoder block in the IC performs the addition and subtraction to produce the separate left and right signals. RIO and CIO form a de-emphasis network that compensates for the 75 /is pre-emphasis that the left channel underwent; R12 and Cll perform the same function for the right channel.




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