THOMSON VK231PS, Technical Manual

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111
CONVERTER 2
PRODUCTION OF 40 Fh + 1/8 Fh signal.
The 40 Fh + 1/8 Fh signal is generated based on a 321 Fh (5.01 MHz) VCO. To ensure that
the converter output signal is perfectly in phase with the playback signal, the VCO frequency is
controlled by the error voltage output by a phase detector.
The phase detector receives:
• Delayed line sync time signals,
• 4.43-MHz signal from the oscillator,
• Playback signal re-transposed to 4.43 MHz.
Phase correction occurs only during bursts. The resulting error voltage is sent back to the
VCO.
The VCO output is sent to an 8-way divider, which brings the signal to 40 Fh + 1/8 Fh (626, 953
kHz).
It is then sent to the phase shift circuit. This stage is necessary to correct crosstalk, and shifts
the signal by +P/2 at each line when a Track 2 is played back. To do this, it receives the 25-Hz
flip flop, which tells it when a Track 2 has been played, and the line sync time signals to
increment the phase shifter. The 626,953-Hz signal is applied to the first input of Converter 2.
PRODUCTION OF 4.43 MHz
This signal is generated by a 4.43-MHz free oscillator. It is sent to the second input of
Converter 2.
The sum of two frequencies is thus obtained at the converter output:
4.43 MHz + (40 Fh + 1/8 Fh)
as well as the difference between two frequencies:
4.43 MHz - (40 Fh + 1/8 Fh)
The sum component is selected by the band pass filter with central frequency 5.06 MHz.