THOMSON VK231PS, Техническое руководство

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103
LUMINANCE PROCESSING DURING PLAYBACK
The signal sent out by the playback preamplifier board is available on Pin 8 of Connector
BV001. T
wo traps, 627-kHz for P AL and 1.1 MHz for SECAM, eliminate the chroma.
The luminance is sent to a correction amplifier . This circuit corrects the response curve of the
play heads, so that a linear response is obtained whatever the signal frequency deviation.
The Y FM signal enters at IV001 Pin 20. It is applied to the dropout detector and the double
limiter via an AGC stage.
DOUBLE LIMITER, DEMODULATOR, AND DE-EMPHASIS
The double limiter stage, internal to IV001, reconstitutes the high frequencies of the luminance
signal. These high frequencies, generated mainly by the pre-emphasis peaks, were strongly
attenuated during the head-to-tape transfer
.
To do this, the low frequencies are separated from the high frequencies. The high frequencies
are strongly amplified and limited (clipping). The two components are mixed again, and
clipped. Signal Y FM is then found at the output, with is high-frequency components restored in
the form of a square signal. This allows the demodulator to be approached directly
.
The demodulator supplies a signal whose level is directly related to the frequency of the input
signal.
The demodulated Y signal is applied to a low-pass filter to eliminate modulation residuals. It is
then sent to IV001 Pin 26 via the main de-emphasis circuit.