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TEA7650H Scheda tecnica(PDF) 7 Page - NXP Semiconductors |
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TEA7650H Scheda tecnica(HTML) 7 Page - NXP Semiconductors |
7 / 23 page ![]() September 1990 7 Philips Semiconductors Preliminary specification Video signal processor for CD-video/laser vision TEA7650H FUNCTIONAL DESCRIPTION Figures 1, 2 show the block diagram of the Video Signal Processor (VSP) including the peripheral circuitry for the video signal processing. The pulse-width modulated FM signal from the preamplifier is fed, via a DC blocking capacitor, into the IC (pin 7) at the input to the Modulation Transfer Function (MTF) circuit which corrects for the characteristic of the optical reading system. MTF correction Due to the finite diameter of the laser beam spot and the tangential velocity of the track of pits on the disk, the MTF of the optical system acts like a radius-dependent low-pass filter for the FM input signal. Although the video signal can be recovered without correction, the ratio of the amplitudes of the chrominance and luminance signals would not then be the same at the most inner and the most outer part of the disk. This influence of the disk radius is automatically corrected by the Video Signal Processor. The principle of correction is to use the deviation of the demodulated burst signal to generate an error voltage in order to control the frequency selective MTF circuit. The burst measurement operates as follows: A burst-key generator is triggered by the line synchronization pulse (CS1) to generate a burst-key pulse which activates the burst gate and rectifier stage. The signal at the rectifier output (pin 6) is used to control the amplification of the MTF circuit. The carrier frequency in the PAL standard is different to that in the NTSC standard, therefore two separate resonant circuits are required on pins 8 and 9. They are selected by the PAL/NTSC system selector (pin 37). The MTF-corrected FM signal at pin 10 is amplified ( +8 dB) and fed, via the external filter which removes the audio frequency components from the signal, into the demodulator at pin 14. Demodulation The FM signal is first fed into a limiter circuit (pin 14) with automatic slicing level control to suppress the main carrier in the demodulated signal. The demodulator has two outputs. The first (internal) clamps the demodulated video signal on peak-sync by controlling the transconductance of the demodulator. The FM signal can now be demodulated during disk start-up, thereby facilitating fast run-in. The second output signal from the demodulator (pin 18) is passed through an external 5 MHz low-pass filter to extract the CVBS signal. The CVBS signal is then fed into the de-emphasis network to compensate for the pre-emphasis of the video signal recorded on the disk. De-emphasis The de-emphasis circuit consists of an internal inverting amplifier and an external RC feedback network. Since the pre-emphasis on the disk in the PAL standard is different from that in the NTSC standard, the time constants are switchable. When PAL is selected, the first arm of the feedback network is active, otherwise both operate in parallel. The de-emphasized video signal is fed into an AGC stage (pin 20) where it is clamped on its black level and amplitude-controlled to a constant level. The signal is then fed into the data slicer and the drop out switch. Data slicer Coded signals on the video disk are extracted by the data slicer (output pin 25) when the Data Request input is activated (pin 17). Drop out compensation The drop out detector (DOD) in the IC is triggered by every positive or negative transition of the FM signal. A drop out is detected when the half-cycle period is outside the limits. Protection against a drop out is achieved by use of a video signal delayed by one line. The signal at the output of the drop out switch is fed out of the IC via a buffer (pin 29) and then through a delaying device (CCD) before being fed back into the IC (pin 27). The delayed video signal appears at the input of an AGC circuit to compensate for gain tolerances of the delay line and avoids the need for an external adjustment. When a drop out is detected, the drop out detector activates the video switch so that the lost information of the line is substituted by the information of the preceding line. The drop out pulse is also present at pin 3 and can be used for different purposes. This pin can also act as an input to control the drop out switch by an external signal for test purposes. Time error compensation In a videodisc player timing errors are caused by deviations of the rotational speed of the motor, imperfections in the disk and unavoidable tolerances in the centering of the disk on the turntable. Track eccentricity is the main cause of timing errors. To minimize timing errors, it is necessary in the first place to keep the rotational speed of the disk as constant as possible. Referring back to the output of the switch in Fig.1-2, the video signal is also fed into a sync separator and a chrominance separator with its external resonant circuits tuned to the chrominance subcarrier |
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