Video-8 format chrominance
The basic principle of the colour system of Video-8 format is the same as for VHS already described, in that the chroma phase recorded on tape is manipulated to ensure that cross-talk signals come off the tape in antiphase over a two-line period. The colour-under frequency for Video-8 is (47–1/8) X fh, which is 732 kHz. It is locally generated, but in order to implement de-jittering during playback, is locked to incoming line sync in a PLL incorporating a ÷375 stage.
Signal processing chip
Throughout this chapter, we have used block diagrams to illustrate the conversion and conditioning of the luminance and chrominance signals for their recording on tape. In practice, dedicated chips are involved in providing all the necessary processing. A 2-chip solution incorporating a head drive/preamplifier chip mounted in or close to the head drum and a ‘Y/C’ chip on the main PC board. A typical chipset is shown in Figure 21.35 (a) for record and (b) for playback. Looking first at diagram (a) there are three video inputs, selected by a switch at chip pin 34 under bus con- trol. Luminance noise reduction uses an electronic 1-line signal delay in IC302 at main chip pins 40/42, after which the Y carrier signal passes through three stages of emphasis on its way to the modulator. Emerging at IC pin 18, the carrier enters IC801 on pin 15 for application to the heads. The chrominance signal undergoes the processes described earlier to pass from pin 14 of IC301 to pin 16 of IC801, where it meets the luminance car- rier en route to the video heads. IC801 also handles the drive to the hi-fi audio heads, whose operation we shall meet later in this chapter.
Turning now to the same chipset working on replay (Figure 21.35b), we can see on the left that this VCR is a dual-speed type with separate video head-pairs for SP and LP. The selected FM envelope signal passes into IC301 on pin 15 and splits two ways: up to the luminance limiter, demod- ulator and noise reduction stage (again using delay chip IC302); and down to the chroma up-converting stage, bandpass filters, etc. Luminance and chrominance are reunited in the Y/C mix block to emerge from the chip as a composite video/chroma signal at IC pin 38. The colour phase rotation switching pulses enter IC301 at pin 66 (diagram a); the colour crystal is hooked to pins 59 and 60, while an I2C bus passes user- and system-control commands into the IC on pins 63 and 64. Two separate SW25 pulse trains enter head amp/switch IC801, one for video and a second for audio, as will be described in the next chapter.