Video-8 tape-signal spectrum
The DTF pilot tones are recorded at the lowest part of the frequency spectrum as shown on the left of Figure 21.25. Here, they do not interfere with the ‘signal’ components of the tape recording. The other parts of the tape- signal spectrum will be dealt with later.
Luminance recording and replay
The luminance (or black-and-white) signal is dealt with separately from the chrominance (or colouring) signals in home VCRs. For luminance, the basic idea is to modulate the signal onto an FM carrier for application to the recording head and demodulate it to baseband during the replay process. As the simplified block diagram of Figure 21.26 shows, however, there are several other processes undergone by the luminance signal and these will be described in turn.
The basic luminance signal that we wish to record may come from a TV camera or other local video source, or more likely, a broadcast receiver built into the VCR. In either case, it will be positive-going for white and will probably contain a chrominance signal modulated onto a 4.43-MHz carrier. It is important that the signal recorded on the tape is within the limits of the recording system, so the luminance signal is first passed through an AGC amplifier with a sufficiently wide range to compensate for signal inputs of varying amplitudes. This works in a similar manner to the AGC system of a radio or TV, by sampling its output level to produce a d.c. control potential, and applying this to an attenuator at the amplifier input. Thus, the output from the AGC stage will be at constant (say) 1 V amplitude.
In low-band (e.g. VHS) VCRs, luminance bandwidth is restricted to about 3 MHz, gives or takes a hundred kHz or so between the formats. If higher frequencies than this are allowed to reach the modulator, they will make mischief with sidebands, as explained earlier. A low-pass filter with a quite sharp cut-off around 3 MHz is incorporated in the record signal path, then, and this also eliminates all the chrominance components of the signal, which are based on a sub-carrier of 4.43 MHz. When recording in monochrome, more bandwidth can be allowed occupying the space nor- mally reserved for the chrominance signal, and many machines have an automatically switched filter characteristic for monochrome and colour recordings as shown in Figure 21.27.
During replay, the off-tape signal from the video heads is very small, and to maintain the necessary >40 dB S/N ratio in the reproduced picture, low-noise amplification is necessary. The main replay processes after the preamplifiers are head switching, equalisation, limiting, drop-out compen- sation, demodulation to baseband, de-emphasis, crispening and amplifica- tion, after which the luminance signal is resorted to its original form, usually 1 V peak-to-peak, negative-going syncs. A block diagram showing the order of the replay circuits is given in Figure 21.28 incorporating two heads (A and
B) with a head switching arrangement. During replay, the signal from the tape head falls at the rate of 6 dB/octave. HF losses due to the approach of fex are largely compensated for by a resonance circuit associated with the replay head. To even-out the response on replay, further equalisation is pro- vided, and in practice it takes the form of a ‘boost’ in the 2–3 MHz region, the major area for the lower FM sideband of the luminance signal. This is often catered for by the provision of a reactive component in the collector circuit of the preamplifier or a following stage within the IC. Equalisation is followed by dropout compensation (DOC).