Digital picture zoom
In addition to the optical zoom facility provided on all camcorders, those fitted with a digital field store are able to offer a further ‘zoom’ feature in which only the memory data corresponding to the central section of the picture is read out at normal field and line scanning rates. The effect on screen is of fewer but larger pixels rather than real image magnification, so it counts more as an ‘effect’ than as a true extension to the zoom range.
Electronic viewfinder
Electronic viewfinder (EVF) systems invariably include colour LCD screens of the sort described in Chapter 11, some types of which are on a side- mounted flip-out panel, and others built onto the camcorder’s backplate, in either case removing the need for an eyecup magnifier system. They have the advantage of giving the operator some idea (though not as accurate a one as a conventional colour monitor) of the colour balance of the pictures being recorded, and can be viewed by more than one person at a time. On the debit side is their relative lack of picture definition; and their vulnera- bility to being ‘washed out’ by sunlight. Some camcorders offer the best of both EVF systems with both a monochrome tube and a colour LCD screen.
Audio section
All camcorders have an integral microphone, which may be a mono or— with hi-fi camcorders—a stereo type. Many have external microphone sockets for greater versatility and an opportunity to eliminate the motor- and handling noise which is almost inevitable in quiet situations where the automatic level control (ALC) drives up the gain of the audio amplifier. The action of the ALC circuit is necessary to enable the camcorder to cope with the huge range of sound levels it may encounter, from the rustle of leaves to the roar of a jet aircraft. Where a hi-fi sound recording system is provided in the more expensive models, the dynamic range for sound recording is much greater and the ALC action can be less harsh. The cam- era sound is passed out of the camcorder during record along with the composite video signal for monitoring if required: very often a headphone socket is also provided for audio monitoring ‘on the hoof’.
D-8 signal processing
As stated earlier, the recording medium for modern camcorders may vary. Figure 22.22 shows the various stages of a typical Digital-8 recording sys- tem. The CCD imager’s output is cleaned up, gain-controlled and then A/D converted within IC502 into a 10-bit bit stream, sampled at 13.5 MHz. IC251 separates Y and RGB data to derive Y, Cb and Cr values, all at 8-bit depth and the latter two at 3.375 MHz sampling frequency. Also within this chip is carried out the ‘steady-shot’ process in which the image is sta- bilised by a fast pan/tilt reading operation from a picture memory bank with a larger effective area than the useful picture size. The next chip in line, IC351, performs blocking, data shuffling, digital picture effects and D-A conversion for use in the viewfinder and to produce an electronics-to- electronics (E-E) output monitoring signal.
IC301 is the MPEG codec concerned with the bit-reduction process. Here the data undergoes DCT, quantisation, VLC and framing to achieve data compression to about one-fifth of the original content. The com- pressed video is joined by the audio bitstream inside IC301 for passage into IC302, where error correction ECC, deshuffling and encoding takes place into a form suitable for recording on tape. Further processing takes place inside IC104, primarily to convert the data from 5-bit parallel form to a serial (one line) data stream for application via recording amplifier IC102 to the video heads. For this application an FE (flying erase) head is also present on the drum to wipe off previously recorded data.
D-8 playback
During replay of digital video tapes, many of the same ICs are used, with their roles reversed to perform the inverse functions to those carried out during record; the same applies to memory chips. Thus in Figure 22.22, IC102 now operates as head preamplifier; IC104 carries out clock conver- sion, synchronous detection etc.; IC302 shuffles the data and provides error-correction and concealment; IC301 takes care of I-DCT, de-interleaving and data expansion; and IC351 de-shuffles the data, D-A converts it and encodes the Y and C components into PAL (or other analogue) form to produce signals for the viewfinder and the AV-out ports. The digital in/out port interface is IC303, shown at the top of Figure 22.22.