Video 8: Not Just A Pretty Picture
The revolutionary Video 8 format has resulted in a new generation of domestic video recorders that offer high quality pictures and up to 6 tracks and 18 hours of stereo digital sound. Francis Rumsey explains how.
Whether you know it or not, we are in the midst of a revolution, for the widespread adoption of the Video 8 format has resulted in a new generation of domestic video recorders that offer recording/playback of not only high quality pictures but, more importantly, up to six tracks and 18 hours of stereo digital sound. Francis Rumsey explains how.
It is quickly becoming impossible to divide the consumer and professional recording worlds into 'audio' or 'video', because so much of the available equipment gives you the best of both worlds. Video 8 is no exception to this, and indeed provides users with a most versatile medium of good quality, allowing a number of levels of sophistication in the audio recording region.
The audio side of Video 8 cannot be discussed without first understanding the video section, so let us investigate what's different about it, and what makes the format so appealing to the modern musician.
Before you set out to buy a Video 8 recorder, it is most important to be aware that not every possibility catered for in the format will be implemented in each model, especially where audio is concerned, so be careful about the limitations of the machine you buy.
The Video 8 format caters for the recording of a video picture using a rotary head, in roughly the same manner as a Betamax or VHS format recorder, at one of two speeds for either Standard Play (SP) or Long Play (LP). Two conventional linear analogue tracks are provided, one running down each side of the 8mm video tape, but when you compare the linear speed of the tape, which is 1cm/sec in LP mode and 2cm/sec in SP, with a reel-to reel recorder running at 15ips (that's 38cm/sec), it's relatively easy to see why these are not generally used for recording audio!
Since the tape speed is very low, two other methods of recording sound are allowed for, the first one working in a way which is very similar to the sound on a Beta or VHS Hi-Fi recorder, in that it is modulated onto RF carriers and inserted between the chrominance (colour) picture information and the luminance (black and white) part of the recorded spectrum (NTSC format only).
With Beta Hi-Fi, the FM sound is stereo and gives a frequency response extending from 20Hz-20kHz with a dynamic range exceeding 80dB, and the same technique is used with Video 8, except that there will only be one audio channel. In order to achieve such a wide dynamic range, compansion techniques are used with pre-emphasis (similar to the dbx noise reduction system) which seem to give minimal side effects.
The thing to remember about FM sound is that it can only be recorded at the same time as the picture, because it is part of the helically recorded signal, which will prevent you from overdubbing sound at a later date. Should you desire this, the next and most important option is PCM sound.
Conventional Betamax recorders wrap the tape 180 degrees around the video head, and record the picture using two diametrically opposite heads on the same drum. Video 8, on the other hand, extends the tape-wrap a further 40 degrees around the drum, to provide recording space for a segment of digital (PCM) audio put down by the rotary head (see diagram).
Two PCM audio channels can be recorded in a 30 degree segment, allowing margins on either side to prevent overlapping or interference with the picture, and the advantage of this over Beta Hi-Fi is that the PCM channel can be overdubbed at any time, allowing the user to provide high quality stereo sound with video recordings.
"It could be applied to the storage of sounds for subsequent loading into the many computer-based sampling keyboards..."
From this brief description you can probably see that it would be necessary to be able to turn the record and erase currents on and off for short segments of the helical scan, and this is indeed what happens, providing the basis for the fourth and most interesting sound recording option.
The Sunday supplement advertisements for Video 8 all emphasise its ability to record up to twenty-four hours of digital audio on certain machines, and this is done by sacrificing the picture-recording area of the helical scan. We have already seen that a stereo PCM channel occupies about 30 degrees of scan and that the total tape-wrap is just over 220 degrees, so it would seem that if the tape were used solely for audio then seven channels could be inserted across one scan. This is not quite the case, however, due to the protective margins which must be placed between segments, reducing the resulting number of channels which can realistically be recorded to six, but this still allows for excellent capacity.
Depending on which segment has been set to record, the rotary heads will switch on for the relevant segment of the scan, using the flying erase head to erase just that portion. The same happens on replay, whereby only the relevant area of data is turned into audio, which has the effect of replaying two tracks of a multitrack tape recorder. Looking at the maximum tape length available, which currently gives three hours in LP mode (soon to be four hours), you will see that this can be used completely for each of the six stereo segments, giving a total capacity for audio of eighteen or twenty-four hours.
We'll look at the applications of this in a moment, but before we do it is necessary to look at the limitations.
Firstly, it seems that it is only possible to record in one direction: in other words, there is no auto-reverse at the moment which would allow continuous replay of all six tracks; and secondly, it is not possible to use the machine as a true multitrack recorder, recording and replaying twelve audio tracks simultaneously.
With all the people moaning that 16 bits is not enough resolution for compact disc, you may wonder how on earth the makers of Video 8 have managed to squeeze anything remotely acceptable quality-wise out of their 8-bit system. Even so, a remarkable compromise has been arrived at between storage capacity and sound quality which seems to give the best of both worlds.
To start with, it's not just 8 bits, it's 10 bits linearly quantised, which are subsequently reduced digitally depending on the level of the signal. This is known professionally as non-linear compansion, and tests have proven that a considerable reduction can be made in the overall number of bits required without seriously affecting the audio quality. As the audio signal level increases, the resolution to which each sample is represented is reduced, which naturally increases the quantisation error causing noise or distortion; but the technique relies upon the fact that high level audio signals will mask random noise to some extent, thus making it audibly acceptable. Low level signals are represented to full resolution, as the quantisation distortion would be more noticeable here.
"Even with two channels of digital (PCM) audio, the unit makes a very good everyday cassette recorder with three hours of non-stop playing time..."
Further analogue methods are used to increase the dynamic range of the PCM audio, and these amount to pre-emphasis and analogue compansion in the same way as the FM sound track. All this adds up to a very acceptable performance for the price, although one must realise that the sampling frequency is only 31.25kHz (PAL), which will give a usable audio bandwidth of about 15kHz - more than adequate for most applications.
Pretty good actually; superior in quality to almost every cassette machine on the market, with the only noticeable side effects being an increase in harmonic distortion at high levels and high frequencies, coupled with slight noise modulation if noise was present on any programme material copied onto the unit. By way of justification for the compansion methods used, it would be worth pointing out that you have probably been listening to non-linearly companded digital audio on BBC transmissions for years now, although without necessarily noticing it!
Low level signals recorded well, although it would be advisable to use the highest possible recording level in order to utilise the dynamic range to its full, and high levels (here I mean signals recorded well over the top) did not seem to clip but to enter a soft limiting, on the particular Sony model which I tested.
Naturally, if you want 16-bit sound quality then there is no substitute for a 16-bit processor like the Sony PCM 701 or PCM-F1, but, unless you have very critical ears, it is likely that the PCM sound on Video 8 will serve for all domestic and most semi-professional applications.
Picture-recording aside, a multi-PCM Video 8 deck has numerous applications for the musician, not least in its ability to store possibly twenty-four hours of digital audio in stereo. It is possible to see how this could be applied to sound archives or the storage of sound effects, and of different electronically or naturally-generated sounds for subsequent loading into the many computer-based sampling keyboards available, for which the resolution of Video 8's PCM sound would be quite acceptable. The operation of the system makes it possible to switch between any one of the six tracks at a particular point on the tape, making time spent accessing your stored digital samples much shorter, for example, than if continuous, rather than parallel recording methods, were used.
Being also a conventional video recorder, a portable Video 8 recorder would make an ideal partner with the PCM-F1 processor for full 16-bit digital audio recording, replacing the now obsolete SL-F1 Betamax. This obviously takes up the recording space on a Video 8 tape of five companded PCM tracks (ten tracks of audio), so you can see from this what a saving in bandwidth has been made with Video 8's own audio compared with the PCM-F1, for a relatively small sacrifice in sound quality.
The flying erase head on Video 8 recorders allows for frame accurate editing, and I do not see why this could not be used for basic editing of Video 8's own PCM audio, although for editing of EIAJ format material (PCM-F1) a system such as HHB's CLUE would still be required to achieve click-free edits to frame accuracy.
PCM adaptors are available for units without a PCM facility, and these allow recording and overdubbing of the two channel segment with a picture, but do not automatically mean a multi-PCM deck being created, because this would normally require a dedicated machine. Even with two channels of digital (PCM) audio, the unit makes a very good everyday cassette recorder with three hours of non-stop playing time, having the advantage over Beta Hi-Fi that the audio can be recorded separately to the picture. This would mean that a video recording with mono FM sound and a totally different stereo digital recording could co-exist on one tape! The applications of that in bi-lingual training videos and other A-V productions are manifold.
So the upshot of all this really is that, although the sound is not faultless, the Video 8 format has allowed for remarkable flexibility, long playing time, and the ability for the user to decide what proportion of audio to video is right for him. Its success lies in your hands...
Feature by Francis Rumsey
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