HiFi Sound - A New Video Standard?
At last! A new system that gives you hi-fidelity sound from a standard video recorder. Could it become the stereo reel-to-reel machine of the future? Judge for yourself.
Whatever will they think of next? First the Revox, then the compact cassette. We've had the Portastudio and now there's digital recording and compact discs. Then there's computers and the MIDI system. Digital delays and drum machines abound: next thing you'll he saying is that video machines can he used for audio recording! They can, as Carl Anthony found out when he went in search of a new video recorder and found an unexpected bonus.
Whilst Dad's been asleep in front of the telly and the musicians of this land have been hard at it with their 8-track and Portastudio recordings, something of a minor miracle has been taking place with the humble domestic video recorder. It's called HiFi Sound, and it promises to be a truly exciting breakthrough not just in video but for the audio world as well. Before we take a closer look at this new development, let's first take a look at what happens in a conventional video recorder.
One of the big problems any tape recorder designer faces is to try and obtain a flat, noise-free extended high frequency response. The slower the tape travels the more difficult the problem. When you consider that 30Hz-25kHz is about the limit for ½ inch machines running at 30ips, you can see the problems facing the video engineer.
In order to record a picture machines need to record and playback frequencies up as high as 5-6MHz. Now that is high when you consider that 1MHz is the equivalent of 1000kHz. Right from the start they realised that they couldn't have a tape transport system whizzing tape across the heads at several thousand inches per second. Imagine the amount of tape it would take just to record a four minute song if a reel of 10½" tape went by in 12 seconds. But they were smart. They realised that if they rotated the heads against the tape it would amount to more or less the same thing as having the tape travelling at supersonic speeds. In fact, you could make the heads rotate extremely fast and still allow the tape to just amble along at a snail's pace. And that's exactly what they did. In the UK, the heads rotate at 1500rpm and, depending on the video system (Beta, VHS), the tape travels either at 1.87cm/sec (Beta) or 2.34cm/sec (VHS).
Now this is all very useful for the picture information, but what about the audio? Until now that's been the Catch 22. In a conventional video recorder the audio track has always been recorded separately along the edge of the ½ inch wide tape. And as the tape is only physically moving at a very slow rate (compare the figures with audio cassettes, for example, that are recorded at a standard speed of 4.75cm/sec), it is easy to see why the sound quality on conventional video recorders has always been regarded as pretty dire. Noise reduction helped of course as far as hiss was concerned, but it didn't really do much for the dynamic range or the frequency response. Like a lot of good ideas, necessity was the mother of invention...
With the coming of stereo video tapes and the introduction of half-speed recording on some video machines, it was becoming quite obvious that something had to be done before the sound quality became totally objectionable, even to non-critical ears.
On paper at least, what the manufacturers have called HiFi Sound, is very impressive. This new development claims to provide an audio soundtrack with a frequency response typically from 20Hz-20kHz, wow and flutter less than 0.005% and a dynamic range of more than 80dB.
So, how did they manage to do the seemingly impossible? Well, the VHS machines use a system they call Depth Multiplex Recording. Beta manufacturers call it Audio FM Recording. For the purposes of this article it adds up to much the same thing. The question of whether the two systems are actually compatible or not simply does not arise: the two video formats use totally different cassettes, so even if you recorded a tape on one format there's no way you could physically get the cassette into a machine from the other format.
You may well be thinking that all this has something to do with digital recording. After all, the Sony PCM F1 digital system uses a domestic video recorder. No, it's not digital - at least not at the moment. For the time being all HiFi video recorders use analogue techniques for the audio soundtrack and they still retain the old system as well.
What in effect happens, is that the tape travels through the machine at the same old snail's pace and a conventional audio track is laid down along the edge of the tape. This is so that recordings made on a HiFi machine will still be compatible with non-HiFi machines of the same format (Beta or VHS, whichever system you have got). It also means that you can still replay conventional video programmes or films on your HiFi machine.
In the HiFi mode, a totally separate audio recording is made underneath the video recording! Yes, you did read it correctly. When you think about it, it is really very clever.
In order to record a colour picture you need basically two components (you need lots of other bits but we can ignore these for the moment). These are the Luminance signal and the Chroma signal. Luminance is basically the black and white part of the picture. In other words, areas of lightness and darkness. Chroma is the colour part.
The Chroma is recorded as a signal in the low MegaHertz area. The Luminance is recorded at a slightly higher frequency. Between the two sets of frequencies is a slight gap. It's a bit like a crossover point where the Chroma signal starts to tail off and the Luminance starts to come in. This is the area exploited by the HiFi recording technique (Figure A).
By taking the incoming audio signal and converting it to FM (like an FM radio in other words), the whole mono or stereo audio programme can be recorded onto tape just like a video signal. Now this in itself doesn't give you HiFi sound. In fact, if you just left it at that, the video replay head wouldn't know whether the audio signal was meant to be part of the picture information or something totally separate. Knowing the laws of nature it would in all probability decide it was part of the picture and you would end up with some pretty weird effects on your TV screen. So the designers did a second clever thing...
In a conventional video recorder, two tiny video heads are mounted on opposite sides of a revolving drum. These record diagonal lines across the width of the ½ inch video tape.
About a millimetre of tape is left at the top for the conventional audio track and at the bottom of the tape is a small space left for a series of control pulses (Figure B). These pulses basically reconstitute the diagonal lines of information which you see on your television.
To record a series of diagonal lines, the video heads have to be set at an angle with respect to the tape. In effect, this is no different to the conventional azimuth setting on an ordinary tape recorder. But instead of setting the heads parallel with the tape, they are tilted at an angle. Two heads are also required so that as one comes round on the drum and travels across the tape and 'off the top', the second one comes in at the bottom. This creates a continuous series of diagonal lines. Now the tricky bit!
In order to record the HiFi audio, a second set of record/play heads are mounted in the revolving drum. These are tilted in the opposite direction to the two video heads. This reduces the amount of crosstalk between the audio and video signals. To really keep the crosstalk to a minimum (and confuse matters even more!), the two video heads are also set at slightly different angles to each other; as are the two audio heads. Now you start to see why, not only is it quite clever, it's almost a miracle.
So what do we have so far? Well, the audio has been converted to FM. The video is recorded as diagonal lines going one way and the FM audio is recorded as diagonal lines going the other way. This can only mean one thing. That's right- two different recordings on the same piece of tape! How can that be possible?
When a HiFi sound recording is made, the first thing to happen is that the audio heads are energized. The audio head gaps are quite wide and the stronger signal forces itself deep into the magnetic coating of the tape. As the drum spins round so the first video head reaches the tape. This has a much smaller gap and records its higher frequency FM signal (Luminance) and AM signal (Chroma) into the upper part of the magnetic coating. Naturally, the audio signal is erased on the surface of the tape but the deeper part of the audio recording is left unaffected by the video head.
When it comes to replaying the tape, the video heads read only the top part of the recording and the audio heads delve deep down into the magnetic coating to retrieve their recorded information. As all four heads are at different angles, each searches out its own 'track' ignoring the other three. All in all a pretty amazing feat of engineering and, in practice, it seems to work extremely well.
So why should this be of interest to anyone but a video buff? Well, the beauty of the system is that you don't have to record just video. There is no reason why it can't be used purely as an audio recorder. You have the choice. HiFi Sound with pictures or simply audio by itself. Some companies are already claiming that not only are their HiFi video recorders better than the best cassette decks you can buy, but they also rival the quality of reel-to-reel machines. Tape is cheap - a three hour cassette costs less than a standard reel of ½ inch tape. There is also talk of making HiFi Sound a standard feature on all future video recorders although that still remains to be seen.
Even so, for the price of a top cassette deck or a reel-to-reel machine, you can now buy a tape deck that will not only record your stereo masters but can be used to record the band's video as well! And if you haven't got any cash and someone in your family is looking for a video machine, a gentle push in the right direction could mean that they get their video player and you get a brand new mastering machine for nothing.
The story doesn't end there either. If you are familiar with the Yamaha CX5M, you'll know that it's a computer-controlled synthesiser. What you may not know is that the computer part of the package has been made to the new Japanese MSX computer standards. Most of the major Japanese HiFi companies are also involved in the project and it shouldn't be too far into the future before we start seeing computer-controlled HiFi systems.
Now this you may regard as a mixed blessing. One of the spin-offs from these developments, however, is the very real possibility of seeing electronic video editing being made available for domestic video. If the same techniques can be applied to audio recordings made on HiFi machines, you start to get some idea of just how exciting the future for home studio recording may become.
As I say, this is all some way off in the future but with the growing number of HiFi machines appearing on the market, at least the first major obstacle has been overcome. Now it just seems a question of time.
Feature by Carl Anthony
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