Bits and Pieces
And you thought PCM was a medical complaint...
Chris Everard takes a logical stance and gets high on some digital recording equipment
Here's a short exercise to take your mind off what's on TV tonight; name an electronic musical instrument aimed at the serious musician which has been brought out during the last few months that doesn't employ digital technology in one form or another? Difficult, isn't it? The point I'm trying to make is that the market has been one of the few so far to revolutionise and throw away all (or nearly all) its design and construction policies in favour of developing and exploiting digital technology to its limits.
However, whilst Roland, Casio, SCI, Korg, Yamaha et al have been fighting for the Lion's share, a company which we all associate with hi fi more than anything else has been developing digital equipment for the professional audio/visual recording industries with great success... Sony of Japan (where else!).
Sony are not only manufacturers producing digital recording equipment; some of you may remember the recent Barclay James Harvest album 'Ring of Changes' that was released on record, cassette and compact disc all at the same time. That album was recorded on the 3M digital recorder; JVC and Philips are two other companies who have been developing equipment for the digital market.
Before we go into the details of the new digital technology it would be a good idea to briefly go over how 85% of professional recordings are still made — via analogue means.
Conventional analogue recording works along the principles of having a tape coated with a magnetic substance. This tape is then used to store electrical impulses as variations in the field of the magnetic coating. The electrical impulses are derived from natural air vibrations (sound) passing into a microphone. There are of course other means — direct injection, with guitars etc, but we'll leave that. When played, the tape passes magnetic heads which reconvert the electrical impulses. You then amplify these and feed them into loudspeakers which complete the cycle by converting the signals they receive back into air vibrations (sound) via a moving cone.
In digital recording, a digital processor converts audio, or analogue signals, into a series of binary pulses. It takes small amounts of the signal it's being fed with, analyses the sound in terms of the frequencies it contains and converts it into pulses of 'on' and 'offs' or 1's and 0's. The number of times per second that the digital processor changes the audio signal into a binary pulse is governed by the very highest signal needed. If you've got a nose which is constantly wet, a bowl, and a basket to sleep in, the chances are that you're probably a member of the canine family and can quite easily hear 20kHz and possibly more. However, we humans can rarely hear above 16-17kHz. Just to be on the safe side though, most audio equipment works up to 20kHz which means that to get an accurate signal, it must be sampled at least at twice this rate, of frequency, and this is usually 40-50kHz. This has been termed the 'sampling frequency'.
As mentioned before, the language used in digital processing is binary — 1 or 0. Every pulse is divided into a series of vertical steps which convey to the memory of the processor the exact height of each pulse. The more steps you have, the more accurately the system will be able to recreate a pulse of the correct height when reading or playing back the information from digital tape or disc. A professional digital set-up will usually use anything between 16,000 and 65,000 steps, depending on the number of bits that the processor can handle. 16,000 steps will need 14 bits and will produce a signal corresponding to a noise ratio of 84 decibels, while 65,000 steps will need 16 bits and can give a noise ratio of 96 decibels.
There are quite a few kinds of digital processors around, for use with Sony and JVC's professional three-quarter-inch (U-Matic) video cassette recorders and the conversion of analogue to digital in the production of compact discs from quarter inch analogue masters (hence the arrival in your local record shop of compact discs from recordings made before digital recording was a twinkle in a research department's eye!). The state-of-the-art processor (at the moment) is the Sony PCM 1610, which is used widely in video and audio production houses throughout the world. Among its many features is a built-in SMPTE time code generator which allows you to locate the programme contents precisely on the tape.
Multitrack digital though is where most interest is shown nowadays due to the fact that, because of the digital technology at the sound engineer's fingertips and the recent introduction of compact discs, the recorded material stays in digital form right the way through the chain to where you — the consumer — play it on your hi fi. Several multitrack recorders are available. The 3M machine from America, mentioned before, uses one-inch digital tape running at 45ips and can record up to 32 tracks. However, the machine which has really made a big impact is the Sony PCM 3324 machine which uses half-inch tape running at 30 inches per second and contains not only 24 digital tracks, but also a control track, an external data track and two analogue audio tracks for normal recording.
The PCM 3324 uses Sony's own brand of half-inch tape on 14 inch spools. It has switchable sampling rates; 44.1kHz at 66.5 cm/s and 48kHz at 72.38cm/s and all the features you expect to find on a top quality 2-inch 24-track recorder, including punch in and punch out facility and remote control.
The mixing desk hasn't been neglected either, in the quest for a fully digital recording system. You may think — as do a lot of people — that having a digital desk is going too far, as the current breed of high tech consoles are so advanced and noise free that digital multi-tracking and mastering machines are all you need to produce the ultimate in sound recording.
The first thing that springs to mind when comparing a digital desk to a standard one is size — you don't need a Moped to go from one end to the other and this makes the task of clipping the tape-op round the ear'ole for wiping the whole of that day's recording a lot simpler and less strenuous. The entire desk can be operated from just a couple of faders, equalizers and other switches, as the desk's operations and control functions are all assignable to every track of the tape machine. A colour video monitoring system is used by the engineer to display the required information.
Another interesting advantage is the ability to have a small remote control unit that can control 85% of the desk's functions through a single fibre optic cable. This comes in handy for people who work alone on recording projects and who need to change desk controls while playing instruments etc. Data storage and automation during mixdown can all be stored on a floppy disc and the scope for expansion in the field of digital mixing is incredible.
Leaving the world of pro-recording for just a minute, it's time to mention the fact that you don't have to be super rich to go in for digital multitracking. The Sony PCM F1 digital processor enables you with the use of two Betamax video recorders to do digital track sound on sound recording. So what? I hear you ask... well, one of the great advantages of recording in digital is that you can make huge numbers of copies without any audible degradation. This means with some good monitors and a decent mixing desk it's quite possible to produce fantastic results for around fifteen hundred quid.
The ability to make countless copies 'before the signal deteriorates by enough' so that you can hear it is a big plus in favour of any form of digital recording. This amazing achievement is due mainly to the fact that the binary pulses (numbers, not magnetic variations in an emulsion) are recorded at levels well above the residual noise level and well below the saturation level, therefore, because you haven't got the limitations set by normal analogue recording on magnetic tape with magnetic heads, a much improved dynamic range (around 90dB) can be attained. Digital editing can be a problem but using the equipment Sony are marketing makes it virtually effortless. When editing using their DAE 1100 digital audio editor and two of their U-Matic VCRs you gain the extra advantage of not having to physically change, splice or cut the tape at all. However, in all fairness it must be pointed out that any recording system employing tape as a recording medium will now and again fall foul of tape drop-outs. When tape has a momentary loss of coating during playback or recording on digital equipment, this is heard as a short 'hole' in the sound. To safeguard this happening, error correction systems have been developed that fill in the 'holes' so that you no longer notice them. These systems are now so advanced that physical splicing and editing of digital tape is possible without the joins being audible.
Coming back to digital desks for the minute, it's also possible using the floppy disk to program from an external source the onboard compressors (for instance) to sound like any other compressors on the market by just feeding their characteristics into the control system. This is just the tip of the iceberg, and I'm sure that it'll be possible to feed in 'phantom' FX derived from analyzing the characteristics of various digital systems. With Lexicon, Ursa Major and AMS wanting big money for their digital FX units a desk with the capability to mimic the FX they can create will easily earn its living — especially in a busy recording studio where time is (lots of) money.
With such a vast subject, as this, it's not really possible to cover every aspect, especially with so many new developments. All the digital recordings I've heard have been pure and in a couple of cases, almost impossible to distinguish from the original sound. Not having the extra worry of noise reduction on multitrack and still being able to bounce several tracks together without getting dreadful amounts of hiss or pumping is like a dream come true. The quality is stunning. Even though the prices being asked for the pro equipment make you go dizzy and give you sharp stabbing pains in the back of the legs, there's nothing stopping the average person from getting involved at home. So, get out there and start buying Sony PCM F1s and CD players — the world will never sound the same again!
Feature by Chris Everard
mu:zines is the result of thousands of hours of effort, and will require many thousands more going forward to reach our goals of getting all this content online.
If you value this resource, you can support this project - it really helps!