Why are top recording artists like Toyah, Haircut 100 and Christopher Cross using Digital recording systems? The reason is simple — the recording quality is as near perfect as you can get as Joe Clerkin of 3M explains.

There aren't too many homes without some kind of quality hi-fi system to be proud of these days, and though it may be only the family pet who gets anything like a good centre stereo position, it's fair to say that the general public have some idea of what good quality sound should be. A visit to London's Tottenham Court Road, the hi-fi Mecca of Europe, gives a taste of the penetration of the boom in consumer electronics over the past ten years.

The recording industry for its part has played an important role in improving sound quality. It's hard to believe that nearly twenty years ago, George Martin was recording the Beatles on four track and in mono! The big milestone for the recording world was, of course, the development of multitrack recording in the late sixties, with much of the pioneering work being done by the 3M company. The M79 recorder, no longer in manufacture, is still the workhorse of many recording studios throughout the world. Multitrack recording, the technique that allows each individual part of a musical work to be put on tape in an analogous method to the way music is set out in a full composer's score, was a real quantum leap in the music business. Firstly, it enhanced creativity: once the basic tracks were down on tape, they could be endlessly played with — mixed and remixed until the producer was satisfied with the result.

Secondly, you don't need to have all the musicians available at the same time for recording. This has proved especially useful for the classical music producers. If the soprano goes down with 'flu on the week of the studio recording, there is no need to cancel the orchestral session. You just record the orchestra without the soprano and add her later.

As musicians and producers have become more confident with the techniques so the number of tracks used has grown. The early eight track machines were superseded by 16 then 24, and finally 32 track, the maximum track density for 2 inch wide recording tape. It's still possible to go further, by syncing two or more multitrack machines together. 32 track recording is more than adequate for most commercial studio recording and has virtually become an industry standard, and was where the industry stood in the early seventies.

There was now only one more hurdle to overcome — the problem of tape noise. All the systems described up till now are analogue machines. As well as recording the audio signal, tape noise and other undesirable factors are registered as well, especially in quiet musical passages. Though highly sophisticated noise reduction systems have been developed including DBX and Dolby, they cannot eliminate the degradation in quality of the signal that occurs between the stages of signal transfer, from multitrack tape to two track master and then, finally, to disc cutting.

Digital recording was developed to overcome this problem. The result is that it is now possible to produce studio mastertapes with an incredible 90dB plus signal to noise ratio, a transparent recording which is as authentic as the real sound.

Multitrack digital recording was first achieved by 3M in late 1978 and the first 3M 32 track and four track recorders were installed in studios in the US in February 1979.

The 3M System

Whilst other systems are being developed and are commercially available the 3M system was the first to be so giving one channel of audio per track of tape. In effect the 3M system was designed so that it could fit in with studio equipment in the same way as the multitrack analogue recorder. The one track per channel configuration was developed as a result of joint co-operation between 3M and the BBC, the latter of which have had considerable experience with digital techniques in their PCM systems, used for sending broadcast material from studio centres to transmitting sites.

The recording process works by sampling the analogue waveform at a frequency of 50, 48 or 44.1 kHz. Each 1/50,000 second point in the waveform is assigned a numeric amplitude value using a 16 bit binary word. Two words are created, each representing the same sample of analogue information and a further parity word is created by comparing these first two words. All the data is spatially separated on the recording tape.

The reason for this is very simple. Every square inch of digital tape contains 890,000 separate magnetic bits of information. At a tape speed of 45 ips the number of write-ins and readouts could be as high as forty million. A small quantity of dirt or a tape blemish could cause a drop out of vital data which on playback would manifest itself as distortion or clicks.

By spatially separating the two sample words and the parity word, it is statistically unlikely that a blemish or dirt on the tape will be able to prevent reconstruction of the encoded data.

Playback of data to reconstruct the analogue signal is not dependent on the mechanical variations of the tape transport system. A time base corrector with a crystal controlled clock gates the digital words out of a playback memory providing precise even timing and completely eliminates wow and flutter. Minor variations in tape speed are servo controlled by the electronics of the readout circuitry which prevents lags of data input or pile-ups of data output.

Although recording in the digital domain enabled much higher standards to be achieved in terms of recording quality, the technique prevented conventional editing methods from being used. In conventional studio recording the multitrack master tape is mixed down to two track stereo tape — of the quarter inch format. The editing process consists of assembling takes by a cut and splice method — a tricky process and choosing suitable edit points is a matter of experience. It is easy to cut on something percussive such as a drum beat for instance but you never cut in the middle of something stringed, or on reverberation.

Editing: Initially all digital recordings had to be transferred to analogue for editing until a suitable electronic editing process could be developed. The process resembles the dub editing technique which is used to edit video tape where again cut and splice techniques are not applicable.

Electronic editing has the advantage that the tape remains untouched and unaffected by the magnetic properties of the razor blade and even the best edits using the analogue approach are subject to some degree of "drop out".

The other advantage to electronic editing is that the edit point can be auditioned and refined with variations as small as one millisecond. Two machines are necessary for the editing process linked by an editing unit. When a pair of edit points are thought to be final, the edit can be previewed. The preview edit function causes the first machine to play up the first edit point, mutes its sound whereby the sound then comes from the second machine. The tapes can be programmed to move backwards and forwards across the edit point to allow the edit to be further refined if necessary before committing the edit.

Performance: As described the 3M digital recording system has an outstanding signal to noise ratio of 90dB plus, which means a complete lack of tape noise. The dynamic range is also very great, which enables very low levels of sound to be recorded with high; level sound without resorting to the use of compressors and eliminating the distortion associated with these devices.

Improved Disc Cutting: The digital delay process employed in the 3M system has lead to improved disc cutting. Where there are particularly loud passages in music the disc cutting lathe pitch has to be set so that the disc grooves are more widely spaced than in quiet passages. It is possible to feed a direct signal to the pitch control and a delayed signal to the disc cutter so that the cutting process is automatically regulated. This leads to more efficient use of the disc surface.

Uptake

There are currently more than sixty 3M digital recorders at work in recording studios in the USA and Europe. Studio rates on digital are higher than on conventional analogue recordings but then the results are superior. It's hardly surprising that in the Rock Music World a handful of top artists including Rod Stewart, Christopher Cross and Abba have used the system. On the other hand classical recording companies have been very keen to use the system. The reason for the outstanding quality of the recording process is the superior life of the master tape. The average rock album is deleted after a year but classical recordings have a life of ten years or more and it's important to produce a master tape of superior quality if several disc cutting masters need to be derived from it.

In the UK, once the heart of the music business, the uptake of digital has been confined to just two studios, Roundhouse Studios in London and now Lodge Studios in Suffolk. This attitude will have to change very soon. Most of the top Japanese hi-fi manufacturers have been demonstrating compact digital audio disc players and are committed to launching these products on the market within the next year. The compact digital audio disc, optically scanned and digitally encoded, enables the sound to be reproduced with the highest possible fidelity. What's more the disc is free of problems like static, scratching and dust so it won't wear out.

In order for recording companies to get the best possible results from the compact audio disc, it will be necessary for them togo digital and master using a digital recording process. According to Peter Gallen, Studio manager at Roundhouse Studios, this is where the 3M system will do well. "The 3M digital mastering system is the only digital system that has really been tested in studio conditions and it's the only one that is really compatible with multitrack analogue systems," he said. Roundhouse have been using the 3M digital system since 1980 and in two years their business has increased, despite the general downturn in the pop music business. They've also achieved a considerable degree of artistic success with the band 'Haircut 100' who've recorded their hit album and singles on the digital system.

Lodge Studio manager Lester Mortimer is also convinced that the digital boom is just a short way off. At the recent Association of Professional Recording Studios Exhibition he said that multitrack analogue had been pushed as far as it could go and that digital was now the only way forward.