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The Hard Edge

Direct-to-disk recording explained

Article from Music Technology, August 1993

MT's guide to the theory and practice of the tapeless studio

Wot, no tape? Recording acoustic sounds straight into a computer will soon be as common as MIDI sequencing and 4-track taping. So here's MT's beginners guide to the future, from basic principles to the latest available systems. On the case: Ian Waugh.

Sync up those guitars with Cubase: Steinberg's Cubase Audio

It's a funny old world, innit? For years musical instrument manufacturers have been beavering away to give us better sound courtesy of digital synthesis - and now fashion dictates that we buy up old analogue instruments at grossly inflated prices. The fact that you can produce all these old analogue sounds - and more! - on virtually any digital synth seems to have escaped notice. But let's not allow the facts to get between a fool and his money.

The recording industry, too, has experienced something of a back-to-analogue backlash. Having busied itself for years developing low noise, high clarity digital recording systems (so that the record companies could busy themselves ripping off the CD-buying public) musicians and producers - demonstrating their customary perversity in these matters - respond by going back to using analogue tape because, they say, it's inherently 'warmer'. What they actually mean is they don't like the high frequencies in their music which tape conveniently masks, and they find the clarity embarrassing. Ah well.

For most of us, still struggling with personal multitrackers and budget 4-and 8-track tape systems, digital recording is a luxury we only wish we could afford. But the fact is, it's a luxury which is becoming ever more affordable - particularly in the area of direct-to-disk recording - with systems now retailing for just a few hundred pounds.

Before we get down to the nitty gritty, let's sort out the difference between digital multitrackers and direct-to-disk recording. Although both types of system convert audio data to digital data and back again, digital multitrackers such as the Alesis ADAT and Tascam DA88 record the data in a linear fashion onto video tape much like a traditional analogue multitracker - except, of course, the data is in a digital format. Direct-to-disk recording writes the data directly to a hard disk.

Unlike tape, you can access any part of a disk - and therefore a song - almost instantly. This has many benefits when it comes to editing and well be looking at these later. But for now lets consider the other principle advantage of direct-to-disk recording - the high quality sound.

Digital recording does away with the background noise which is an inescapable part of any tape recording. Good AD converters will ensure a broad, flat frequency response with highs and lows that aren't subject to the vagaries of tape characteristics. You can also back up a recording with no loss in quality and 100 percent accuracy.

Other traditional audio problems which simply disappear with the switch to digital recording include crosstalk, drop outs, noise reduction colouring, counter slippage, timecode problems (through dropouts and/or timecode leaking through to adjacent tracks) and tape speed problems such as wow and flutter.

Impressive as the sound quality argument is, however, the real benefits (for most people) of direct-to-disk recording are to be found within the list of editing features. These are akin to many of the facilities you'll find on a sampler, but unless your sampler can take upwards of 200Mb of RAM (which, at a conservative estimate, would cost about £7000!), direct-to-disk recording lets you work with much larger files.

There are three main advantages - accuracy, quality and versatility. As you're no doubt aware, digital data is comprised essentially of numbers - huge strings of numbers, in fact, but each one individually accessible and capable of being changed. You can't get much more precise than that. No faffing about rocking reels past tape heads armed with a razor blade. And none of the problems associated with editing stereo or multitrack tape recording. Most direct-to-disk systems allow you to edit individual tracks - or just one side of a stereo recording.

And of course, numbers don't deteriorate with editing. You can shuffle sections of the recording around ad infinitum (if you've got that long) with no loss of quality. You can also bounce tracks - repeatedly - on a multitrack system with absolutely no degradation of signal quality.

Another huge advantage is the non-destructive nature of the editing. You could look at this rather like a software jukebox which lets you specify the order in which you want a series of MIDI files to play. Simply select a portion of the recording and drop it into a playlist. Totally new recordings can be built up in this way: all the system does is to play back sections of the recording in a different order using a series of markers. The original recording stays intact.

If the first vocal chorus is better than the second, you can create a playlist which uses the first one whenever the chorus occurs. You can record several takes of a section of music and paste the best one into the final version. In fact, you could paste in that excellent high note the singer hit in an otherwise naff verse, recording into a better recording of the verse in which the high note was fluffed.

In many ways, you can think of a direct-to-disk recording like a MIDI recording - editing is possible down to the level of individual notes through a variety of different functions. Of course, the precise nature of these will vary from one system to another, but cut, copy and paste should be pretty standard. It's also likely that there will be invert, fade in and out, and crossfade facilities and indeed, on-board digital effects which may be applied in real time during playback.

Other useful functions might include EQ and timestretch/compression which would let you, for example, squeeze a 33 second jingle into 30 seconds without altering the pitch.

Direct-to-disk recording has uses other than the creation of a song. It can be used, for example, to select the running order of songs on an album. Say you have a DAT master but want to re-jig the order of the songs. Load the tape into the system digitally (there will be no loss of quality) create a new playlist and save it back to DAT. You might also decide to crossfade the end of one song into the start of another. Again, no problem.

Direct-to-disk is also becoming the preferred medium in film dubbing. As the recording can tie in with SMPTE very easily, its a simple matter to sync film and sound.

At this point it's perhaps worth including a few words about digital inputs. Once a sound has been converted to a digital format it makes sense to keep it in that format during any edit or transfer procedures. To convert it into analogue audio to transfer it to another device will inevitably result in a loss of quality (albeit a very small one). Most samplers have a digital interface or a provision for adding an optional one. DATs have digital in/outs and some CD players include them too. Many sample CDs now also feature digital data tracks which you can load directly into a sampler via a digital interface - and, of course, into a direct-to-disk system.

As the production of music is carried out more and more in the digital domain, digital inputs on equipment will become more prevalent. At the Frankfurt show earlier this year, Akai even announced the imminent launch of a digital patchbay. Notwithstanding my comments at the beginning of this feature, when such high quality becomes a possibility, few people would want to yield an inch to the ravages of analogue systems.

But all this versatility does require a degree of power from the system playing host to it. For example, the hard disk itself will need a pretty fast access time so the system can read the data off it fast enough to prevent hiccups. A computer-based system will also demand a certain amount of power from the computer's processor - although if the system uses an external hardware unit, the computer should have a fairly easy time of it and should be largely free to handle MIDI data, etc.

And this brings us to yet another massive advantage of direct-to-disk recording - the complete integration of digital audio and MIDI sequencing. This, for many people, would be the ultimate recording setup: a system which can play back a MIDI sequence and an audio recording together, with no tape sync problems and the simultaneous viewing and editing of the two types of data.

Most direct-to-disk systems can do this, but you'll need to check the specs as sending out MIDI data in sync with digital audio data requires a considerable amount of processing power and there might be limitations.

It could be argued that we don't actually need direct-to-disk technology. After all, we managed perfectly well for years with vinyl and cassettes. And a good analogue multitrack recorder with noise reduction can produce excellent results. But the plain fact is, digital recording is of a much higher order; indeed, the quality matches that of the CD playback systems which have found their way into between 30-40% of homes in the country. Combine this with the powerful editing facilities and the fact that the technology has now reached the stage where it is affordable for the semi-pro and enthusiastic home user (it's virtually a must for the professional) and I think you have a pretty convincing argument for investigating the possibility of going direct-to-disk.

As technology marches inexorably onwards, more changes will come. There will almost certainly be a move towards 20- and 24-bit systems; smaller, faster more reliable hard disks; and of course, regular drops in price. One day - and it's not too far away - well be using personal direct-to-disk multitrackers costing just about the same as the current tape-based machines which are the cornerstone of home recording.

Of course, as with today's analogue machines, there will always be a noticeable quality difference between these machines and professional systems. But even at its lowest level the quality of the personal direct-to-disk systems will be far superior to present-day tape multitrackers. And given the right number of bits, there's no reason why a personal direct-to-disk system should not give the same quality results as a heavy-weight pro system. After all, even without a direct-to-disk system it's possible to record professional quality music (using a MIDI sequencer) by recording direct to DAT. Direct-to-disk systems will simply extend that ability into the analogue audio domain.

That digital is going to be the preferred recording medium of the future there can be no doubt. The only question to be asked is whether to opt for a digital tape system, such as ADAT, or go direct-to-disk. You might also find yourself mulling over whether to dive in now or wait until your chosen system becomes even cheaper. In real terms, a direct-to-disk system today is cheaper than those early Teac 4-track machines (I had an A-3440 which cost around a grand over 12 years ago!). But you may think it's worth waiting a year or so to see how the situation pans out. Trouble is, by then, everyone will have one. Not that this is a good enough reason to part with your money now, but it is worth considering just how much experience of direct-to-disk systems you could gain in 12 months and how much of a creative - and commercial - advantage that could prove to be, particularly in the semi-pro arena. You may even be able to hire out your direct-to-disk facilities to song writers and semi-pro studios.

The choice, as ever, is yours. But rest assured that MT will be doing its utmost to keep you abreast of new developments and new systems as and when they appear. There's no doubt in our minds that one day all recording systems will be made this way.

System selective

Roland DM-80R: hard disk recording without a computer!

The most popular direct-to-disk systems are still based around the Apple Mac, but the PC is quickly becoming a popular alternative. There are also systems for Atari's Falcon and Commodore's Amiga.

The following list does not claim to be exhaustive but aims simply to point you in the right direction if you're looking into the possibility of buying a direct-to-disk system...

Digidesign were probably responsible for the launch of direct-to-disk systems with its ProTools package for the Mac. The company has a current list of about seven direct-to-disk systems which run on the Mac and PC and it's well worth getting further info if you have one of these machines.

Opcode's Studio Vision (now down to £499.95 from MCM) for the Mac can integrate with Digidesign's Audio Media card and the two are available for £1173.83.

Steinberg's Cubase Audio from Harman should soon be available for the Atari ST/Falcon, the PC and Mac. This integrates direct-to-disk recording with Cubase and is bound to prove popular with Cubase users. It can support up to 16 tracks of audio with ProTools and it can also work with Yamaha's new CBX-D5 hard disk recorder (around £2500).

Emagic's Notator Logic Audio from Sound Technology will follow fairly soon, too, first on the Mac and later possibly on the Falcon. Sound Tech also handle Mark of the Unicorn's Digital Performer which is supported by MotU's own digital board and Yamaha's CBX-D5.

Roland has a range of DM hard disk recorders and Akai launched the DR4d (£1599), a 4-track hard disk recorder at Frankfurt this year. You need a hard disk in addition to the unit but the system is not based around a computer.

d2d's software and hardware for the Falcon has already been mentioned and has been reviewed on page 62.

The Soundscape system offers 4-track direct-to-disk recording for the PC for £2500 and was reviewed in our July '93 issue.

Studio Audio & Video also has a 2- and 4-channel up-market PC direct-to-disk system called SADiE, reviewed in our January '93 issue.

The hidden cost of direct-to-disk

Compared to the cost of a personal multitracker, a direct-to-disk recording is still a relatively expensive business. The Falcon promised to bring affordable direct-to-disk recording to the masses with its built-in direct-to-disk hardware, but still the cheapest system, including a monitor and software will cost around £1500 - more if you want a greater recording time and better quality. And whichever direct-to-disk system you plumb for, there are hidden costs which should be take into account when working out your budget.

The first is the size of the hard disk. Assess your requirements carefully in terms of recording time. If you need 400Mb of disk space, remember that after formatting and partitioning, a drive could lose 20Mb, so a physical 400Mb drive could end up only giving you 380Mb.

If the direct-to-disk system can run in conjunction with a sequencer, you may also need to invest in extra RAM.

But the most vexed question is that of backing up your data. When you've filled a disk with a song or album, you have to wipe it if you want to record new material. You could backup to floppies, but apart from the cost it would take an age - you'd need over 150 high density floppies to back up a 200Mb hard disk!

A far more convenient option is to use a tape streamer which is used for backing up traditional computer disks, or to backup to DAT. As you'd probably be mastering to DAT (you couldn't use all that digital technology and master to tape!) this could be best option, though you'll need a direct-to-disk system with digital ins and outs (most have these, but Falcon users will need extra hardware).

Another alternative is to backup to optical disks. These typically store about 120Mb of data and cost about £40 each, although the drives are currently around £1000! Optical drives are too slow to be used for direct-to-disk recording although they can be used successfully for other computer applications.

Quality CD

In any discussion of digital recording technology, the phrase 'CD quality' inevitably crops up. But what precisely is CD quality and just how good is it? CDs have a sampling resolution of 16 bits and a sample rate of 44.1kHz. However, these specifications alone are not enough to guarantee a 'CD quality' performance.

The number of bits refers to the data storage format - the more bits, the greater the accuracy of the digital representation of the sound. In an ideal world, 16 bits would provide absolute accuracy (or as near as damn it), but the data has to get into the system and out again. There is no point in storing data to a resolution of 16 bits if the playback circuitry can only output with an accuracy of 14 bits.

To put it another way, a 16-bit machine should have a signal-to-noise ratio of 96dB - which is commonly accepted as being CD quality. That's the theoretical dynamic range. If the converters aren't up to scratch and were operating at, say, 14 bits, then the SNR would be down to a theoretical maximum of 82dB.

Because of the AD and DA conversion process, a 16-bit system will actually have a SNR of around 90dB. However, if you leave a little headroom while recording, the SNR drops even further.

What this boils down to is that in order to guarantee a 16-bit resolution, the equipment should actually use more bits. This is why you will now often see high quality equipment quoting an accuracy of 20 and 24 bits. There's little doubt that in time, all high-end equipment will be built to this specification and then we will truly have CD or 'real life' quality.

Until then, check the specs of equipment carefully and don't assume that anything which offers 16-bit resolution and a 44.1kHz sampling rate will automatically give you 'CD quality'. The Atari Falcon is a case in point. Although the specs quote 16-bit resolution and high sample rates, the resulting output falls somewhat short of this ideal. (See the review of d2d's 4T/FX on page 62.)

Hard disk, big bucks

As with samplers, direct-to-disk recording is a trade off between quality and space. Assuming you are recording at 16-bit resolution and at a sample rate of 44.1kHz, 1Mb of hard disk space will be required to store 10 seconds of a 1-track recording. A typical 4-minute stereo recording, therefore, will need about 50Mb.

If you want to use direct-to-disk to re-order the tracks on an album in one take, say for a CD, and the album is 45 minutes long, you'll need a 600Mb hard disk. For the Mac this will typically cost around £900, for the ST around £900 and for the PC about £700 - plus VAT! Why the difference in price? A combination of tradition, competition and supply and demand.

One thing you must bear in mind is the speed of the drives. Most of the larger drives will have an access time of around 15ms which you'll probably need if you are using a multitrack system with more than two tracks. The manual for the direct-to-disk system in question will tell you what the minimum speed of the drive should be. Some stereo systems are happy with 20 or 28ms drives.

The conversion business

Direct-to-disk recording saves sound to disk in digital format, but what exactly happens to the sound during the conversion process?

Recording works on the same principle as a sampler. An analogue-to-digital (AD) converter measures or takes a sample of the sound at regular intervals. The frequency with which this occurs is referred to as the sample rate and expressed in kHz or so-many-thousand samples per second. CDs use a sample rate of 44.1kHz.

In a 16-bit system each sample is represented by a number (the digitisation process) between 0 and 65536. This is the sample resolution. For the wireheads, a 16-bit number consists of 16 binary bits - 1111111111111111 - which translates as 2^16 or 65536. But you knew that already. Didn't you?

If you use a lower resolution, say 8 bits, then the numeric range would be between 0 and 256 which will not give as accurate a picture of the sample as 16-bit resolution. Being in numerical form, the data can easily be handled by a computer and this allows for extremely accurate editing.

Once complete, the numbers are squirted out through a digital-to-analogue (DA) converter which translates them into sound again.

You can copy digital data with 100% accuracy as, again, all you are doing is copying a stream of numbers. This is what caused the fat men in the plush record company offices to sweat when DAT machines arrived - suddenly technology had made it possible for anyone to copy a CD with no loss of quality. This lead to the inclusion of SCMS (Serial Copy Management System) into many consumer DAT machines which allowed an original recording to be digitally copied, but prevented further copies being made from that copy. It was a futile act, as SCMS override units soon appeared.

This has nothing to do with direct-to-disk recording, by the way, unless you master the recording onto DAT. As, of course, you will.


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Publisher: Music Technology - Music Maker Publications (UK), Future Publishing.

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Music Technology - Aug 1993

Feature by Ian Waugh

Previous article in this issue:

> The A-Z of Analogue

Next article in this issue:

> The MT Ambient Quiz

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