Alesis ADAT XT20

Paul White tests the 20-bit Alesis ADAT XT20, to see whether it wil really help us make better recordings or just allows us to record our low-level hum and noise more accurately!

When Alesis' original ADAT digital tape recorder was launched way back in 1992, SOS hailed it as "one of the landmark products in the development of hi‑tech music and recording." It brought affordable digital recording into the home studio and was rapidly adopted by musicians all over the world, both project studio owners and professionals who found that they could assemble a serious studio at a lower cost than ever before.

Alesis didn't rest on their laurels, though, and in early 1996 they introduced the ADAT XT, featuring improved converters, a faster transport, and access from the front panel to features previously only available via the optional BRC Big Remote Control. The ADAT format continued to flourish, and Alesis' proprietary ADAT 8‑channel optical digital interconnection format began to find its way into products from other manufacturers — a sure sign of success.

However, the hi‑tech music world has a habit of never being satisfied — why settle for 16‑bit digital audio if it's possible to have 20‑bit? Alesis have responded once again, and as of now, production of all 16‑bit ADATs will cease, to be replaced by a new range of 20‑bit models. The top of the range is the M20 (the ADAT formerly known as Meridian), which is aimed primarily at professional users, while the original XT has been replaced by the XT20 reviewed here. There's also a lower‑cost model, the LX20, that will be reviewed in a later issue of this magazine.

The XT20

As far as I can verify, the majority of the XT20's hardware, including the transport, is identical to that of the original XT, but the recording system is now 20‑bit and the A‑D and D‑A converters are actually 24‑bit. A new recording format, known as ADAT Type II, has been adopted to accommodate the additional information recorded to tape. The front‑panel metalwork is anodised with a pale champagne finish, to differentiate it from the regular XT, which has a blue/grey tint.

The 20‑bit ADAT Type II format has been implemented without reducing the play time of a standard ADAT tape, and it's still possible to play and record with the new machine using the original 16‑bit ADAT Type I format where backwards compatibility is necessary. However, a Type II (20‑bit) tape cannot be played on an original Type I ADAT.

Alesis ADAT XT20 rear panel connections.

Why 20 Bits?

Because of the way in which digital data is encoded in a linear PCM (Pulse Code Modulation) system, a 20‑bit recording has 16 times the resolution of a 16‑bit recording, but if 16‑bit is OK for making CDs, why do you need any more at the multitracking stage? The simple answer is that when you make your original recordings, you have to leave a little safety margin, or headroom, to prevent clipping, so with a 16‑bit system, you're only likely to be using 14 or 15 of the total bits, resulting in less than optimum resolution. Note that one bit is equivalent to 6dB of level, so if you record peaking at 15 bits, your signal‑to‑noise ratio and dynamic range is worsened by 6dB. A commercial CD, on the other hand, will be normalised so that the loudest peaks reach digital full scale — at the editing stage, your 14‑ or 15‑bit recording has to be scaled up in level, but this only increases the level, not the resolution.

Perhaps more seriously, every time a digital signal is processed in some way, whether it's being mixed, compressed, equalised or whatever, the process will invariably involve increasing or decreasing the level of the original signal, and every time this happens the subsequent mathematical rounding up and down of the data causes further loss of resolution. To obtain the optimum 16‑bit CD master, therefore, it's necessary to start off with more bits than you need, so that after all the data processing has been done the resolution of your signal does full justice to those final 16 bits. Furthermore, if you're planning to use a noise‑shaping dither system at the end of the mastering process, to improve the low‑level resolution of sounds close to the noise floor, the process requires that you start off with more bits than you intend to end up with.

...your old tapes will still play on the new machine.

That sounds fine on paper, but what are the audible benefits? To be honest, most people recording pop music are not going to perceive any significant difference, as pop music rarely includes the sort of low‑level passages that might benefit from the extra dynamic range of 20‑bit recording. However, if you're recording live you can leave yourself at least 10dB more headroom (in theory, it should be 4 x 6 = 24, but converter design isn't that perfect), and still end up with the same quality of recordingas you'd get with a 16‑bit machine just peaking at 0dB DFS (Digital Full Scale). A Type I ADAT has a dynamic range of around 92dB, A‑weighted, but Type II claims 102dB A‑weighted.

To hear how much better 20‑bit is than 16‑bit at low signal levels, such as may be encountered as the room ambience dies away in the quiet section of a classical recording, I listened to a comparison of the same signal (a piano recording) recorded on both formats at a level of ‑60dB. In the days of analogue tape, that would have been right down in the noise. On the 16‑bit machine, the sound of the notes decaying was extremely rough, with the background noise almost as loud as the wanted sound. It was quite unusable, but then you wouldn't normally record with the record level meters showing ‑60dB, would you? The same recording on the 20‑bit machine, while still not perfect, was reasonably quiet and significantly smoother, decaying politely into silence rather than grating its way into oblivion. In everyday terms, this means that decaying reverb tails, and such‑like, remain clean down to much lower levels. Those with golden ears also claim that stereo imaging is better preserved by systems recording with in excess of 16 bits.

In reality, you'll only benefit from 20‑bit recording if you work with very high‑quality sound sources, otherwise all those extra bits will simply go into making a much more accurate recording of your hum and noise! Ground‑loop hum, circuit hiss, tape hiss and digital noise from synths will void any benefit you might have gained. As ground‑loop hum is so difficult to eliminate completely (not least due to the different ways in which manufacturers handle their internal grounding systems), it may be true to say that unless you have one of the very best analogue desks, you might be better off using a digital desk connected via the ADAT optical format, in order to avoid this source of signal contamination.

One of the exciting aspects of the Type II format is that instead of mastering to DAT, you could mix to two spare tracks of your ADAT, thus preserving the 20‑bit signal resolution, pipe this into a 20‑bit‑compatible computer editing system via an ADAT‑compatible interface or card, do all your editing and level changing, then finally use a noise‑shaped dither process to reduce the file to a pristine 16‑bit master.

Impressions

Aside from the 20‑bit implications, the feature set and operation of the ADAT XT20 are essentially identical to those of the XT, and given that the XT20 costs less than the original XT, there's no arguing that it represents good value, whether you feel you need 20‑bit audio or not. Personally, I welcome the ability to record peaking at ‑15dB DFS so that my desk meters match those on the ADAT without losing quality, and for live work the extra headroom is a godsend.

While tape may be seen as a little old‑fashioned, you can't argue with the fact that the media are cheap and archivable, and no matter what else goes wrong, the tracks will always remain in sync with each other — something that isn't always true for tapeless computer‑based systems. Even so, I do feel that Alesis have missed a trick for the second time running. Back when the original XT was launched, I complained that despite the fact that nearly everyone has a sequencer, no MTC (MIDI TimeCode) output was provided, and now Alesis give us the XT20 with the same glaring omission. Many people don't need a BRC, or even the added features of a JL Cooper DataSync, so for them a simple MTC output referenced to the ADAT's real‑time subcode would have done the job perfectly adequately. All that's needed is a menu to select the frame rate — offsets could be dealt with in the sequencer. In this day and age, when every digital multitracker from entry‑level upwards has an MTC output, leaving off this vital function just gives more ammunition to those critics who claim that tape is a dinosaur.

Summary

Other than the 20‑bit signal path, the ADAT XT20 is still a traditional ADAT, with all the strengths and frustrations that embodies. The lock‑up time between two or more machines is still exasperating when you're trying to do a series of complicated punch‑ins, but you can't deny that the system is robust, the media are cheap, and you won't suddenly lose half your tracks on playback, to be greeted by an 'UNEXPECTED ERROR TYPE 42 HAS OCCURRED' message. And, perhaps just as importantly, in this confusing world of changing recording standards and removable media that become obsolete overnight, ADAT is about as close to a true standard as we're likely to come.