Food for Thought on A-D/D-A Conversion
When converting audio from analog to digital, we are trying to preserve what we already have, but does it really stay the same? Despite the fact that I’m a huge analog fan, I believe that digital storage and processing are very useful and do have their places, especially in mastering. Even in our studio we have four DAT players including a classic Studer, but some people will question the idea of converting their digital masters back to analog for processing.
We convert to digital, thinking our audio will be as safe as it was before conversion. However, the conversion back to analog brings with it a perceived “analog degradation”. Was the program material perceived to be in a state of degradation before the initial A-D conversion? Probably not. This perceived “analog degradation” is not really an analog problem at all. In almost every case, it can be traced to inadequate conversion to and from digital. Not the state of being in digital, but the conversion process itself.
Contrary to the big-budget marketing hype of various tape and equipment manufacturers, get ready now . . . “There is no such thing as digital sound.” All sound is analog.
Some sounds are generated in a digital environment, but most sounds we hear and record are analog sources (i.e., piano, drums, violins, guitar amps, horns, etc.). Because our ears are not a digital device but an analog transducer, all sound is heard . . . “analog”. So what’s the problem with analog? There really isn’t one. It’s the first and last step in almost every recording session and it’s a great recording medium (how many studios wouldn’t swap their ADATs for a 2″ Studer if they could?), but in all the comparisons I’ve been involved with, the weakest link is still getting in and out of digital with some degree of accuracy.
Most DAT players’ A-D and D-A converters lack the precision of high-end converters such as those available from Apogee, Wadia or Prism. Considering this grade of converter shows up at the 4K-8K price point and higher, and a common DAT player like the Tascam DA-30 costs about 1.5K — only a fraction of this 1.5K goes into the construction of its converters. Imagine a 1/2″ 15 ips Dolby SR master transferred to one of these machines. This would result in a digital recording, but with comparatively low fidelity because of the inaccurate A-D and D-A conversion.
So go ahead and record your tracks to digital, but understand that using a set of high end converters will eliminate the weakest link in the chain, providing you with a precision copy of your work.
Analog ‘0′ vs. Digital ‘0′ – Are They the Same?
In mastering, many digital masters are submitted with different regard to audio levels and their relative reference level tones. While there is no officially recognized standard that bridges the two, many audio engineers (including the author) have found a few simple rules that work well. Most engineers already know these but for those who are just starting out, here are a few points:
Do not put a 1 kHz tone at ‘0′ digital full scale. A tone at this level will be from 12 to 16db higher than it should be, it has virually no relationship to the RMS audio value that it’s supposed to represent and is brutal on your speaker cones and signal path, not to mention your ears. Can you imagine audio levels hovering nicely around 0 VU, preceded by this killer 1 kHz tone ripping your ears off at +15 VU? It happens!
A level reference tone on a DAT, just like on an analog recorder, is supposed to represent RMS audio values, not peak values. On a DAT recorder, a respectable RMS audio value and its level reference tone will be around -12 to -16 ppm (peak program meter). Tracking usually requires more headroom, so -14 to -16 ppm or 14 to 16 decibels below digital full scale works well. Example: your analog ‘0′ VU reference tone would appear at -14 ppm, leaving a margin of 14db peak headroom above 0 VU. Mastering engineers utilize tighter dynamics control, so a margin of -10 to -12 ppm is usually enough. Now after all that, I’m going to tell you that tones of any kind are not necessary for DAT recording.
The use of 10 kHz and 100 Hz tones are also not necessary for DAT recording. These tones are used for azimuth alignment, and high and low frequency playback equalization on analog tape recorders. Such alignment is not user definable on DAT recorders, so using these tones might send the wrong message regarding your experience with digital recording.
Bud Bremner owns and operates Coastal Mastering in Vancouver, BC.