Tech Tip:Fact and Fiction About Digital Audio
By Craig Anderton
Maybe it’s just the contentious nature of the human race, but as soon as digital audio appeared, the battle lines were drawn between proponents of analog and those who embraced digital. A lot of claims about the pros and cons of both technologies have been thrown back and forth; let’s look at what’s true and what isn’t.
A device that uses 16-bit linear encoding with a 44.1 kHz sampling rate gives "CD quality" sound.
All 16-bit/44.1 kHz systems do not exhibit the same audio quality. The problem is not with the digital audio per se, but interfacing to the analog world. The main variables are the A/D converter and output smoothing filter, and to a lesser extent, the D/A converter. Simply replacing a device’s internal A/D converter with an audiophile-quality outboard model that feeds an available AES/EBU or S/PDIF input can produce a noticeable (and sometimes dramatic) change.
The main differences that people perceive occur in the high frequencies and the width of the stereo field. Lower frequencies seem far less affected, which would imply that the output filtering is a very important part of a digital system’s sonic character.
16-bits gives 96dB of dynamic range, and 24 bits gives 144dB of dynamic range.
There are two things wrong with this statement. First, it’s not really true that each bit gives 6dB of dynamic range; for reasons way too complex to go into here, the actual number is (6.02 X N) + 1.76, where "N" is the number of bits. Based on this equation, an ideal 16-bit system has a dynamic range of 98.08dB. As a rule of thumb, though, 6dB per bit is a close enough approximation for real-world applications.
Going from theory to practice, though, many factors prevent a 16-bit system from reaching its full potential. Noise, calibration errors within the A/D converter, improper grounding techniques, and other factors can raise the noise floor and lower the available dynamic range. Many real-world 16-bit devices offer (at best) the performance of an ideal 14-bit device, and if you find a 24-bit converter that really delivers 24 bits of resolution...I want to buy one!
Also note that for digital devices, dynamic range is not the same as signal-to-noise ratio. The AES has a recommended test procedure for testing noise performance of a digital converter; real-world devices spec out in the 87 to 92dB range, not the 96dB that’s usually assumed. (By the way, purists should note that all the above refers to undithered converters.)
Digital has better dynamic range than analog.
With good components and engineering, analog circuits can give a dynamic range in excess of 120dB—roughly equivalent to theoretically perfect 20-bit operation. If you want the maximum dynamic range possible with today’s technology, quality analog tape running Dolby SR beats standard 16-bit linear digital recording systems.
With digital data compression like MP3 encoding, even though the sound quality is degraded, the original data remains intact. So if you want to increase quality, resave it at a higher bit rate.
Data compression programs for computers (as applied to graphics, text, samples, etc.) use an encoding/decoding process that restores a file to its original state upon decompression. However, the data compression used with MP3, Windows Media, AAC, Minidisc, etc. is very different; as engineer Laurie Spiegel says, it should be called "data omission" instead of "data compression." This is because parts of the audio are judged as not important (usually because stronger sounds are masking weaker sounds), so the masked parts are simply omitted and are not available for playback. A copy of a compressed file can never exhibit higher quality than the source.
Don’t ever go over 0 VU when recording digitally.
The reason for this rule is that digital distortion is extremely ugly, and when you go over 0 VU, you’ve run out of headroom. However, as any guitarist can tell you, a little clipping can do wonders for increasing a signal’s "punch." Sometimes when mixing, engineers will let a track clip just a tiny bit—not enough to be audible, but enough to cut some extremely sharp, short transients down to size. It seems that as long as clipping doesn’t occur for more than about 10ms or so, there is no subjective perception of distortion, but there can be a perception of punch (especially with drum sounds).
Adding just a tiny bit of clipping to digital can actually work well with some types of tracks. No, I’m not advocating the widespread use of digital distortion! But if a mix is perfect except for a couple clipped transients, you needn’t lose sleep over it.
And here’s one final hint: With any piece that contains intentional distortion, or unintentional distortion that’s judged as not being a deal-breaker, it’s a good idea to include a note to let "downstream" engineers (e.g., those doing mastering) know it’s there, and supposed to stay there. Otherwise, they might reject it.