What's the standard for Digital Audio?
In analog audio it was easy - 0 VU was the target and a little excursion over that was acceptable, but with digital audio going into distortion over 0 dB, what's the matching setting? Is it -20, -18, -14 or -12 dB?
The great thing about standards is that there are so many of them!
Practically speaking, either -18 or -14 dB are considered to be equal
to zero dB in analog audio. The difference is that analog audio is
forgiving, so a few dB over zero isn't a problem. It takes a lot of
overload to cause distortion, and with analog audio the onset of
distortion happens slowly. With digital audio, any level that exceeds
zero dB is irretrievably distorted. Digital audio is very unforgiving.
 What level you choose as your standard, depends on were you
are going with this. If you set tone at -20 dB, that will match the
digital levels of most pro VTRs, using digital levels. In other
words, -20 tone over SDI or AES audio into a Digital Betacam deck will
equal -20 dB at unity (preset) input levels. The level of -20 dB is the
accepted standard for VTRs and that level is equal to 0VU on an
analog mixer. The recorded tone will then be at the correct level if
you don't adjust the playback knobs. It also gives you 20 dB of
headroom, rather than only 12dB.
DV25 formats define analog-zero-equivalent as -12 dB, so if
you were mastering to DV the workflow would be consistent with tone at
-12 dB, audio peaks on that meter at -6 dB. Avid NLEs defined -14 dB
early on as being equivalent to 0 dB analog, then switched to -20 dB.
If you always master to BetaSP or DigiBeta, then -20 dB works fine
for tone, and peaks at -14 (basically one notch up). Just make sure
you're reading on a Peak meter (like in Final Cut Pro or Avid) and
these numbers should work fine when viewed on a VU meter on an analog
deck. Notice there's roughly 6 dB difference in either example.
The reason why you set tone
differently than audio peaks is that there are no peaks in tone, just
steady sound, and peaks belong...well, peaking above reference. And the
reason why you might set tone at a different level for different
mastering decks is that the poor schmoe who will be
dubbing/broadcasting it will have one of those deck formats as well. To
convince yourself of correct levels when mastering to any deck format,
play back your tone & program audio from your master and see if
they both read correctly on the deck's meters. If not, change something.
Recording field audio is debatable. You'd get more headroon safety
when
field recording on DV if you used -20 dB as your reference. But why
might you use -12 dB instead? Because in Final Cut Pro or Avid, you
only have +12 dB of gain in your timeline (without pulling some
tricks). If your sound was recorded using a "safer" -20 dB instead of
DV's -12 dB reference, you just ate up 8 dB of your possible
post-boost, and you would only have another 4 dB of gain available in
Final Cut Pro.
Final Cut Pro color codes the audio meters from green to red. Simply
put, keep it in the green! Aim for an average of -18 and keep maximum
levels to -6 dB and you'll generally be OK. If audio is compressed, as
it usually is for commercial soundtracks, the average can be set
higher, but never peaks over -6 dB.
Thanks to members of the Avid-L2 for their contributions to this Tip.
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