Tuesday, October 26, 2010

How much headroom do you need when recording? Part 1

Some years back, I recorded a Stravinsky symphonic work to analog tape running at 15 ips half-track, dbx type 1. When I transferred the recording to digital for archiving, the big orchestral bass drum gave me problems. Had I been recording to digital on location, it would have been a mess: I would have needed to set 0 VU at -25 dBFS to keep the digital meter out of the red. That's how much energy the bass drum was putting out. 

Standards organizations specify how much headroom should be available above operating level (0 VU) in the digital domain. In my experience, the European EBU standard of 0 VU = -18 dBFS does not afford nearly enough headroom, and neither does the North American SMPTE standard of -20 dBFS. Granted, these were "reasonable" compromises in the days of 16-bit technology, when 93dB dynamic range was about all you could expect to get in the real world (as opposed to the theoretical 96 dB, calculated at 6 dB per bit).

Analog headroom of 24 dB—which many manufacturers of professional grade equipment achieve with maximum output levels of +28 dBu (ref 0 VU = +4 dBu)—should be considered the minimum standard during production. Even then, the Stravinsky would have been into overload by about 1 dB, so you might occasionally require even more headroom. 

In our current 24-bit world, I would say that it's not unreasonable to demand 28 dB headroom when recording wide dynamic range program material, such as symphonic works. It still gives you a working signal-to-noise ratio of better than 100 dB, and 28 dB of headroom includes a small comfort margin so you can enjoy the program without stressing over the levels during recording, knowing that you will most likely never go into the red.

Incidentally, I see a lot of "analog channels" being marketed as quality front end processing for recording into ProTools and other DAWs. Many of these boxes include a compressor after the mic preamp, probably because few recordists stop to consider how much analog headroom is really needed in a given situation. Instead of backing the level off to allow for enough headroom without compression, they tend to run the ProTools meters high up into the yellow, recording with compression on individual tracks at 24-bit resolution. It's as if a little bit of green flickering at the low end of the meter must be avoided at all costs. This is foolishness.

24-bit technology allows you to record at a moderate level with 28 dB of headroom and still accumulate no perceptible noise in the recording. Save the compression for mixing and mastering, when it becomes a creative tool rather than a protective device. And then, whatever else you do, don't normalize! Oversampling digital-to-analog converters, which are pretty much the norm these days, routinely create signal peaks greater than 0 dBFS between samples that measure 0 dBFS on disc. But that's another subject for another time.

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