[sdiy] "Standard" signal levels and maximum voltages? How much headroom is expected?

[Steve Ridley]

Hello Mattias

I've worked in professional audio for many years, and have never found the "+4dBu" thing useful, and measuring consumer levels in dBV instead of dBu seems plain daft.

Over those years, the main measurements of interest were reference level (so you can set your levels), peak level (how high your program should go) and headroom (how far over your program can go before significant unwanted distortion comes in).

In Europe, the reference level was generally 0dBu, programs were allowed to peak to +8dBu (generally measured on a PPM - which under-read peaks) and there was 10db of headroom above that before it all went a bit square - so peak level before significant distortion was +18dBu. There were various national and manufacturer's variations on this, with anything from 16 to 24dB of headroom above the reference level and a bewildering variety of line-up tone levels and meter scales, but a reference of 0dBu = 0.775V rms and 18dB of headroom above that was fairly standard.

In recent times, loudness and it's trust sidekick true peak have become the final measure of acceptability for broadcasting in the shape of EBU R128. That's probably not critical to synth DIY, but worth being aware of.

But that's only part of the story - equipment has to deal with the dynamics of real life - whisper to gunshot - dynamic processing can keep levels under control, but only if the signal path before can handle the unprocessed signal levels. There's no point in "riding the levels" at the faders if it's already been clipped to blazes. That's where higher headroom in comes in - a mic amp which throws away too much headroom to give a better noise figure can look good on paper and be a nightmare in real life. Resonant EQs and filters need headroom too, so pro analog gear can often go well over +18dBu, and digital gear may have 24 bit ins and outs but work internally at a much higher bit depth.

Does that help?


Steve