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

Michael E Caloroso mec.forumreader at gmail.com
Wed Mar 16 02:09:13 CET 2016

Anyone who studies schematics of pro audio gear will find that they
operate on +/-18 volt rails for a reason.  The top of the line studio
desks - Neve, API, SSL, Trident, et al - run on higher rails.


On 3/14/16, Steve Ridley <spr at spridley.freeserve.co.uk> wrote:
> 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
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