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

[Mattias Rickardsson]

Hi,

I've been trying to find a good explanation of this, but it's proven
to be unproportionally difficult - so now I turn to you, my dear
expertise. :-)

There are "standard" studio signal levels like "pro-audio" +4 dBu and
"consumer" -10 dBV. But these are nominal levels and I guess they are
RMS values of signals with potentially much higher transients. So what
voltages are we really expected to handle if we say that we are one of
these levels? I can do my internal work much much better (noise wise)
if I don't need to have lots of spare headroom above the nominal
level. If I claim to be an effects unit, a mixer, a recorder, or a
soundcard - of if I'm supposed to handle the signals coming from a
mixer or a soundcard - how much headroom is needed?

"Professional" +4 dBu corresponds to 1.7 V peak. Many units like
mixers have a headroom above this, often to +22 dBu. This is probably
no magic number, since it just translates to 13.8 V peak, which is
what you can cram in between standard +/- 15 V supplies. But how about
the +4 dBu "standard" level, are we really expected to have enough
headroom to be able to handle 13.8 V peak? Or no headroom, meaning
just 1.7 V peak? Or something in between?

"Consumer" -10 dBV corresponds to 0.45 V peak. But how hot signals are
we supposed to expect there? I guess that CD players and today's
battery-powered gadgets have a peak voltage limit very close to this
nominal level, but historically there must have been much more variety
with higher voltage rails and yesteryear's sources like magnetic
tapes, turntables, microphones and God knows what?

When searching for this info primarily on Wikipedia, where I'd expect
all of this to be neatly described but remarkably hard to find, I just
came across this German article:
https://de.wikipedia.org/wiki/%C3%9Cbersteuerungsreserve
(I include it here just because it's in German and because it doesn't
link to an article in any other language. Serious stuff.) ;-)

I'm sure some of you can shed some light on this!
Thanks,

/mr