Broadcast Audio -10db - why?

[cdd]

I've consistently noticed (and seen corroborated) that audio levels for broadcast should never exceed -10db. What I haven't been successful in finding out is why - why attenuate it at all?

I know a little attenuation is always needed in every medium to compensate for people who apply equalisation etc (which boosts the volume of certain frequencies): to avoid clipping, there has to be a bit of head room. But 10db!?

Surely you get lower quality audio that way anyway as the sampling resolution isn't using the full spectrum of the wave (a bit like saying "we're only using integers, now rate these between 1 and 10%" - you get much more detail by offering 1 and 100, the full range).

I bet someone here knows! (hopeful smile )

[Nick Harvey]

I'll leave this to somebody who's all digital and up-to-date because I'm sure the reasons have either changed or been lost in time.

Long gone are the days of PPM4 for music and PPM5 for speech because the analogue bits of wire got hot if you tried to put more than .7 of a volt down them for any length of time.

(Am I right in remembering .774 of a volt, peak to peak, at PPM4, or something like that?)

[interstiti-al]

It's a long time ago but I was always taught at Wood Norton that it was to do with providing enough headroom for live recording and transmission lines on digital equipment...

In Analogue terms, PPM 4 (0dBu) was the normal peak and you were allowed to peak to PPM (+8 dBu). You should never exceed PPM6 but you did, in the real world, transient peaks would exceed PPM6 and may not even be displayed as exceeding PPM6 on the PPMs if they were really quick. Analogue equipment would distort when these peaks occurred but often in an "inoffensive" manner - i.e. you could get away with it. Early digital systems were not so tolerant of overloads and would often "crack" in a very obvious manner.

Since digital systems were giving better signal to noise levels compared to some of the standard analogue recorders / lines, it was decided to lower the nominal level on the digital systems to protect against transient peaks. So they set the line up level to -18dB full scale - i.e. PPM4 = -18dB fs which means that PPM6 (the nominal peak level) is -10 dB fs.

Like I say, I could be wrong, and it was a very long time ago but it kinda makes sense I suppose.