In article ,
Jim Lesurf wrote:
It's interesting (at least to me!) that although this problem has
existed for as long as digital audio has, in the early days of digital,
we always left a few dBs headroom, even as much as 10 dB if the levels
couldn't be precisely controlled. (1)

FWIW when recording anything from an analogue source onto CDR I always
use a peak headroom of over 6dB - typically more like 10dB. Until
recently I wasn't sure how much was needed for dealing with this
specific problem, but I would now certainly recommend avoiding allowing
the peak samples above about -6dBFS. 10dB seems wise as an aiming point
to allow for operator error, etc.

10dB over peak is still used by broadcasters. Standard line up tone which
is 8dB below peak is set at -18dBFS on video recorders, etc. And used in
the main for lining up things like FreeView and DAB transmitters. Several
of the radio stations on there peak to 0 dbFS, though. Leave any headroom
anywhere and someone will exploit it. ;-(

The reason for this 'headroom' is easily explained. The standard measuring
device in the UK for broadcast is the PPM - a device invented before WW2.
It reads peaks - but not absolute ones. It can't react fast enough to
measure them all. Of course in this day and age you can make one which did
- but then it wouldn't be such a useful tool for those balancing actual
sound. In analogue days these fast edged peaks didn't matter much - the
tape or whatever just absorbed them without audible effect. But digital
systems without a brick wall limiter will make *very* nasty noises if over
modulated.

--
*In "Casablanca", Humphrey Bogart never said "Play it again, Sam" *

Dave Plowman London SW
To e-mail, change noise into sound.