In article , Serge Auckland
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.

[snip interesting report]

As Jim says, this might account for why some CD players sound different
to others. There is a supreme irony here that all CD players *would*
sound the same if CDs were mastered properly. Ah well....

From some other measurements I have seen (but are as yet unpublished so I
can't give details) it is clear that the resulting waveform *do* vary from
one player to another. It is also clear that quite a number of 'remastered'
pop or rock CDs have sample sequences that imply a need for 0dBFS peaks in
the reconstructed waveforms. So the problem is a real-world one.

What isn't clear at present is how audible this may or may not be, and how
this varies from one player/disc combination to another. As you say, it
could mean that some players would be indistinguishable with well recorded
disc, but show up differently with lousy discs. Or the differences may not
normally be audible. Hard to say at present.

Some discs may be crudely clipped. Others may be OK in that the samples
correctly and unambiguously define the waveform even during the 0dBFS
excursions.

This means that if an audible difference shows up in some cases, then it
could be said to be due to a 'faulty' disc. Allowing all involved to play
'pass the parcel' with any responsibility. The nice thing about my
'Waveform From Hell' is that it is legitimate in Information Theory terms,
so a player should play it without blunting the 5dBFS spikes.[1] I wonder
if any player on Earth would do so, though... But if they can, I doubt a
mere 1 or 2 dB over 0dBFS would bother such a player. :-)

So far as I can see, the common problem discs are ones that have been
clipped, and in those cases distortion is unavoidable. But in other cases
it may not... Hard to tell at present. The conclusion I have at present is
the one most liked by academics - futher investigation is required. ;-

I have however started to regard 'remastered' on a pop or rock CD as
probably being a synonym for 'buggered'. :-/

Slainte,

Jim

[1] Unless some part of the 'Red Book' I don't know about specifies that
the recordings have to be such that no part of the reconstructed waveform
should exceed the 0dBFS level. But I doubt Philips/Sony thought of this as
they'd just given the world a 90dB dynamic range for the carrier. They
probably didn't dream that those making pop/rock CDs would be so dumb as to
lead us into this possibly becoming a real-world problem.

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