Hi,

I have now produced a new webpage which looks at the way signals from CD,
can exceed the nominal 0dBFS level. The page is at

http://www.audiomisc.co.uk/HFN/OverTheTop/OTT.html

The material is an expanded form of what was previously published in the
August 2007 issue of Hi Fi News. It shows some examples of the waveforms
reconstructed from commercial CDs exceeding 0dBFS, and also shows a test
waveform that should (if reproduced without distortion) reach to above
+5dBFS.

Happy Christmas! :-)

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.audiomisc.co.uk/index.html
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html

Jim Lesurf wrote inter alia in
:

Hi,

I have now produced a new webpage which looks at the way signals from CD,
can exceed the nominal 0dBFS level. The page is at

http://www.audiomisc.co.uk/HFN/OverTheTop/OTT.html

Interesting stuff;- thanks for posting this!

Just one small typo;- eh? :=})

"it is hard to answer this question with any confidence as we lack the
necessary date"

Happy Christmas! :-)

Jim

Best to you and yours!

Ross

In article , RdM
wrote:

Just one small typo;- eh? :=})

"it is hard to answer this question with any confidence as we lack the
necessary date"

I must have been thinking of Christmas... No mention of walnuts, though.
:-)

Cheers,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.audiomisc.co.uk/index.html
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html

"Jim Lesurf" wrote in message
...
In article , RdM
wrote:

Just one small typo;- eh? :=})

"it is hard to answer this question with any confidence as we lack the
necessary date"

I must have been thinking of Christmas... No mention of walnuts, though.
:-)

Cheers,

Jim

--
Electronics
http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.audiomisc.co.uk/index.html
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html

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)

Finished CDs still left several dBs headroom, and never used to be
normalised to 0dB as they are now. Consequently, we never experienced this
sort of problem, and it was then acceptable for digital players to clip at
just over 0dBFS, albeit for the wrong reason.

(1) BBC practice is still to leave 10dB headroom by using the EBU stanrdard
of 0dBFS=+18dBu for their converters. Peaking the signal at +8dBu on BBC
PPMs leaves the theoretical 10dB. However, it is now recognised that the BBC
PPM underreads on very short term peaks by something like 4dB depending on
programme content when compared to a true-peak meter, so the actual headroom
left will be closer to 6dB.

I have recently put our local Hospital Radio Station on the Internet, and
had to put a hard-limiter before the encoder as even though the PPMs peak
at 6 (+8dBu) on music which is hard-disk or CD derived, the real levels go
over and clip the encoder. The gain-reduction meters on the limiter indicate
2-3dBs gain reduction fairly frequently even though this doesn't show on the
desk PPMs. It also helps with some of our more enthusiastic presenters who
don't watch their levels....

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....

S.


--
http://audiopages.googlepages.com

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.

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.audiomisc.co.uk/index.html
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html

On Wed, 19 Dec 2007 15:42:16 +0000 (GMT), Jim Lesurf
wrote:


[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.

Darwinism operates very quickly in this area. Make something foolproof
and the fools will evolve to be smart enough to beat it in a single
generation.

d

--
Pearce Consulting
http://www.pearce.uk.com

Jim Lesurf in
:
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. :-)

There's an interesting thread over at rec.audio.tech I'm reading at the moment
on "Digitising Vinyl"; I'm only part way through the (rather long) thread;
this post by Peter Larsen seems relevant to these issues: - use
http://www.google.co.uk/advanced_group_search?hl=en
to search for Message ID

--
RdM

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.

"Serge Auckland" wrote in
message

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)

Still good practice for live recording.

Finished CDs still left several dBs headroom, and never
used to be normalised to 0dB as they are now.

Agreed.

Consequently, we never experienced this sort of problem,
and it was then acceptable for digital players to clip at
just over 0dBFS, albeit for the wrong reason.

Thing is, these FS overs are very rare.

As Jim says, this might account for why some CD players
sound different to others.

I doubt that. First, you have to find the good CD players that actually do
sound different from others.

There is a supreme irony here
that all CD players *would* sound the same if CDs were
mastered properly. Ah well....

Since I generally record with lots of headroom, the problem of FS overs
takes on a different form. I end up with recordings that have very
infrequent excursions that are 3-4 dB above the norm. If they weren't
there, I could up the average level of the recording by 3-4 dB without
clipping. Since my distributed recordings are typicaly uncompressed, I'm
already producing recordings that sound far softer than most commercial
releases.

In article , Arny
Krueger
wrote:
"Serge Auckland" wrote in message



Consequently, we never experienced this sort of problem, and it was
then acceptable for digital players to clip at just over 0dBFS, albeit
for the wrong reason.

Thing is, these FS overs are very rare.

Depends on the context. I have found various commercial CDs that generate
excursions above 0dBFS. In some case, on many occasions during the CD.
However these are almost all rock/pop music CDs. Much rarer with classical
or jazz in my experience. The difficulty here is checking enough CD issues
to results that have any statistical meaning, though.

As Jim says, this might account for why some CD players sound
different to others.

I doubt that. First, you have to find the good CD players that actually
do sound different from others.

At present that is the unanswered question which would need testing in this
context. There are CDs that generate the excursions, and others don't. I
have also see test results which show that different players can give
visibly different results on something like a scope when given 0dBFS
material. But have no evidence that this leads to audible differences.

There have been comparisons where the players seemed indistinguishable.
That might mean this doesn't matter. But because of the above we'd now
really need to check what CDs were used as source material, and if they
produced any 0dBFS excursions or not. And if they did, the effects might
vary from case to case, which makes this difficult to track down, except in
terms of a generalisation like that is 'usually' has no audible effect, or
whatever...

FWIW I don't yet feel any urge to change my DACs despite having some reason
to suspect they can't cope with the possible excursions without some amount
of waveform distortion. Simple reason being that they produce fine results
to my ears, and I've not yet noticed any problem when listening.

But we could null out all such concerns if the bods at the commercial music
companies simply avoided clipping the CD material and kept the level down
so that no 0dFBS excursions could be required on replay.

There is a supreme irony here that all CD players *would* sound the
same if CDs were mastered properly. Ah well....

Since I generally record with lots of headroom, the problem of FS
overs takes on a different form. I end up with recordings that have
very infrequent excursions that are 3-4 dB above the norm. If they
weren't there, I could up the average level of the recording by 3-4 dB
without clipping. Since my distributed recordings are typicaly
uncompressed, I'm already producing recordings that sound far softer
than most commercial releases.

I find much the same when doing home recordings, but it does not matter as
I am not trying to get anyone else to listen to the results. So can just
turn up the replay volume control a few steps. But as you say, this does
tend to make even more obvious just how loud many commercial CDs are!

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.audiomisc.co.uk/index.html
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html