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Looper's Delight
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Interesting webpage I discovered on the audible differences between
CD and DVD recordings at higher resolutions. It was written a while
back
but I think it is pertinent
for all of our recordings:
High resolution digital audio -- can you tell the difference?
by Stephen Dawson
Ever since the compact disc was introduced in the early 1980s, it has been
subject to criticism from many quarters, mainly for alleged inadequate
resolution. By that I mean that the sampling frequency of 44.1kHz has been
accused of both restricting the frequency response of music (it can't go
much above 20kHz without the introduction of digital artifacts) and
necessitating a difficult to implement steep filter which could,
potentially, have audible effects. Likewise, the 16 bits of resolution
(providing samples of up to 65,536 voltage steps) has been accused of
introducing a 'graininess' to CD music, as though one can hear the steps.
(These steps, incidentally amount to 0.0003dB at high modulations, and
around -60dB come to 0.27dB.)
To be fair to the CD format, it was pushing the limits of the technology
available at its introduction. The maker of one of the first two CD
players
released effectively recognised this by limiting it to only the most
significant 14 bits of resolution!
Since then, a number of strategies have been employed to overcome these
problems, including oversampling (using a higher sampling frequency and
then
mathematically deriving the signal levels for the newly invented samples
from the adjacent ones) which allows a gentler filter to be used at the
top
end, although it adds nothing to the actual resolution.
DVD finally promises, though, to break through the CD format's bottle
neck.
It provides sufficient capacity to hold a full CD worth and more of music
at
a 96kHz sampling rate and 24 bits resolution (which requires around 3.3
times more data than the CD standard). The 96kHz sampling rate means that
a
rather more leisurely acting filter can be applied, well clear of 20kHz. A
resolution of 24 bits means that for each sample step of a CD, there are
256
steps on the DVD. So, even at -60dB, a sample step means a jump of just
0.001dB, and it's not worth counting up the decimal places to calculate
the
step at 0dB.
The question is: is this difference audible?
Comparability
One of the problems with comparing formats has been the difficulty of
finding comparable sources. LP was clearly LP, and not comparable to CD,
if
only because of surface noise. But even without such give-aways direct
comparisons would be impossible. After all, how much difference in sound
is
due to a difference in between the frequency response characteristics of
the
cartridge and phono preamplifier (with its RIAA equalisation) and the CD
player? Even a brand new well-pressed LP has around one per cent
distortion
created during manufacture. Direct comparisons are impossible.
For this DVD vs CD test, though, direct comparisons are indeed possible
subject to certain conditions being met. The first is that the only
difference between the two formats is no more than that the formats are
actually different. I selected the recording Ink by Livingston Taylor from
the US audiophile label, Chesky (CD version; DVD version apparently no
longer available). I questioned the record company about the comparability
of the two formats. The original recording was made in 96/24 format, live
to
a two track digital recorder. Near minimalist recording techniques were
used, with a stereo tube microphone used for most of the music, a single
tube microphone used for Taylor's vocals, and some additional spot
microphones. A valve mixing console was used to combine the signals for
the
two track.
The DVD is a direct copy of this original two track. The CD was created by
digitally down-sampling to 44.1/16. There was no limiting, compression or
any other processing used, nor any changes in the transfer to CD other
than
the down-sampling.
The next step in a comparison is to ensure that the equipment used is not
imposing its own characteristics on the sound quality. One way to do this
is
to use just one player, swapping discs. I have found this method very
unsatisfactory unless the audible differences are quite marked. For this
reason I chose to conduct A/B testing by using two players, roughly
synchronising playback from both sources, and then switching between the
two. But this brings in the possibility of the different players stamping
their marks on the sound. To reduce this as much as possible, I used two
DVD
players: a Philips DVD725 ($1,099) for the CD and a Pioneer DV-717
($1,499)
for the DVD. Both use dual lasers, so the laser wavelength was appropriate
to the medium in both cases. In both cases, to avoid any subtle
differences
created by the players' analogue circuitry, I used them simply as
transports, feeding their digital output primarily by electrical digital
connections to a home theatre amplifier.
The amplifier I selected is one that is difficult to beat: the Denon
AVC-A1D
home theatre amplifier ($5,999). This uses separate high quality 96/24
DACs
for each of its five channels (I only used two, of course) and was
perfectly
happy decoding the PCM output from both the CD and DVD. I should add that
the Denon is kind to CDs. It uses some processing (for the front stereo
channels) which Denon calls AL24. In essence, this takes the 16 bit CD
data
and calculates intermediate steps to generate a bit depth of 24 bits. This
lowers the height of the steps between CD samples, of course, but the
since
the intermediate information is calculated from what precedes and follows,
it cannot fully emulate the resolution available on DVD. Nevertheless it
does a good job and the processing, along with its Alpha predecessor, is
well regarded.
The reason for using one amplifier was to ensure that all analogue paths
were identical for both formats, and in the hope that with digital feeds,
the analogue output would be identical.
Measurements
To confirm this, and to confirm that the DVD and CD are as close to
identical as Chesky advises, I recorded the first track from both discs
('Isn't She Lovely') onto my computer. I used the left and right
preamplifier outputs from the Denon amplifier for the signal (the Denon
wouldn't deliver to its tape outputs an analogue version of a digital
input)
and left the volume control on the same setting for both recordings. I
recorded onto my computer at its maximum capacity (48kHz and 16 bits,
slightly better than the CD standard) and was able to analyse and compare
the outputs from the Denon for the two media.
Any changes performed by Chesky during the down-sampling to CD standard
would apparent from just a little examination of the wave forms and some
basic analysis. In particular, any compression would show up as a smaller
difference between the average output levels and the peaks.
The wave forms superficially looked very similar. So similar as to obviate
the possibility of any remixing. And what I shall call the peak range
(that
is, the difference between the average sound level and the highest peak)
soon demonstrates whether there was any compression. There was not. In
fact
the decibel differences between the highest sample value and the average
was
identical for the two formats on the right channel and out by 0.01dB on
the
other (an insignificant amount likely to be caused by variations between
the
places on the peak curves the samples of each happen to fall). Likewise,
the
range between the overall average and the maximum RMS level, using a 100ms
measuring window, was identical for both channels.
In the absence of any marked audible difference in the mix, I am satisfied
that the music is identical on the two formats, other than any effects
resulting from the different sampling rates and bit depth.
I should also note that the analogue output levels for the two media from
the Denon amplifier were no more than 0.11dB apart from each other. For
listening tests, the outputs should be identical. Conventional wisdom
suggests that a difference of less than one decibel is largely inaudible.
This is largely correct if one is asked to say which signal is the
loudest.
However smaller discrepancies can produce a subjective effect of making
the
louder signal sound 'better'. I was not equipped to adjust the signals by
such a small amount, unfortunately. However as it turns out, it was the CD
which was a touch louder than the DVD, so any bias caused by the slightly
different levels should be in favour of the CD.
Also, over the four and a half minutes of the song, the CD came in at
0.003
seconds longer. This is most likely due to slight differences between the
clocking crystals of the two players and at 0.001% is totally inaudible.
The other reason for copying the analogue outputs to my computer was to
see
if there was any measurable difference between the two media. I was
somewhat
surprised to see that there were. My surprise was due to the basic
inadequacy of using my computer as the test instrument. It tends to suffer
from fairly high noise levels in the sound card and I was, after all,
using
little better than a CD standard digitisation process.
The main differences concerned low level noise and high frequency
performance. While the gap between the average level and the RMS maximum
and
peak levels were consistent between the two, the gaps between the average
and the minimum RMS levels were not. Indeed, the DVD scored a 1.08dB
greater
range than the CD on the both channels. This reflects the lower noise
levels
on the DVD format. Had my computer lower noise levels itself, this
difference would have been greater, reflecting the ability, with the
greater
bit depth, of abandoning dither noise, or at least reducing its level
considerably.
More interesting was to analyse the frequency spectrum from both signal
sources. These were for all purposes identical from the bottom up to
around
17,000 hertz. The DVD's trace (Fig 1) for this track then continued on its
gentle decline (almost all music exhibits an average output level falling
by
around 9dB per octave from 100 or 200 hertz to the limits of the upper
frequency response, which is why tweeters can get by with such small voice
coils), steepening a little after 22,500 hertz (most likely due to my
sound
card). However, the CD version (Fig 2) showed an actual increase in high
frequency output, reaching a 4dB boost by 21,000 hertz, and a rapid
fall-off
beyond that point. Okay, the drop above 21,000 hertz can be expected for
CD
with its much lower sampling frequency, but what about the rise? I
speculate
that this is the result of dither noise in the CD, designed to mask
quantisation noise in low level signals.
Zooming in closely on a low level wave, such as the fade out section of
the
track, shows something else of interest. The CD version (Fig 3) shows much
wilder variation than the DVD (Fig 4) in the wave form, oscillating above
and below the basic trend of the wave quite markedly. Admittedly the
extent
of the oscillations are around 25 quantisation steps out of the 65,000+
the
CD format offers, but it would suggest the masking of low level
information.
It is likely to be these oscillations that show up in the high frequency
boost in the spectrum analysis.
Well, enough of that. How did the DVD sound compared with the CD?
Listening
First, my preconceptions: I was inclined to think that there would be no
audible differences between the two media. For me at least. On the other
hand, I earnestly hoped that I would hear some difference between the
media.
Failing to do so tends to put one's credibility as a writer on such
matters
on the line. So to the extent that our subjective impressions are
influenced
by our expectations and hopes, I think I was fairly balanced.
I set myself up in my office after midnight, lights switched off, computer
off, no traffic around and listened. The first thing to note is that
whether
CD or DVD, this album is magnificently recorded. Indeed, so well recorded
that it somewhat paradoxically exposed a problem with the recording
technique. The use of a stereo microphone for the band has produced, in
both
formats, an incredibly well delineated and detailed sound, with the
backing
vocalists on 'First Time Love', although standing close to each other a
little left of centre, being clearly separated from each other. I can't
emphasise enough the naturalism of this recording.
Except ... the vocals were recorded with a separate microphone (allowing
Mr
Taylor's vocals to be more forward than the band) and are poured in
apparently identical amounts into the left and right channels. Aside from
the prominence of his voice over the band, this had an occasional odd
result, due to the very magnificence of the band recording. On
'Hallelujah,
I Love Her So', the brushes are tangible, almost to the point of each
strand
of the brush striking the drum being individually discernible. The only
problem is that this is offset from centre by just a tiny distance to the
left. In the meantime, Mr Taylor's voice is right at centre. The aural
result is that the voice sounds as though it's coming from just a little
above the line drawn between the speakers, while the brushes sound like
they're coming from the front of the singer's chin. I suspect that Mr
Taylor
was not standing in front of the drum kit during the recording, so while
the
geometry of the sound delivery makes it sound as though he is, the absence
of bodily muffling of the kit denies this.
This leads me into DVD vs CD. I cannot confidently say that there are no
differences in the sound delivery of the two media. But I can insist that
anyone who uses language of the 'chalk and cheese' variety is fooling him
or
herself. The differences, if there are indeed any, are very subtle indeed.
So before equivocating further, I'd better write what I felt I noticed.
First, there were no marked differences in frequency balance. On 'Isn't
She
Lovely' there is a quietly played, somewhat distant and restrained
tambourine. On the CD this sounded just a trifle sharper than the DVD,
showing perhaps the tiniest little bit more emphasis on the upper
harmonics.
I switched back and forth throughout this section, and my conviction faded
as I continued. Nevertheless, I think the result was that the DVD was a
little more natural.
Second, I listened closely to lateral stereo imaging and stage depth. I
could detect no difference between the two for stage depth, but did
occasionally sense that the stereo image was a little more tightly focused
with the DVD than the CD.
And there you have it. This was the extent of any variation between the
two
formats discernible by me with this music. In all other respects, I could
pick no differences. So, for example, I paid particular attention to the
air
around instruments. Yet as far as I could tell, they were identical.
Conclusion
My Ink CD cost me $30 from Rockian Trading, the Australian Chesky
distributor. My Ink DVD cost $60 from the same source. Clearly the latter
is
not twice as good as the former. But it is slightly better ... I think!
The
audible differences really are so very subtle that I am by no means
convinced that it was not my imagination conjuring them.
There is, though, one clear advantage of the 96/24 recording process,
whether finally delivered as 96/24 on DVD or as 44.1/16 on CD. That is to
do
with headroom. Live to tape recording is problematic. How much headroom do
you allow? In a digital recording you dare not allow 0dB to be exceeded.
Digital formats produce the hardest clipping known to humanity. The wave
forms are chopped off at 0dB with a ruler flat line. But if you keep the
modulation levels well down, you are not making full use of the 65,536
quantisation levels available with 16 bit recording. Remember, if your
whole
recording doesn't get above -6dB, you are effectively recording at 15
bits.
So you want to turn up the level as much as you can, without clipping.
But how do you get the level just right? You can set it during rehearsals,
but when the band plays the track to be laid down, a moment of enthusiasm
on
its part can easily throw you into clipping. Or it may decide on a more
restrained performance, and give you that 15 bit recording you were trying
to avoid.
Then, if the recording is a little low, should you 'normalise' it? --
digitally amplify it so that the average level is more or less the same as
other CDs? If you do this, you increase quantisation noise along with the
level of the music. If you don't, you end up with something like my Telarc
CD of Vivaldi's The Four Seasons, extremely quiet throughout and incapable
of delivering a high level from a portable CD player to headphones.
Using 24 bits of resolution avoids these problems completely. You can
allow
plenty of headroom for momentary orchestral expressiveness, yet retain
plenty of resolution. Remember, those extra eight bits give you an extra
48dB to play with! You can normalise the signal during the transfer to CD
without any loss of CD resolution or increase in quantisation noise.
But whether the extra cost over a CD for the at most subtle improvement
provided by a 96/24 DVD is a matter for each listener to decide.