DVD audio versus CD audio

["loop.pool" <looppool@cruzio.com>]

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.