At 11:40 PM 7/17/99 -0700, you wrote:
>Let me de-lurk here for a moment and ask a totally newbie question:
>
>What is PCM Audio? Isn't it the format that DVDs use for audio?
It's a fancy name for signed linear uncompressed audio. The typical sample
rate is 44.1kHz, with the waveform represented by 15 bits of amplitude and
one bit of sign. It is the format used by CDs, and while such sounds are
possible on DVD, they do limit the bit rate available for the video, they
are usually retired in favour of compressed formats like MPEG, DTS and
Dolby Digital, which also have the advantage that they are well adapted to
presenting surround sound.
>Also, does anyone know what Super CD and DVD-Audio is? My company
>will begin carrying Super CD and DVD-Audio decks in late 99-2000 (I
>think). I've heard Super CD and DVD-Audio are supposed to sound much
>better than CDs (and MDs, I assume). How is that possible? I thought
>the regular CDs sampling rate already picked up every possible
>frequency that the human ear can hear. How will these new formats
>improve upon the current? And -- most importantly -- will they be
>compatible with MDs (no copyright lockout)?
Well, DVD Audio has only just been formalized, and details are only readily
available to members of the "standardization" organization. It is
supposedly part of the "acoustic audio renaissance". For years, devoted
audiophiles have been complaining that CDs are too "sharp" sounding and do
not present the "warmth" of vinyl records. I personally find this
ridiculous- vinyl is clearly a distorted impression of the sound- and it is
my belief that a vinyl record, suitably filtered, and recorded onto a CD
(or MD), would exhibit no difference to a listener accustomed to this
warmth. i.e. The effect is probably a desirably familar distortion, not an
advantage of analogue sounds over digital sounds.
However, there may be a case to answer in terms of distortion present on
CDs and other 44.1kHz 16 bit PCM audio media. There are two forms of
distortion here- quantization distortion (the 16 bit part) and waveform
distortion (the 44.1kHz part). For my money quantization distortion is
vanishingly low at CD quality, because 66536 steps should be more than
enough to reproduce a smooth waveform to all ears (caveat being that soft
sounds have fewer steps available in this linear scheme). However, there
may be a case to answer in terms of waveform distortion.
A 22kHz sine wave is on the fringe of human heaing. However, a CD will
convert this to a square wave, and waves slightly above and below this
sound level will be indistinguishable, wheras there are perhaps perceivable
differences to analogue waveforms around this level. This extreme example
(which is hardly relevant to your average listener, who has more problems
in their amp than from digital artifacts) does also present an effect in
all high frequencies, though to a significantly lesser extent. A steep sine
wave simply cannot be accurately represented at a 44.1kHz sample rate,
which has the number crunching audiophiles up in arms.
Whether the reality is that the human ear percieves these mishmashed square
waves as different from corresponding sine waves is debatable. However, to
appease the audiophiles, and to present a possibly new wave of the
lucrative "I already have that on media X, but on media Y it will sound
better" rebuying scenario, the record companies are taking advantage of the
density of DVD media to present sound at the limit of what is digitally
feasible today.
"DVD-Audio supports a wide range of digital audio options: PCM and
MLP decoding are mandatory for all players. Discs may use either on a
track-by-track basis. Sampling frequencies may include 48 kHz, 96 kHz,
192 kHz, and 44.1 kHz, 88.2 kHz, 176.4 kHz. DVD-A can have bit
resolution of l6-, 20-, or 24-bit. With MLP the precision may be selected
between 16 and 24 bits in 1-bit steps. Up to six channels are available for
multichannel recording, with a transfer rate of 9.6 Mbps. Many provisions for
providing 2-channel and multichannel mixes. High compatibility with the
DVD-Video and DVD-ROM formats."
- http://avacademy.com/news/stories/06_03_99_01.html
There are also possibilities for directly encoded 6 channel sound (akin to
surround schemes like DTS and Dolby 5.1, except there is no compression),
which is a definite benefit over CD. This is somewhat ironic given that
such high sample rates would have been sufficient to allow a highly
directional form of Dolby ProLogic to be used, even for high frequency
effects- phase differences would be much more readily represented given the
increased resolution.
How DVD-A is used in practice... well, we'll have to wait and see. And that
wait could be quite significant- we have already waited some 5 years for a
standard to emerge. Which leads us to yet another "competing formats"
scenario, although it may be true that it has already been won and lost.
As you have alluded to, Sony in partnership with Ph*l*ps has a little beast
called Super Audio CD. Essentially, a DVD by another name, this format
ditches PCM altogether in favour of something called DSD...
"The Super Audio CD employs Direct Stream Digital as
the fundamental recording technology. DSD uses a
sampling rate of 2.8224 MHz to record a 1-bit signal
directly, thereby eliminating the need for decimation
filtering in the recording process and interpolation filtering
in the playback process. Thus, according to Sony, the
resultant digital signal very closely resembles the shape
of the original analog waveform.
Sony said it has achieved a theoretical playback
frequency range of up to 100 kHz and a dynamic range
in excess of 120 decibels across the audible range."
- http://www.techweb.com/wire/story/TWB19990409S0004
Which is probably lifted directly from Sony/Ph*l*ps press kit. I notice
they have used a scheme not directly comparable to linear PCM (as found in
CD and DVD-A), which could bambuzle the public was there to be a media war.
But it seems like Sony and Ph*l*ps have already resigned to failure, as
both will support DVD-A immediately. The big advantage that Super Audio CD
was to have over DVD-A has been ditched- that is, a CD compatible layer
which would allow backwards compatibility with existing CD equipment.
Super Audio CD looks set to be a novelty or very high end format at best,
and a stillborn disaster at worst. Then again, DVD-A is not guaranteed of
instant uptake either, and it too may fall flat. I have a suspicion that
DVD-A might be popular for another reason- if it is possible in the spec to
have 44.1kHz PCM at 16 bit resolution (which seems to be the case), I think
it may be popular to have "super compilations" which run for many hours,
perhaps rivalling CD changers. The problem, of course, will be the
licensing hurdles and how such media are priced.
Anyway, sorry for the OT nature of all this, but it was asked. And it is
kind of relevant to MDers, who record from a variety of audio media...
Cheers,
Kade, who's getting a DVD player this week- digital recordings at last!
--
Archer
http://www.geocities.com/SoHo/6413/
End.
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