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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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In Reply to: If I had to guess. . . posted by JimF on January 2, 2003 at 09:53:06:
. . . are a heavily-researched field, BTW. They're
the secret behind the "lossy compression" schemes
for getting "CD-quality" (as the mainstream newspapers
are incorrigibly wont to call it) sound out of low-bit-rate
audio (as in MPEG audio, MP3, Dolby AC-3, Sony's ATRAC,
etc.). You look at the spectral content of the incoming
signal, decide on psychoacoustic grounds that some
components are masked (hidden) by other components,
and allocate your bandwidth to digitize the psychoacoustically-
prominent components at a higher resolution than the
psychoacoustically less-relevant ones.It's not at all implausible that similar principles,
along with those "analytic continuation" techniques
mentioned earlier, could be used to clean up selected
spectral components of an incoming signal to produce
better-than-CD quality from a CD datastream (a sort
of reverse-MP3 process, generating **more** data from
a (relatively) low-bit-rate data stream).Also, Perpetual may in fact be using the Discrete Wavelet
Transform rather than the more familiar Fourier Transform
to do this, though I know even less about wavelets than
Fourier transforms (at least I was introduced to the
latter in college math), so I'll keep my mouth shut
about it.We may actually see a lot more of this sort of thing in
the future. Audio Alchemy may have been a little ahead
of its time here. Also, it was crippled by the fact
that few DACs, until about 1998, were capable of passing
24 bits of data all the way through the system, from input
receiver through digital filter and on to the DAC itself.Harley describes the situation as it was in 1996
(_Stereophile_, Vol. 19 No. 3, March '96, p. 143):"Choosing the correct output word length is crucial to
getting the best performance from the DTI Pro 32. Let's
say you have a digital processor with 18-bit DACs, and
you set the Pro 32's output word length to 20 or more
bits. The DACs will simply truncate (cut off) any bits
below 18, introducing noise and distortion. As JA
described in his review of the Meridian 518 Mastering
Processor in the January '96 _Stereophile_, truncation
also hardens midrange textures and reduces the sense
of space.Conversely, setting the Pro 32's output word length
to 18 bits if you have a true 20-bit processor prevents
you from experiencing the full benefits of Resolution
Enhancement.Just because your digital processor has 20-bit DACs doesn't
mean that it will pass 20-bit data from input to output.
The NPC 5813 digital filter, for example, truncates the
incoming data to 18 bits. The older Yamaha YM3623
input receiver chip will pass only 16-bit data. For
comparison, the Crystal Semiconductor CS8412 and
UltraAnalog AES21 input receivers will pass up to 24-bit
data, as will the Pacific Microsonics PMD100 HDCD
decoder/filter.There's another trap to be aware of. Digital processors
with the Crystal or UltraAnalog input receivers, the
PMD100 filter, and 20-bit DACs still may not pass 20-bit
data. Some processors that have been retrofitted with
the PMD100 weren't redesigned to pass the PMD100's full
resolution to the DACs. Two such examples are the
PS Audio UltraLink Two and Enlightened Audio Designs
DSP7000 Series III. Each of these processors has
only a 16-bit data path from the digital filter to
the DACs. Consequently, they'll truncate 20-bit input
data, even though they have a 24-bit input receiver,
24-bit digital filter, and 20-bit DACs [hopefully
not "truncate" -- the PMD100 could be programmed to
dither 24 bits down to 16 bits, after all. But who
knows! JF]. Any processor designed from the ground
up around the PMD100 (as opposed to an existing design
retrofitted with the PMD100) should pass at least
20-bit data. The only way of finding out which '20-bit'
processors truly pass 20-bit data is by the measurements
included in reviews. Reports from readers suggest
that some processor manufacturers either aren't aware
of their products' capabilities, or provide misleading
information."Pretty sad state of affairs! And not too auspicious
for folks looking to get the most from the Audio Alchemy
DTI Pro 32 -- it was probably a crap-shoot whether it
would improve or degrade the sound (even assuming the
owner could interpret the manual -- setting the output
word length was a matter of 'press and hold the "Phase"
button until the three power-indicating LEDs switch
from power indication to operational status indication.
Still holding the Phase button, press the Input button
to scroll through the output word-length options.'
Just as cumbersome as the Perpetual P-1A! As Harley
remarked in his review, "Without the owner's manual
(or a great memory), you won't be able to correctly
set up the Pro 32 for your system."These days, thanks to the new DVD-Audio-spec chips,
this isn't a problem, and folks with newer 24/96-capable
DACs can take full advantage of the Perpetual P-1A's
full 24-bit output word length. I find it extremely
ironic, BTW, that devices developed for DVD-Audio will
make it possible to hear CDs at higher fidelity
than heretofore imagined. This is not something the
record companies intended. Of course, they have themselves
to thank for delaying the introduction of true 24-bit
DVD-Audio disks -- what with the furor over encryption,
watermarking, and so on. By the time the DVD-As are
on Tower's shelves (or Amazon's Web-site), a lot of folks
will have discovered how to make their CD playback quite
sonically competitive with the new media.Jim F.
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Follow Ups
- Masking and psychoacoustics. . . - JimF 01/2/0311:47:22 01/2/03 (0)
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