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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: Re: Fluff posted by Todd Krieger on January 1, 2003 at 20:29:35:
Todd Krieger wrote:> That graph, for example... What was actually measured to
> deduce this "improvement?")I thought that was reasonably clear from the excerpt
I quoted from Audio Alchemy's "Manufacturer's Comment".A synthesized 16-bit undithered sine wave, at a given
frequency and level, was fed to the DTI Pro. The input
sine wave, if converted to analog by an ideal D-to-A
converter, would contain the sine wave's fundamental,
plus a series of other frequency components due to
the quantization noise inherent in the 16-bit resolution.
The level of this noise can be summed and computed,
presumably entirely via mathematical operations (the
article mentions the fast-Fourier transform).If the 16-bit sine wave were padded out to 24 bits by
adding zeroes in the 8 least-significant digits, the
level of quantization noise would remain the same.
However, after processing by the DTI Pro, the lower 8 bits
contain numbers manufactured by the algorithm running
on the DSP. If the algorithm is doing what it's claimed
to do, then the quantization noise in the 24-bit output
from the DTI Pro would be **less** than that in the
16-bit input. Audio Alchemy claims that they actually
computed the quantization noise in the 24-bit output
(again, using FFT math).The graph therefore shows the reduction in the level of
quantization noise in the 24-bit output generated by the
DTI Pro compared to the 16-bit undithered input (converted
from dB to an equivalent number of bits of added
resolution at the exchange rate of one bit per 6 dB of
quantization-noise reduction), at various frequencies and
levels of sine-wave input.How this test correlates with the DTI Pro's effect on
music, which is not a pure sine wave, I do not know.
However, the existence of this measurement procedure (a pretty
time-consuming one, according to Madnick and Allsop -- 6 hours
on a Unix workstation at 1994 computational speeds) at least
shows that Audio Alchemy was using some sort of objective test
methodology in addition to subjective listening tests to
refine their "resolution enhancement" algorithm.> I just have a problem with all this phony technobabble
> being tossed around, that not only deceives potential
> buyers of products, but also confuses those who are
> seeking improvements in DAC design. . . Perpetual's
> "technologies" (no pun intended) have not spread
> throughout the industry. . .Well, given the suspicion and resistance that have always
surrounded mysterious black-box add-ons in audio, maybe
that's not too surprising. As far as deceiving potential
buyers is concerned -- if you're the kind of audio consumer
who **has** to have a water-tight technical explanation
for something before believing your own ears, then you're
out of luck in this case. As far as the process being
a genuine improvement, over and above the design of a
DAC itself, well -- Robert Harley got a surprise during his
March, '96 _Stereophile_ review of the DTI Pro 32
(Vol. 19 No. 3, pp. 144 - 145):"If you think that a Mark Levinson No. 31 transport
driving the Spectral SDR-2000 Pro [D-to-A] processor couldn't
benefit from an Audio Alchemy processing device inserted
between them, you'd be wrong -- as I was. The Spectral's sound
was taken up another full notch in quality by adding
the Pro 32. . . And this was with what I consider to
be the finest digital front-end extant: the mighty
Mark Levinson No. 31 transport and Spectral SDR-2000
Pro digital processor. . .When the Pro 32's resolution enhancement was bypassed
(meaning the Pro 32 was acting only as a jitter filter),
I thought that the Pro 32 slightly degraded the sound
of the SDR-2000. . . Engaging the resolution enhancement
turned the tables, however, making the Pro 32 an asset
to the system's sound. This experience . . . suggests
that the resolution enhancement processing does provide
benefits. . .It's hard to imagine that a world-class digital front-end
such as the Mark Levinson No. 31 transport and Spectral
SDR-2000 Pro digital processor could be improved by a
little Audio Alchemy box, but it was."You know, if I were a thoroughgoing cynic, I might think
that something like the Audio Alchemy/Perpetual "resolution
enhancement" technique might be nothing more than an
implementation via digital filter of an old trick used
in the analog domain to "sweeten" the sound of CDs by
applying little frequency-response irregularities -- a slight
dip in the 3-5 kHz range to reduce "glare", a boost in the
> 10 kHz range to increase "air", and a slightly bumped-up
bass to increase "warmth". In fact, IIRC, some of Carver's
early CD players had a switch to do this. If done via a
DSP-implemented digital filter, the output could very well
have more than 16 bits (since when you multiply a 16-bit
sample by a filter coefficient, you end up with more than
16-bits). Voila -- "resolution enhanced" "analog" sound
from CDs. Of course, if that's what's really going on,
it would be easy enough to expose with frequency-response
measurements (though, come to think, I've never seen any
such frequency-response measurements in any Audio Alchemy
DTI Pro or DTI Pro 32, or Perpetual P-1A, review (Aha!
Must be a conspiracy! The reviewers must have been taking
bribes! ;-> ).I don't really believe that anything that simple is going
on, but even if it were -- heaven help me, I still love
the sonic result! Though it would be much more embarrassing
to have to admit that my audiophiliac pleasure center could
be tickled by such a simple trick, rather than requiring a
more "respectable" technique like "multiresolution wavelet
filtering".Anyway, YMMV. ;->
> Wadia is in fact one of few companies that apparently
> wasn't afraid to state what upsampling truly was.)For all the good it did them. They got a bit of a black
eye in the August, 2000 issue of _Hi-Fi News & Record
Review_ (see link).Jim F.
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- That graph - JimF 01/2/0308:39:00 01/2/03 (2)
- If I had to guess. . . - JimF 09:53:06 01/2/03 (1)
- Masking and psychoacoustics. . . - JimF 11:47:22 01/2/03 (0)
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