28 Australian
FEATURE DSD vs. PCM — Which is the Best?
Often this is represented as a picture of a square
wave, with startlingly fast rise times for DSD
and slower ones for PCM. Mathematically, DSD
may indeed be capable of superior resolution
on this front, except for one thing...analogue
to digital to analogue conversion, or ADA.ADA Conversion
So DSD’s virtues, perhaps, lie not in the intrin-
sic accuracy of the format, but in the accuracy
of conversion from analogue to digital and
back to analogue. This would have been an
extremely strong argument back in the early
1980s, when PCM was just fi nding its place.
The fi rst Philips CD player was well-reviewed
because it was considered that compared to the
16-bit DAC of the fi rst Sony, its 14-bit DAC was
a safer choice.
But engineers are clever. As an analogy, we
should note that today’s computer technology
is in just about every countable respect three
orders of magnitude bigger, better and faster
than it was back then. Literally, one thousand
times. So while 16-bit digital-to-analogue
and analogue-to-digital PCM conversion may
have been hard back then, 24-bits is easy now.
Especially since a straight conversion is rarely
used. These days there’s oversampling, multibit
Sigma-Delta conversion and other techniques.Anti-aliasing fi ltering is performed digitally,
eliminating phase shift, and with 96kHz and
192kHz sampling frequencies, sharp fi lters are
largely avoided anyway.
At least, that’s the case for PCM.
DSD has a signifi cant weakness, though.
Especially DSD64. One ought to be reluctant to
pour high levels of ultrasonic noise into one’s
tweeters. Who knows what effect it may have
on the actual treble signal, and indeed on the
longevity of the tweeter.
So DSD has to be fi ltered. And unless you
are going to convert it to PCM before decod-
ing—which rather defeats the point of it—that
fi ltering must be performed in the analogue
domain. How that is implemented is up to the
DAC maker. As we’ve seen, SACDs are supposed
to have a fi lter of –3dB at 50kHz.
Perhaps the DSD encoder can capture that
transient beyond the capability of high bit-rate
PCM. Perhaps DSD can hold it. But it’s unlikely
that it will survive intact a –3dB fi lter at 50kHz.
Pictured above are the frequency responses
of three different DSD-capable DACs, using
a test signal designed for 192kHz sampling
(converted to DSD).
In all three cases the PCM version (white
trace) is more extended than the DSD128
version (blue trace) which in turn is moreextended than the DSD64 version (green trace).
The differences between the three depend upon
the DAC designers’ choices. The fi rst two units
retail at $5K+, the last is the Korg unit, which
costs less than $400. All three are –3dB at well
under 50kHz for DSD64.Conclusion
Now one can claim that these various measure-
ments and illustrations fail to capture an es-
sence of DSD that is somehow superior to PCM
yet which is still able to be detected by the
human ear. That indeed may well be the case.
But what we can see is that in every respect
in which these things can be measured, and
in every respect in which they can be visually
illustrated, DSD is less accurate than high-
resolution PCM. That should make one at least
cautious about claims which are signifi cantly
less easy to demonstrate.
Indeed, the argument I am making is not
that PCM is superior to DSD, or that DSD
doesn’t sound as good as PCM.
I am in fact proclaiming the wondrous fact
that we now live in an age where the delivery
formats—whether DSD or high-resolution
PCM—so far exceed the resolution of the
human ear that they are, for our purposes, prac-
tically perfect! Stephen DawsonGraph 4. Frequency Response Comparison: PCM (white trace) vs DSD128 (blue trace) vs DSD64 (green trace) using Astell & Kern AK380Graph 5. Frequency Response Comparison: PCM (white trace) vs DSD128 (blue trace) vs DSD64 (green trace) using Chord Hugo TTGraph 6. Freqency Response Comparison: PCM (white trace) vs DSD128 (blue trace) vs DSD64 (green trace) using Korg DS-DAC-100m