THE CD PLAYER PLUS - Ultra High Fidelity Magazine

We’ve been aware for
a while that many of
our hard core readers
are transferring their
digital music onto hard disc. That
makes it available for placement
on an iPod or other player,
of course, but having instant
access to all your music is frankly
addictive. See Music Through the Air
in this issue.
Of course the music most people are
transferring is on Compact Disc, but
perhaps you’ve also built up a collection
of SACDs. What about those? Can they
be added to your playlist?
The answer is yes, but with how
much of a compromise? We set out to
find out.
The bad news is that there is no direct
way to transcode the contents of an
SACD to the pulse code modulation used
for other digital music processes, including
the CD. That’s because SACDs are
recorded not with PCM but with Direct
Stream Digital (DSD), which works
totally differently. We’ll leave aside for
now the claims for or against DSD, but
let us see how it works, and how it is
different from the more familiar pulse
code modulation used in such systems
as the Red Book CD.
Let’s look first at PCM.
Here the signal level is sampled at a
regular, pre-determined interval, 44,100
32 ULTRA HIGH FIDELITY Magazine
Nuts&Bolts
SACD on Hard Disc?
t i m e s
per second
in the case of
the CD, more often
with modern professional
systems. The values
gleaned are encoded as binary
digits, from which a jagged version of the
original waveform can be reconstructed.
It is then smoothed with a low-pass filter.
This works well providing (as per the
Nyquist theorem) the sampling rate is at
least double the highest signal frequency
to be encoded.
The DSD system, by contrast, is
asynchronous, which means it doesn’t
pay attention to the clock. The system
monitors the waveform as it arrives. If
the level of a sample is higher than the
previous one, it generates the binary
digit “one.” Otherwise it generates a
zero. Silence is represented by alternating
ones and zeros.
Sony, which developed the DSD
recording system that is at the heart of
SACD, likes to say that this is the system
closest to analog, and that, indeed, if
you simply take the DSD code and run
it through a low-pass filter, you will
actually hear the analog signal,
albeit with a lot of noise.
What is certain is that
moving from DSD to
PCM in the digital
domain is not simple,
and for the audiophile there
is but one way to accomplish
it, and that is by making a detour
through the world of analog.
It’s not ideal, we know. Going from
analog to digital or vice versa is like (we
imagine) getting beamed up by Scotty.
You arrive nearly intact, but you don’t
want to do it too often. We’ve seen Star
Trek, but we’ve seen The Fly as well.
But how bad is the process? Here’s
how we set out to evaluate the method.
To get the music out of the proprietary
DSD format, we played our test
discs on our Linn Unidisk player, whose
performance appears to be at or near
the state of the art. We bypassed our
preamplifier and went from the Linn’s
analog outputs directly into our Edirol
UA-25 audio interface box, which you
can see overleaf. The Edirol, in turn,
was connected to a MacBook Pro laptop
computer with a USB cable. We did the
recording work using the free opensource
program Audacity, set to record in
24-bit resolution with a 96 kHz sampling
rate.
If you have a newer DAC that can
handle 24/96, you can stop right there,
because you now have your music right
where you want it. For the purpose of
this test, however, we did something you
may not: we burned our newly-created
PCM files to a blank DVD. Since 24/96
reproduction is part of the basic DVD
standard, that disc will play fine on any
DVD player. We listened to it with the