Manual

1.27
Operation Manual
© 2016 Prism Media Products Ltd
Revision 1.01Prism Sound Callia
6.5 DSD (Direct Stream Digital) conversion
DSD was conceived in an age when monolithic sigma-delta converters were relatively new. The
quantisers in the converter chips of the day were one-bit and ran at 64x the eventual PCM rate
('64fs'); their digital decimation and interpolation filters were real-estate-limited and so had very
non-ideal behavior in terms of flatness and stop-band attenuation - as well as being confined to PCM
rates below 50kHz. The low sample rate meant that brick-wall filters were needed, with their
attendant time-domain dispersion problems.
Although the idea of transacting and storing pulse-density-modulation (PDM) signals (like DSD)
directly, without down-conversion to PCM ('decimation') was not new, it had not been used for audio
until the mid 1990s. Experiments were made by connecting the 64fs one-bit product of the front end
of a sigma-delta ADC directly to the one-bit back end of a DAC, bypassing the decimation and
interpolation filter stages. The result, unsurprisingly, sounded better than with the filters in circuit (no
processing, be it analogue or digital, or even up-sampling, can make a signal more like the original -
but the filters of the day were also very far from perfect). So it was that DSD and the SACD were
born.
By the time the SACD format came to fruition, higher quality monolithic PCM converters had become
available with longer wordlengths and higher PCM sample rates. These formats were specified for
DVD Audio (DVD-A), and a format war ensued which killed them both (or perhaps it was the lack of
enthusiasm amongst punters for ANY new higher-cost format whose main selling point was better
sound quality than the CD, which most people found quite acceptable). Although SACD and DVD-A
never caught on, DSD and high-resolution PCM audio are now enjoying a renaissance amongst
quality-conscious listeners in download and streaming form.
So which is better? Although Prism Sound is a manufacturer of no-compromise ADCs for the music
recording industry, it is not necessarily for us to say: whilst we may have an opinion on the best way to
convert analogue audio to digital, and maybe even an opinion on the best format in which to store it,
we must give our customers the means to produce whatever format they want: both! And as a
manufacturer of no-compromise DACs we must strive to give them the best conversion to analogue
from either format - after all, there are some excellent recordings in each format! In the case of Callia
this involves converting incoming DSD streams to something with a longer wordlength and a lower
sample rate prior to conversion. But surely that sacrifices the wide bandwidth which is the whole point
in DSD? Let's look at the pros and cons:
Since the days when DSD was conceived, converter technology has moved on - dynamic
performance and linearity have greatly improved, largely owing to the use of multi-bit front and back
end circuits running much faster than 64fs - the limitations of one-bit quantisers at 64fs were
surpassed not long after the initial DSD experiments. This presents us with an awkward choice in
converting a DSD stream to analogue: do we present the DSD directly to a lower-quality 64fs one-bit
back end, or do we decimate the signal to produce a longer wordlength which we can then re-process
for a faster state-of-the-art multi-bit back end? We might prefer the latter.
All this is borne out by studying the datasheets of today's highest-performing audio DAC chips: they
nearly all support both DSD and PCM inputs, so surely we will see that the dynamic performance and
linearity is worse for DSD than for PCM? Actually, no! Most of them seem to have exactly the same
performance for both! OK, then surely we can use them in DSD mode without losing all those DSD
advantages like super-wide bandwidth and inherently linear one-bit quantisation? Sadly though,
further study reveals that these devices are actually decimating the DSD to a longer wordlength and
lower sample rate internally - that's why the performance is identical, since the decimation process
can be done with minuscule degradation compared to that of the D/A conversion process itself. In
many cases, the fact that the chip's digital volume control works in DSD mode gives the game away!
So I'm afraid that there are few DACs which really do convert DSD to analogue without PCM in
between, although many designers who adopt them will not realise it. Like many things lurking in the
audio woodshed, this reality is anathema to the purist. But we should remember that nearly all DSD
releases have been processed somewhat in the recording and mastering studio, even if it's just a tad
of levelling or a smidgeon of EQ - and that NONE of this can be done without going via PCM or