Manual

1.28
Operation Manual
© 2016 Prism Media Products Ltd
Revision 1.01Prism Sound Callia
(worse) back to analogue, so maybe we shouldn't be prissy about the DAC chip. Having said this,
there are a few rigorously-produced minimalist DSD recordings which haven't been processed at all,
but one wonders whether the ADC had a one-bit 64fs front end: if not, we're back to DSD via PCM, if
so, performance was presumably limited.
Actually, there are a few DAC chips out there which DO allow the DSD stream to be presented
directly to the back end DAC, without prior conversion to some form of PCM - but their datasheets
tend not to quote the direct-mode performance, only the via-PCM figures - which are usually (you
guessed it) the same as for PCM. The rest tend either to admit to worse performance in direct DSD
mode, or else their performance may not be such that we care much either way.
So, having decided that we may prefer to sacrifice some bandwidth for linearity by going via PCM,
what will be the price? Even a basic DSD64 signal, which is sampled at 2.8224MHz, could represent
frequencies up to nearly the Nyquist frequency of 1.4112MHz, whereas the current state-of-the-art
PCM signals are only sampled at 192kHz (with a bandwidth up to only about 90kHz), or perhaps
384kHz (with a bandwidth up to maybe 180kHz). We'd better hope that what we're sending to the
back end DAC isn't sampled anything like that slowly!
In reality, though, there is no evidence that anybody can hear much above a few tens of kHz so
perhaps the higher PCM rates might not sound too awful? Also, it turns out that we really don't want
super-high frequencies entering our other items of analogue audio equipment - which are so rarely
capable of either rejecting them thoroughly or transmitting them intact, so the consequence is often
that they are 'demodulated into the baseband', which is audio-speak for making odd noises which
weren't there in the first place. This problem was addressed in later versions of the SACD
specification (the so-called Scarlet Book), which added the requirement for DSD DACs to incorporate
a 50kHz analogue low-pass post-filter in order to protect subsequent equipment. So maybe we don't
need to worry about limiting ourselves to higher PCM rates.
So, to sum up, it seems that since those days, reduction in silicon geometry and resulting increases in
speed and complexity have meant that DACs' interpolation filters have become far closer to the ideal,
and can work at much higher PCM sample rates, meaning that brick-wall responses are no longer
necessary; whilst at the same time, faster multi-bit front and back ends have pushed back the limits of
linearity and dynamic range - all of which are moving PCM quality ever onwards and upwards. For
DSD, those same digital filters can convert to PCM on the way to the back end DAC, whilst perhaps
providing the stipulated 50kHz low-pass filter at the same time - surely this is the ideal solution?
Actually, not quite. We think that it's nice to do the conversion process ourselves so that we aren't
limited to the arbitrary response provided by a particular DAC device (which is often inscrutable for
obvious reasons); thus we can achieve exactly the response we want. It also means that we can
devote a suitable silicon budget to doing a really nice job of it. I might just add that we haven't
adopted this approach lightly: Prism Sound and SADiE have been researching such processing and
conversion technologies since the earliest days of DSD. Our Prism Sound ADA-8XR A/D D/A
converter, which has long been the DSD conversion tool of choice in many of the world's top SACD
mastering studios, uses exactly the same architecture as Callia.
I hope that this discussion has made it clear why Prism Sound approach DSD conversion in the way
that we do. When all is said and done, the proof is hopefully in the listening. If you are minded to
look further, you could Google Prof. Stan Lipshitz, and find out why he thinks that "1-bit Sigma-Delta
Conversion is Unsuitable for High Quality Applications". Or you could search "DSD PCM myths" -
and be sure to stop by Eelco Grimm's excellent analysis. I've just been scanning the "Direct Stream
Digital" page on Wikipedia, which seems to address some of the same issues I've been discussing
above, and is a good starting point for researching this interminable debate.
By the way, on the subject of the SACD specification: there was also at one point an amendment to
exactly what constituted the maximum permissible DSD signal level. Early SACDs were prepared with
a maximum peak signal level of 0dB SACD, whereas a later annex permitted maximum peak levels
up to 3.1 dB SACD. So in designing a DSD DAC, we need to accommodate the higher level without
clipping, but that means that for DSD recorded in observance of the lower maximum level we are
needlessly squandering 3dB of our hard-won dynamic range. So we have provided a setting (see the
Callia hardware section above) to allow you to choose how best to deploy the DAC's available
dynamic range.