PS Audio DirectStream Junior Owner's Reference Manual page 4

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excellent results.
• DSD soft clips when overdriven, more like magnetic tape: signals which exceed the nominal full scale
value only get slightly compressed, if at all. With PCM, you either have fl at tops which induce extra
energy at the squared off edges or, worse, you can have wrap around, which is very
audible.
• All bits in a DSD stream have the same weight: a single bit error anywhere is barely measurable, let
alone audible. Some bits in PCM carry a lot of weight and would make a very loud pop if changed.
PCM needs more error recovery to keep to a given signal to noise ratio (S/N) with a slightly corrupted
digital signal.
• Ironically, most sanely priced PCM players actually use DAC chips that utilize a sigma-delta modulator
(DSD) to get a DSD-like signal anyway. Similarly, many (most) A/Ds are sigma-delta based. The typical
PCM path is analog -> DSD -> PCM -> disc -> PCM -> DSD -> analog. The DSD path can skip the
conversions to PCM. Those conversions can't be perfect, and artifacts of the steep anti-aliasing fi lter
or the reconstruction fi lter aren't considered benign by many.
• DirectStream Junior handles the PCM conversion from AES/EBU, S/PDIF, TOSLINK, I2S and USB
without recovering a clock by simply watching for the edges and making decisions about what they
mean in context. The result is that any jitter present on the input is lost entirely in the FPGA.
There is little difference between TOSLINK or I2S because the output clock's rate only depends on the
long term average rate of the inputs not on any edge or other local feature.
The heart of DirectStream Junior is the DSD engine itself. Regardless of input format, whether PCM
The Heart of
or DSD, all data are upsampled to 30 bits running at 10 times the standard DSD rate and then back
Junior
down again to double rate DSD for noise shaping.
The internal volume control keeps complete precision: every bit in the input affects the output of the
DAC for any volume level. Except for the sigma-delta modulation process itself, there is no rounding,
dither, or other trimming - not to 24 bits, not to 32 bits, not to 48 bits, but rather a full 50. The incoming
PCM signal is 30 bits from the upsampling fi lter, and the volume control is 20 bits wide, so all 50 bits
of the output are used throughout the sigma-delta conversion, requiring more than 50 bits of precision.
DSD only requires a nominal 20 bit signal to noise ratio. This design utilizes a minimum width of 24
bits with wide fi lter coeffi cients and 144dB S/N. Use of full precision everywhere and many guard bits
in the IIR fi lters and the sigma-delta modulator help maintain our goal of perfecting the audio output.
While some designs may run out of headroom or approach saturation levels, depending on the source
material, the new design opts for an extra top bit everywhere in the digital path coupled with an extra
6dB of head room in the analog path beyond the 6dB of headroom that SACD uses. The top bit keeps
PCM from saturating, even if that PCM was not properly bandwidth limited in the initial recording
process.
The output of the DSD engine is fed directly into the output stage, based on high speed video amplifi ers
and a passively fi ltered solid state output stage.
Most output schemes for DSD modulators are active low pass fi lters, covering the required
120dB S/N ratio from 10Hz to 220MHz and have a number of design challenges and
problems associated with even the best designs.
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Introduction
Owner's Reference
Introduction
15-073-01-1
Rev A
DirectStream Junior
iii