Being Dangerous with DSD

DSD can be like TNT in the audiophile world.  Even as accepted as it is these days, it still evokes controversy in a lot of circles.  Less so now than in the 'Great DSD Wars' of the previous decade.  But there indeed was a time when having a strong opinion one way or the other about DSD was a path to anathema from someone or some group.  No one was pleased.  In those days and in the days since, I have studied and studied the pros, cons, ins and outs of Direct Stream Digital.  (1 bit PDM, or is that 1bit PCM?  Depends on who you talk to I suppose).  I have learned enough to be very dangerous.  Which means I have crossed the point where I know how truly complex this subject is, and I know I am nowhere near an expert.  At the same time, there are a few things that I do know fairly well, with decent confidence.  So let us start there as we continue to stir up a little controversy in the name of FUN here at Euphonic Review.  

How do you like your DSD?  

Personally, I will take mine rare, as straight from the cow as possible, presuming the germs are all killed.  I prefer "NATIVE DSD", but what does that even mean??  It seems every product maker has their definition, usually all in the name of selling products over accuracy.  If you poll 1000 audiophiles, you will probably get 10,000 different answers.  Not even the actual experts can agree, either.  

What is MY definition?

​Native DSD (not counting the production side, that is another can of worms) is when a DAC maintains the native, bit-perfect 1-bit signal, and the only conversion occurs at the end of the chain via a filter that leaves behind the music and cuts out the noise.  

This can be divided into two camps, but only one of them seems to be realistically achievable, although it hasn't stopped many from trying.  The 'purest' form I speak of, which is still out there like the Holy Grail waiting to be found, looks something like this (poorly put together) graph.  

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This extremely minimalistic design has only one bitswitch, which is usually the DSD output of a USB audio processor.  Amanero Asynchronous USB input card is a common choice when attempting to build one of these 'dac-less' DACs.  This 1 bit 'logic' output, can be treated the same as the unfiltered analog DSD square wave.  It is of low signal level, but can be filtered by a RC analog filter to remove the carrier frequency  and some of the noise-shaping.  A good result will be more achievable with a very high DSD sample rate due to presumably less ultrasonic noise, but as of 2023, I know of no one who has been able to make this work well enough to produce a commercial example.  I believe Lampizator came up with a version than used a very particular style of antiquated FM radio to do something similar, but it was too noisy to be viable.  

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The next 'camp' of Native DSD conversion is ubiquitous in modern Digital to Analog Conversion.  I am sure earlier versions of this exist, but the Burr-Brown DSD1700 chip is what I think of as ushering in the SACD age.  ​

DSD1700 NATIVE DSD CHIP

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This is what you will find in Native DSD products by iFi.  It is the technique used by ROHM chips in DACs such as the S.MS.L D300.  AKM and Cirrus/Wolfson chips use a similar technique in their Native DSD 'Bypass' Modes.  It is also how the Signalyst DSC1 DAC works.

NO, it is not what ESS uses.  ESS in this case reminds me of that old Sesame Street song, 'One of these things is not like the other'.  More on ESS later.  Back to our first two Native DSD 'camps'.  

How are they different from one another?  In principle, not very.  In practice, however, yes, there is a difference, and whether or not this is 1-bit or multi-bit conversion is probably a fruitless argument, because in this configuration, the 'multi-bit' properties exist post coefficients, and are fully in the analog domain of this 'hybrid digital/analog' Fixed Impulse Response (FIR) filter.   The output of the filter isn't a digital word, rather, it is an electrical current, whether that be from the switches/coefficients or at/after the accumulator.  Again, we are not in the 'standard digital' realm.  

FIR filter.  There you have it.  That is the difference.  Rather than a single bit switch and an RC filter, the 1 bit DSD goes through what is a digital FIR filter, however it is implemented in the analog domain, either partially or fully.  This FIR filter IS the Digital to Analog conversion.  Pretty cool huh?  Filter and converter all in one step.  Here we still have the simplicity of no DSP, a 1-bit signal all the way until the very end when it reaches the Fixed Impulse Response 'Moving Average' filter.  The 1-bit signal moves though a delay line, and the taps/coefficients ARE the bitswitches themselves, their outputs being summed up as the analog signal output that goes to your ears.  (Note many of these type DACs also have a secondary RC filter to help deal with the ultrasonic noise).  

Note, the graph is crude, but the basic concept of how it works is here, so don't go too hard on me...

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So what is next??  What about ESS?  Well, ESS is super quiet about DSD.  They protect their proprietary info.  It is VERY closely guarded, kind-of like a Fort Knox.  And that is not a terrible exaggeration.  All I can say, it is NOT QUITE like what you read above in the 'analog' FIR filter example.   But we can make some educated guesses based on measured behavior, and the experiences of those who work with the chip in their kit.  

​What about AKM in non Native DSD mode?  Cirrus?  Well, they all use some variations on the same theme.  1 bit DSD is internally converted to a multi-bit signal.  Sometimes it may be decimated to PCM, sometimes it may maintain the same sample rate.  It is VERY common to convert 1-bit DSD into a multi-bit 'DSD derivative' via a digital filter.

However, unlike the FIR filter described above, this Filter is in 100 percent digital domain.  These fully digital DSD filters are excellent and precise at filtering out the unwanted ultrasonic noise, and due to their multi-bit nature, and being in fully digital domain, other DSP is easily applied in this state.  I.E. Sample Rate Conversion, Volume Control, further filtering such as IIR etc.  One thing that MUST occur, though, is a re-modulation to some form of Delta-Sigma before the output.  ESS would call this their Hyperstream DAC.  DCS has a 5 bit (32 levels?) Delta Sigma modulator before its "Ring DAC", which is simply their take on a thermometer/unary code converter using scramble code/dynamic element matching.  AKM has various types of Delta Sigma modulators in their various chips.  Same for Cirrus/Wolfson and others.  

I personally am not as big a fan of this because of all the DSP.  Yes, it is true that PCM often goes through lots of DSP, too.  But I am personally consistent in not liking that either.  I like my PCM at high resolution with Non-Oversampling.  Also, simplicity of conversion is one of the 'promises' of DSD.  When you bring lots of DSP into the equation, it isn't so simple anymore.  Transient response can suffer.  The original 1-bit input and whatever is at the output are very, very different.  This CAN be a good thing, though, as advanced Delta-Sigma modulators like the ESS Hypersteam can and often do convert DSD with a lower noise floor, lower distortion, and better linearity.

As I always say, in the end, let the ear be your guide!!  

For a visual example of these types of DACs, I have gone old school, and will take a stab at showing how Sony converts 1 bit DSD into their 8 bit PCM 'DSD-WIDE'.  

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I hope you found this trip through DSD land edifying.  Thank you as always for reading Euphonic Review!

Until next time,

​Andrew