Monday 31 March 2014

Real-Time DSD Conversion

BitPerfect user Eugene Vivino writes: “As you may be aware, PS Audio is releasing the DirectStream - a product that upscales all signals to 2xDSD.  They say they do this because all PCM processors mask the sound in some way, while DSD outputs a more realistic and believable signal.  And reviewers say that it sounds ‘right’.  Now that BitPerfect supports DSD, would it be possible to add real time DSD conversion?”.

Actually, we will soon be taking delivery of a DirectStream.  We are getting one of the first batch of production units.  We continue to hear great things about this product, from both public and private sources, and as the hype has been quite intentionally built up I feel justified in expecting great things of it.  But does DirectStream’s contribution to the state-of-the-art derive from the fact that it upscales to 2xDSD?  I have received both private and public communications from Paul McGowan and his team on this subject, and I have no intention of commenting on any aspect of any information communicated privately.  But I am prepared to comment on some of the publicly-acknowledged elements of DirectStream’s design.

One of the defining aspects of the DirectStream design lies in the fact that it does not use anything that you would describe as a conventional DAC chip.  So I need to recap briefly on what a ‘conventional DAC chip’ actually does.  What these devices do is to convert the incoming PCM signal to what is often erroneously described as DSD, primarily for the entirely laudable motive of avoiding customer confusion.  This is because DSD is 1-bit 2.88MHz, and nothing else.  To create DSD it is necessary to pass an incoming signal - whether analog or digital - through a thing called a Sigma-Delta Modulator.  These SDMs can be configured to produce all sorts of different outputs, and 1-bit 2.88MHz (DSD) is just one possibility.  More specifically, SDMs can produce a multi-bit output as well, with as many bits as you like.  It turns out that there is at least one seriously good reason to use a SDM with a multi-bit output.  This is that SDMs can be unstable, and, in particular, a 1-bit SDM can be seriously unstable.  SDM design is not for the faint of heart.  A typical commercial DAC chip will take the incoming PCM and pass it through a SDM to create a multi-bit, even higher sample rate, version of “DSD”, which it then converts to analog.

What Paul’s team have done is to focus on one of the fundamental benefits of single-bit SDM-based DACs.  With a multi-bit output - such as PCM itself - the DAC has to create an output voltage which has a magnitude determined by the bit pattern.  The more bits, the more different possible output levels there are.  With 1-bit, there are only two levels - encoded as 1 and 0 - and from an electrical perspective, all a DAC has to do is switch its output between two fixed voltage sources represented by those numbers.  These voltage sources can be, for example +1V and -1V, and can be controlled and regulated with fantastic precision, and with extremely low noise.  The job of the DAC is then simply to switch the output signal line between one voltage source and the other.  This is something you don’t need a chip to do, and is furthermore something you can employ a lifetime of audio electronics circuit design experience to realizing in the best possible manner. 

Of course, DirectStream has some other quite unique elements to its design, such as its approach to jitter rejection.  But all that aside, the only thing that counts is what it sounds like, and I am as intrigued as the next guy on that score, and quite impatient to boot!

So when Eugene Vivino observes that DirectStream upscales all incoming PCM to 2xDSD, the truth is that virtually all DACs do something very similar, except that it isn’t necessarily 2xDSD.  And there is nothing particularly special about 2xDSD, other than that it is a bit better than 1xDSD.  Unless you are using what is termed a ladder DAC, a vanishing beast with only a few examples still in production, the process of Digital-to-Analog conversion fundamentally involves converting the PCM data to one of the “DSD-like” SDM-produced formats.  This in and of itself is unlikely to be the source of any differentiating performance attributes that might emerge from DirectStream.

Now, as I mentioned earlier, SDM design is not for the faint of heart.  It is based on some massively complicated signal-processing mathematics.  A breakthrough in SDM design will often earn you a PhD.  There is also a lot of untapped potential in SDM design waiting patiently in the wings for computing power - always inexorably moving forward - to reach a level necessary for these process-intensive SDM designs to be brought into real-world implementation. 

Yes, BitPerfect could implement PCM-to-DSD conversion on-the-fly to enable your PCM content to be delivered to your DAC in DSD form.  But simply converting PCM to DSD is no panacea.  It will not suddenly release trapped information in your PCM data stream.  The only reason you would want to do that is if BitPerfect’s PCM-to-DSD conversion algorithm would produce a significantly better result than the equivalent functionality inside your DAC’s DAC chip.  Make no mistake about it, these chips contain algorithms designed by teams of dedicated mathematicians, who know their stuff, and are implemented on silicon designed expressly for signal processing.  So, at BitPerfect, we do not have an SDM of that quality, and with all respect to the other fine player Apps out there, some of which offer this capability, none of them do either.

The SDM technology we are working on requires seriously impressive amounts of processing power, so when we reach the point where we are able to implement it, it will not be introduced in a real-time system, but likely as an option in a forthcoming version of DSD Master.