Converting DSD to PCM, bad?

Hi i have read and watch the things with TED and your direct stream dac.

I came across this one

http://www.audiostream.com/content/qa-thorsten-loesch-amrifi

It states that conversion between format dsd and pcm is an unwanted thing.

Can you explain how your DAC benefits from the conversion rather than not?

Answer:

Well converting DSD to low rate PCM is bad.  In fact, in spite of that, many SACDs are actually mastered with a workstation that converts DSD to 32 bit samples at 384kHz (DXD) and I would contend that that sample rate is too low.  But allowing single bit DSD to widen when you do math on it isn’t a problem at all.  There MAY be a problem when you requantize (use a sigma delta modulator) to get back to one bit.  Being clear: there is no loss of information when doing sample widening in PCM or DSD.  There CAN be loss of information when you do math (or dither) carelessly in DSD as well as in PCM.

In the DirectStream we need to convert single rate DSD to double rate DSD so the DirectStream only needs one passive output filter designed for one frequency.  The analog filter required for single rate conversion either is steeper than one would like or would have a cut off that’s lower in frequency than one would like.  Double rate DSD gives the freedom to have a less aggressive output filter that starts rolling off at a higher frequency.  It also allows less aggressive noise shaping in the sigma delta modulation process which lowers the ultra sonic noise introduced during noise shaping.  This allows the well known “DSD noise hump” to be lowered by 40 or more dB and also allows it to start nearer to, say, 60kHz instead of 20kHz.

If one were to repeatedly requantize single rate DSD the extra noise that would be introduced could eventually rise above, say -120dB FS at 20kHz and would certainly add more noise that that at 30kHz or 40kHz.  A few requantizations (say from passes back thru an analog mixing board in SACD mastering) aren’t a practical or audible problem, but too many would be.

On the other hand things are materially different for double rate DSD.  The only thing that changes in with multiple requantizations at double rate is the particular details of the noise that’s generated by noise shaping.  That changed noise is at a very low level over the audio band (quieter than, say, -144dB FS).  It also or only grows over -120dB FS well above the audio band, say at 60kHz.  (That higher frequency noise is also filtered by the analog output filter.)  The number of requantizations required to have audible noise growth is quite large.  Over the audio band this noise is less than the noise used in PCM processing to dither back to 24 bits from wider intermediate PCM results.  Some practitioners even recommend not dithering back to 24 bits when processing PCM which actually adds even more noise if any non-trivial math is done.

Perhaps in an ideal world we might handle the special case of double rate DSD input specially, but no-one would ever be able to tell that we did.

-Ted