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Liberating DSD

Posted by Tony Lauck (A) on December 18, 2008 at 09:12:53

In Reply to: RE: DSD playback and Sony Patents posted by rich121 on December 17, 2008 at 21:54:35:

It is possible that there are patents on DST files, as the particular lossless compression of delta sigma data may be novel. (I don't know, I haven't looked into it.) However, who cares? One can simply store the DSD data in plain form as a DFF file. If storage space is at a premium for seldom played files, one can compress them using WinZip, which did a fairly good job on the one DFF file I have (from 2L). It compressed to 67% of original size, compared to 60% of original for the typical 24 bit FLAC file.

To play back DFF files on a computer the easiest way is to convert them to PCM, which one can do, but I haven't found any working software available on the web at a reasonable price. It would be (and was) an undergraduate project to do this. One has to extract the raw delta sigma from the DFF file, define it to be a PCM file at an equal sample rate (e.g. define the bits as + - 0 dbFs in some number system, such as 64 bit floating point) and then, presto magic, you have PCM, albeit a huge amount of PCM. (And as you can see, the transformation in this direction hasn't lost any information.) To play it you will then have to downsample it to a lower rate, using a normal SRC. And then you will have to convert it to a standard file format, such as WAV or AIFF. There are several software vendors who sell the necessary software, but the prices are absurd, given that the work for decoding is really nothing more than an afternoon's work if you already have PCM SRC library routines. Because of the high sampling rates involved, a high quality implementation would probably use a lot of compute power, not something that really matters if one does this offline.

Going the other way is much more difficult, because it involves constructing a delta-sigma encoder. It's not that the software is terribly complex, it's that there are many possible ways to do this, and these do not have any sound theoretical basis for their construction. Indeed, it is likely that the various commercially available encoders have different sonic properties (and different stability performance under overload). In addition, Sony has patents on some modulators, so it is possible that your result will be infringing unless you pay them off. And even if not, you can expect to hear from their lawyers and be subject to a lot of FUD, to say the least. (It can cost you $1 million to successfully defend a patent lawsuit.)

Since a decoder is not useful without a source of corresponding files, and since the copy protection on SACD disks has proven difficult, there is little market for decode software to play DFF files. I believe this explains the lack of inexpensive converters from DFF files to PCM.

A quick glance showed that Sony also has patents over serial transmission of delta sigma modulation over wires (multiplexed clock and data). These may be why we don't see PC interfaces at low cost. It would be my guess that these patents are weak, because DSD is really not different for transmission purposes than any other digital data. However, expect FUD at least if you market unlicensed devices.

If you want to liberate DSD from SONY, a D.I.Y. approach would seem best for now.




Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar

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