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RE: Try using Google

Posted by Tony Lauck (A) on December 18, 2008 at 19:58:52

In Reply to: Try using Google posted by Christine Tham on December 18, 2008 at 19:03:04:

"However, higher order converters are prone to idle tones, which you need to handle."

Perhaps I am confused, but I believe you are talking about problems in PCM to DSD conversion, which as I stated before is a much more difficult problem involving, as it must, high order modulators. These are feedback circuits that are prone to various undesirable behaviors, particularly with high level input signals. These multi-order coupled non-linear feedback circuits may be simple to build, but they are impossible for anyone to understand. This is evident in the research literature, where people boast of the virtue of their "chaotic" circuits. It is also evident in user documentation for DSD encoders, e.g. Weiss Saracon software.

However, going the other direction is quite simple and doesn't require any complex feedback circuits, just a FIR filter, which can be a single stage device if enough compute power is available. (As described in your Matlab reference.) Hopefully, if the encoder that originally created DSD has idle tones they will be well out of the audio range and so will be filtered out by the low pass filter as part of the conversion to PCM. If the encoder has idle tones in or near the audio range then there is little that can be done that will result in good sound. I doubt that this is a problem with encoders used for commercial DSD recordings.

It is possible to avoid idle tones completely with multi bit delta sigma modulation if proper dither is used. Unfortunately, proper dither of one bit delta sigma (DSD) is impossible. Once again, Sony has screwed up in their technology choice. The first time it was reducing the sample rate of RBCD down to 44100 when it should have been 48000. This time it was the choice of 1 bit delta sigma modulation, when it should have been multibit. Their choice of 1 bit might have made sense in 1990, but technology has passed this by. ADC and DAC designers have gone to multi-bit because it offers better noise performance and can be properly dithered, thereby avoiding noise modulation and idle tones, among other artifacts of 1 bit encoders.

Tony Lauck

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

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