Losses in 1-bit DSD vs multi-bit PCM
by, 05-22-2015 at 06:06 PM (1478 Views)
Modern PCM sigma-delta converters produce much lower error signals than1-bit sigma-delta DSD converters. The errors in the DSD system are due to the1-bit quantization that occurs in 1-bit sigma-delta DSD converters. Multi-bit PCMsigma-delta converters can be fully dithered and do not suffer from this un-dithered truncation. Plus, every added bit reduces the noise signal by 6 dB. A 4-bit sigma-delta converter is 24 dB quieter than a 1-bit sigma-delta DSD converter. Right from the start, 1-bit DSD signals have much higher losses than multi-bit PCM signals.
Conversion from 1-bit DSD to multi-bit PCM is a lossless process inside the audio band. The only thing that is removed is the out-of-band noise above the Nyquist limit of the PCM system. Nothing else is lost. Don't believe the DSD marketing hype.
In contrast, conversion from multi-bit PCM to 1-bit DSD is always a lossy process.The loss is due to the 1-bit truncation. This truncation introduces a very large ultrasonic error signal that makes the ultrasonic region unusable for audio. But remember the ultrasonic region of DSD is always unusable for audio because of the high noise levels. This ultrasonic noise produced by 1-bit DSD systems must always be removed before reaching power amplifiers and tweeters. When the noise is removed, the ultrasonic audio content is also removed.
Processing a 1-bit signal to create a 1-bit signal is also always alossy process. A volume control is one of the simplest processes in a multi-bit PCM system, but it creates a large error signal when applied in a 1-bit DSD system.The same is true for any other 1-bit to 1-bit DSP process. The lossy part of these DSP processes is the quantization back to 1-bit. Cascaded 1-bit truncation processes can rapidly degrade the audio quality. For this reason, DSD is almost always processed as multi-bit PCM.
Any DSP process applied to a 1-bit DSD signal produces a multi-bit PCM signal. No loss of information occurs until this multi-bit signal is quantized back to a 1-bit signal. Why incur the loss by going back to a 1-bit signal after the processing inherently produces a multi-bit PCM signal?
All practical DSD systems require some sort of DSP processing (gain control, mixing, filtering, etc.) and all of these processes produce multi-bit PCM results. Taking these lossless multi-bit results and adding loss by truncating them back to a 1-bit DSD signal makes absolutely no sense. DSD complicates the signal processing and adds unnecessary losses in several places along the signal path. DSD does not simplify the signal path.
There is absolutely no truth to the marketing hype that claims that 1-bit DSD is a simpler system than multi-bit PCM. The exact opposite is true. 1-bit DSD is a lossy system.
John Siau, VP Benchmark Media Systems, Inc.[/QUOTE]