PCM and DSD are considered as different formats, as rule. But author think, that PCM and DSD are two side of a single phenomenon. Read here why.
DSD (sigma-delta modulation) is 1-bit audio format that use noise-shaping to increasing of dynamic range.
PCM (pulse code modulation) is multibit music format that use number bits to expanding of dynamic range.
Dynamic range is the difference between maximal and minimal allowable level of transmitted signal.
The maximal signal level cause overload.
Minimal signal level less than or equal to quantization noise level (a.k.a. noise floor). Signal "under" noise have bad recognizing.
Simplified dynamic range
Simplified dynamic range is difference between maximal level and noise floor.
Warning: It is not technically correct definition. But in frame of the article we will use it for easier understanding.
We can discuss dynamic range as common base for both PCM and DSD. Dynamic range is single matter for different bit depth.
Maximal level in the digital audio is accepted as 0 dB. It is not depend on bit depth. But noise floor depend on the bit resolution.
As example, we have a certain noise level for some PCM bit depth.
After the bit resolution reducing, the noise level is higher and dynamic range is reduced.
Bit depth truncating (from PCM to DSD)
However noise level may be kept into limited useful frequency band. Noise energy, that grown due bit depth truncating, may be pushed out of the band via noise shaping.
Noise shaping, PCM to DSD transformation
It is very like to DSD. And, yes! It is real multibit DSD!
Read about PCM to DSD conversion and DSD to PCM converters here > and here >
Now let me try to come from DSD to PCM. Here we also have given noise level.
Noise shaping may be characterized by the steepness. Both total and useful audio signal bands define the steepness.
Noise shaping steepness
Steeper noise shaping may be used to expand the useful band. So DSD 64 vs 128 (5.6 MHz) is matter of the steepness.
Higher steepness can cause lesser stabilty of sigma-delta modulator (noise shaper). So lesser steepness may be good.
Adding bit to sample word decrease noise level. So noise shaper's steepness may be decreased too. Because noise energy, that need to push out of the useful band, is lesser.
Adding bit - lesser the steepness, DSD to PCM transform
At last the noise floor become flat. So DSD signal is transformed to PCM.
DSD have significantly higher sample rate, than PCM, because band reserve is need to push excess noise energy out of the useful frequency band.
However, even simple band expanding cause lesser noise level. Because quantization noise energy is constant and distributed in full band. Graphically noise energy is square of the noise spectrum. It like to rectangle. If its width is expanded, height (noise level) diminish.
Sample rate and quantization noise level
Two times band expanding decrease noise level at 6 dB.
So we can say, that DSD decrease noise level twice:
Yuri Korzunov, 2017
Copyright © Yuri Korzunov
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