[linux-audio-dev] Direct Stream Digital / Pulse Density Modulation musing/questions

Tom Browne lad at tbrowne.demon.co.uk
Mon Jul 28 10:45:01 UTC 2003


Anders Torger wrote:
> On Monday 28 July 2003 14.59, Alfons Adriaensen wrote:
>> On Mon, Jul 28, 2003 at 02:00:41PM +0200, Anders Torger wrote:
>>> It becomes even more interesting when you bring in dither. Then you
>>> can represent signals whose amplitude is less than one bit, and you
>>> can increase time resolution to infinity.
>>
>> Time (phase) resolution *is* already infinite, even without dither.
>> The quantisation of time and amplitude are two completely different
>> beasts.
>
> Well, not exactly. I'm am by no means an expert on dither, so if I
> have wrong, let me know.
>
> Think that you are sampling a sine wave, without dither. Truncating to
> an even sample can result in a small phase shift after the
> reconstruction, which is not there if you make a properly dithered
> recording. Thus - dither improves temporal resolution, through
> improvement of amplitude resolution.
>
> Am I right?

Sounds spot-on from here...

It's important to remember that dithering "works" through averaging, and as
such can only provide a finite level of benefit.  I do not believe there is
such a thing as infinite phase resolution with PCM... only effective phase
resolution, which varies depending on the dither, signal content and length
being analysed.

See how many different answers you can get from the FFT of a 997Hz sine wave
dithered to 16-bit and generated at 44100Hz by changing the window length
and type... :)

- Tom.




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