CS8421 upsampling in a DAC

[Glassman]

the oversampling filter internal to DACs (or external chips like the DF1704/6 or PMD200 mentioned) do the processing in three steps - three halfband 2x oversampling filters in cascade.. the single most critical part is the first 2x step. This is confirmed both theoretically and practically by our digital output module.. said module had 2x upsampling enabled and fed various external DACs including the flagships from Crystal (CS43122) and Analog (AD1955/AD1853) with great results compared to upsampling disabled. This is just to say that there is a difference well worth it even though the original oversampling filters are still in signal path, however it is of course favorable to do the whole processing with such precision as our 2x upsampler. we've done it and the results were stunning. You might have a luck feeding the DAC a 176.4k signal in external digital filter mode, with a little trick or two - that way you get rid of the internal filters completely..

[Wingfeather]

That sounds interesting. Upsampling to 192kHz "nicely" should let me do away with the first two oversampling filters. Sounds like something worth investigating.
Taking a quick look at the datasheet for the TI PCM1796 (presumably the situation is the same for the 1794 and the 1792) it appears that even in the External Digital Filter Mode the DAC still applies some oversampling. Looks like oversampling in the DAC is generally quite hard to get away from! Perhaps the Analog Devices chips are different. Will have a look at some point.

Also...

This is probably a very silly question, but how did you get at the 8x oversampling figure? Werner also said that 44.1kHz will end up at 352.8kHz for the 4398 - which is of course also 8x higher. So you both agree.
My point is that the 4398's datasheet says that it does 128x oversampling for a 44.1kHz input (section 4.4). Now, the idea that the DAC is actually upsampling to 5.6448MHz and D/A converting THAT sounds a bit crazy... but what else could "128x oversampling" mean? Or, to put it another way, if the oversampling is really 8x (which I'm sure it is if you've said so) then why do they say "128x"?

Thanks.

[ezkcdude]

Originally posted by Wingfeather

Taking a quick look at the datasheet for the TI PCM1796 (presumably the situation is the same for the 1794 and the 1792) it appears that even in the External Digital Filter Mode the DAC still applies some oversampling. Looks like oversampling in the DAC is generally quite hard to get away from! Perhaps the Analog Devices chips are different. Will have a look at some point.[/B]

I am using PCM1798(4), and you can bypass the DF, but only in mono mode (which I'm not doing).

[Werner]

Originally posted by Wingfeather
Or, to put it another way, if the oversampling is really 8x (which I'm sure it is if you've said so) then why do they say "128x"?
[/B]

128 x is the product of


-8x oversampling and digital-domain reconstruction filtering at high internal accuracy (this signal, reduced to 24 bit) could conceptually be used to drive directly a multi-bit DAC such as the PCM1704.

-16x further oversampling to the 5.6MHz base rate of the DAC's delta-sigma modulator (i.e. a bitstream, low-bit, whatever, ... DAC).
Any filtering done in this step is irrelevant to our discussion here.


Glassman, can you disclose any more on your filtering method? If not in public then perhaps privately? Are you following Craven's ideas (but warped to operate for 44.1kHz)? Or just a massive overkill implementation of Sinc(x)? Something in-between?