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Old 29th May 2006, 01:35 PM   #1
Asgard is offline Asgard  Sweden
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Question Sample rate into BB PCM1798?

I'm almost finished with a DAC like many others, that is CS8416 -> AD1896 -> PCM1798 -> OPA2134 (I/V).

The sample rate converter is only for jitter reduction and not perticulary important for the design itself. But I've started to rethink what sample rate I should pump my D/A with. The BB DAC has an internal 8x oversampling filter, and the chip supports up to 192kHz rates.

I could set the ASRC to output 48kHz thus performing only a minor upsampling of the decoded SPDIF signal and thus lowering THD (in the DA) with some small amount. Is it really interesting to pump the DA with 96kHz or higher since the filter already works with a high 8x oversampling, even at ordinary rates.

Cheers for your feedback !
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Old 29th May 2006, 01:41 PM   #2
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Old 29th May 2006, 02:52 PM   #3
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As one of the "many others" working along very similar design paths, I too would like to see a schematic.
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Old 29th May 2006, 09:51 PM   #4
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Hmm, I've planned in documenting the DAC when it's finished and sitting in it's case. Though I'm a bit afraid of receiving a bunch on hate-mails criticizing the design (like qmarch got for his headphone DAC/amp ). I'll create a new thread for this later on.

This thread concerns only the need of a high samplerate into any sigma-delta DAC that consists of a oversampeling digital filter.
So what do you say, are there any gains in letting the DA work with 192kHz instead of 44.1k at the input? 8x oversampling would push any images far away even at 44.1k and I doubt that the DA is actually upsampling the incoming 192k at all? (192x8=1.536MHz )
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Old 29th May 2006, 10:08 PM   #5
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I thought gmarsh's stuff was very well received...

Anyway, as for upsampling, you should do it with the chips you've chosen, because the PCM1798 and AD1896 are both going to apply filtering no matter what, although the PCM1798 DF can be bypassed, but only by using it in mono.
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Old 29th May 2006, 11:21 PM   #6
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He mentioned that he got a bunch of highly negative PMs due to his design (mine is much uglier with half-wave rects ), but as you said most of us loved his box and got inspired.

Since I can see no use of using 192k between the ASRC and DA I guess that it would be nice to have the sample rate as low as possible due to THD+N and plain sence (EMI). The lowest rate while still supporting 96kHz input rate would be around 52kHz. This isn't a very nice multiple of 44.1 but then again, what is =)

Currently I'm looking at using 96k since some DVDs/soundcards use 48k output rate. That would sum up nicely to 96/2 (though I strongly doubt that any small integer-mismatch in the interleaving would result in anything audiable). Anyhow, 96k seems more favourable than 192k
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Old 30th May 2006, 12:32 AM   #7
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Just looked at the PCM1794 data sheet again. Most of the measurements are best at 96 kHz, so that's probably a good sign.
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Old 30th May 2006, 04:43 PM   #8
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Hi! Don't be afraid! I think major of us do look forward to the schematic......
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Old 31st May 2006, 02:55 PM   #9
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Heh ok, I'll just have to clean up the schematics first and send the PCB for fabriation. Hopefully I'll post something in a few weeks or so.
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Old 31st May 2006, 02:59 PM   #10
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Do you go straight from schematics to PCB, without prototyping? That would be impressive.
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