Why no THD measurements for SM5847 ? - diyAudio
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Old 23rd April 2009, 04:52 PM   #1
percy is offline percy  United States
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Default Why no THD measurements for SM5847 ?

I had recently contacted NPC after looking at the datasheets of the SM5847 DOSF chip. I asked them why there are no THD or THD+N charts in the datasheet, and also requested that information from them. But I was told that "thd does not apply" to oversampling filters, and no one has ever asked for such information for that chip.

I tried arguing that an oversampling filter is pretty much an SRC, except the fout/fin ratio is always integer, and if manufacturers of SRC can publish THD charts in their datasheets(like src4392 or Cirrus), why not NPC for their DOSF ? Didn't work. They maintained it "doesn't apply".

Oh well. Maybe I not understanding something but would anyone please care to explain to me why it does not apply ??

Wouldn't the design and the internal precision used in the filter have a direct and most significant impact on distortion and noise in band ?
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Old 24th April 2009, 02:15 AM   #2
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http://focus.ti.com.cn/cn/lit/an/sbaa055/sbaa055.pdf
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Old 24th April 2009, 01:33 PM   #3
percy is offline percy  United States
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care to explain your point ? and also what in the document answers my question ?
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Old 24th April 2009, 07:45 PM   #4
Wodgy is offline Wodgy  United States
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Digital filters are linear systems. THD only measures nonlinear distortion. That's the basic reasoning behind their position. This is why you don't see distortion measurements in the datasheet for DF1704 either.

The only things that are potentially nonlinear in digital filters are quantization and dither.

ASRC is not an appropriate comparison because ASRC is inherently mixed analog/digital (time being the analog component). Pure digital filters are just digital devices.
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Old 25th April 2009, 02:05 AM   #5
percy is offline percy  United States
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Quote:
Originally posted by Wodgy
ASRC is not an appropriate comparison because ASRC is inherently mixed analog/digital (time being the analog component). Pure digital filters are just digital devices.
I always thought of ASRCs as Digital Oversampling Filters followed by a decimation filter. Or in other words the DOSFs dont have/need a decimation filter.
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Old 25th April 2009, 02:35 AM   #6
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Those "thdn" results are dependent on the dsp power thrown at the task. Here are some more :

http://src.infinitewave.ca/

the one called Izotope is widespread nowdays and it has 64bits at its disposal.

-TI has polyphase filter to get 16x OS for cheap, purists dont like OS filters like that.
-NPC sells the same old (15y. at least) sm5842 in disguise, as the 24/96 capable 5847... Both are the same 2x2x2 FIR , recent integrated efforts (ESS ) outclass it . Again, purists dont like cascade 2x2x2 filters like that, but to jump to 8/16 OS result at once, depending on your requirments this way can be very expensive in terms of computing power.
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