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Non-oversampling delta-sigma DAC?
Non-oversampling delta-sigma DAC?
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Old 19th December 2016, 11:43 PM   #1
hollowman is offline hollowman  United States
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Default Non-oversampling delta-sigma DAC?

Asked this in another thread, but perhaps the topic deserves its own thread:

Quote:
If the internal oversampling filters of a delta-sigma DAC (IC; e.g. Wolfson) are bypassed (not used) and no external digital/OC filters are used prior to it the DAC IC, does this make the D/A processor "non-oversampling"?
I don't think I've seen this config. in a commercial or DIY DAC before.
To clarify, this query refers to delta-sigma DACs that offer user-defeat of the DAC's internal oversampling (digital filtering).
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Old 20th December 2016, 01:22 AM   #2
Richidoo is offline Richidoo  United States
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The DDDAC uses the TI PCM1794 with defeated digital filters, claiming to be a non-oversampling DAC. More info.
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Old 20th December 2016, 03:33 AM   #3
Ken Newton is offline Ken Newton  United States
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Quote:
Originally Posted by hollowman View Post
...To clarify, this query refers to delta-sigma DACs that offer user-defeat of the DAC's internal oversampling (digital filtering).
That is an excellent question. Following, is my highly oversimplified and non-authoritative explanation of what is essentially happening. While both sigma-delta-modulators (SDMs) and oversampling digital reconstruction filters (ODFs) operate at multiples of the input sample rate they have distinctly different functions.

The function of an SDM is to reduce the quantization noise (the quantization error) of the converter. This is achieved by spectrally relocating the quantization noise/error out-of-band, which, for audio, is to the ultrasonic band. Now, imagine a 1-bit quantizer based SDM DAC reproducing some arbitrary D.C. level. The DAC's 1-bit quantizer can correctly produce only two specific levels. All levels in-between would exhibit extreme amounts of quantization noise/error if it were not for the SDM unit relocating that noise/error to the ultrasonic band. If instead of a D.C. level the DAC were to output an A.C. signal, the SDM would dynamically adjust the ultrasonic quantization noise so that each next sample is accurately converted in analog level.

The function of the ODF is to remove (well, mostly remove) the repeating bands of ultrasonic images created by the sharply switching output of the DAC quantizer unit when reproducing an A.C. signal. These ultrasonic images are not to be confused with the ultrasonic noise created by an SDM unit. These images are repeating copies of the desired signal except shifted up in frequency and falling in amplitude. The ODF's job is to remove these repeating ultrasonic images. It does this by sharply low-pass filtering the desired signal band which results in additional samples being sent to the quantizer and converted. All that these additional samples do is to reconstruct the fundamentally sine based original audio signal.

Should an SDM based DAC convert a signal without benefit of an ODF (NOS) the output will have the same non-ringing impulse response, the same repeating ultrasonic image bands, and the same uncorrected high treble response droop characteristics of any other NOS based zero-order-hold DAC. However, there will also be SDM created ultrasonic quantization noise present. That's my conception, but as I said, it's non-authoritative.
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Old 20th December 2016, 08:41 AM   #4
hollowman is offline hollowman  United States
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I did know about dddac's TDA1543 NOS, from years back. It has that charac. "NOS sound". But I'd prefer carefully-implemented OS, even with older DFs such as SAA7220.

Don't know why dddac would pursue NOS for delta-sigma DACs other than for novelty or proof-of-concept. I've never heard such a setup, so I'll refrain from further comment. I can say that dddac seems to be a lone supporter of this topology.

Oversampling, with delta-sigma DACs, is partly used for the same reason OS is used for multi-bit DACs: gentler analog filtering.
In addition (I suspect), to increase the sampling rate so that the higher-speed DS modulator stage more easily "accelerates" to speed. E.g., for PCM Red Book (at 44.1khz) data to be "accelerated" to >1MHz speed of DS modulation is probably asking too much (= noise). But if one up/ovesamples before the DS mod stage, this eases the transition to the higher-speed DS modulation.
Non-oversampling delta-sigma DAC?
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Old 20th December 2016, 01:52 PM   #5
Ken Newton is offline Ken Newton  United States
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Quote:
Originally Posted by hollowman View Post
I did know about dddac's TDA1543 NOS, from years back. It has that charac. "NOS sound". But I'd prefer carefully-implemented OS, even with older DFs such as SAA7220...Don't know why dddac would pursue NOS for delta-sigma DACs other than for novelty or proof-of-concept.
I believe that his objective was to obtain NOS sound character from a SDM based D/A chip. Why from a SDM based chip? That's simply because they are the only type of audio application specific D/A in current production, as far as I know. The key insight behind the ddac, IMO, is the recognition that oversampling digital signal reconstruction filtering is a function different from sigma delta modulation.
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Old 20th December 2016, 04:44 PM   #6
hollowman is offline hollowman  United States
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Default Why no support from major chip manufs?

Quote:
Originally Posted by Ken Newton View Post
I believe that his objective was to obtain NOS sound character from a SDM based D/A chip. Why from a SDM based chip?
Right. However, while the major chip manuf's allow end users of hardware manufs to choose between certain OS filters ("Sharp", "Slow", "MP", etc.) they do not allow "OFF". Why? Because the lab measurements (at "OFF") start to suck? Or do their own* subjective tests reveal flaws/compromises with NOS?

*I know that ESS does conduct fairly extensive subjective evals.
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Old 20th December 2016, 09:16 PM   #7
Ken Newton is offline Ken Newton  United States
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Quote:
Originally Posted by hollowman View Post
Right. However, while the major chip manuf's allow end users of hardware manufs to choose between certain OS filters ("Sharp", "Slow", "MP", etc.) they do not allow "OFF". Why? Because the lab measurements (at "OFF") start to suck? Or do their own* subjective tests reveal flaws/compromises with NOS?

*I know that ESS does conduct fairly extensive subjective evals.
I feel fairly certain that the engineering departments of major DAC chip producers all accept the proven scientific principles of sampling theory - which is that the objectively correct reconstruction filter has a sharp frequency cut-off, and is preferably linear phase if a digital filter, to boot.

Soft filter roll-off modes were initially provided on-chip because they offered a shortened latency of the filter processing engine and intended for use in mastering. The specialty audio industry came to feature the soft filter modes as often subjectively preferable to the sharp. The DAC chip marketing folks picked up on that customer use and, like any good supplier, they began addressing that general market desire with the additional option of minimum phase filter modes.

I have no knolwedge of what the various DAC producers do or don't do as far as subjective chip evaluation. Having once been a middle manager in the IC industry I strongly suspect that there is no formal DAC chip subjective evaluation program within most vendors.

Last edited by Ken Newton; 20th December 2016 at 09:18 PM.
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Old 20th December 2016, 10:58 PM   #8
goodjingan is offline goodjingan
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is that possible ?

i don't have EE background so correct me if i made any mistake. my understanding of sigma delta DACs is they need 2x oversampling to get 1 bit of resolution. modern delta sigma DACs are probably 4 - 8 bit so that's why they call it "multibit delta sigma". because reducing bit depth made quantitation noise, they use noise-shaping technique to push the noise out of audio band and send it to ultrasonic area. then, by using some filters (some are very steep which IMHO made new problem) they remove the noise. and that's how they achieve higher bit depth.

http://www.diyaudio.com/forums/digit...-dac-work.html
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Old 20th December 2016, 11:45 PM   #9
Ken Newton is offline Ken Newton  United States
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I suspect that confusion (possibly, my own ) stems from the fact that there are two different oversampling ratios involved in SDM + ODF DACs. The ODF unit has the lower rate of the two, often x8. The SDM has the higher rate, x64 for example, which then would be 8 times the oversampling digital filter's output rate. So, while both the digital filter and the sigma delta modulator operate at some multiple of the native input rate the job of the ODF is to REMOVE ultrasonic image content, while the job of the SDM is to ADD (relocate) ultrasonic quantization noise. I don't know whether that expalanation helps.
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Old 21st December 2016, 02:44 AM   #10
hollowman is offline hollowman  United States
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Default DF better for RedBook?

With respect to an ODF used prior to a DS dac (PCM1794, etc.), I can understand why an ODF would be needed for RedBook PCM (16/44.1).
But at higher rez (24/192), I'm not sure how much benefit an ODF will have.
Then, again, I do not have the years-accumulated wisdom or theoretical/R&D experience of the major players (TI, AD, Cirrus, ESS).
Frankly, I think the multi-bit hype, that has crescendo'd the past several years (especially on diy and vintage-audio forums), is strange and mostly unsupported by the audiophile community at large.
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