I've somewhat studied this using band-limited 7 kHz square wave. And in addition this can create notable issue on transient (step) response too. Then again, whenever you limit bandwidth in analog domain using traditional analog filters, in an amp or such, you also tend to exhibit phase shift at top audio octaves. One dominating factor is how the amp's THD/IMD vs frequency profile looks like. We cannot know what kind of amps there are after the DAC, so we need to account for all kinds of possibilities, such as analog class-D amps, etc. But if/when something happens with any of the higher frequency output we'd want that to be something that disturbs the listening experience the least? In any case there shouldn't be any images that would indicate incomplete/imprecise reconstruction. Which, of course, is only part of the story skipping over all the reconstruction filter details and such. Noise HF -> less bad, because potential IM tones are also noise (random) -> less audible and if audible just background hiss (like tape or radio noise)īut overall, for proper reconstruction, both audio band and wide band output must be clean. Correlated/discrete HF -> bad, because it potentially generates discrete IM tones For example RME ADI-2 Pro's noise floor looks pretty much exactly the same regardless if you run it at 705.6/768k PCM or DSD256 (with compliant noise filter). I have not seen many that would do PCM at 352.8/384k with flat noise floor and flat frequency response up to 176.4/192k. Now most "PCM" ADC's based on SDM look like DSD128 from noise profile point of view. I think suitable way to look at bandwidths is like:ĭSD64: flat noise floor bandwidth equivalent of 44.1/48k PCM, but without AA-filter effectsĭSD128: flat noise floor bandwidth equivalent of 88.2/96k PCM, -"-ĭSD256: flat noise floor bandwidth equivalent of 176.4/192k PCM, -"-ĭSD512: flat noise floor bandwidth equivalent of 352.8/384k PCM, -". So 100 kHz band measurement still shows some difference between the two at that rate. Only above that you can see a difference if analog filter doesn't cut all the noise away.įor DSD128 noise corner is around 50 - 60 kHz (like pretty much for most ADC's too). I've done quite a bit of measurements where in < 100 kHz band both ASDM5 and ASDM7 looks exactly the same. But at higher DSD rates usually DAC's analog noise floor dominates and the digital noise floor of DSD is way below that. So in the end between the two it is largely system-dependent.ĪSDM7 is more aggressive and complex so it puts more demand on ultrasonic filtering capabilities, but is otherwise cleaner and pushing the noise down more in audio band. If you look from wide-band perspective it is at DSD512.
From audio-band performance perspective Holo Spring's optimal point is DSD256. For Spring1, 20-bit TPDF ditther at 352.8/384k is fine for the R2R section. However, for example on macOS one is limited to 16-bit output at those > 1MHz rates, but using NS5/NS9 noise shaper and 16-bit at 1.4112/1.536 MHz rate gives practically same audio band SNR as 20/24-bit at same or lower rates. As PCM (R2R) example, Holo Spring (2) gives optimal results with 20 bits, up to 1.536 MHz. For PCM inputs, linearity sweep is good starting point to see how much accuracy DAC actually has to set exact output dithering resolution.
For 16-bit and high rates noise shaping is good. For PCM, if DAC can do 20 bits or more resolution, TPDF or Gauss1 is fine for dither. For DSD, depending DAC's DSD filters, either ASDM7 or ASDM5 modulator. Nowadays I do most measurements and listening with "poly-sinc-ext2" filter.
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