Digital Drive

Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it.

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RE: Just a thought ...

Posted by Charles Hansen on June 12, 2017 at 00:48:59:
>> Interesting, the Bryston DACs, engaging it makes the sound a bit softer, but very pleasing, and a bit more textured. <<

It appears that you are referring to the Bryston BDA-3. According to the owner's manual, "the internal sample rate converter upsamples incoming 44.1kHz and 88.2kHz digital audio to 176.4kHz. All 48kHz and 96kHz digital audio upsamples to 192kHz. The Upsample feature does not affect HDMI or USB."

That unit uses dual mono AKM AK4490 DAC chips, which have built-in 8x digital "oversampling" filters. (The correct technical term is interpolation filters.) When "upsampling" is engaged (again, the correct technical term is "interpolation"), a separate interpolating digital filter is inserted prior to the interpolating digital filter built into the DAC chip - nothing more and nothing less. There are three things to note in this situation:

1) Concatenating digital filters is done all the time, almost always to save money. Virtually all 8x interpolating digital filters are a concatenation of three 2x interpolating digital filters. Nothing new here.

2) The first digital filter in the chain has the greatest sonic impact. It is possible that the sonic differences are simply due to the different characteristics between the first stages of the external ("upsampling") and internal ("oversampling") interpolating digital filters.

3) The other factor that may come into play is the rate at which the modulators in the DAC chip are operating. Depending on the specific internal architecture of the DAC chip, the modulators may or may not be operating at different rates when presented with different input rates (eg, single-, dual-, or quad-rate signals). I've not seen any that are so affected (the modulators typically operate at the master clock rate set by the local crystal oscillator), but it is conceivable that there are exceptions with which I am unfamiliar.

The bottom line is that the "upsampling" feature inserts an extra digital filter into the signal chain. This is very much the situation with MQA, as well. The degree to which the sound is affected (for better or worse) in either situation is simply due to the effect of the extra digital filter.

Just out of curiosity, how would you describe the magnitude of the sonic difference created by the addition of the "upsampling" digital filter in the Bryston DAC?

As always, strictly my own personal opinions and not necessarily those of my employer or other digital engineers.