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RE: What about 32 bit DACs

Posted by Thorsten (M) on February 11, 2016 at 10:28:16

In Reply to: What about 32 bit DACs posted by Duster on February 10, 2016 at 19:17:01:

Hi,

I think we need to consider what terminology we apply.

If a DAC accepts a certain word-length (16/18/20/24/32 Bit) does this "qualify"?

Or do we expect a DAC to be able to show true performance (ENOB - equivalent number of bits)?

If the first then "number of bits" is essentially meaningless as a measure of Quality. For example a current commercial "32-Bit" DAC chip offers 106dB A-Weighted SNR, which is around 17 Bit ENOB, with the best at around 138dB or 23 Bit ENOB.

If the second is applied, then neither 32 Bit nor 24 Bit DAC's exist and a 32 Bit DAC is literally impossible, as it would require > 194dB analogue SNR, this may require immersion in liquid nitrogen to get anywhere close and we are likely to still be many bits short.

Another question is if we use the DAC for music, what sort of dynamic range can actually be used?

Typically, recording microphones have around 26dB(A) self noise, the very best have around 10dB(A) but are not necessarily the best sounding microphones. Peak SPL from a large orchestra in the front rows is around 105dB, above the rostrum (where recording mikes often are hung) it may be another 10dB.

So a minimalist classical recording using the lowest noise microphones may have a real usable dynamic range of around 105dB. Multi-tracked recordings may have microphones closer to instruments and thus higher SPL and potentially greater dynamic range.

The other side is how much dynamic range can we get even in a treated and silenced listening room? Peak SPL's much above 105dB (and average SPL's much above 85dB) are likely to lead to hearing damage. A little headroom perhaps at 110dB would be nice.

If we presume "normal" speakers (87dB/2.83V/1m) and a reasonable size room it around 3m listening distance needs at least 400W, but with most speakers this level of power leads to compression and huge distortion. So we may need to get very high power handling speakers with well above average efficiency to even get 110dB undistributed peaks (and an Amplifier/Preamplifier with sufficiently low self noise).

And then we still only have 110dB usable dynamic range if there is absolutely no background noise. And these 110dB is around 18 Bit ENOB.

What does it all mean?

Extending DAC performance much past 18 Bit real world analogue performance has little practical use for music reproduction.

This level of performance was reached reliably in the late 1980's and is now fairly easily and cheaply attained. More dynamic range in itself, more "bits" in themselves are unlikely in themselves to offer any audible improvement.

What matter much more to how we perceive digital music replay is the way digital filtering and other digital processing (e.g. noise-shaping) impact the music. Part of this is the sample rate which in some ways can never be high enough and realistically my experience tells me that the less digital domain manipulation the better.

So for me - I'd rather have a good old-fashioned 18 Bit real (multi-bit) DAC at 176.4kHz than a modern noise-shaped low bit DAC that is marketed as "32 Bit". But others may disagree...

Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?

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