Re: 64bit processing - Why?
Re: 64bit processing - Why?
- Subject: Re: 64bit processing - Why?
- From: Brian Willoughby <email@hidden>
- Date: Thu, 30 Jul 2009 20:51:37 -0700
On Jul 30, 2009, at 20:23, William Stewart wrote:
Point(2)
AUHAL
As for the output stage. The best DAC/ADC performance I've seen
that is commercially available is within the 110 to 120 dB range.
Its hard to get up to the 120dB mark. That still leaves 4 bits of
dynamic range unused from a 24 bit signal path (which single
precision floats can represent). So, most (if not all shipping at
least) available converters use 24bits and as Ethan explained, this
is quite a wide dynamic range (bleeding ears 'n all)!
Ok, so going from the single float to the 24bit int. You have NO
loss or precision when doing this conversion. There is no
truncation or other loss of resolution in this transformation
(provided that the float is within the nominal range of -1 < 1).
If you have a double precision signal chain, you can do the
dithering yourself to single precision float. That is then
faithfully preserved through the rest of the signal chain to the
24bit converters.
So, I don't see how using AUHAL poses a problem here. But, maybe
I'm just stupid!
I think you're correct. Certainly not stupid. :-)
While I still stress the point that the DAC analog performance of 110
dB to 120 dB cannot be trivially compared to the bit depth (i.e. you
need way more digital dynamic range than analog), it's somewhat of a
moot point so long as there is a 24-bit clean path from host
application to hardware (and there is).
There are, however, emerging converters which claim 30-bit or 32-bit
performance, but their techniques are not necessarily high-quality.
It does present the possibility that we may soon desire to go beyond
24-bit precision when passing data from host app to hardware.
Today's 32-bit DACs are questionable, but perhaps we're right around
the corner from something which truly goes beyond 24-bit.
Since the full exercise of 144 dB is not comfortable for humans, the
precision beyond 24-bit is all about reducing objectionable
quantization noise by placing it further and further below the analog
noise. The question is how quiet does the quantization noise have to
be before absolutely nobody can detect it. With some people who I
assume are qualified making the statement that 48-bit busses should
be dithered, that means they believe the quantization noise should be
more than 280 dB down.
Brian Willoughby
Sound Consulting
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