linuxaudio.org

On Mon, Jul 18, 2011 at 9:38 AM, Erik de Castro Lopo wrote:

quoted text
> Moshe Werner wrote:

>

>> Dale Powell to me

>>

>> > 64bits is double (comes from 32bit computer architecture, so double uses

>> > two words per value.)

>> >

>> > Single generally uses 24bit values and 8bits of exponent.

>> > Double uses 53bit values with 11bits of exponent.

>>

>>

>>

>> Oh I see. So whats this Pro tools 48bit mixer talk about that goes on in the

>> industry?

>

> That 48 bit thing could hark back to Protools when it used the Motorola

> 56k DSP chips:

>

>    https://secure.wikimedia.org/wikipedia/en/wiki/Motorola_56000

>

> which are fixed point DPS chips.

>

> According to this:

>

>   https://secure.wikimedia.org/wikipedia/en/wiki/Protools

>

> Protools uses 48 bit fixed point arithmetic (possibly doubled 24 bit

> registers of the Moto 56k or possibly custom hardware).

>

>> Why would it be better or more precise than the Ardour mixer (which i prefer

>> over Protools).

>

> If it is 48 bit fixed point I would think that it is inferior to

> Ardour which almost certain uses double floating point which has

> a 53 bit significand.

>

AFAIK everything Jack (including Ardour) uses single precision 32 bit

floating point samples. (Not 64 bit double precision as Erik suggests

- or am I wrong here?)

32 bit floating point gives a dynamic range of ~192dB, well above the

dynamic range of our hearing or any analog audio hardware, leaving

ample headroom for rounding errors to disappear.
I would not speak of inferiority or superiority when comparing this

and 48 bit integer calculations of pro tools. Single precision floats

as jack uses them will not be the bottleneck of SN ratio or any other

performance measure, the same is most likely true for anything pro

tools does. (I'm sure it is, just writing "most likely" 'cause I don't

*know*)
Giving this[1] paper a quick look, they use the term "double

precision" for 48 bit integer, probably relating it to the 24bits of

the DA/AD converters. All that

bit-shifting/truncation/extra-headroom-bits-stuff mentioned there is

related to the integer format and does not apply to floats.
[1] http://akmedia.digidesign.com/support/docs/48_Bit_Mixer_26688.pdf
best,

d

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