[LAU] About Algorithms

Renato rennabh at gmail.com
Mon Jul 18 10:13:11 UTC 2011


On Mon, 18 Jul 2011 11:53:47 +0200
David Adler <david.jo.adler at gmail.com> wrote:

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

isn't it useless going way above 24 bits, since that's what in the end
the DAC (sound card) will use? as far as I can remember, delta-sigma
DACs don't go over 24 bit, do they?

cheers
renato


More information about the Linux-audio-user mailing list