Re: Double precision

On 6/30/2013 10:10 PM, Gabriel Bouvigne wrote:
> Le 22/06/2013 16:47, Thomas Orgis a écrit :
>> It would be interesting to see if raised internal precision has any
>> effect at all with 16 bit input. Disclaimer: I didn't work on LAME code
>> myself, so don't know details on how the filters are implemented.
>
> Using double instead of float has some effect, for sure, as the 
> resulting file will be different. The question is then to know if this 
> has any audible effect or not.
>
> Changing compilation options, using assembly or not, or compiling for 
> a different architecture (x86 vs x86_64) all have some impact on the 
> resulting bitstream.
>
> I actually asked about the reverse question (changing from double to 
> float) in 2004:
> http://osdir.com/ml/audio.mp3.lame/2004-05/msg00052.html
>
> ...so I suspect that I might actually be the one who changed the 
> default to float latter on (that was a long time ago, so I'm not sure 
> anymore).
>
> Advantages of using floats over double were then :
> * ability to use the vector unit (SSE) instead of FPU, even for 
> processors which did not featured SSE2 instructions
> * faster binary
> * ability to use some SSE assembly/intrinsics (not sure if it's still 
> there nowadays)
>
> Even nowadays, with proper use of SSE you can crush twice as many 
> floats as you could process double.
> I'd even think that float over double might enable to get some 
> significant speed boosts through proper use of SSE or GPGPU. 
> Unfortunately, we don't have much SSE code in Lame, so this advantage 
> is still mostly theoretical.
>
> Regarding the quality difference, I suspect that double instead of 
> float would only make a negligeable difference, which would be 
> significantly less important that other changes (like improving our 
> resampling filter, as an example).
>
Well the reason I raise the topic, is mainly one of principle. I think 
there is a small benefit, and that we can afford. Also see my post on 
not needing a filter and bitrates etc.  I think it would be a good 
contemporary refresh of the encoder. Not that I am going to be snapping 
any whip here.

I actually DID a search and replace of some floats, and managed to 
compile atleast the encoder. And first impression is that it does sound 
a bit more natural. That is typical for EQ´s aswell, etc. I always use 
double, and find floats to give a bit quantized sound, unless one is 
doing very little processing, no IIRs etc.

Anyway it was just a suggestion, and most playback equipment will not 
benefit much.

Peace Be With You!

-- 
Ove Karlsen,
www.ovekarlsen.com


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