Re: [Bluez-devel] Advanced Audio Distribution Profile (A2DP)

[Marcel Holtmann <marcel@holtmann.org>]

Hi Brad,

> >>i was hoping to get the timing problem hammered by using setitimer. 
> >>unfortunately, there still seems to be some dependency on the cpu speed 
> >>which is surprising to me. there's a place in the code where the time 
> >>period reported by libsbc is multiplied by 87/100. that 87 has to be 
> >>tweaked for different cpus :(
> > 
> > this is because the encoding itself takes some time. For that we need to
> > encode up to 6 SBC frames in advance.
> 
> the way a2play is now, we do preencode. right now it's set so the 
> encoder will get ahead by 3 packets (typically 6 frames each?). sigalarm 
> fires off regularly to initiate the transmission of one packet.

maybe I should look at the code again and test it.

> as far as integer stuff, should we be doing something like a short float
> 
> struct sfloat {
>   int16 mantissa;
>   uint8 exponent;
> }
> 
> or a fixed point 32-bit value? any idea what part of the 32 bits would 
> be best to have in front and behind the decimal?
> 
> i guess i should start by having libsbc keep track of ranges of the 
> different floats so we can figure out where the precision needs to be.

Look at how the MAD project does it:

http://www.underbit.com/products/mad/

I think they have the fastest MP3 decoder at the moment.

Basically using a fixed point uint32_t value should be faster than
dealing with a struct. This is only my impression and it is not verified
by any performance tests.

Regards

Marcel




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