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

[Brad Midgley <bmidgley@xmission.com>]

Henryk

> So now you're going to bigger-than-32-bit integers anyways?

We don't need the full 32 bits. I want to get this working and then play 
around with slimming it down.

> Some time ago I already stated that using 32 bit for the floating point 
> presentation with 16 bits before the decimal point and 16 bits after 
> should be enough. (PCM and reconstructed subband samples should not exceed 
> 16 bits and all intermediary values should only be fractions of these 
> samples. I might have forgotten one bit for the sign, though.)

yes, i remember your suggestion. the libmad guys took an interesting 
approach... they only use 3 bits for the decimal and 28 for the 
fractional part. i will look at the range of our computation but it may 
be possible to similarly dedicate most of the range to the fractional part.

libmad produces 24-bit audio, another indication it is overkill for us 
to have 32-bit numbers.

> But then intermediate results during multiplication need an additional 16 
> bits and your platform therefore needs to be able to handle (at least) 48 
> bit integers without falling back to floating-point emulation speeds.

multiplying two n-bit numbers results in a 2n-bit product. if we have 
16-bit numbers, then the multiply/divide intermediate result would fit 
in a tidy 32 bits.

> If it can do that, then of course doing fixed point should be easy: 
> Addition, multiplication and shift work unchanged. Each multiplication 
> must be followed by a shift right 16 bits. Conversion integer -> fixed 
> point or fixed point -> integer is shift left 16 bits or shift right 16 
> bits respectively.

yes, exactly. I put inline functions in sbc.h to do the multiply and 
divide and add/sub "just work."

Brad


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