[mad-dev] Quantizing mad_fixed_t?

[Rob Leslie]

On Thursday, July 3, 2003, at 04:26  AM, Arve Knudsen wrote:
> I see that the algorithm for 32 bit output in madplay quantizes to 24 
> bit; I'm
> a little daft when it comes to fixed point representations, but why 
> are some
> of the fractional bits discarded even when you're outputting in full 32
> bit?

This is mostly the result of convenience of uniformity. I think some of 
the 32-bit audio hardware out there ignores the least significant byte, 
so it seemed appropriate to round/dither to the 24th bit -- not that 
the bits past the 24th are terribly important or accurate anyway.

There's nothing wrong with using all available bits for 32-bit output.

> There's also one more thing I don't understand; the original sign bit 
> of the
> fixed point number isn't kept (as far as I can see), instead the least
> significant whole part bit occupies the sign position. Does this whole 
> part
> bit represent the sign (a little confused here)?

In two's complement notation, the least significant whole part bit (and 
all intervening bits) are guaranteed to be the same as the sign bit 
when the absolute value is less than MAD_F_ONE. Since clipping ensures 
this condition, shifting will not affect the sign of the value.

-- 
Rob Leslie
rob@mars.org