[Ffmpeg-devel] ac3enc.c modifications

Justin Ruggles jruggle
Thu May 19 18:30:32 CEST 2005

Simone Karin Lehmann wrote:
> sample_t was intended to switch between doubles and integers at compile time, 
> but I didn't finish to fully implement the integer code. This is why you get 
> silence.

Oh, ok.  Now it makes sense.

> ... but the calculations to get these values will be more accurate if you use 
> doubles. In mdct() there are plenty of multiplications using  sin and cos 
> factors. Only using integers will give some really great rounding errors. 
> I've done a lot of testing to this. First I encoded a wav file only using 
> integers, decoded it back to wav, took audicity to give me spectrum analysis. 
> Second I did the same procedure with doubles.
> The spectrum resulting out of the doubles encoding was more closely to 
> original than only encoding in integer arithmetics.

I can see where that would work.  Since it uses summations, the rounding
errors would just keep adding up.  And if you're going for accuracy over
speed, using floating-point would help.  How does the spectrum vary
between using floats vs. doubles?

I was going to suggest keeping mdct_coefs as int32 and just using
doubles internally in the mdct, but the rematrixing would get a very
slight accuracy gain with floating point as well.


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