[vdr] Re: "No accelerated IMDCT transform found"

["Rienecker, Fa. Evenio, ITS P, M" <C.Rienecker@deutschepost.de>]

> > "No accelerated IMDCT transform found"
> > 
> > with the following one:
> > 
> > "Using djbfft for IMDCT transform"
> > 
> > The follwing patch will help in any case. Apply with -p0 to 
> imdct.c in the
> > liba52 directory:
> 
> Why do you want to remove those messages ? As far as I have read it
> this djbfft lib does accelerate something. I don't know if it will 
> improve something for vdr but if it does, I'd like to see the message
> telling me I should install it.

That's O.K. but in a production environment (like living room) this message is annoying. Anyway, it depends on the compile option, which branch is executed. This cannot be change during runtime.

> Can anyone with more knowledge shed light on it. Will the usage of djbfft imrove
> anything ?

It's said to be one of the fastest (if not the fastest) (inverse) fast fourier (fft)transform available for x86 CPU.

In general the fourier transform is a special case of the 'harmonic analysis' which deals with bijectivly mapping certain spaces onto each other with gage conservation (de: Masserhaltung). IOW the conservation of distances and integrals.
The fast fourier transform (fft) is a discrete approximation of the fourier transform that is well suited for computer implementations.
This stuff, in general, is used to compress video and/or audio data. Since fft is discrete you'll have a rather simple way to quantisize data that would otherwise be continuous. It will result in an set of fourier coefficients. Before compressing that set it will be quantisized again by mapping real numbers to a discrete set of integers.
On heavily copmressed mpeg2 movies you'll especially see this quantization at small color gradients like skin colors (e.g. the Rocky movies on 13th Street).

Actually VDR uses the liba52 which in turn is meant to decode AC3 (from the DVDs) which in turn uses the djbfft as accelerated fft.

@Andreas (or Andy):
liba52 has been introduced with the nav10 patch for vdr-0.99.
Why is it used for replaying simple vdr recordings and MP3 files ?

CU,
Christian.


> 
> Thilo
> 
>