[FFmpeg-devel] [PATCH] E-AC-3 spectral extension

Benjamin Larsson banan
Mon Jun 1 12:57:43 CEST 2009

Michael Niedermayer wrote:
> On Sat, May 30, 2009 at 11:43:12PM -0400, Justin Ruggles wrote:
>> Michael Niedermayer wrote:
>>> On Sun, May 17, 2009 at 02:23:34PM -0400, Justin Ruggles wrote:
>>>> Hi,
>>>> I was recently made aware that some French TV station(s) will soon (if
>>>> not already) start using E-AC-3 streams in their broadcasts which
>>>> utilize spectral extension.  I was also given some samples (thanks j-b
>>>> and Anthony), which I uploaded to mphq:
>>>> http://samples.mplayerhq.hu/A-codecs/AC3/eac3/csi_miami_*
>>>> So I decided to revisit my SPX patch.  The previous version was done
>>>> with all integer arithmetic, but it turns out that it's really not
>>>> accurate enough for spectal extension processing.  The resulting decoded
>>>> output had a max bandwidth of about 2kHz less when using 24-bit fixed
>>>> point vs. floating point, and was only slightly higher than without any
>>>> SPX processing at all.  Making just the square roots floating point
>>>> raised the bandwidth about 1kHz, and making the rest (noise/signal
>>>> scaling, spx coords, and notch filters) floating point added about
>>>> another 1kHz.
>>>> I was able to compare the output to Nero's E-AC-3 decoder (thanks
>>>> madshi), and the results are very close considering that AC-3 uses
>>>> random noise for zero-bit mantissas:
>>>> stddev:  131.16 PSNR: 53.96
>>> i wouldnt call 131.16 close
>> Well, considering I don't know how the Nero decoder differs, it's not
>> bad.  I don't know how the Nero decoder ends up with higher bandwidth
>> than it should, it very likely uses a different random noise generator,
>> and it could do dithering in the float-to-int16 conversion.
> dither in float2int might account for ~1.0 stdev maybe but we are 2
> magnitudes above that.
> about the PRNG, well just decode a AC3 with 2 different PRNGS and compare
> by how much they differ
> also you can take neros output and ours and create a wav file with the
> sample wise differences.
> looking at that / listening to it might provide a hint about what is that
> differs.

My guess is a sample delay. I know Siarhei wrote some patch for 
tiny_psnr to calculate that.

Benjamin Larsson

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