# Re: Fast Fourier Transform Queries

*From*: "Maarten Wiltink" <maarten@xxxxxxxxxxxxxxxxxx>*Date*: Mon, 16 Jan 2006 09:59:46 +0100

<alanglloyd@xxxxxxx> wrote in message

news:1137309700.621468.135260@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

> "Hans-Peter Diettrich" <DrDiettrich@xxxxxxxxxxx> wrote in message

news:42u7b2F1jtb11U2@xxxxxxxxxxxxxxxxx

>> Maarten Wiltink schrieb:

>>>>> The file is in GSM format of 65 bytes/block, 320 samples/block.

>>>> 320 samples in 65 bytes? ;-)

>>> Of course. Start with 320 physical, time-domain samples. Transform

>>> to frequency domain. Throw away precision (low-order bits), phase

>>> information, and accuracy (higher harmonics) until you have 65 bytes

>>> left. Transmit, transform back, be annoyed at poor audio quality.

>>> It's not magic, it's compression.

>> I.e. the file does already contain kind of "FFT" data?

> [...] I had already tried doing FFT on the unconverted GSM audio,

> and there was no significant difference between correct and

> corrupted blocks. And there was a fair amount of level in higher

> harmonics, with a fairly smooth envelope. So I would suppose the

> compression algorithm is more complex than Maarten's simple

> description.

It _is_ a simple description of course, more a statement of intent

than a fullfledged algorithm.

The devil is in the details. It might be a DCT instead of an FFT, or

something else still, and anyway if you don't know the exact format,

you can't expect to make anything out of it. The above description

is not that far from JPEG - but it skips differential encoding and

Huffman compression. There may be pre- and postprocessing.

Higher harmonics have a tendency to creep back in. Bandpass filters

aren't perfect and everything that is done in the time domain has

repercussions throughout the frequency spectrum.

Groetjes,

Maarten Wiltink

.

**References**:**Fast Fourier Transform Queries***From:*alanglloyd@xxxxxxx

**Re: Fast Fourier Transform Queries***From:*Hans-Peter Diettrich

**Re: Fast Fourier Transform Queries***From:*alanglloyd@xxxxxxx

**Re: Fast Fourier Transform Queries***From:*Hans-Peter Diettrich

**Re: Fast Fourier Transform Queries***From:*Maarten Wiltink

**Re: Fast Fourier Transform Queries***From:*Hans-Peter Diettrich

**Re: Fast Fourier Transform Queries***From:*alanglloyd@xxxxxxx

- Prev by Date:
**Re: Identifying Sender: TObject** - Next by Date:
**Re: Help reducing object instance size** - Previous by thread:
**Re: Fast Fourier Transform Queries** - Next by thread:
**Re: Fast Fourier Transform Queries** - Index(es):