On Sun, Sep 24, 2000 at 10:57:39AM +0200, Gabriel Bouvigne wrote:
>
> Minidisc also uses mixed windows. Perhaps mixed windows would help in our
> case.
> I've got another question about window sizes: are the short ones really
> essential in VBR? Would it be possible to only use long ones, and then
> allocating a lot more bits in the case of transcients?
>
A long window have a duration of up to 36 ms (32 kHz).
So the worst case pre-echo's are:
* - 5 dB for dt = 12 ms (32 kHz) or dt = 9 ms (44.1 kHz)
* -12 dB for dt = 18 ms (32 kHz) or dt = 13 ms (44.1 kHz)
* -24 dB for dt = 24 ms (32 kHz) or dt = 17 ms (44.1 kHz)
Because this is much more flat than the human pre-masking, you really need a
huge amount of more bits. Often 320 kbps are sounding worse.
For post-masking I found values around 1 dB/6 ms for 1...5 kHz.
What's the value for pre-masking?
> > > 5.
> > > Spectral prefiltering to get nearly constant ATH in every CB.
>
> Why can we read in the literature that humans got 25 CB but mp3 uses only
> 22?
>
I think it is because the low frequency CBs are larger than the in the
literature. You have two problems:
* MP3 uses only CB width which are a multiple of 4, perhaps to make
use of the Intel SIMD instructions ;-)
* So all CBs have sizes of multiples of 111 Hz/153 Hz/167 Hz, which can't
be mapped to the CBs often found in literature.
* The exact width of an CB is a little bit arbitrary, you can found
values from 40 Hz...120 Hz for low frequencies. It depends on
the exact definition of the item "CB". A lot of people say that
100 Hz for low frequencies is much too large.
* Note, that Zwicker also splits the 3 low frequency CBs into several
subbands to compensate the ATH frequency dependencies (see Table 1 in
DIN 45631).
See also ISO 532: "Methode de calcul du niveau d'isononie"
Another question: I have some C++ programs generating test signals.
The programs are around 1...2 KByte large and generating WAV-Files in the
range from 1...10 MByte.
Some of them are really nasty for MP3. Should we collect such programs?
--
Frank Klemm
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