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!AudioMPEG
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1995-05-23
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MPEG Audio encoder and decoder
==============================
Version 0.02 by Henrik Bjerregaard Pedersen.
This program is Public Domain, and is provided 'as is'. The author cannot
accept any liability for any damage or loss of any kind as a result of the
use of this program.
You are allowed to copy, distribute and use this program for any
non-commercial purposes, as long as this file is included unaltered.
Bug report, suggestions (and donations!!!) to
Henrik Bjerregaard Pedersen
Stengaaards Alle 13B
DK-2800 Lyngby
Denmark
email: henrik@login.dknet.dk
Encoder
-------
The encoder compresses a raw sample file (16 bit linear signed) in either
mono or stereo. The samples must be stored with the low byte first.
Usage: Encode [switches] <input> <output>
Switches: -m <mode> m (mono) or s (stereo)
-s <freq> source sample rate in kHz.
-b <kbps> bitrate in kbits/second
Default is -m s -s 44.1 -b 384
If the sample file is a stereo sample, the channels should be interleaved
LRLRLRLR...
Legal sample rates are 32kHz, 44.1kHz and 48kHz. However, this doesn't mean
that you cannot compress samples recorded at other sample rate; only that the
encoder assumes that the frequency is either 32, 44.1 or 48 kHz.
You should select the sample rate closest to the actual samplerate and you
should be careful when encoding samples recorded at frequencies very much
lower than 32kHz. The encoder uses various psychoacoustic tricks to compress
the sample; eg. it assumes that the ear is less sensitive to variations in
volume at some frequencies than at others. If you encode a 10kHz sample by
specifying a sample rate of 32kHz, the algorithmes will actually be applied
to frequencies only 1/3 of what they're supposed to be, and so will have a
different effects than what was intended.
The possible bitrates are:
0,32,48,56,64,80,96,112,128,160,192,224,256,320,384.
If you want to compress a sample to 15% of the original, you should select a
bitrate which is 15% of the sample rate you specify. So if your sample was
recorded at 28kHz, you specify 32kHz as the sample rate and a bitrate of
80 kbps (0.15 * 32000 samples/second * 16 bits/sample = 80000 bits/second).
Example: Encode -m m -s 32 -b 80 samplefile ampegfile
Decoder
-------
The decoder outputs a file holding 16 bit linear signed samples. If the
sample is in stereo, the channels will be interleave (LRLRLRLR...).
Usage: Decode [switches] <input> <output>
Switches: -h output high byte first
Example: Decode -h ampegfile samplefile
The frontend
------------
When you run !AudioMPEG, it installs an icon on the icon bar. Click
SELECT on this to open the encoder window. To encode a sample file, simply
drag the file to this window or to the AudioMPEG icon on the iconbar.
You may then alter the parameters (bits-per-second, sample-rate, number
of channels) and when this is OK, clicking on the 'OK' icon will open a
standard save-box which lets you specify where the encoded file should be
saved.
The encoding may optionally be run multitasking by selecting the 'Multitask'
icon in the encoder window.
To decode a file, simply drag it to the AudioMPEG icon on the iconbar which
will open the save window.
The file MUST have filetype &1AD (AMPEG). Decoding is done multitasking (in
a taskwindow) if the 'Multitask' icon in the encoder window is set.
Speed
-----
The current decoder runs at about 2-4% of real-time, which is actually
quite fast compared to the encoder. So the encoder and decoder should only
really be used when you need to store/transmit large samples.
ftp-sites
---------
For examples of MPEG audio files, try
sunsite.unc.edu
in the directory
pub/electronic-publications/IUMA/music/Clapton,_Eric
pub/electronic-publications/IUMA/music/Madonna
pub/electronic-publications/IUMA/music/Prince
pub/electronic-publications/IUMA/music/Buffalo,_Grant_Lee