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lpc10-15为美军2400bps语音压缩标准的C语音源代码。

Tue Feb 13 10:47:25 CST 1996
Andy Fingerhut ([e-mail])


File formats:

.au

Sun/NeXT audio file format.  These have headers specifying the
sampling rate, number of channels, sample format, and perhaps a few
other things.

.spd

Extension of the sample audio files that came with the LPC10
distribution.  They are raw audio data (i.e., no header), sampled at
8000 samples/sec, 1 channel, where the samples are 16 bit 2's
complement signed values, with the most significant byte first.  This
appears to be the same as Sun's .au format with the same parameters,
except that it doesn't have the .au header.

.dsp

An extension that appears to be common for Linux audio files, because
the device name for recording and playing audio is /dev/dsp.  They are
also raw audio data (i.e., no header).  They might have any of several
sampling rates, numbers of channels, or sample formats.  When working
with the LPC10 coder, it is desirable to use the same raw .spd format
as the sample sound data files are.

Unfortunately, when I have compiled the Fortran version of lpcsim
using f2c and GCC on my Linux machine, it appears to expect 16 bit
samples with the least significant byte first for input files
specified with lpcsim's -is option, and to produce such files for
output files specified with lpcsim's -os option.  This might have
something to do with the "endian-ness" of the Sun machine architecture
vs. the Intel machine architecture.

Similarly, the programs srec and splay for recording and playing sound
with 16 bit samples on Linux machines (using the -b 16 option of those
commands) also use files in this byte order.  It is easy to convert
between .spd and .dsp files using the following command to swap bytes
(that it what the 'swab' abbreviation means):

dd conv=swab < dam9.spd > dam9.dsp


----------------------------------------------------------------------
spd2au  - convert LPC10 .spd files to Sun .au files
----------------------------------------------------------------------

Example of use:

spd2au < dam9.spd > dam9.au

The shell script spd2au contains a command that may be used to convert
the raw audio files used by the LPC10 coder (with extension .spd) to
Sun format audio files (with extension .au), with the same sample
format (8000 samples/sec, 1 channel, 16 bit linear samples encoded as
2's complement signed numbers).

It uses the program audioconvert, which is available on at least some
Sun machines.  I have found it on siesta.wustl.edu, a Solaris 5.3
machine, but not on leia.wustl.edu, which runs SunOS 4.1.4.  It might
be merely a difference in the optional Sun software installed, rather
than the OS version difference.

I created this command, and then when I found that audioconvert
doesn't exist on the machine on my desk, I dug a little deeper into
what audioconvert actually does, and created playspd.  See the
description for that command.  I leave this script around mostly for
documentation purposes, and for others that can use it.


----------------------------------------------------------------------
playspd  - play LPC10 .spd file(s) on a Sun workstation
----------------------------------------------------------------------

Example of use:

playspd dam9.spd dam9_lpc55.spd

While siesta.wustl.edu has the audioconvert program, its audio speaker
does not sit on my desk.  The audio speaker of leia.wustl.edu does.
From my own simple testing, it appears that all the spd2au command
above does is prepend the data with a 32 byte header, which is the
same for all audio files with the same sampling rate, channels, and
sample format.  Therefore, I put these 32 bytes into a file called:

8khz-16bit-1channel-sun-au-header

and created a small shell script called playspd that prepends this
header to the raw audio samples, and pipes the output to the play(1)
command available on Sun's.  This can be used to play the raw audio
files on leia.wustl.edu, without having to go over the network to
seista.wustl.edu, run audioconvert, copy the result back, and then
play it.  Much more convenient.

It would be even nicer to have one command that works on any of the
file formats .dsp, .spd, and .au, distinguishing between them by their
extensions (or, for .au files, by their "magic number", i.e., the
first four bytes of the file), and for Linux, Solaris, and SunOS
machines (using splay for Linux, and play for Solaris or SunOS, and
using some environment variable or command to determine what kind of
machine it is running on).  I'll probably do something like this in
the near future.  That command could be called just 'play', or maybe
some other name that isn't already taken.


----------------------------------------------------------------------
dump-spd  - show decimal values of LPC10 .spd file samples
----------------------------------------------------------------------

Example of use:

dump-spd dam9.spd

This command merely runs the GNU version of od with the appropriate
options.

Note: This command also appears to use the 'native' byte ordering of
the machine on which it is run.  Running this command on a .spd file
on a Sun machine produces the desired results, but running it on a
.spd file on a Linux/Intel machine produces bad results (i.e., the
2-byte words are interpreted in the wrong order).  It would be nice to
have this command work on all desired machines with all of the .spd,
.dsp, and .au extensions.

It is becoming clear to me that a nice solution would be to make play
and record programs for Linux that use Sun/NeXT .au files, with
headers, and store 16-bit samples with the same byte order that Sun
workstations do.  Then, I could also modify the lpcsim source code so
that it used .au files, and performed byte-swapping internally (if
necessary) on Linux/Intel machines.  This would mean that all audio
files would be in the same format on both types of machine, and could
be FTP'd between them with no harm done.  Plus, I wouldn't have to put
case-checking code in all of these short little scripts to handle the
different formats.


----------------------------------------------------------------------
s2bits - Compress a .spd or .dsp file with lpcsim to a .bits file
----------------------------------------------------------------------

Example of use:

s2bits dam9.spd

This script has a bit of case-checking code in it to determine what
type of machine it is on, and to determine the extension of the input
file.  It performs byte-swapping on the input file, if necessary, into
a temporary file, and then runs lpcsim to produce a compressed
bitstream file (using lpcsim's -ob option).

If I modified lpcsim to accept .au files, all of this case checking
would go into the LPC source code, and out of this script.  That would
be nice.