ref: d365f98fa5311c8f55b939d36a3d6b6d26e87657
dir: /libst.txt/
ST(3) ST(3)
NAME
libst - Sound Tools : sound sample file and effects libraries.
SYNOPSIS
cc file.c -o file libst.a
DESCRIPTION
Sound Tools is a library of sound sample file format readers/writers
and sound effects processors.
Sound Tools includes skeleton C files to assist you in writing new for-
mats and effects. The full skeleton driver, skel.c, helps you write
drivers for a new format which has data structures. The simple skele-
ton drivers help you write a new driver for raw (headerless) formats,
or for formats which just have a simple header followed by raw data.
Most sound sample formats are fairly simple: they are just a string of
bytes or words and are presumed to be sampled at a known data rate.
Most of them have a short data structure at the beginning of the file.
INTERNALS
The Sound Tools formats and effects operate on an internal buffer for-
mat of signed 32-bit longs. The data processing routines are called
with buffers of these samples, and buffer sizes which refer to the num-
ber of samples processed, not the number of bytes. File readers trans-
late the input samples to signed longs and return the number of longs
read. For example, data in linear signed byte format is left-shifted
24 bits.
This does cause problems in processing the data. For example:
*obuf++ = (*ibuf++ + *ibuf++)/2;
would not mix down left and right channels into one monophonic channel,
because the resulting samples would overflow 32 bits. Instead, the
‘‘avg’’ effects must use:
*obuf++ = *ibuf++/2 + *ibuf++/2;
Stereo data is stored with the left and right speaker data in succes-
sive samples. Quadraphonic data is stored in this order: left front,
right front, left rear, right rear.
FORMATS
A format is responsible for translating between sound sample files and
an internal buffer. The internal buffer is store in signed longs with
a fixed sampling rate. The format operates from two data structures: a
format structure, and a private structure.
The format structure contains a list of control parameters for the sam-
ple: sampling rate, data size (bytes, words, floats, etc.), encoding
(unsigned, signed, logarithmic), number of sound channels. It also
contains other state information: whether the sample file needs to be
byte-swapped, whether fseek() will work, its suffix, its file stream
pointer, its format pointer, and the private structure for the format .
The private area is just a preallocated data array for the format to
use however it wishes. It should have a defined data structure and
cast the array to that structure. See voc.c for the use of a private
data area. Voc.c has to track the number of samples it writes and when
finishing, seek back to the beginning of the file and write it out.
The private area is not very large. The ‘‘echo’’ effect has to mal-
loc() a much larger area for its delay line buffers.
A format has 6 routines:
startread Set up the format parameters, or read in a data
header, or do what needs to be done.
read Given a buffer and a length: read up to that many
samples, transform them into signed long integers,
and copy them into the buffer. Return the number
of samples actually read.
stopread Do what needs to be done.
startwrite Set up the format parameters, or write out a data
header, or do what needs to be done.
write Given a buffer and a length: copy that many samples
out of the buffer, convert them from signed longs
to the appropriate data, and write them to the
file. If it can’t write out all the samples, fail.
stopwrite Fix up any file header, or do what needs to be
done.
EFFECTS
An effects loop has one input and one output stream. It has 5 rou-
tines.
getopts is called with a character string argument list for
the effect.
start is called with the signal parameters for the input
and output streams.
flow is called with input and output data buffers, and
(by reference) the input and output data buffer
sizes. It processes the input buffer into the out-
put buffer, and sets the size variables to the num-
bers of samples actually processed. It is under no
obligation to read from the input buffer or write
to the output buffer during the same call. If the
call returns ST_EOF then this should be used as an
indication that this effect will no longer read any
data and can be used to switch to drain mode
sooner.
drain is called after there are no more input data sam-
ples. If the effect wishes to generate more data
samples it copies the generated data into a given
buffer and returns the number of samples generated.
If it fills the buffer, it will be called again,
etc. The echo effect uses this to fade away.
stop is called when there are no more input samples to
process. stop may generate output samples on its
own. See echo.c for how to do this, and see that
what it does is absolutely bogus.
COMMENTS
Theoretically, formats can be used to manipulate several files inside
one program. Multi-sample files, for example the download for a sam-
pling keyboard, can be handled cleanly with this feature.
PORTABILITY PROBLEMS
Many computers don’t supply arithmetic shifting, so do multiplies and
divides instead of << and >>. The compiler will do the right thing if
the CPU supplies arithmetic shifting.
Do all arithmetic conversions one stage at a time. I’ve had too many
problems with "obviously clean" combinations.
In general, don’t worry about "efficiency". The sox.c base translator
is disk-bound on any machine (other than a 8088 PC with an SMD disk
controller). Just comment your code and make sure it’s clean and sim-
ple. You’ll find that DSP code is extremely painful to write as it is.
BUGS
The HCOM format is not re-entrant; it can only be used once in a pro-
gram.
The program/library interface is pretty weak. There’s too much ad-hoc
information which a program is supposed to gather up. Sound Tools
wants to be an object-oriented dataflow architecture.
October 15 1996 ST(3)