ref: 96538801bd79896dacfe05dbbfe5e858a8e54755
dir: /libst.txt/
ST(3) ST(3)
NAME
libst - Sound Tools : sound sample file and effects libraries.
SYNOPSIS
#include <st.h>
ft_t st_open_input(const char *path, const st_signalinfo_t *info, const
char *filetype);
ft_t st_open_output(const char *path, const st_signalinfo_t *info,
const char *filetype, const char *comment);
st_ssize_t st_read(ft_t ft, st_sample_t *buf, st_ssize_t len);
st_ssize_t st_write(ft_t ft, st_sample_t *buf, st_ssize_t len);
int st_close(ft_t ft);
int st_seek(ft_t ft, st_size_t offset, int whence);
cc file.c -o file libst.a
DESCRIPTION
Sound Tools is a library of sound sample file format readers/writers
and sound effects processors. It is mainly developed for use by SoX
but is useful for any sound application.
st_open_input function opens the file for reading whose name is the
string pointed to by path and associates an ft_t with it. If info is
non-NULL then it will be used to specify the data format of the input
file. This is normally only needed for headerless audio files since
the information is not stored in the file. If filetype is non-NULL
then it will be used to specify the file type. If this is not speci-
fied then the file type is attempted to be derived by looking at the
file header and/or the filename extension. A special name of "-" can
be used to read data from stdin.
st_open_output function opens the file for writing whose name is the
string pointed to by path and associates an ft_t with it. If info is
non-NULL then it will be used to specify the data format of the output
file. Since most file formats can write data in different data for-
mats, this generally has to be specified. The info structure from the
input format handler can be specified to copy data over in the same
format. If comment is non-NULL, it will be written in the file header
for formats that support comments. If filetype is non-NULL then it will
be used to specify the file type. If this is not specified then the
file type is attempted to be derived by looking at the filename exten-
sion. A special name of "-" can be used to write data to stdout.
The function st_read reads len samples in to buf using the format han-
dler specified by ft. All data read is converted to 32-bit signed sam-
ples before being placed in to buf. The value of len is specified in
total samples. If its value is not evenly divisable by the number of
channels, undefined behavior will occur.
The function st_write writes len samples from buf using the format han-
dler specified by ft. Data in buf must be 32-bit signed samples and
will be converted during the write process. The value of len is speci-
fied in total samples. If its value is not evenly divisable by the
number of channels, undefined behavior will occur.
The st_close function dissociates the named ft_t from its underlying
file or set of functions. If the format handler was being used for
output, any buffered data is written first.
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.
RETURN VALUE
Upon successful completion st_open_input and st_open_output return a
ft_t (which is a pointer). Otherwise, NULL is returned. TODO: Need a
what to return reason for failures. Currently, relies on st_warn to
print information.
st_read and st_write return the number of samples successfully read or
written. If an error occurs, or the end-of-file is reached, the return
value is a short item count or ST_EOF. TODO: st_read does not distigu-
ish between end-of-ifle and error. Need an feof() and ferror() concept
to determine which occured.
Upon successful completion st_close returns 0. Otherwise, ST_EOF is
returned. In either case, any further access (including another call
to st_close()) to the handler results in undefined behavior. TODO: Need
a way to return reason for failures. Currently, relies on st_warn to
print information.
Upon successful completion st_seek returns 0. Otherwise, ST_EOF is
returned. TODO Need to set a global error and implement st_tell.
ERRORS
TODO
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 32-bit integers and return the number
of samples 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 (8, 16, or 32 bits), encoding (unsigned,
signed, floating point, etc.), number of sound channels. It also con-
tains other state information: whether the sample file needs to be
byte-swapped, whether st_seek() 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.
BUGS
The HCOM format is not re-entrant; it can only be used once in a pro-
gram.
On errors, the effects currently invoke st_fail and rely on that call-
ing exit(). They do not currently gracefully fail.
The program/library interface is pretty weak.
September 26 2005 ST(3)