NAME¶
swgen - a simple swept frequency signal generator
SYNOPSIS¶
- swgen [-2] [-s samples] [-8/-16|-b 8/16] [sweepwaveform]
- sweepfreq [sweptwaveform] minfreq maxfreq
- swgen [-2] [-s samples] [-8/-16|-b 8/16] [sweepwaveform]
- sweepfreq [sweptwaveform] centrefreq percent%
- waveform, either sweep or swept,
- is sine, cosine, square, triangle, sawtooth, noise
for full list of options see below.
DESCRIPTION¶
swgen generates a swept frequency waveform on the LINUX /dev/dsp device.
The swept and sweep waveform can be separately specified, as can the sweep
frequency range and the sweeping frequency. Sweep frequency range can be
specified either by giving the minimum (start) and maximum (end) frequency in
Hertz; or by giving the centre frequency and the percentage frequency
variation below and above. The percentage is given as an integer value from 0
to 100.
The default sweep waveform is a sawtooth (ramp), and the default swept waveform
is a sine. If the soundcard can do 16 bit samples, swgen will do 16 bit by
default.
8 or 16 bit samples can be generated, in mono or stereo. In stereo, one channel
carries the swept frequency signal, while the second channel carries the
sweeping signal. This can be useful fed to the X input of an oscilloscope when
displaying frequency response curves etc.
The samples can be written in raw or WAV format to files instead of to the sound
device.
The frequency is specified as an integer number of Hertz. Fractional Hertz
frequencies are not supported. Of course, only frequencies less than half the
samplerate (number of samples/sec) can be generated.
The waveforms that can be generated are:
- sine
- A standard sine wave
- cosine
- a sine wave with a 90 degree phase shift
- square
- a standard square wave with a 50% mark space ratio
- sawtooth
- a ramp waveform with 'infinitely' fast flyback (:-) An ideal oscilloscope
timebase signal.
- triangle
- shaped like equally spaced teeth on a saw (:-)
- noise
- This is weak. All it consists of is one second of pseudo-randomly
generated samples, played repeatedly. I'd love to do proper white/pink
noise, but I don't know enough, and I don't think the structure of the
program is conducive to accurate noise generation.
swgen creates one second's worth of generated output in a buffer and
plays the buffer repeatedly, until it is terminated.
A lot of thought has gone into the algorithms for generating the waveforms. I
believe the sin/cos wave to be very pure (modulo your sound card :-), but I
don't have access to a THD meter to measure it. For best signal accuracy
NEVER use the gain factor option (-A). The generator will then make the
wave's peak value fit the maximum digital values allowed. Use a mixer program
to control the output volume, or an external attenuator.
The gain factor option can be useful for simulating a signal that has been
subject to clipping. Specify a gain of > 100%. In fact a trapezoid signal
can be made by generating a clipped triangular wave. The greater the gain, the
closer the signal approaches a square wave (the rise and fall times decrease).
- Defaults
- output to /dev/dsp, 22050 samples/sec, mono, 16 bit samples if possible,
else 8 bit.
OPTIONS¶
- -h
- display usage and help info
- -v
- be verbose
- -f,-a
- force overwrite/append of/to file.
- -C file
- use "file" as the local configuration file (see below).
- -o file
- write digital sample to file ('-' is stdout)
- -w file
- as '-o' but written as a WAVE format file. -a (append) is not valid with
this option.
- -s samples
- generate with samplerate of samples/sec
- -8/-16 or -b 8|16
- force 8 bit or 16 bit mode.
- -1,-2
- mono (def), or special stereo mode (see above).
- -A n
- scale samples by n/100, def. n is 100 (i.e. percentage of full scale
output)
- -t N|Nm
- generate output for either N secs or Nm millisecs only.
- -x10 or -x100
- Scale frequencies down by a factor of 10 or 100. This allows fractional Hz
values to be generated. See EXAMPLES below for its use. It is a
Kludge.
EXAMPLES¶
- swgen -v 2 100 1000
- sweep a sin wave from 100Hz to 1000Hz using a sawtooth wave twice a
second, at 22050 samples/sec, 16bit samples on 16 bit card, 8 bit samples
on an 8 bit card.
- swgen -v -s 44100 -w sweep.wav 2 100 1000
- as above but at a samplerate or 44100/sec and save one second of samples
as a WAVE file in sweep.wav
- swgen -v -2 squ 10 1000 20%
- generate a sine wave switched by a 10Hz squarewave between 800Hz and
1200Hz. The swept signal is on one channel and the 10Hz square wave is on
the second channel.
- swgen -v -x10 5 4400 4500
- generate a swept sine wave from 440Hz (4400/10) to 450Hz (4500/10), being
swept at a frequency of 0.5Hz (5/10). Yes it's a royal pain remembering to
scale all freqs. up by a factor of 10, but I needed it in a hurry and
didn't have time to do it better.
CONFIGURATION FILES¶
Three possible configuration files can be used: a LOCAL config file (usually in
current directory), a HOME config file in user's $HOME directory and a GLOBAL
config file.
All the siggen suite of programs are compiled with the names of the config files
built in. By default the configuration files are:
- ./.siggen.conf
- is the LOCAL config file.
- $HOME/.siggen.conf
- is the HOME config file.
- /etc/siggen.conf
- is the GLOBAL config file.
- swgen -h
- will indicate which config files will be searched for.
The config files do not have to exist. If they exist and are readable by the
program they are used, otherwise they are simply ignored.
The config files are always searched for configuration values in the order
LOCAL, HOME, GLOBAL. This allows a scheme where the sysadmin sets up default
config values in the GLOBAL config file, but allows a user to set some or all
different values in their own HOME config file, and to set yet more specific
values when run from a particular directory.
If no configuration files exist, the program provides builtin default values,
and most of these values can be set by appropriate command line switches and
flags.
See
siggen.conf(5) for details of the configuration files.
swgen looks for configuration values CHANNELS, DACFILE, SAMPLERATE,
SAMPLESIZE, VERBOSE.
- CHANNELS
- sets either mono or stereo mode like the '-1|-2' options.
- DACFILE
- allows the name of the DAC/DSP/PCM device to be changed from /dev/dsp
- SAMPLERATE
- sets the number of samples/sec for the DAC device
- SAMPLESIZE
- sets whether 8 or 16 bit samples to be generated
- VERBOSE
- sets whether or not to run in verbose mode.
SEE ALSO¶
- siggen.conf(5)
-
BUGS¶
COPYING¶
Copyright 1995-2008 Jim Jackson
The software described by this manual is covered by the GNU General Public
License, Version 2, June 1991, issued by :
- Free Software Foundation, Inc.,
675 Mass Ave,
Cambridge, MA 02139, USA
Permission is granted to make and distribute verbatim copies of this manual
provided the copyright notice and this permission notice are preserved on all
copies.
Permission is granted to copy and distribute modified versions of this manual
under the conditions for verbatim copying, provided that the entire resulting
derived work is distributed under the terms of a permission notice identical
to this one.
Permission is granted to copy and distribute translations of this manual into
another language, under the above conditions for modified versions, except
that this permission notice may be included in translation instead of in the
original English.
AUTHOR¶
Jim Jackson
Email: jj@franjam.org.uk