swgen(1) 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.

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: [email protected]am.org.uk