man signalgen (1): a simple signal generator program

SYNOPSIS

signalgen [flags] waveform freq: waveform is sine, cosine, square, triangle, sawtooth, noise
signalgen [flags] pulse freq [Mark/Space]: pulse has extra param Mark/Space % - def. is 10 (%)

DESCRIPTION

signalgen is a simple signal generator program, that can digitally generate standard waveforms on the LINUX /dev/dsp device. 8 or 16 bit samples can be generated, in mono or stereo. In stereo the two signals can be in phase or in anti-phase (180 degrees). 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 'infinately' 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.
pulse: A square waveform where the mark/space ratio (as a percentage) can be specified. The default value is 10% (mark/space ratio of 1:9).

signalgen 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
-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.
-f,-a: force overwrite/append of/to file.
-C file: use "file" as the local configuration file (see below).
-s samples: generate with samplerate of samples/sec
-8/-16 or -b 8|16: force 8 bit or 16 bit mode.
-1,-2,-2a: mono (def), stereo or stereo in antiphase
-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

signalgen -v sin 440: generate a sin wave of 440Hz at 22050 samples/sec, 16bit samples on 16 bit card, 8 bit samples on an 8 bit card.
signalgen -v -s 44100 -w sin444.wav sin 440: as above but at a samplerate or 44100/sec and save a one second of samples as a WAVE file in sin440.wav
signalgen -v -A 500 saw 1000: generate a 1000Hz sawtooth wave severely clipped. The waveform will look like a square wave with a rise and fall time of one tenth of the wave period. (I think that's what the scribbled calculation on the back of this envelope gives :-)
signalgen -v -2a sin 1000: generate 2 1000Hz sine waves out of phase by 180 degrees, one on each stereo channel.
signalgen -v pulse 1000 95: generate a 1000Hz pulse wave with the 'on' period being 95% of the waveform period, i.e. a mark/space ratio of 19:1.
signalgen -v -x10 sin 2616: generate middle C 261.6Hz (2616/10) from the equally tempered scale. Yes it's a royal pain remembering to scale all freqs. 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.
signalgen -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.

signalgen 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

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]