The Signal Generator can produce signals of the following types, output simultaneously on both the left and right channels of the selected soundcard output:
- Sine Waves
- Pink Noise
- Full range (spectrum to below 10Hz)
- Speaker Calibration
- Subwoofer Calibration
- Custom filtered
- Pink & White Periodic Noise
- Full range (spectrum to below 10Hz)
- Length to suit RTA FFT length
- Adjustable crest factor
- Sine Sweeps
- Changing frequency and/or level
- Looping to repeat the sweep
- Square Waves
- Variable duty cycle
RMS Signal Level
The RMS signal level can be set for any of the signal types with 0.1dB resolution relative to digital full scale. The arrow buttons on the RMS Level spinner change the value in steps of 1dB, or any required value can be typed directly into the level box.
Sine waves can be generated with frequencies between 10.0 and half the soundcard sample rate, e.g. 24kHz for a soundcard operating at 48kHz. The frequency is controlled by entering a value in the Frequency box, or using the arrow buttons to increment or decrement the value in steps of 0.5Hz for frequencies below 200Hz and steps of 1Hz thereafter. The generated sine wave sequences have an integer number of samples at the selected sample rate, the sequences are made long enough to ensure the frequency is accurate to better than 0.1Hz below 2kHz and better than 1Hz thereafter. The exact frequency that has been generated is shown at the bottom right corner of the frequency display when the generator is running.
Frequency Tracks Cursor
The frequency can also be controlled via the graph cursor by checking the "Frequency tracks cursor" box. When this box is checked the signal generator frequency is linked to the position of the graph cursor and will change to follow the cursor frequency as it is moved - the changes are smooth with no phase discontinuities.
Add dither to output
When the "Add dither to output" box is selected the generator adds 2 lsb pk-pk triangular dither to the output to remove quantisation noise spikes. This can be beneficial if making very precise distortion measurements of an electronic device such as a receiver, processor or equaliser. There is an additional processing load when dither is being applied, so you may wish to turn this option off if the computer's processor loading is high when running REW. Dither is usually not required when making acoustic measurements as the quantisation artefacts it removes are far below the acoustic noise floor. The Graphs below show the effect of the dither option during a loopback test of a soundcard playing a 1kHz tone at -6dB FS. The first plot is without dither, the second plot is with dither. Addition of dither cleans up much of the noise that was apparent below -120dB FS, especially at high frequencies, making the true harmonic distortion levels more visible.
The RMS level can range from -90 to -3.0dBFS (-3.0dBFS is the maximum RMS level for a sine wave before clipping, at this level the peaks are at 0dBFS).
The Pink Noise generator uses white noise filtered through a -10dB/decade filter generated from a weighted sum of a series of first order filters, as devised by Paul Kellet circa 1999. Stated accuracy is within 0.05dB above 9.2Hz at 44.1kHz sample rate.
The Full Range option outputs the filtered noise directly, giving the widest bandwidth and the greatest low frequency content. The Speaker Calibration option applies 2nd order (40dB/decade) filters at 500Hz and 2kHz, producing a signal with its energy centred on 1kHz. Subwoofer Calibration applies filters at 30Hz and 80Hz. Both are broadly in line with the THX test signal recommendations. Custom Filtered allows low and/or high cut filter frequencies to be set arbitrarily, subject to a minimum bandwidth of 1 octave.
REW automatically adjusts the signal levels for the various options and filter settings so that the RMS values reflect the setting in RMS Level. Note that as Pink Noise has random variations some clipping of peaks will occur at RMS levels above approximately -10dB.
Pink and White Periodic Noise
Periodic Noise (PN) sequences are ideally suited for use with spectrum and real time analysers (RTA's). They contain every frequency the analyser can resolve in a sequence length that matches the length of the analyser's FFT. Their great benefit is that they produce the desired spectrum shape without requiring any averaging or windowing, so the analyser display reacts much more rapidly to changes in the system than it would if testing with Pink or White random noise, making them ideal for live adjustment of EQ filters. Use Pink PN when measuring with an RTA or White PN with a Spectrum analyser.
The Length control must be set the same as the length of the FFT used by the analyser. If it is set shorter than the analyser FFT there will be notches in the analyser display, as the periodic noise will not contain some of the frequencies the analyser is looking for. If it is set longer the extra frequencies will give a noisy display requiring more averaging. The images below show the effect of correct and incorrect settings of the PN length for a loopback measurement with 1/48 octave RTA that is using an FFT length of 65536 (64k).
Length 32768, shorter than FFT
Length 131072, longer than FFT (no averaging)
Length 65536, matching FFT
When using the REW Spectrum/RTA the PN length is automatically set the same as the FFT length.
The Save PN to WAV file button generates a 16-bit stereo wave file containing the PN sequence in both channels. The file duration is approximately 1 minute, the level is per the RMS Level setting of the signal generator. This file can be used to generate a test disc to be played on a system whose response is to be measured. Make sure that the current soundcard sample rate corresponds with the format of the disc to be made - for example, 44.1kHz should be used if generating a CD, or 48kHz for a DVD. When measuring the system the sample rate and FFT length must be the same as used for the test disc.
Linear Sweep, Log Sweep
The Signal Generator can produce sweeps with configurable start frequency/ level, end frequency/level, duration and linear or logarithmic progression. Sweep duration can be up to 60 seconds. If the "Loop" box is checked the sweep will repeat continuously.
The Measurement Sweep signal is used by REW when measuring system response. It consists of a logarithmic sweep from the start frequency to the end frequency. The sweep duration is set using the Length control. If the start frequency is below 20Hz the signal begins with a linear sweep from DC to 10Hz, followed by a logarithmic sweep from there to the end frequency. This signal is selected automatically to make sweep measurements.
The square wave generator allows duty cycles between 1% and 99% in 1% steps.
The generator places two constraints on the generated signal: the frequency must have a period that is an even number of samples at the soundcard sample rate to ensure that exactly 50% duty cycle square waves can be produced (the spectrum of the signal will then contain only odd harmonics of the frequency) and the duty cycle is adjusted to ensure there are an integer number of samples in both the high and low periods to avoid jitter in the edges of the signal. These constraints mean the generated frequency will usually not exactly match the requested frequency. The exact frequency that has been generated is shown at the bottom right corner of the frequency display when the generator is running.
Setting the audio output is described in Getting Started.