Signal Generator


Signal Generator

The Signal Generator can produce signals of the following types:

RMS Signal Level

The RMS signal level can be set for any of the signal types with 0.1 dB 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.
RMS Level control

Saving signals to WAV files

The Save button generates a stereo wave file containing the selected signal in both channels. The file duration is approximately 1 minute for sine, square, dual tone, CEA-2010 burst, pink noise and Pink/White PN signals. The sweep signal durations are per the settings for the signal. Signal level is per the RMS Level setting of the signal generator. The signal sample rate is the same as the current soundcard sample rate. To save a measurement sweep it is best to go through the process of making a measurement with the desired sweep settings first, that ensures the generator is configured correctly for REW's subsequent analysis. Note that the measurement sweep may change between REW versions so always use a sweep generated with the REW version being used for analysis of the captured response.
RMS Level control

Output

Setting the audio output device is described in Getting Started. The output on the chosen device can be selected in the signal generator. When using Java drivers the signal can also be generated on either or both channels of the output if it is stereo.

Sine Wave


Sine Wave Generator

Sine waves can be generated with frequencies between 1.0 Hz and half the soundcard sample rate, e.g. 24 kHz for a soundcard operating at 48 kHz. 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 0.5 Hz for frequencies below 200 Hz and steps of 1 Hz thereafter. If the option to lock the frequency to FFT bins has been selected the actual frequency that has been generated is shown at the bottom right corner of the frequency display when the generator is running as it will typically differ slightly from the entered frequency.

The RMS level can range from -90 to -3.0 dBFS (-3.0 dBFS is the maximum RMS level for a sine wave before clipping, unless the View preference Full scale sine rms is 0 dBFS has been selected, in which case the maximum is 0 dBFS).

Lock frequency to FFT

When the "Lock frequency to FFT" box is checked the generator output frequency is locked to the nearest FFT bin centre for the current RTA FFT length (meaning the signal is periodic within the FFT length). This allows a rectangular FFT window to be used for maximum spectral resolution of the RTA plot. The exact frequency that has been generated is shown at the bottom right corner of the frequency display when the generator is running.
Actual frequency shown below requested frequency

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 harmonic distortion

When the "add harmonic distortion" box is checked a window is displayed allowing control over the levels and phases of harmonics from the 2nd to the 9th. Each harmonic can be enabled or disabled using the check box. The levels are adjusted in dB relative to the level of the fundamental, with the equivalent percentage value shown in a label alongside. Above the controls is a display of the maximum signal level that can be used with the current distortion settings without clipping the output. Note that only harmonics that fall within the range supported by the current sample rate are generated.

Harmonic distortion controls

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. The level at which the dither is added is controlled by the sample width selector to the right of the check box. N.B. When using the Java drivers audio data is usually limited to 16 bit precision. Dither is beneficial if making very precise distortion measurements of an electronic device such as a receiver, processor or equaliser. It 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 1 kHz tone at -6 dBFS. 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 -120 dBFS, especially at high frequencies, making the true harmonic distortion levels more visible.

1kHz -6 dBFS sine without dither

1kHz -6 dBFS sine with dither

Square Wave


Square Wave Generator

The square wave generator allows duty cycles between 1% and 99% in 1% steps. The frequency that is generated is adjusted to ensure there is an even number of samples in the period, so that the spectrum of a 50% duty cycle square wave will only have odd harmonics. The actual frequency that has been generated is shown at the bottom right corner of the frequency display when the generator is running, at higher frequencies this can be significantly different to the frequency that was entered.

Dual Tone


Dual tone Generator

The dual tone generator is to facilitate intermodulation distortion measurements. It has presets for SMPTE, DIN and CCIF signals and allows custom signals to be generated with ratios of 1:1 or 4:1. Note that for valid IMD results with custom signals f2 must be higher than f1. The level of f1 is the signal generator rms level, the level of f2 is according to the signal ratio. Note that this means clipping occurs at levels above -9 dBFS for 1:1 signals or -6 dBFS for 4:1 signals.

CEA-2010 Tone Burst


CEA-2010 Burst Generator

The CEA-2010 Burst generator produces a 6.5 cycle, Hann-windowed tone burst at the selected frequency. This signal is used for testing the maximum power output of subwoofers by using an RTA in Spectrum mode to observe the levels of the distortion components produced when the signal is playing, usually testing at 63, 50, 40, 31.5, 25 and 20 Hz. The limits for the distortion components are shown in the table below, where f0 is the test signal frequency.

Start Freq (Hz)End Freq (Hz)Limit (dB)Comment
161.59*f00Fundamental
1.59*f02.52*f0-10 (32%)2nd harmonic
2.52*f03.78*f5-15 (18%)3rd harmonic
3.78*f05.61*f0-20 (10%)4th and 5th harmonic
5.61*f08.50*f0-30 (3.2%)6th - 8th harmonic
8.50*f010 k-40 (1%)Higher order harmonics

If the Repeat the burst checkbox is selected the burst will be repeated at intervals of not less than 1 second (the actual interval is chosen to align with the RTA block length). The Previous 1/3rd octave frequency and Next 1/3rd octave frequency move the generator to a 1/3rd octave centre frequency.

The highest level of the fundamental for which none of the harmonic limits are exceeded is the maximum output level at that test frequency. The reference level for the limits is the maximum level within 3 Hz of the test frequency. When the CEA burst signal is playing the RTA shows the limits and the peak level at the test frequency as an overlay, provided the RTA is in Spectrum mode and the frequency of the burst is not more than 1,176 Hz. The peak level is shown in red if the limits are exceeded.

RTA overlay for CEA burst

If the RTA data from a CEA-2010 signal is saved as a measurement the peak level and limits overlay will be shown on the SPL & Phase graph and the CEA-2010 test frequency and peak level figure will be recorded in the measurement notes.

The recommended RTA settings for 44.1 kHz or 48 kHz sample rate are:

RTA settings for CEA burst

For 88.2 kHz or 96 kHz use FFT length of 131,072. Refer to the CEA-2010 standard for details of the measurement procedure, or search for guides on the Internet.

Pink Noise


Pink Noise Generator

The Pink Noise generator uses white noise filtered through a -10 dB/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 0.05 dB above 9.2 Hz at 44.1 kHz 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 (40 dB/decade, 12 dB/octave) filters at 500 Hz and 2 kHz, producing a signal with its energy centred on 1 kHz. Subwoofer Calibration applies filters at 30 Hz and 80 Hz. 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 -10 dBFS.

Pink and White Periodic Noise


Pink Periodic Noise Generator

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. The PN sequences REW generates are optimised to have a crest factor (ratio of peak level to rms level) that does not exceed 6 dB for full range sequences, narrower sequences should have crest factors less than 6.5 dB. Use Pink PN when measuring with an RTA or White PN with a Spectrum analyser.

The Sequence Length control must be set the same as the length of the FFT used by the analyser. If it is 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. When using the REW RTA the sequence length is automatically set the same as the FFT length.

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
Periodic Noise controls

Length 131072, longer than FFT (no averaging)
Periodic Noise controls

Length 65536, matching FFT
Periodic Noise controls

The Full Range option generates noise over a span from 10 Hz (Pink) or the lowest frequency for the selected FFT length (White) to half the sample rate, giving the widest bandwidth and the greatest low frequency content. The Speaker Calibration option generates noise from 500 Hz to 2 kHz, producing a signal with its energy centred on 1 kHz. Subwoofer Calibration generates noise from 30 Hz to 80 Hz. Custom Filtered allows low and/or high cut frequencies to be set arbitrarily, subject to a minimum bandwidth of 1 octave. In all cases frequencies outside the selected range are suppressed to the extent the soundcard permits (very sharp roll-offs).

REW automatically adjusts the signal levels for the various options and filter settings so that the RMS values reflect the setting in RMS Level. Clipping of peaks will occur at RMS levels of -6 dBFS or above.

The Save 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.
RMS Level control

Linear Sweep, Log Sweep


Sine Sweep Generator

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 sweeps have a 10 ms raised cosine fade in and fade out.

Measurement Sweep


Measurement Sweep Generator

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. To save a measurement sweep it is best to go through the process of making a measurement with the desired sweep settings first, that ensures the generator is configured correctly for REW's subsequent analysis. Note that the measurement sweep may change between REW versions so always use a sweep generated with the REW version being used for analysis of the captured response.

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