RTA Window

RTA Window

The RTA window allows Real Time Analyser (RTA) or spectrum analyser plots to be generated, updating as the input signal is analysed. It is shown by pressing the RTA button in the toolbar of the main REW window.
RTA button

The RTA trace is activated by pressing the record button record button in the top right hand corner of the graph area, after which it will continuously analyse blocks of input samples and display the frequency spectrum of each block. Sometimes the analyser would be used without a test signal, for example to look at the frequency content of background noise, but more often it would be used together with the REW generator or an external generator or signal source. If the generator is playing a pink noise signal (or even better, pink Periodic Noise) the RTA display will show the frequency response of the room, updated live so that the effects of changing EQ settings can be immediately seen.

Playing a sine wave test tone on the generator allows the levels of the tone and its harmonics to be observed on the analyser and distortion percentages to be calculated, whilst using the dual tone generator allows intermodulation distortion measurements.

The RTA plot shows the currently selected measurement as a reference and the live RTA or spectrum. A Peak trace is also available, which is reset by the Reset Averaging button. If Inverse C compensation is being applied the icon is shown after the trace value. If Mic/Meter calibration file or soundcard calibration file have been loaded they are applied to the results. The current Input RMS value is shown to the left of the record button, in dB SPL or dBFS according to the setting of the Y axis. This figure excludes any DC content in the signal. A and C weighted values are shown below the unweighted rms figure. If clipping is detected in the input the RMS value turns red.

Spectrum/RTA controls

The controls for the plot are shown below.
Spectrum/RTA Controls

The Mode can be set to Spectrum for a spectrum analyser plot or to various RTA resolutions from 1 octave to 1/48 octave. The difference between spectrum and RTA modes is how the information is presented. In spectrum mode the frequency content of the signal is split up into bins that are all the same width in Hz. For example, with a 64k FFT length and 48 kHz sample rate the bins are 0.732 Hz wide. The plot shows the energy in each of those bins. In RTA mode the bin widths are an octave fraction, so their width in Hz varies with the frequency. For example, a 1 octave RTA plot has bins that are 70.7 Hz wide at 100 Hz (from 70.7 Hz to 141.4 Hz) and 707 Hz wide at 1 kHz (from 707 Hz to 1.414 kHz). The plot shows the combined energy at each frequency within each bin. This is closer to how our ears perceive sound. The different presentations mean signals with a spread of frequency content will look different on the plot. The best known examples are white noise and pink noise. White noise has the same energy at each frequency. On a spectrum plot, which shows the energy at each frequency, the white noise plots as a horizontal line. On an RTA plot it appears as a line that rises with increasing frequency, as each RTA bin gets wider it covers more frequencies and so has more energy. The bin widths double with each doubling of frequency so the energy also doubles, which adds 3 dB on the logarithmic plots we use to show level. White noise sounds quite 'hissy', we perceive it as having more energy at higher frequencies.

Pink noise has energy that falls 3 dB with each doubling of frequency. On a spectrum plot it is a line that falls at that 3 dB per octave rate, on an RTA plot it is a horizontal line as the energy in the signal is falling at the same rate as the bins are widening. We perceive pink noise as having a uniform distribution of energy with frequency.

Single tones are a special case, they will appear at the same level on either style of plot as their energy is all at one frequency, so on a spectrum plot they show as a vertical line, on an RTA plot they show (typically) as a bar of the width of the bin at their frequency, but the height of the bar is the same as the height of the line on the spectrum as all the energy is at that one frequency.

In Spectrum or RTA modes the plot can either draw lines between the centres of the FFT bins or draw horizontal bars whose width matches the FFT bin or RTA octave fraction width, this is controlled by the Use bars on spectrum and Use bars on RTA check boxes.

In Spectrum mode smoothing can be applied to the trace according to the setting of the Smoothing box. Smoothing is not applicable for RTA modes.

FFT Length

The FFT Length determines the basic frequency resolution of the analyser, which is sample rate divided by FFT length. The shortest FFT is 8,192 (often abbreviated as 8k) which is also the length of the blocks of input data that are fed to the analyser. An 8k FFT has a frequency resolution of approximately 6Hz for data sampled at 48kHz. As the FFT length is increased the analyser starts to overlap its FFTs, calculating a new FFT for every block of input data. The degree of overlap is 50% for 16k, 75% for 32k, 87.5% for 64k and 93.75% for 128k. The overlap ensures that spectral details are not missed when a Window is applied to the data. The maximum overlap allowed can be limited using the Max Overlap control below to reduce processor loading at higher FFT lengths.


The FFT resolution is also affected by the Window setting. Rectangular windows give the best frequency resolution but are only suitable when the signal being analysed is periodic within the FFT length or if a noise signal is being measured. The Rectangular window should always be used with the REW periodic noise signals. Most other signals, e.g. sine waves from the REW generator or test tones on a CD, typically would not be periodic in the FFT length. Using a rectangular window when analysing such a tone would generate spectral leakage, making it difficult to resolve the frequency details - the plot below shows an example of a 1kHz tone from an external generator with a Rectangular window.
1kHz tone, rectangular window

Here is the same tone analysed with a Hann window.
1kHz tone, Hann window

The window allows the harmonics of the tone to be resolved. However, the tradeoff is that windows cause some spreading of the signal they are analysing, which reduces the frequency resolution. To use a rectangular window with the REW signal generator use the generator's Lock frequency to FFT option.

The Hann window is well suited to most measurements, offering a good tradeoff between resolution and shoulder height. If very high dynamic range needs to be resolved (very small signals close to very large signals) use the 4-term or 7-term Blackman-Harris windows. If the spectral peak amplitudes must be accurately measured use the Flat Top window, this will provide amplitude accuracy of 0.01 dB regardless of where the tone being measured falls relative to the bins of the FFT. The other windows only show the spectral amplitude accurately if the tone is exactly on the centre of an FFT bin, if the tone falls between two bins the amplitude is lower, with the maximum error occurring exactly between two bins. This maximum error is 3.92dB for the Rectangular window, 1.42dB for Hann, 0.83dB for the 4-term Blackman-Harris and 0.4dB for the 7-term Blackman-Harris.

Max Overlap

The spectrum/RTA plot can be updated for every block of audio data that is captured from the input, overlapping sequences of the chosen FFT length. This can present a significant processor load for large FFT lengths. The processor loading can be reduced by limiting the overlap allowed using this control.

Update Interval

The spectrum/RTA plot is updated by default for every block of audio data that is captured from the input. This can cause a significant processor load, particularly if the RTA window is very large or for large FFT lengths. The processor loading can be reduced by updating the plot less often, which is set by the Update Interval control. An update interval of 1 redraws the trace for every block, an interval of 4 (for example) only updates the trace on every 4th block.

Peak Hold and Peak Decay

The Peak Hold and Peak Decay controls set how long, in seconds, a peak value is held and how quickly, in dB per second, the peak values decay. If Peak Hold is set to 0 the peak values are not held at all. If Peak Decay is set to 0 the peak trace does not decay.

Distortion High Pass and Low Pass

The Distortion High Pass and Low Pass are used to set the lowest and highest frequencies that will contribute to the calculation of THD and THD+N. They are only applied when the Enable high pass and Enable low pass boxes are selected and the Distortion button is pressed. Either can be enabled individually. When they are active the region of the plot which is excluded from the calculations will be greyed out and the THD and THD+N figures will show the range over which they have been calculated.

Adjust RTA Levels

The RTA plot shows the energy within each octave fraction bandwidth. As the RTA resolution increases, from 1 octave through to 1/48 octave, the octave fraction bandwidths decrease and, for broadband test signals such as pink noise, the energy in each octave fraction decreases correspondingly. Whilst the RTA is correctly showing the actual level within each octave fraction, this variation of trace level with RTA resolution can be awkward when using the RTA with a pink PN noise signal to adjust speaker positions or equaliser settings. The Adjust RTA Levels option offsets the levels shown on the RTA plot to compensate for both the bandwidth variation as resolution is changed and the difference between a sweep measurement at a given sweep level and a pink PN RTA measurement at the same level, allowing direct comparison between RTA and sweep plots. Whilst the levels shown are not the true SPL in each octave fraction, they are more convenient to work with. N.B. This option should only be used with broadband test signals, such as pink noise or pink PN.

Show phase of harmonics

If this option is selected the distortion data panel includes the phase of each harmonic relative to the fundamental.

Harmonic Phases

Use 64-bit FFT

If this option is selected the RTA uses a 64-bit FFT to process the incoming data instead of 32-bit. This is useful when analysing purely digital 24-bit data paths to view behaviour below -160 dBFS. It has no visible effect when analysing signals that have an analog connection at any point along the data path or when dealing with 16-bit data, as in those cases noise and quantisation effects far exceed any numerical limitations of 32-bit processing. Here are some examples showing the difference the 64-bit FFT makes when analysing undithered and dithered 24-bit data over an S/PDIF loopback connection from REW's signal generator producing a 1 kHz sine wave at -20 dBFS. Note that the 2nd harmonic spike at -173 dBFS in the dithered data appears to be an artefact of data handling within the S/PDIF loopback connection (via Windows 10). The vertical divisions are at 20 dB intervals, the bottom of the plot is at -220 dBFS.

32-bit FFT undithered data
64-bit FFT undithered data
32-bit FFT dithered data
64-bit FFT dithered data


The plot can be set to show the live input as it is analysed or to show the result of averaging measurements, according to the selection in the Averaging control. Selecting a number for averages results in that many measurements being averaged to produce the result, with the oldest measurement being removed from the average as each new measurement is added. There are several Exponential averaging modes, which give greater weighting to more recent inputs. The figure shown in the selection box is the proportion of the old value which is retained when a new measurement is added, the higher the figure the more heavily averaged the display becomes. There is also a Forever averaging mode which averages all measurements with equal weight since the last averaging reset.

The Reset Averaging button above the graph restarts the averaging process (keyboard shortcut Alt+R). Averaging is needed when measuring with pink noise or when there is noise in the signal being measured. Note that if measuring a response using pink noise the best results are obtained using REW's periodic noise signals, which can be exported as wave files from the signal generator to produce a test disc for the system to be measured if direct connection to the PC running REW is not possible.

The Save button converts the current display into a measurement in the measurements pane (keyboard shortcut Alt+S). It is converted in the current mode of the analyser, so if the analyser is in Spectrum mode the measurement shows the spectrum, if it is in RTA mode it shows the RTA result. The saved measurements can be used as references for subsequent spectrum/RTA measurements. If distortion data is available it is copied to the comments area of the saved measurement.

Distortion Measurements

When the Distortion Panel button (keyboard shortcut Alt+D) is selected the analyser calculates and displays harmonic or intermodulation distortion figures for the input, including THD, N (noise and non-harmonic distortion), THD+N and the relative levels of the 2nd to 9th harmonics.

Harmonic Distortion

Harmonic distortion results are only valid when the system being monitored is driven by a sine wave at a single frequency. If the REW signal generator is playing a sine signal the generator frequency is used as the fundamental frequency of the input, otherwise the highest peak is used to determine the fundamental. The fundamental and its level are displayed. In calculating the power for the fundamental and harmonics the energy in the FFT bins within the relevant span of the nominal frequencies appropriate for the RTA window selection is summed and then corrected according to the window's equivalent noise bandwidth.

The THD figure is based on the number of harmonics whose levels are displayed and is calculated from the sum of those harmonic powers relative to the power of the fundamental. Individual harmonic figures are also calculated from their power relative to the power of the fundamental. The THD+N figure is calculated from the ratio of the input power minus the fundamental power to the total input power (note that it is possible for THD+N to be lower than THD using these definitions). The N figure is calculated from ratio of THD+N minus THD to the total input power. The upper limit for data used in distortion calculations is 95% of the Nyquist frequency (i.e. 95% of half the sample rate) or the Distortion Low Pass, if enabled. The lower limit is the first FFT bin or the distortion High Pass, if enabled.

The example below shows data for a 1 kHz sine input. The positions of the harmonics are shown on the spectrum or RTA plot. The Distortion High Pass and Distortion Low Pass have been set to 20 Hz and 20 kHz respectively, hence results are based on data from the span 20 Hz to 20 kHz.

Distortion Results

Intermodulation Distortion

Intermodulation distortion results are only valid when the system being monitored is driven using REW's Dual Tone test signal. The generator provides preset signals for SMPTE, DIN and CCIF intermodulation measurements and a 'Custom' option allowing a user-selected pair of frequencies at a 1:1 or 4:1 ratio. When the signals are in 1:1 ratio the IMD figure is calculated from the level at f2-f1 (also called Difference Frequency Distortion or DFD), the reference level for the percentage figure is twice the level at f2. For signals with 4:1 ratio the IMD is calculated from the 2nd order (d2) and 3rd order (d3) components, the reference level for the percentage figure is the level at f2. REW displays the overall IMD figure and, where appropriate, the individual d2 and d3 levels, labelled as follows:

d2Lf2 - f1
d2Hf2 + f1
d3Lf2 - 2*f1
d3Hf2 + 2*f1

Intermodulation Results

Intermodulation Distortion Plot

Stepped Sine Distortion

When the Stepped Sine Measurement button is pressed a dialog appears to configure and run a stepped sine distortion measurement.

Stepped Sine

REW's signal generator is used to produce the measurement frequency. Note that the currently applied signal generator settings are used, so dither will only be applied if selected (it is selected by default and is recommended). The frequency can be stepped in intervals of between 1 and 96 points per octave over the selected span. To avoid scalloping loss effects the test frequencies use the closest FFT bin frequency, that ensures the peaks of the fundamental and all harmonics are captured in the plots.

At each frequency step all the distortion data is captured, when all points have been captured a new measurement is generated. The levels of the fundamental (brown), 2nd harmonic (red) and 3rd harmonic (orange) are shown on the RTA graph while the measurement progresses. If the Stop button is pressed a measurement is generated from the data collected up to that point. Stepped sine measurements typically take many minutes. The Pause button pauses the measurement, turning off the signal generator. Press it again to resume the measurement. The Back button removes the last point measured and re-measures it. Back can be used when the measurement is running or paused. Cancel discards the measurement.

If Capture spectrum at each frequency is selected a 96 PPO log spaced copy of the spectrum data at each measurement frequency will be captured. That spectrum data can be viewed on the Waterfall or Spectrogram graphs or exported to a text file. Note that exported spectrum data should not be used to try and calculate distortion levels as it does not have sufficient frequency resolution to accurately calculate the energy in each harmonic.

If Stop measurement if excessive clipping occurs is selected the measurement will be stopped if more than 30% of the samples in an input block are clipped. That corresponds to an input level about 2 dB above the clipping threshold.

The minimum start frequency depends on the FFT length and the sample rate - for example, for an 8192 point FFT and 44.1 kHz sample rate the minimum start frequency is approx 60 Hz, for a 32768-point FFT at 44.1 kHz the minimum start is approx 15 Hz. The spreading effect of the RTA window would obscure the 2nd harmonic level at frequencies lower than the minimum and prevent a valid reading of distortion. The measurement frequencies are chosen so that they correspond to bin frequencies for the selected FFT length, this prevents window scalloping loss from affecting the amplitudes of the fundamental or harmonics.

At the beginning of each measurement the noise floor is captured and used to (optionally) mask distortion results that are below the noise floor (see the distortion graph help).

The progress bar shows the approximate time remaining to complete the measurement. Stepped sine measurements are faster when using ASIO drivers as the input and output buffers are smaller than when using Java drivers, hence less time is required for the buffers to flush through when changing frequency. The distortion results can be viewed on the Distortion graph. Note that, as with the log sweep distortion measurements, calibration files are not applied to the stepped sine distortion results, but they are included in the SPL values for the resulting measurement.

Although much, much slower than a log sweep the stepped sine measurement captures N (noise and non-harmonic distortion) and THD+N (neither is available with a log sweep) and can measure low distortion levels more accurately than a sweep, particularly at high frequencies and for higher harmonics. This makes it well suited to measuring the distortion of electronic components. The plots below show a soundcard loopback measurement at -12 dBFS measured with stepped sine (64k FFT, 24 ppo) and with 8 repetitions of a 1M log sweep. Note the rise in the levels of harmonics with frequency when measuring with the sweep. This reflects the rise in the noise floor of the device (as can be viewed with the RTA in RTA mode), the sweep cannot separate distortion from noise as well as the stepped sine measurement.

Stepped sine distortion measurement

Log sweep distortion measurement

Stepped sine spectrogram

Stepped sine waterfall

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