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.
The RTA trace is activated by pressing the 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. In RTA mode 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 dB FS according to the setting of the Y axis. This figure excludes any DC content in the signal. If clipping is detected in the input the RMS value turns red.
The controls for the plot are shown below.
The Mode can be set to Spectrum for a spectrum analyser plot or to various RTA resolutions from 1 octave to 1/48 octave. 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.
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.
Here is the same tone analysed with a 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.
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.
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.
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.
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, pink noise or pink PN.
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.
When the Distortion button (keyboard shortcut Alt+D) is selected the analyser calculates harmonic or intermodulation distortion figures for the input, including THD, THD+N and the relative levels of the 2nd to 9th harmonics.
Harmonic distortion results are only valid when the system being monitored is driven by a sine wave at a single frequency. The highest peak is used to determine the fundamental frequency of the input, this is displayed with the level of the fundamental. 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. 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 example below shows data for a 1kHz sine input. The positions of the harmonics are shown on the spectrum or RTA plot.
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:
|d2L||f2 - f1|
|d2H||f2 + f1|
|d3L||f2 - 2*f1|
|d3H||f2 + 2*f1|