The Real Time Analyzer (RTA) feature in REW is very easy to use and allows for accurate results quickly. This method takes into account the actual signal chain and allows you to make changes on the fly and instantly see their affect (if your system lets you make changes on the fly during playback). It is useful for checking changes made to Room Correction, Parametric Equalizer, or Convolution.
The RTA measures with exactly the same accuracy as the sweeps. I tested it several times and the results were always identical. The downside is I don't think you can use the measurement captured by the RTA for the waterfall, impulse or other REW features. You are only using it for SPL across the frequency spectrum.
Since you aren't using sweeps, you need to have a signal for REW to measure. Using the Generator module in REW you can save the Pink PN with a length of 131072. Click on "Save PN to WAV file..." and save it somewhere. If you have an HTPC connected to your receiver, just use that to play the file. Otherwise you need to burn the file to a CD for playback on a CD, DVD, or Blu-ray player.
From the REW manual:
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 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.
The file is a two channel file so you will need bass management on in your receiver or JRiver for it to be sent to your sub.
With the file playing through your signal chain, calibrate the SPL in REW by opening the SPL Meter module and clicking "Calibrate." In the signal source windows select "Use an external signal" and click "OK." Use an SPL meter at the mic location to measure the SPL. Increase your system volume to about 75-85 dB. Now enter the reading in the SPL Meter module to calibrate REW.
With the Pink PN file still playing, now open the RTA module. Click the setting button in the top right of the module and set it to the following:
Mode: RTA 1/48 Octave
FFT Length: 131072
Averages: None
Window: Rectangular
Max Overlap: 87.5%
Update Interval: 1
From the REW manual about the Window:
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.
John, the developer of REW, also said the following:
A couple of things on the RTA: when you use a noise signal as a stimulus it is best to set the window to Rectangular. You can then also take advantage of a nice property of the Pink PN signal, which is that it gives very good results with no averaging (because, unlike random pink noise, each complete sequence contains all frequencies at the correct levels and matches the FFT length, with random pink noise the levels are only correct over a (very) long average, especially at the low end).
Click the settings button again to close the settings flyout. Now click the red start/stop button in the RTA. You should see a frequency readout appear. I like to window it from 10 Hz - 200 Hz for subwoofers and over 20,000 for speakers. This is showing the frequency response at the mic position. You can change from "Averages: None" to something like "32." This will use the last 32 readings. You can move the mic around to different seats to get an average of a larger area.
If you want to save the measurement just click the "Save" icon at the top of the screen. You can now use the measurement for generating filters or to compare to other measurements. If you are averaging an area, you need to click save as soon as you finish moving the mic around.
After your sub is in the location with the best response, you can unmute one of the mains and look at the crossover region on the RTA. Adjust your distance setting until you get the smoothest response at the crossover region. I have found that sealed subs needed about 2 ft more than mains, ported subs (folded port) 5-7 ft, and horn subs 14-17 ft. You can also make changes to the crossover frequency and see which has the smoothest response and which sounds best to you.
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Topic: How to use the RTA feature of REW with JRiver (Read 17244 times)