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post #1 of 2 Old 04-09-11, 10:21 AM Thread Starter
Shackster
Steve
 
Join Date: Nov 2008
Location: Arlington, TX
Posts: 73
RTA settings

I have a couple of questions regarding the RTA feature of REW 5.

The first is what window, FFT length, etc. settings (including any smoothing) should I use in conjunction with a 31 band graphic eq to flatten response of some vocal monitors my son's band uses in their practice room.

The other thing I need to do is use the RTA to check out what changing my sub location and/or main listening position does to my FR. I have a pretty big dip around 126 Hz at my main LP now - see graph. What settings would be used for this task?

RTA settings-main-lp.jpg

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post #2 of 2 Old 04-09-11, 11:32 AM
Elite Shackster
 
Join Date: Jan 2010
Location: GTA, Ontario
Posts: 1,400
Re: RTA settings

Quote:
JohnM wrote:
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).

As a final tip, if you tick the "Adjust RTA levels" box the RTA graph will be drawn at levels that are comparable to your measurements.

Quote:
page 98 of the PDF manual wrote:

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

Page 67-68
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).



Quote:
JohnM wrote:
Again the behaviour depends on the FFT length and averaging mode, and in this case on the window setting also. For noise measurements set the window to Rectangular. If you use REW's Pink PN test signal you can use either no averaging or very little averaging, if you use random pink noise you will need to use a lot of averaging if you want to see anything meaningful at the low end as random pink noise only has the correct spectrum when averaged over a very long period. To see detail in the response you need a long FFT length and high RTA resolution, e.g. 1/48 octave
Hope something there helps .

<> EarlK
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