Spectrum/RTA feature

23538 Views 28 Replies 10 Participants Last post by JohnM, Nov 23, 2011

nick72

Discussion Starter · Mar 9, 2008

Hi everyone!

First let me say that the new REW 4.10 is really quite amazing....Thank you John!!!

The RTA/Spectrum tab is very, very cool.
I have to admit though that I'm a big novice at this stuff so I was wondering a few things as I was trying the new Spectrum Tab features..

- What is the difference between RTA and Spectrum Analysis? In a sense, the graphs are similar yet different. I guess more to the point is, what uses would RTA have over spectrum analysis and vise versa?

- I noticed that when using the DB Y axis, my RTA and spectrum graphs are 20+ DB below the 75db reference when sending noise from the Generator, even though the SPL Meter reads 75 DB. Is this normal? When I take a standard measurement, it is graphed correctly. Here's a snapshot below with the two on the same graph.
The caption of the RTA reads:
"65536-point 1/48 octave RTA using Blackman-Harris 7 window and exponential averaging
Input RMS 94.3dB"

Thats all for now I guess. I'm sure I'll have more questions as I play with it a little more...

Thanks again for a really cool new version of REW!!!

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klinemj

· #21 · Jun 9, 2010

I calibrated the SPL using the normal approach within REW, so it was set "about" 75 dB.

The main part of the frequency spectrum I am interested in for the other things I want to measure will be between ~60 and 75 dB. So, perhaps what I should do is to keep the calibration set at ~75 dB & ignore anything below 0 dB. And, unless I am trying to measure things down closer to 60 dB, I could calibrate at ~60-65 dB and then measure.

Also, thanks - I did find the dB vs. dBFS pull down. There's so many little things to find in the software - great to have those!

Mark

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jazzcat

· #22 · Jul 9, 2010 (Edited)

I am having a tough time with the RTA feature. Can someone help me to take a simple measurement please? The measurement I get is very different from the usual measurement I get under the measured tab. I am not sure if I've configured it properly. Please have a look at the graph and the actual parameters that are set. I understand that the db will be lower but the measured response is different all across the board. I pasted the parameter settings from the spectrum tab to the main measurement. The graph shows 3 sweeps I did with the REW software. Once I get this setup, then I can measure my bluray player.

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JohnM

· #23 · Jul 9, 2010

Set the mode to RTA rather than Spectrum, e.g. RTA 1/24 octave. What is the test signal you are feeding the system? Best would be to use "Pink PN" from the REW siggen.

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jazzcat

· #24 · Jul 9, 2010 (Edited)

Thanks John, that helped me to take measurements. All of that was already posted previously by you, for some reason I didn't grasp it. Long week I guess. I was using the sweep from the measure tab. Will the pink noise on the avia or DVE disks be a close approximation to the pink PN on REW? I need to test my bluray player now.

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JohnM

· #25 · Jul 11, 2010

jazzcat said:
Will the pink noise on the avia or DVE disks be a close approximation to the pink PN on REW?

No, they will be pink filtered random noise so if you use them you will need to use averaging on the RTA to see what is happening at low frequencies.

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jianhua1975

· #26 · Nov 16, 2011

brucek said:
ause you can balance the right channel input against the looped back left channel calibrate input signal. brucek

It’s mentioned that we don’t need calibration file if we do measurement through R channel, while loop back L channel, does that mean I don’t need to perform the soundcard calibration steps to get a calibration file? Just loop back L channel while doing measurement is enough?
Also it’s mentioned to balance right and left channel input level, How to balance left and right channel when loop back one and use another for real measurement? What I understand in this case, R channel input is from Phantom powered Mic, and L channel input is directly from output whose signal is generated from REW. So what I shall do in order to balance them?

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JohnM

· #27 · Nov 16, 2011

jianhua1975 said:
It’s mentioned that we don’t need calibration file if we do measurement through R channel

Not anywhere I've seen

does that mean I don’t need to perform the soundcard calibration steps to get a calibration file? Just loop back L channel while doing measurement is enough?

How to balance left and right channel when loop back one and use another for real measurement? What I understand in this case, R channel input is from Phantom powered Mic, and L channel input is directly from output whose signal is generated from REW. So what I shall do in order to balance them?

If each channels has its own input gain then adjust each gain as required. If the channels do not have separate gain (unusual if one is mic and one is line) you could alter the windows mixer balance control for the input.

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jianhua1975

· #28 · Nov 21, 2011

JohnM said:
Not anywhere I've seen

No

If each channels has its own input gain then adjust each gain as required. If the channels do not have separate gain (unusual if one is mic and one is line) you could alter the windows mixer balance control for the input.

so one another question that bothers me is that, i do calibration for both R and L channel for my Tascam US 144MKII soundcard, that will generate two Cal files. so when i am doing the measurement, i think i shall load the R channel cal file if i am using R Mic in and R line out for measurement , then what for Left channel, what i can do is only loop back, right? so why i need to also calibrate L channel if i decided to loopback this channel for time reference?

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JohnM

· #29 · Nov 23, 2011

jianhua1975 said:
so why i need to also calibrate L channel if i decided to loopback this channel for time reference?

You don't

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Spectrum/RTA feature

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