RTA delayed reaction time - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 11 Old 03-28-11, 10:00 AM Thread Starter
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RTA delayed reaction time

I'm just starting to play around with REW and I have a strange problem with the RTA. It has a delay of about 2 seconds from the onset of a sound to its reflection in the graph. If the sound is too short, it might not show up at all. Also there is the same delay in the graph dropping down when the sound ends. I've searched but I can't find the explanation for this.

I also downloaded TrueRTA, and there is almost no delay in that program. However, the free version only has 1-octave resolution so I only have 10 bars.

Dan
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post #2 of 11 Old 03-28-11, 10:07 AM Thread Starter
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Re: RTA delayed reaction time

While I'm at it with the newbie questions, here's another one. The graph of pink noise doesn't reflect the results of playing individual test tones. I stuck the microphone in my car and played pink noise and got a fairly smooth graph, sloping down toward the midrange and disappearing in the high frequencies. Then I played individual test tones and got a very different response picture, with peaks in the mid and high range, which agreed with what I was hearing. Why is the pink noise measurement so different and how do I make sense of it?

Dan
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post #3 of 11 Old 03-28-11, 10:30 AM
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Re: RTA delayed reaction time

Quote:
earthtodan wrote: View Post
I'm just starting to play around with REW and I have a strange problem with the RTA. It has a delay of about 2 seconds from the onset of a sound to its reflection in the graph. If the sound is too short, it might not show up at all. Also there is the same delay in the graph dropping down when the sound ends. I've searched but I can't find the explanation for this.

I also downloaded TrueRTA, and there is almost no delay in that program. However, the free version only has 1-octave resolution so I only have 10 bars.

Dan

While mine has also a delay, it's not that big. Also, my RTA is dead on accurate.


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earthtodan wrote: View Post
While I'm at it with the newbie questions, here's another one. The graph of pink noise doesn't reflect the results of playing individual test tones. I stuck the microphone in my car and played pink noise and got a fairly smooth graph, sloping down toward the midrange and disappearing in the high frequencies. Then I played individual test tones and got a very different response picture, with peaks in the mid and high range, which agreed with what I was hearing. Why is the pink noise measurement so different and how do I make sense of it?

Dan
Of course you got a different response in the car. It's all about the room acoustics. Also, in your car the mic was really close to the speakers, what, about 3 feet?
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post #4 of 11 Old 03-28-11, 11:21 AM Thread Starter
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Re: RTA delayed reaction time

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ddgtr wrote: View Post

Of course you got a different response in the car. It's all about the room acoustics. Also, in your car the mic was really close to the speakers, what, about 3 feet?
Everything was done in the car. Pink noise and test tones were all played off the same CD in the same session with the mic wedged under the headrest.

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post #5 of 11 Old 03-28-11, 03:10 PM
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Re: RTA delayed reaction time

Quote:
earthtodan wrote: View Post
I'm just starting to play around with REW and I have a strange problem with the RTA. It has a delay of about 2 seconds from the onset of a sound to its reflection in the graph. If the sound is too short, it might not show up at all. Also there is the same delay in the graph dropping down when the sound ends. I've searched but I can't find the explanation for this.
How the RTA reacts depends on the FFT Length selected and the averaging setting. For fastest reaction use no averaging and a short FFT. The RTA is showing you the frequency content of segments of input that correspond to the FFT length, the longer the FFT, the more input signal needs to be captured to correspond to it - for example, a 128k FFT at 48kHz sample rate corresponds to a 2.7s segment of input.
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post #6 of 11 Old 03-28-11, 03:14 PM
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Re: RTA delayed reaction time

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earthtodan wrote: View Post
While I'm at it with the newbie questions, here's another one. The graph of pink noise doesn't reflect the results of playing individual test tones. I stuck the microphone in my car and played pink noise and got a fairly smooth graph, sloping down toward the midrange and disappearing in the high frequencies. Then I played individual test tones and got a very different response picture, with peaks in the mid and high range, which agreed with what I was hearing. Why is the pink noise measurement so different and how do I make sense of it?
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.
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post #7 of 11 Old 04-02-11, 03:53 PM Thread Starter
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Re: RTA delayed reaction time

Thanks, I set the FFT to the smallest value and now I'm getting better response.

One more question though, which I can't find in the help file. I plugged my car's head unit RCA output into my preamp and into my computer, playing test tones. When I turn up the volume past about 1/3, which registers ~82 dB, the dB meter turns red. What does the red indicate?
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post #8 of 11 Old 04-02-11, 05:21 PM
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Re: RTA delayed reaction time

When you say "turns red" do you mean the red "over" indicator turns on? That indicates that the input is clipping, to allow measurement of higher levels you would need to reduce the input volume setting and recalibrate the meter, at the end of the calibration REW tells you the maximum level that can be measured without clipping.

If the digits are red that indicates the meter has not been calibrated, go through the Calibrate process and when you have finished the digits should be white.
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post #9 of 11 Old 04-03-11, 01:49 PM Thread Starter
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Re: RTA delayed reaction time

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If the digits are red that indicates the meter has not been calibrated, go through the Calibrate process and when you have finished the digits should be white.
The digits in the dB meter turn red. In this case I'm using it without a microphone; head unit RCA output to Audiobuddy Preamp to REW.
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post #10 of 11 Old 04-03-11, 01:56 PM
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Re: RTA delayed reaction time

If by "dB meter" you mean the dB reading in the RTA plot turns red, that also indicates clipping. To allow measurement of higher levels you would need to reduce the input volume setting and recalibrate the SPL meter. I'd generally use "meter" to refer to this:
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