TrueRTA-for-dummies - Page 5

Can you tell me what 8x more is in dB?

The bandwidth is increased 8x - thus there is 8x more signal than there was before. The only way there could be the same amount of signal would be to reduce the per frequency amount by 8x. This is a real time signal analyzer - you increased the bandwidth of the filter and have 8x more data - it is not simply a matter of changing a chart resolution. It has nothing to do with what your SPL meter said - which is not using bandwidth filters to look at the signal as individual pieces. The only reading that should correlate with the SPL meter in your hand - is the SPL meter in the software. This chart is NOT an SPL meter it is an RTA curve in units of SPL. What if that was volts per frequency - would you expect a voltmeter to be the same reading? How is it that you decided that 92SPL at 100Hz was the SPL meter reading - why can't it be that the answer is the 86SPL at 50Hz? Why not correlate the reading at 80SPL at 400Hz? How much frequency range is there in the 24th octave centered on 125Hz vs 3rd octave?

How much more dB do you think there should be? You said you are a professional using this signal analyzer - what is the answer?

For those that realize the earth is not flat just move along --there is nothing to see here.

Otherwise: Many well intentioned individuals have tried to explain how this measurement works. I will now use the method of cows.

There are 24 cows in each octave. With 10 octaves across the audio band, that means there are a total of 240 cows.

First I will make a plot of cows using a 1/3 octave resolution. Lo and behold there are 8 cows in each 1/3 octave.

Next I will make a plot of how many cows there are in each 1/24 octave. Well my goodness, there is only one cow in each 1/24 octave.

Is my second plot right or wrong compared to my first plot? If I add up all of the cows in each instance did the total number of cows change?

Of course the total number of cows did not change. Each measure of cows in a fraction of an octave was also correct.

What exactly did you want to measure?

Yes, the graph "jumps up" ~10dB when I switch between resolutions, but I understand why it does so, as you have so much mure total energy in each bin for a 1/3rd octave analysis vs a 1/24th.

However, I do agree it would be nice to have the option in the Software for an automatic "adjustment" of the SPL axis when switching resolutions. A simple check-box in the Preferences/Options dialog box or some such.

/Jonas

If you are going to invalidate the measurement, what erroneous levels would you choose for which particular resolutions. Of course if would not be an RTA at that point.

That reminds me, I have both a laser interferometer and a mass spectrometer and neither one of those silly things reads the way I want them to. Gotta remember to call the manufacturers and ask them to trash those measurement instruments for me.

Jeez, don't you just hate those standards organizatons? Everyone measuring the same things the same ways all over the planet. How boring

Well, that was certainly a helpful contribution.

What I cannot understand is, why the graph jumps +10 when the actual levels have not changed.

What I wanted to see was the data points interpreted on a 1/3 octave basis. Since that doesn't happen in TrueRTA, that make TrueRTA useless for my needs. I cannot measure a speaker cabinet and publish graphs that are inflated +10dB. Bass players certainly won't understand bins, etc.

I wanted to measure the height of the cows.

One cow, or 10 ten cows, the height does not change. I want to know the height of the cow standing at a particular point. I don't care about the weight of the other cows around it, nor how many cows there are.

If I have a cow standing at the 100 Hz point and change octave resolution, the cow does not get taller.

Hi,
I am testing the free version but first of all it is only outputting sinewave and pink noise.The sinewave level is frequency dependent!Although clean it may be eg 2000mv at 400hz and then at 800hz it drops to something like 1000mv then it goes up again at 1khz to 1500mv then drops again and so on , at freq above 5khz i cannot even read the signal because it drops too low.On square wave or triangle wave I see a small sine wave residual and occasinally some kind of impulse on the screen.Is this a bug because it cannot be the way it functions?

The cows represent the energy in an octave.
The RTA functions as it should and as they all do.

sheep?

It is actually very simple. The signal levels in each bin DO change precisely as indicated by TrueRTA. Narrower filters measure a lower signal level. Maybe another example will help make it clear why the shift is both necessary and correct.

The sequence below shows what happens when you change resolutions after measuring a sine wave at 0 dBu. Notice that the peak at 500 Hz remains at the same level as we change resolutions. But the broadband noise floor decreases as the resolution is increased...as it must. Each 1/24 octave bin is measuring only a fraction of the signal measured by the 1 octave wide frequency bins. But the bin at 500 Hz measures a constant signal level as that bin is made narrower. This is because the 500 Hz bin is being stimulated primarily by a single frequency. So the 500 Hz tone remains constant around 0 dBu even as the resolution is changed. The remaining bins are measuring mostly white noise and as such they fall in level as the bin is made more narrow.





For those engineers seasoned in such measurements this is a normal and expected behavour. But occasionally someone less experienced in electronic measurements will question why the noise floor shifts.

Regards,

John

Make sure you have any sound card special effects or "monitoring" turned off as this can lead to the kind of response you describe. Also try reducing your generator level. You may be overdriving the sound card output which can lead to erratic results.

Regards,

John

Review your Vista "Sound" Control Panel settings making sure that the Mobile Pre is selected as the default sound system both for playback and recording.

Regards,

John

I don't have a problem with the noise floor. I have a problem with the peak shifting +10dB. My sources are in the 100 to 136 dB range, so the noise floor is inconsequential to the intensity of the sine wave.

As I understand a swept sine, each point is driven at full voltage. This is an automated method of taking discreet measurements at single frequencies. When I sample at point at 1/24 it correlates with the external meter. The same point at 1/3 is higher by 10dB than the meter.

Your example above shows the 500 Hz peak does not change amplitude with resolution. This is exactly what I wanted from Level 4, but did not get. I notice you are using dBu, and I was using SPL.

bgavin

Please answer why you chose to sample the point that correlated your SPL meter - rather than another frequency on the curve which does not.....

Perhaps.

My concern is not with dogma, but with finding an understanding why a peak point value changes +10 dB between resolutions.

I understand the QuickSweep is a series of 1/24 bins, individually sampled. Therefore, the amount of energy in the octave is irrelevant. Only the energy in each bin is relevant. This displays correctly at 1/24.

Changing to 1/3 evidently sums all the bins, instead of charting between points relevant to 1/3 resolution. This summing boosts the graph by +10dB, yet the measured bin data does not change.

If the chart were drawn between points at 1/3 octave, i.e. 80, 100, 125, 160 Hz using the measured bin data, this would produce a chart at the sample amplitude, but at 1/3 resolution.

Thanks John but I have followed the recmmendations about setting up the audio card i.e.no effects etc.I have adjusted windows and the outputs mixers of the card (Sounblaster live) which are VERY sensitive (just a couple of percent and the output jumps to double on the mv range!) and as I said the sinewave looks clean i.e. no clipping.I have played with the generator level but it does not affect the results that much unless I overdrive it of course.Also the fact that I cannot see any form of square wave is strange.I am thinking of buying a full version but only if I am sure I can sort this out.

I am playing around with various settings but no go.First of all if I am seeing a nice sinewave it means the input output settings are correct.I can see a triangle (although it is not really sharp at the point)and pink noise and saw waves but i cannot see a square wave even though the mv meter is registering something like 2000mv on this setting.The sine wave is good now up to like 3khz then drops to zero level.I have not set any equalizer or effects.On the square wave if I set the oscilloscope to to very gross ms divisions i can see something like an impulse response i.e. a thick line with intermittent spikes but no square waves or similar on any setting.I guess the frequency aberration i.e. nothing above 3khz is affecting the square wave production since even low hz square waves need higher harmonics , right?

The other weird thing on the sinewave is that as I increase the gen level up to +10 dbu i get about 200mv approx. for every db increase then after +10dbu the level instead of increasing starts dropping even though there is no clipping evident.In fact at +20 dbu the level drops to almost zero.
Also if I go down to 100hz sinewave and change the output level by 1dbu ie from 18 dbu to 17 dbu the output level jumps from 2600mv down to 1600mv!

Let's go over it again. These measurements are not trivial and take a while to understand. This time I'll use the SPL mode to demonstrate that a measured peak does not change in level as you go from 1 to 1/3 to 1/24 octave resolution. The measurement depends strongly on the nature of the signal present in each frequency bin. A bin with a single strong frequency will remain constant in level but a bin with noise (many frequencies) will fall with increasing resolution as the filter is narrowed. Note that a bin with a mix of a small peak (say a distortion component) and noise will reveal the peak better as the bins narrow and the noise is reduced to reveal the peak. Here is a measurement of the speaker in my notebook PC along with lots of ambient noise to add character to the measurement.


In the 1 octave resolution shot above we see the peak at 300 Hz only generally defined with a hint of a second peak at 60 Hz. The peak reads close to the broadband SPL level of 80 dB. The noise floor appears around 50 dB SPL when seen through 1 octave wide filters.


The 1/6 octave resolution shot above reveals the presence of a 2nd harmonic distortion component at 600 Hz and perhaps a 4th harmonic at 1200 Hz. A broad peak is seen near 60 Hz. The 300 Hz peak is seen unchanged at 80 dB SPL but the 60 Hz peak has fallen in apparent level from about 57 dB in the 1 octave measurement to about 53 dB in the 1/6 octave measurement. The noise floor appears around 44 dB SPL when seen through 1/6 octave filters.


Finally, in the 1/24 octave resolution shot above we see the 2nd and 4th harmonics sharply revealed. We also see that the 60 Hz peak is actually two separate peaks at 50 and 60 Hz. Note that the 300 Hz peak remains UNCHANGED at 80 dB SPL while the 2nd harmonic level also remains constant as the 4th drops slightly as noise is eliminated from its bin. The noise floor appears around 38 dB SPL when seen through 1/24 octave filters.

Most importantly, note that the peak amplitudes remains constant as the resolution is changed. The noise floor drops at higher resolution to reveal the smaller peaks. It is easier to see what is happening here than it is with a pink noise response so in a subsequent post I will show what happens with a pink noise response as resolution is changed. I'l also show how to shift a pink noise measurement to calibrate it up to a fixed pitch SPL measurement.


A Personal Note to TrueRTA Users:

Meanwhile, I am busily working on a maintenance release of TrueRTA to fix a verifiable bug affecting a major industrial user. When reliable users report bugs that I can reproduce I take them very seriously even if I can't fix them immediately. I'm not sure the minor "import bug" (discussed above) will be addressed in this release but I will be investigating it further.

You may have noticed that as a small software publisher I do all my own customer support. This takes a lot of time and sometimes my responses are delayed more than you (and I) would like. I really enjoy making powerful audio software available at a price the represenst a high value to users. But I am sometimes tempted to charge a separate fee for support! I would much rather spend my time expanding the software. That is why I am highly appreciative of the AVS Forum hosting this thread, and for you TrueRTA users supporting each other here as you do. This allows me to spend more time developing new software. I actually have in the works a powerful new impulse analyzer that will allow analysis of signal arrival times and reflections. But this sort of project requires focused concentration with lots of quiet time to work out all the details.

I want to offer a warm Thank You to all TrueRTA users for helping to make this product sucessful and supporting my small software business.

John

Looking at the spectrum analyzer with pink noise it shows these anomalies.The response is flat to about 1khz witha peak at 2khz and then it drops all the way down.Is this a bug of the freeware or coud it be my soundcard?
I have tested the signal generator on speakers and it seems to have a response up to high frequencies ie 16khz but this is just personal acoustic impression without mic measurements.So the problem is in the input side probably.Either the input is being equalized in some way or what?
By the way how do you attach a screenshot?

I absolutely agree.

You continue to focus on the single peak, but that was not the bug report. I only see 0.3dB change in a single sine peak between 1/24 and full octave resolution. My bug report was concerned with swept sine traces, not single points.

In field use, the whole 1/3 vs. 1/24 issue is avoided by not using 1/3. The 1/24 sweep is accurate, and that did the job. If I wanted 1/3 charts, I can dump to TXT and extract the 1/3 octave data values for plotting.

What is more important to other TRTA owners, is fixing the .TXT import problem that you said can be improved.

The other important one is investigating why a .RTA on a laptop provides wildly different results when moved to a desktop machine. The laptop had the sound system calibration and the USB sound card interface. The desktop had no calibration file, and was not using a sound card. It was for charting purposes only, but the results were very different for the same .RTA file.

I may be the only one who had the data portability problem, but it showed on up three machines. All the machines without sound cards and calibration files were the same. The laptop with sound card and calibration file was different. Right or wrong, the data is not portable between my machines.

The refund from TrueRTA posted to my credit card, so I have removed TRTA and can do no further testing or reporting.

I bet that if you import the calibration files into your desktop PC, it will plot the exakt same graphs as your Laptop does...

/Jonas

Just wanted to let you know that after having gone through hell to get XP installed on my Laptop (it is a HP/Compaq and I think they do something to the hardware to prevent us from installing anything else than what is pre-installed, in this case Vista) - I have been able to calibrate the soundcard for a straight and smooth response.

(XP is still not installed 100% correctly)

But it works good enough so to speak. I can use my Laptop with TrueRTA again!

/Jonas

Hey guys,
I would appreciate some feedback and help setting up my rta.Nobody seems to have a n answer.
I checked my set up according to rta pdf instructions and it seemed correct.I followed set up instructions for soundblaster boards.
After pulling my hair out with the results I started playing with the in out connection cable.You can imagine my surprise that I could still get adjustable sinewaves on the screen etc even with this disconnected completely.This is with wave/mp3 input.On the line in input with the cable in I only get noise on the screen that does not change no matter how I adjust various settings in the mixer or the rta itself.
So all my comments about roll off in high freq apply to measurements that were made using the wave/mp3 input but that seems to be connected internally in the soundcard.
Using the line in input I only get noise on the screen.
I would appreciate some advice.
Thanks.

Check your Machine Configuration / Hardware Config. Make sure nothing besides the Soundblaster is active.

Then, connecting a simple 1/4" line STEREO cable fron line out to line in, really should do it. Are you conneccting it to line in or mic in?

In TrueRTA, set the channels to L+R. See if that changes anything. I sometime get the same type of "noise" on the screen, thats when i realizre I have Left Ch activated and running the mic on right.....

Next is to make sure the levels for input (record) and output is set correctly.

/Jonas

/Jonas

Thanks Jonas.
I tried changing the l/r settings but this does not change anything.The line in is correct because if I lower the mixer volume control the noise goes to zero so obviously it is receiving some kind of input.Very high noise judging by the scope.
Funny thing if I connect amp/speakers to the soundcard output then I get an adjustable tone right up to the high frequencies using the wave/mp3 control.It does not seem to roll off after 3khz.However I do hear a fast clicking sound on top of the tone which must be a digital problem.
Also strange is that while running the output to the speakers as I said I can see the rta sine waves on the scope when wave/mp3 input is on.Just that after 3khz I can see practically nothing on the scope although I can hear clearly up to 14khz on the speakers.

Can somebody explain how to use TrueRTA to measure speaker impedance. Thanks

but at least im trying : )



i have been researching RTA for several months, and even pruchased WinMLs2004 per board member recommendation. Sad to say, i cannot get it to work. But now i would like to try TrueRTA. Will this software do thiS?





btw, isnt this suppose to be a file that u download?




thank you........................Jason

hmmmm

sorry, need to show url