While doing the time alignment, it was beginning to bother me very much that the transient response shown by ARTA was out of absolute polarity.
Numerous previous tests have shown my Acoustats, as well as my whole system, to be in correct absolute polarity.
I was also beginning to be troubled by the apparent rolloff above 21kHz. Response just plummets to the 23kHz cutoff required by the 48kHz sampling rate.
So I thought I would investigate both of these problems with loopback tests, simply feeding the output of the Focusrite interface back into the input, bypassing the system and microphone.
This showed that while (curiously) the response is not perfectly flat, but there is no evidence of rolloff to the highest frequency ARTA looks at, in fact the response is still rising at 23.5kHz where the plot ends. The bass is still rising to its cutoff around 12Hz also. Distortion is negligible compared with the speakers.
So ARTA and the Focusrite interface are not showing any rolloff above 22kHz. Does the computed step response show polarity inversion? Not at all (but actually, I needed to check the transient time record, as that is the speaker measurement that shows inversion, the computed step response never does). This is what the step should look like with close to DC response you will never see on speakers:
OK, so now I remembered to check the time record with a transient:
It's out of polarity, just like my speakers when I measure them this way. It appears somehow the polarity inversion is caused by the measurement, though currently I cannot tell if it is caused by the Focusrite interface or the ARTA program. I suspect the interface most, which may have its XLR inputs wired reverse from US standards. Actually I'm not using the XLR inputs in this test, I'm using a mono phono plug to RCA adapter plugged into the combi plug. I would think the convention of tip being hot would apply regardless of the XLR wiring, but maybe not.
Also notice the digital ringing, which makes this transient similar to the one I recorded on the speakers, except I had the Acoustats in reverse polarity so the transient looks like normal polarity.
I then tried recording the "Speaker Pop" signal, which I have slightly modified. This signal is used by the Speaker Pop app, and consists of a series of steps followed by slow recoveries. In the original version, the pop is full scale and the entire signal is positive voltage. Not all amplifiers will appreciate this kind of signal, nor many speakers. I've modified it by reducing the level to -20dB and eliminating the DC offset. In Audacity, it looks like triangle waves, but it's so slow it's all pops and not waves as such.
Running the time record feature in ARTA, I simultaneously played the speaker pop signal on the Acoustats and the tweeters, which produced this:
It's almost unintelligible, except you can sort of see downward in-polarity ramps like the speaker pop signal. But it could be mistaken. Then I realized I should turn the woofers and tweeters off and just look at the Acoustat response.
First of all, it's a pretty clean transient, despite the crossovers being applied. Once again, the linear phase crossovers. But also, it has been recorded out-of-polarity through the Focusrite. This suggests the inputs to the focusrite are where the polarity reversal occurs. Note that the original signal here is not the ARTA transient, it's the actual modified speaker pop signal, but because of the highpass filter on the Acoustats just quits abruptly rather than ramping down to zero, so it looks like a transient.
To summarize:
1) In normal test the polarity is reversed (using XLR plug from microphone).
2) In loopback test, the polarity is reversed (using 1/4 plug through RCA adapter).
3) With speaker playing positive impulse, Focusrite/ARTA records negative.
Since the loopback reverses polarity just like the normal test, the audio system including speakers does not reverse (unless the XLR and 1/4 inch plug inputs are reversed from each other, which seems doubtful).
This means the polarity reversal almost certainly occurs in the Focusrite input or the ARTA time recorder. To distinguish those two, I'd need to compare different program or interface.
Still, for a publishable result, I'd want to replicate the reversal using an electronic interface. There's also the issue of using the 1/4 jack vs the XLR plug. To be absolutely certain the speaker is not at fault, I'd want to be sure those two have identical polarity.
Now about that tweeter rolloff, I'd long assumed it was caused by the narrowing of the beam of the tweeters. So to test that, I put the microphone in front of the Dynaudio D21AF tweeters, within 2cm, facing up (with 90 degree calibration file):
Tweeter is relatively far far hotter, no doubt because of this being made right in front of the tweeter. However, the decline above 20kHz or so is still plainly visible.
Thinking the D21AF tweeters might have deteriorated from 35 years of age, I tried measuring the VIFA tweeters I use on back.
The result was virtually identical.
Note that the microphone calibration file goes all the way to 23kHz, and was made for this exact microphone by a specialist.
I need to test other tweeters with known response above 23kHz.
Numerous previous tests have shown my Acoustats, as well as my whole system, to be in correct absolute polarity.
I was also beginning to be troubled by the apparent rolloff above 21kHz. Response just plummets to the 23kHz cutoff required by the 48kHz sampling rate.
So I thought I would investigate both of these problems with loopback tests, simply feeding the output of the Focusrite interface back into the input, bypassing the system and microphone.
This showed that while (curiously) the response is not perfectly flat, but there is no evidence of rolloff to the highest frequency ARTA looks at, in fact the response is still rising at 23.5kHz where the plot ends. The bass is still rising to its cutoff around 12Hz also. Distortion is negligible compared with the speakers.
So ARTA and the Focusrite interface are not showing any rolloff above 22kHz. Does the computed step response show polarity inversion? Not at all (but actually, I needed to check the transient time record, as that is the speaker measurement that shows inversion, the computed step response never does). This is what the step should look like with close to DC response you will never see on speakers:
OK, so now I remembered to check the time record with a transient:
It's out of polarity, just like my speakers when I measure them this way. It appears somehow the polarity inversion is caused by the measurement, though currently I cannot tell if it is caused by the Focusrite interface or the ARTA program. I suspect the interface most, which may have its XLR inputs wired reverse from US standards. Actually I'm not using the XLR inputs in this test, I'm using a mono phono plug to RCA adapter plugged into the combi plug. I would think the convention of tip being hot would apply regardless of the XLR wiring, but maybe not.
Also notice the digital ringing, which makes this transient similar to the one I recorded on the speakers, except I had the Acoustats in reverse polarity so the transient looks like normal polarity.
I then tried recording the "Speaker Pop" signal, which I have slightly modified. This signal is used by the Speaker Pop app, and consists of a series of steps followed by slow recoveries. In the original version, the pop is full scale and the entire signal is positive voltage. Not all amplifiers will appreciate this kind of signal, nor many speakers. I've modified it by reducing the level to -20dB and eliminating the DC offset. In Audacity, it looks like triangle waves, but it's so slow it's all pops and not waves as such.
Running the time record feature in ARTA, I simultaneously played the speaker pop signal on the Acoustats and the tweeters, which produced this:
It's almost unintelligible, except you can sort of see downward in-polarity ramps like the speaker pop signal. But it could be mistaken. Then I realized I should turn the woofers and tweeters off and just look at the Acoustat response.
First of all, it's a pretty clean transient, despite the crossovers being applied. Once again, the linear phase crossovers. But also, it has been recorded out-of-polarity through the Focusrite. This suggests the inputs to the focusrite are where the polarity reversal occurs. Note that the original signal here is not the ARTA transient, it's the actual modified speaker pop signal, but because of the highpass filter on the Acoustats just quits abruptly rather than ramping down to zero, so it looks like a transient.
To summarize:
1) In normal test the polarity is reversed (using XLR plug from microphone).
2) In loopback test, the polarity is reversed (using 1/4 plug through RCA adapter).
3) With speaker playing positive impulse, Focusrite/ARTA records negative.
Since the loopback reverses polarity just like the normal test, the audio system including speakers does not reverse (unless the XLR and 1/4 inch plug inputs are reversed from each other, which seems doubtful).
This means the polarity reversal almost certainly occurs in the Focusrite input or the ARTA time recorder. To distinguish those two, I'd need to compare different program or interface.
Still, for a publishable result, I'd want to replicate the reversal using an electronic interface. There's also the issue of using the 1/4 jack vs the XLR plug. To be absolutely certain the speaker is not at fault, I'd want to be sure those two have identical polarity.
Now about that tweeter rolloff, I'd long assumed it was caused by the narrowing of the beam of the tweeters. So to test that, I put the microphone in front of the Dynaudio D21AF tweeters, within 2cm, facing up (with 90 degree calibration file):
Tweeter is relatively far far hotter, no doubt because of this being made right in front of the tweeter. However, the decline above 20kHz or so is still plainly visible.
Thinking the D21AF tweeters might have deteriorated from 35 years of age, I tried measuring the VIFA tweeters I use on back.
The result was virtually identical.
Note that the microphone calibration file goes all the way to 23kHz, and was made for this exact microphone by a specialist.
I need to test other tweeters with known response above 23kHz.
No comments:
Post a Comment