DC offset in measurements can be very frustrating, I thought I'd open this topic to determine best practices and ask what the recommended solutions are.
Equipment here is a Motu M4 interface, and sine sweep measurement in ARTA. Motu claims "DC coupled TRS input" so it tends to include more low frequency noise than other AC coupled equipment.
For the following measurements, I measured with amp, mic, and loopback jig connected to amp output, however no speaker connected to measurement is just background noise. Dual channel sine sweep shows increasing noise towards low frequencies, and DC offset is obvious in the impulse view:
Increasing time scale on the impulse to 2.5s to show the entire measurement, we can see that it is not strictly a DC problem, but a very low frequency wandering noise:
Single channel measurement of just the mic input, shows no low frequency / DC problem, which is expected:
Single channel measurement of the reference channel, shows the low frequency problem clearly:
Zooming out on time scale of the reference input, shows the low frequency information in the reference channel input:
VituixCAD only shows response down to 5Hz, but using ARTA we can show down to 0.5Hz, to see that there is very low frequency content in the measured signal, even though it starts to roll off at 20Hz:
Given this low frequency problem with dual channel measurements, the options I have in mind would be:
Equipment here is a Motu M4 interface, and sine sweep measurement in ARTA. Motu claims "DC coupled TRS input" so it tends to include more low frequency noise than other AC coupled equipment.
For the following measurements, I measured with amp, mic, and loopback jig connected to amp output, however no speaker connected to measurement is just background noise. Dual channel sine sweep shows increasing noise towards low frequencies, and DC offset is obvious in the impulse view:
Increasing time scale on the impulse to 2.5s to show the entire measurement, we can see that it is not strictly a DC problem, but a very low frequency wandering noise:
Single channel measurement of just the mic input, shows no low frequency / DC problem, which is expected:
Single channel measurement of the reference channel, shows the low frequency problem clearly:
Zooming out on time scale of the reference input, shows the low frequency information in the reference channel input:
VituixCAD only shows response down to 5Hz, but using ARTA we can show down to 0.5Hz, to see that there is very low frequency content in the measured signal, even though it starts to roll off at 20Hz:
Given this low frequency problem with dual channel measurements, the options I have in mind would be:
- Use "DC offset" checkbox in VituixCAD IR to FR processing, when using sine sweep meausurements
- Use PN noise measurement instead of sine sweep, to inherently high pass the measured signal.
- Use multiple measurement averaging to lower noise floor
- Place a capacitor on the reference channel input to AC couple it.(?)
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