Forum Replies Created
14 July 2020 at 10:38 pm #223
>> Is it correct to assume that this loudspeaker is displaying non-minimum phase behavior
>> around the crossover region when one looks at the 2-nd order all-pass response of its
>> phase response?
>> Can the phase adjustments being done around the crossover region, to flatten the phase
>> response, be deemed to be “maximum phase” corrections seeing that we are inverting an
>> all-pass response?
Yes. But remember this is just one loudspeaker. Each loudspeaker is different.
>> Is the flattening of both the magnitude (EQ boosting) and phase response around the
>> crossover region not going to introduce audible artifacts like pre-ringing?
Maybe. Linear-phase brick-wall crossovers also have pre-ringing but it’s rare that people say they think they can hear it.
>> Can the same EQ concepts be readily applied in correcting the room transfer function
>> displaying non-minimum phase characteristics at the listening/measurement position?
Yes but I’d suggest doing a literature search on the topics of room impulse response correction and inverting room impulse responses; maybe in the Audio Engineering Society resource library.
EA Support18 June 2020 at 11:35 am #180
Even though your measurement system might be doing specific time windowing, the IR still has both loudspeaker phase and high delay which manifests as high phase slope when exporting as freq/mag/phase TXT. (I’m guessing this high delay is the time-of-flight across your chamber, plus soundcard buffer delays.)
In FIR Designer there is no error to give. The TXT file HAS phase and so the loading process is doing the best it can to interpolate the TXT file to the finer resolution FFT spacing. The high phase slope is the problem, and in this case is causing spurious distortion in the calculated IR. (You can see this on the FIR Designer Import tab if you expand the time axis and zoom the vertical axis.) If you truely want to ignore phase, then you can set all the phase values to 0 deg in the TXT file. (FWIW SMAART spectrum measurements come across to FIR Designer with 0 deg phase.)
In this situation it’s always best to avoid using log-frequency-spaced freq/mag/phase data – no matter how fine in resolution – for transfer between programs. Conversion to this format looses information which then causes problems when trying to reconstructed the impulse response – which is what FIR Designer is doing. It’s always best to transfer using an impulse response or time format, where information doesn’t get lost. (E.g. AFMG ETM files, or the time impulse as TXT or WAV.
>> And since Auto Mag should be trying to work on matching measured magnitude to target only, I do not see the relation to phase.
Auto Mag works with complex input and calculates the magnitude using either power averaging or complex averaging across frequency. You’re correct that power averaging should avoid issues of high phase slope but the additional distorting (added by the import interpolation process due to the high phase) is also causing problems in the Auto Mag.
I guess we could add an option to completely ignore phase on import, but it’s not something people normally want. When doing any filtering on a loudspeaker, most folks want to see both the phase of the filtering and the effect – both magnitude and phase – of the filter on the loudspeaker measurement.
EA Support18 June 2020 at 10:27 am #176
I’ve looked over the project and measurement files. The measurement is a 3 column freq/mag/phase TXT file with very high phase slope. In other words the impulse response of this measurement wasn’t near time=0 in the measurement system, before the TXT file was exported. (In the excerpt below, the phase starts at -76 degrees and ends up < -40,000 degrees.) The file import process in FIR Designer does it’s best to interpolate this TXT file up to the internal FFT size, but very high phase slope causes errors in this interpolation. And both the interpolation errors and high phase slope are causing the Auto Mag problems that you are seeing.
Two suggestions. Either remove any bulk time offset from the measurement before exporting the freq/mag/phase TXT file, or export the impulse response itself as a time signal – TXT or WAV. With time signals, FIR Designer can remove bulk delay without any response error.
Note that freq/mag/phase files are great for plotting, but inherently loose information*, when compared to the time waveform IR.
* unless they are a linear frequency spaced FFT data.
Frequency Magnitude Phase
20 74.9498 -76.0055
20.3 75.0916 -78.8764
20.6 75.2281 -81.1653
20.9 75.2503 -82.5894
21.2 75.3959 -82.2123
18500 88.7457 -40505.1289
18750 88.765 -41041.2812
19000 88.7698 -41577.7695
19250 88.7277 -42114.3984
19500 88.6458 -42650.1914
19750 88.6929 -43186.1406
20000 88.7875 -43758.5703
- This reply was modified 10 months ago by EA Support.