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Okay, sorry to disappoint, but no CSI here exactly. No dead bodies.....yet.
Wanted: critique, commentary or explication of whether something I do makes sense or not.
By forensic I mean determining from musical recordings the relative frequency response of two or more pieces of gear. A couple of examples. Let me say I play the same musical source through two pre-amps and record the output with an ADC. Or perhaps the same digital file through two DACs and record with an ADC. Can you use the music to determine relative frequency response of the gear?
What I do is select a few seconds of the more energetic parts of the music, and run an FFT on it. Often a 4096 point FFT which gets me 2048 frequency bands. I make sure it is done on the same exact parts of two recorded files. I export the values of each band, dump them into a spreadsheet and subtract one from the other. You then get a curve of difference between two pieces of gear.
This seems to work to a fractional db level. Seems quite accurate, repeatable and reliable giving the same results with multiple pieces of music. Also thus far in the few instances I have checked gives the same answer as running a frequency sweep or spot tests of sine waves.
You do have to pay attention to details of level and noise. If the recording has no content at frequency extremes then you can't do a good comparison. You are mostly comparing noise levels so low the results make no sense and are of no use. Conversely the devices under test must be relatively low noise. Otherwise the same issue you are comparing noise rather than response to input signals. So quite often you can only compare over a 50 or 60 hz to 12 or 15 khz range. Still that is most of what we care about.
Also, though this isn't the motivation for me doing this in the first place, it occurred to me later it answers one common critical complaint of measurements. That the frequency response was determined with sine waves and not music. Well so far it seems dynamic music and sine waves or sine sweeps give the same answers.
So what say some of you with more knowledge, training or experience in measurements. Any pitfalls or problems I am not accounting for in this method of measurement?
Wanted: critique, commentary or explication of whether something I do makes sense or not.
By forensic I mean determining from musical recordings the relative frequency response of two or more pieces of gear. A couple of examples. Let me say I play the same musical source through two pre-amps and record the output with an ADC. Or perhaps the same digital file through two DACs and record with an ADC. Can you use the music to determine relative frequency response of the gear?
What I do is select a few seconds of the more energetic parts of the music, and run an FFT on it. Often a 4096 point FFT which gets me 2048 frequency bands. I make sure it is done on the same exact parts of two recorded files. I export the values of each band, dump them into a spreadsheet and subtract one from the other. You then get a curve of difference between two pieces of gear.
This seems to work to a fractional db level. Seems quite accurate, repeatable and reliable giving the same results with multiple pieces of music. Also thus far in the few instances I have checked gives the same answer as running a frequency sweep or spot tests of sine waves.
You do have to pay attention to details of level and noise. If the recording has no content at frequency extremes then you can't do a good comparison. You are mostly comparing noise levels so low the results make no sense and are of no use. Conversely the devices under test must be relatively low noise. Otherwise the same issue you are comparing noise rather than response to input signals. So quite often you can only compare over a 50 or 60 hz to 12 or 15 khz range. Still that is most of what we care about.
Also, though this isn't the motivation for me doing this in the first place, it occurred to me later it answers one common critical complaint of measurements. That the frequency response was determined with sine waves and not music. Well so far it seems dynamic music and sine waves or sine sweeps give the same answers.
So what say some of you with more knowledge, training or experience in measurements. Any pitfalls or problems I am not accounting for in this method of measurement?