Subject: Re: Optimal sweep duration for BRIR measurements From: Brian FG Katz <brian.katz@xxxxxxxx> Date: Tue, 28 Jul 2015 10:37:31 +0200 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>This is a multipart message in MIME format. ------=_NextPart_000_00F9_01D0C921.1C9EFE70 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Hi all, I've been asked to clarify the point I was making in the following part = of my last message > Regarding distortion of the source, this should be an issue for the processing element (different from burning out you speaker). This is = because one of the strengths of the sweep method (when done correctly) is that = any harmonic distortion components of a higher frequency that the excitation signal at the time [are] folded back to BEFORE the direct sound after deconvolution by the excitation signal (as that frequency has yet to be generated). Due to a typing error, one word was wrong which was maybe confusing (corrected above). The principle I can try and explain again, though = without images it is not always easy. So, here goes:=20 Let's assume our reference/excitation signal is a linear frequency = ascending sweep. Looking at a spectrogram this is a nice line at 45=B0. For the = recorded response signal, each frequency excites the room and there is a = reverberant tail for each frequency which can be seen in the resulting spectrogram. = When you perform the deconvolution of the response by the stimulus, the = diagonal of the sweep excitation is the reference and the resulting response is = not an impulse response relative to that excitation. The result is that the sweep signal diagonal is in some sense rotated to a vertical impulse (again, thinking in terms of a spectrogram). With the direct sound for = all frequencies now being time aligned.=20 Now, if we look at transducer distortions, these are generated at frequencies higher than the excitation signal, meaning a speaker with a signal at 1kHz can produce distortions at 2kHz, 4kHz, etc. When the = sweep is rotated or folded back to the vertical through the deconvolution, these distortions are also moved. As they occur BEFORE their corresponding frequency excitation in the stimulus, they will occur BEFORE the direct sound in the RIR, appearing as pre-RIR. These need to be windowed out in = any processing step when the RIR is to be used. This effectively removes the speaker distortion from the measurement.=20 However, if one is interested, one can also analyse these pre-RIR as = they provide information on the distortion of the speaker (or other devices). With a correctly designed sweep, this is a powerful tool, as one can now model a non-linear distorted IR (see the paper I referenced earlier).=20 Of course, one should try and minimize the level of distortion in the = sweep excitation when played, as this isn't too friendly or efficient for the speaker. An advantage of the sweep over other methods is that the = speaker is only generating a single frequency at a time, and is therefore more efficient in level than for the same input at broadband; speakers like = sine waves, and you get more bang for your buck with sweeps.=20 Hope this helped anyone who was interested. -Brian FG Katz LIMSI-CNRS ------=_NextPart_000_00F9_01D0C921.1C9EFE70 Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="smime.p7s" MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIIL2DCCA20w ggJVoAMCAQICAQAwDQYJKoZIhvcNAQEFBQAwLDELMAkGA1UEBhMCRlIxDTALBgNVBAoTBENOUlMx DjAMBgNVBAMTBUNOUlMyMB4XDTA5MDEyMTA4NTExM1oXDTI5MDEyMTA4NTExM1owLDELMAkGA1UE BhMCRlIxDTALBgNVBAoTBENOUlMxDjAMBgNVBAMTBUNOUlMyMIIBIjANBgkqhkiG9w0BAQEFAAOC AQ8AMIIBCgKCAQEAuLp3q1edEiUZFUSKI1XaYw0GbVnrwQSgF3kS8yeCSOqNvjzLNjV4GwTVKQO1 aUgp+6ena8Tc0ikQ8+JOlaUArUl1whzrIjjNmQh/8vEkqvEqTQxss6Iidiv7d7ZgPD2IV6nrrxal 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