Hello Bassik and Katjav,

Kirkeby inverse function creates inverse chirp from forward chirp, before all the trouble begins....

A long sweep from f1to f2has no more bandwidth than a short sweep from f1to f2. Seemingly small FFT can do a lot of windowing. Here is study of room transfer function using FFT bandwidth limited sweeps:

http://waveformfidelity.com/

The windowing produces super smooth spectrum at expense of computational noise, which of course is well below that of the room, speakers, and microphone. The extra fade in method is very quiet, but has a little ripple.... I had not found this prior to the above investigation.

Barleywater

@Barleywater said:

Kirkeby inverse function creates inverse chirp from forward chirp, before all the trouble begins....

Ah, seems I misunderstood your 'before all the trouble begins'. Thought you were talking about correction of the forward chirp, using Kirkeby inverse filtering. The overshoot in a chirp generated with the traditional formula is detrimental to the crest factor. So, the trouble begins with the forward chirp. As Bassik mentioned, this is in Expochirp solved by using an improved chirp formula. The concept was already formulated in an earlier post on this topic, but if you're interested you can find the mathematical definitions in this paper:

http://www.uni-weimar.de/medien/wiki/PDCON:Conference/Pure_Data_implementation_of_an_ESS-based_impulse_response_acoustic_measurement_tool

Also, the C source code of Pd class [expochirp~] is open, so you can even check how it is implemented if you like.

Thanks Barleywater, for your contribution to the topic, and for pointing to interesting articles. It's good to know that Cool Edit Pro has this FFT size limit as well, and that there are ways to get around it with downsampling /upsampling. Although in Pd this would be less trivial as you need to develop your own interpolation routines.

Katja

you need to click 'post reply'

the 'quick reply' box that comes up by default doesn't let you post patches.

it's annoying, but that's the way the forum software works.

Hello,

You can now check a dedicated blog for ExpoChirp:

http://expochirp.tumblr.com/

We are going to update it with the last versions and all the new feature we are planning to include.

Please send us your comments!!!