Current time-stretch algos applied to impulse responses ?

[Krakatau]

Let me apologise first because i already opened a very similar topic at least once in this forum

But i was thinking more closely on the problem and one point that appear to my mind was : what might currently be the best time-stretch engine that doesn't create unwanted artifacts while applied to such spaces ?

Questions of psychoacoustic consideration : specialised time-stretch algorithm suitable for these peculiar types of signal/sounsource...different cases if you want to independently modify monophonic signals or polyphonic instrument signals for instances !

[CinningBao]

This is a slow topic..

But I've also considered this over the years, and to my knowledge, there are no time stretching algorithms which do not produce artifacts or uncontrollable phase-shifts in some or all frequencies allowing for IR modifications, unless one seeks the artifacts themselves! Few enough artifacts to not affect the source in unwanted ways can, I doubt, be achieved. If it were I suspect one or more of the major impulse response players (AudioEase, probably Waves) would have this as a feature.

At most, and Space Designer offers this, is a hard ratio driven multiplication of the file; double, or half, or quarter, which obviously results in the impulse changing character and tone. No phase issues will occur this way, as you probably know.

So yeah, no, one can't stretch/shrink impulse responses succesfully at the moment. It may be that IRCAM or some clever boffins at a DSP research institute somewhere have developed the perfect time-stretching algo suitable for this kind of operation and have yet to work out how to monetise it!

Although I am ALWAYS willing to be corrected

[Krakatau]

interesting answer ...thanks !

[Sampleconstruct]

Meldas convolution reverb has a timestretch function.

[CinningBao]

No probs Krakatau Gotta share what we learn..

Simon, I'm not a Melda user so I didn't realise this.
Would you say it is suitable for the expected detail of convolution reverbs?

[Sampleconstruct]

No probs Krakatau Gotta share what we learn..

Simon, I'm not a Melda user so I didn't realise this.
Would you say it is suitable for the expected detail of convolution reverbs?

The times I used the stretch function, it always produced satisfactory/good results.

[CinningBao]

Interesting - the manual says:

Code: Select all

Stretch
Stretch resizes the impulse response. As a result you may observe not only a change of length, but also a change of pitch.

but also

Code: Select all

Override length
Override length changes the length of the IR to the specified length. It can be used for various creative effects, but the most typical use is to synchronize an IR consisting of a loop to the current tempo. It is ignored when the length is minimum, hence 0ms, or OFF. Please note that since its operation is the same as the Pitch shift parameter, it is strongly advised not to use both parameters at once.

Pitch shift
Pitch shift changes the length of the IR and as a result causes a pitch shift specified in semitones. Please note that since its operation is the same as Override length  parameter, it is strongly adviced not to use both parameters at once.

So my guess is that it's just slowing down/speeding up the IR like a tape/record - it doesn't look like there are any complex time-stretching algos present. Since, though, most reverb IRs just sound like noise, speedingup/slowingdown a little bit wouldn't affect the result too much, but extreme settings will be fun and experimental!

[cturner]

There are folks using PaulStretch on IRs.

[valhallasound]

I would stick with "repitching" the impulse (i.e. interpolation so that it plays back at a slower or faster rate), possibly combined with some filtering and/or nonlinear harmonic enhancement to recreate the original frequency response.

Most time-stretching algorithms, that seek to preserve the pitch of a signal while changing the duration, make the presumption that the signal is largely harmonic. Impulses are usually inharmonic. So the types of tricks used to time-stretch harmonic signals (i.e. tracking the period of the signal to determine best splicing point, resynthesizing the signal using a sum of sinusoids) would fail on inharmonic signals.

IIRC, Jonathan Abel (of Universal Audio fame, currently a researcher at CCRMA) has done some recent work on extending impulse responses by calculating the frequency response in multiple bands, and using Gaussian noise to resynthesize the tail. This approach would probably be the best for standard reverberation purposes, but I don't know if it has made it out of the research stage.

Sean Costello

[Krakatau]

Most time-stretching algorithms, that seek to preserve the pitch of a signal while changing the duration, make the presumption that the signal is largely harmonic. Impulses are usually inharmonic. So the types of tricks used to time-stretch harmonic signals (i.e. tracking the period of the signal to determine best splicing point, resynthesizing the signal using a sum of sinusoids) would fail on inharmonic signals.

exactly...this is the crucial point

IIRC, Jonathan Abel (of Universal Audio fame, currently a researcher at CCRMA) has done some recent work on extending impulse responses by calculating the frequency response in multiple bands, and using Gaussian noise to resynthesize the tail. This approach would probably be the best for standard reverberation purposes, but I don't know if it has made it out of the research stage.

Sean Costello

Thank you Sean for all the relevant infos, instructive and much appreciated

[valhallasound]

IIRC, Jonathan Abel (of Universal Audio fame, currently a researcher at CCRMA) has done some recent work on extending impulse responses by calculating the frequency response in multiple bands, and using Gaussian noise to resynthesize the tail. This approach would probably be the best for standard reverberation purposes, but I don't know if it has made it out of the research stage.

Sean Costello

Thank you Sean for all the relevant infos, instructive and much appreciated

My pleasure! Here's some slides from Nicholas Bryan and Jonathan Abel on extending impulse responses:

https://ccrma.stanford.edu/~njb/researc ... Slides.pdf

[Krakatau]

Here's some slides from Nicholas Bryan and Jonathan Abel on extending impulse responses:

https://ccrma.stanford.edu/~njb/researc ... Slides.pdf

Thank you, seems very appropriate to enlight even relatively profane minds to the subject,



i might find the time to read this carefully...excellent !

[CinningBao]

Great thread, with some great info, thanks Sean, as well.

Fascinating research too, so will UA be the first to implement a good IR stretching technique?