linuxaudio.org

On Thu, September 30, 2010 3:47 am, Robin Gareus wrote:

quoted text
> On 09/30/10 09:40, Patrick Shirkey wrote:

>>

>> On Thu, September 30, 2010 12:01 am, Louigi Verona wrote:

>>> Hey guys!

>>>

>>> I have two questions.

>>>

>>> 1. How does Sound Stretch work? It is incredible the way it can produce

>>> a

>>> tone which has no noticeable vibrations, just a wall of sound. How is

>>> that

>>> accomplished, in layman terms if possible :)

>>

>> It grabs a section of the audio and copies/multiplies it then appears to

>> apply some pretty neat math to smooth it all out funnily enough using a

>> selection of windows to get the best result.

>

> Are you sure? Where did you get that info from?

>

> Just skimming over Stretch.cpp gives me the impression that it's based

> on Fourier analysis and re-sythesis.

> http://en.wikipedia.org/wiki/Fourier_analysis

>

> The process() loop looks like:

> {

>   apply_window();

>   smp2freq(); // Fourier Analysis

>   process_spectrum();

>   freq2smp(); // Fourier Synth

> }

>

> In layman terms:

>

> There's a smart French guy by the name of Joseph F. sitting inside it:

> If you play him some audio: He thinks: "Hey, this is actually just a few

> simple sine-waves added together (superpositioned)", he quickly

> calculates their frequencies and amplitudes and asks "Now, you want to

> change the duration?" easy: "I'll generate some new sine-waves with

> these frequencies and amplitudes, how long did you say you want?"

>

> (The smart thing about this French guy is that he actually speaks fluent

> English - Sorry I could not resist :)

I suppose you missed the various windows he has added. Maybe they are just

there for show?
>

quoted text
>>

>>> 2. Can this program be jackified and is that a lot of work?

>>>

>>

>> It uses portaudio. Doesn't that have a jack output?

>

> Yes, but its implementation is not very well done.

>

>> Otherwise yes, he has designed it so that adding jack support would be

>> fairly trivial.

>>

>

> Indeed, to change the integrated player to output to JACK would be

> trivial. It currently uses the PortAudio's StreamCallback which is very

> similar to JACK's process callback.

>

> Using it to do "live" timestreching with JACK (jack-in -> jack-out) is

> AFAICT impossible because:

>

>

> If you feed it N samples (or seconds) of audio you end up with M samples

> (or seconds) with  M > N.

>

> One could do a kludge:

> eg. for a 1:10 time stretch with continuous output:

>   - read 1 sec of audio from the input

>   - ignore 9 secs of the input

> but I don't think this will be useful.

>

>

But nothing wrong with having it jackified for ease of use. Especially if

the portaudio code is as you suggest. No doubt someone will take the time

if it is a good piece of software otherwise.
--

Patrick Shirkey

Boost Hardware Ltd.
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