Accessing Musics Samples

[Foaly]

Hello everybody,
my questions is if it is possible to access the samples of a music file in real time. I'm not sure if it's clear what I mean, so I'll explain what I want to do. I would like to get the samples of a music file currently being processed, so I can do some sound manipulation in real time (while it's being played). I know this is possible with SoundBuffers, but I want to use bigger files (i.e. SoundStreams / sf::Music). Is there any way I can do this with SFML? And if not is there any way I can do this using as much of SFML convenience as possible?
Thanks in advance
Foaly

I am on a Windows machine using the current SFML2 rc btw.

[Laurent]

class MyMusic : public sf::Music
{
protected:

    virtual bool onGetData(sf::SoundStream::Chunk& data)
    {
        bool result = sf::Music::onGetData(data);
        // ... do something with data.samples ...
        return result;
    }
};

(SFML 2)

But with this code, you'll process samples as they are loaded into the music's internal buffer, which is slightly before they are played.

[Foaly]

Ok, perfect! Thank you for the *really* quick reply!
First of all I wanted to implement a beat detection algorithm i found here: http://www.flipcode.com/misc/BeatDetectionAlgorithms.pdf
It uses a window of 1024 samples to stay in real time. How could I get 1024 samples at a time, while still staying in sync with the playing? (right now the sampleCount in onGetData is 88200, which is like 2 seconds)
thanks again
Foaly

[Laurent]

I guess there's nothing much you can do with the current API.

You could try to modify the internal sample size: at line 145 of Music.cpp, you can try:

m_samples.resize(1024);

1024 is not a lot so I don't know if the music will still play smoothly. And it will still not be perfect, you'll still be 1024 * 2 samples before what's played (SFML streams are triple buffered).
I don't know if true real-time is achievable without specialized audio APIs (or even hardware?), to be honest I have never thought about this kind of use case.

[SoleSoul]

I am also interested in this question.
What I'm trying to do is to create bars visualization of a playing music file. For that I'm intending to use kissfft in order to get the frequency spectrum of the current sounds.

I thought about doing it in onGetData but the long interval between calls does not suit my needs too.
I'm still trying. If I'm able to do it I'll tell you.

[SoleSoul]

After some fiddling I was able to draw the waveform of the loaded chunk.
Since the chunk is much bigger than the time window of a frame I divide the chunk by framerate and draw the waves sequentially.

The problem is it seems that there is a delay between loading a chunk and playing it so if I start drawing the chunk when it loads I draw ahead of time. Do I have to access another buffer of samples?

[SoleSoul]

I resorted to using SoundBuffer and loading the whole file to memory for now. It works and the synchronization is good.

If I'll find a way to do it with a stream in the near future I'll post here.

[Laurent]

If I'll find a way to do it with a stream in the near future I'll post here.

I don't think you can get correct synchronization, there's no way to know when a chunk of samples will be played -- except by looking at the source code and adding one or two ugly hacks to your code.

[SoleSoul]

Thanks.
For this functionality I would have to use another library in conjunction with SFML then. It is ok I guess. Any idea where I should look first? LibSndFile? Portaudio? I don't really know them.

[Laurent]

I have no idea. SFML uses libsndfile + OpenAL, so I guess you won't have better results with these libraries; unless you choose a different strategy, but I don't even know if there's a good alternative.

I don't know other audio libraries, except FModEx which is an audio engine and has many many features.

[SoleSoul]

Ok. Thanks for your responsiveness.

I'll post my findings if I succeed.

[Phanoo]

Look at portaudio for real time applications

[SoleSoul]

Ok, thanks.