Play a sound of indefinite duration on the iPhone

Question

I need to play a "horn" sound when a user touches the screen.

The horn sound needs to play for as long as the user's finger remains in contact with the screen.

The ending of the sound effect will be different. Imagine the horn "petering" out at the end as airflow decreases.

Would I have to stitch 2 sound files together? Do I need to use Remote I/O since the timing will be so critical?

This library looks like a possibility: http://www.surina.net/soundtouch/

It seems like I could play a sound indefinitely, but I haven't implemented it yet, so I am not sure.

I feel like this would be a common problem in game programming, but I have little experience in that area.

I've never used audio to this extent before so I am at a loss of where to begin. (I have only required the use of AVAudioPlayer in my apps until now).

Any suggestions on a library to use or at a technology to use would be great.

Answer 1

I'm not experienced with audio programming on the iphone. I think you can create a good duration sound file that includes the final 'petering out' sound at the end. What you can do is, as long as the user is touching the screen, play the sound file and keep seeking back to the beginning as soon as you get to the beginning of the petering out, effectively looping it. If the user stops touching the screen, then you can stop seeking to the beginning and finish playing the entire sound, which finishes off with the fade out effect (Or for a more immediate reaction, seek to the beginning of the fade out).

I'm not sure if seeking like this is possible, but that would be one way to do it. Sorry if I misunderstood your question.

Answer 2

To make it realistic, you need three sounds:

1. Attack. This is the start of the sound; it is the most important part of the sound, as it is the part that the brain uses to identify what the sound is. The sound then decays slightly, and becomes the...

2. Sustain. This is the part of the sound that you will need to loop continuously until the user lifts his finger. You will have to craft this part of the sound carefully to avoid "ticks" or other artifacts while looping the sound.

3. Release This is the sound that you play when the user lifts his finger.

There are audio tools such as Audacity that will make the creation of these three files easier. You can use an actual recording of a horn to create the three sound pieces.

Answer 3

Achieving this with a recording of a horn (as a WAV or MP3 file or whatever) is a relatively difficult programming task, especially for someone who's never worked with audio before. If the iPhone supports MIDI (or if there's an easy-to-use MIDI component that you can incorporate into your application), this would be a more practical route for you to take. With MIDI, you basically tell the synthesizer to start a note at a particular pitch (like you're pressing a key on piano) and the synthesizer will play that note until you tell it to stop playing (part of the stop instruction involves telling the synthesizer how gradually to "release" the note). MIDI synthesizers all support a basic set of 128 instruments (not counting drums) including various horns.

Answer 4

You will want to checkout Audio Queues.

The IPhone-DJ project on Google Code might provide some inspiration.

And there is are a couple of blog entries introducing Audio Queues here and here.