CODEWITHSUNDEEP
iosobjective-cruby-on-railsruby
professormeowingtons
professormeowingtons

Reputation: 3514

How to synchronize audio playback on 2 or more iOS devices?

I would like to write a web application that allows me to sync audio playback of an MP3 down to ~50ms, or close enough that the human ear can't detect the difference.

The idea would be that two or more smartphones could each be paired to a bluetooth speaker, and two or more speakers would play the same audio at the exact same time.

How would you suggest I go about setting this up, both client-side and server-side? I'm planning to use Rails/Ruby for backend, and iOS/obj c for mobile dev.

I had though of the idea of syncing to a global/atomic clock on the server, and having the server provide instructions to clients on when to start playing/jump in to an already playing track. My concern is that, if I want to stream the audio, that it will be impossible to load a song into memory and start playback accurately on the millisecond level.

Thoughts?

Upvotes: 0

Views: 1096

Answers (2)

professormeowingtons
professormeowingtons

Reputation: 3514

I'm going with a solution that uses an atomic clock for synchronization, and an external service that allows server instructions/messages to be sent to all devices in close sync.

Upvotes: 0

meaning-matters
meaning-matters

Reputation: 22946

The jitter in internet packet delivery will be too large, so forget about syncing over the internet. However you could check the accuracy of NTP which is still used (I guess, I know that older UNIX's used it) by the OS when you switch on automatic date/time in Settings, but my guess is that it won't be good enough either. But perhaps the OS may also use other time sources like GPS; I'm don't know how iOS does it but accuracy within 20ms is not to be expected. You could create experimental app to check it out.

So, what's left is a sync closer to home, meaning between the devices directly. Of course you need to make sure that all devices haves loaded (enough of) the song, and have preloaded it in AVAudioPlayer or whatever you're using, to be able to start playing immediately. (It may actually not be the best idea to use higher level 'AVAudioPlayer` API's as it may give higher delays, and what more important higher jitter, than lower level API's.)

Here are three ideas (one device needs to be master triggering the start play, the others are slaves that are waiting for the trigger):

  • Use an audio trigger pulse, like a high tone of a defined length and frequency. Then use FFT to recognise this tone.
  • Connect the devices via GameKit Bluetooth and transmit the trigger on these connections.
  • Use the iPhone 4+ flash as trigger: flash in a certain pattern. This would require you to sample the video data which is quite doable and can be very fast.

Upvotes: 1

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