What is the best method of synchronizing audio across iOS devices with WiFi?

Question

Basically, for my team\'s app, we need to be able to synchronize music across multiple iOS devices. The first way we did this was by having the music on all the devices already

Answer 1

I think your first way would work if you expand it a little bit. Assuming all the clocks on the devices are in sync you could include a timestamp in your play command. Then each device would calculate the time between the timestamp and when it received the command. You would then play the music and offset it by the time difference.

Answer 2

You'd be amazed at how good the human ear us at spotting audio anomalies...

Sync the time of day

Effectively your trying to meet a real time requirement with a whole load if very variable things in the way (wifi, etc). I strongly suspect the only way you're going to get close to doing this is to issue a 'play' instruction that includes a particular time to start playing. Of course that relies on all the clocks being accurately set.

NTP

I don't know how iPhones get their time of day. If they use (or could use) NTP then you'll be getting close. NTP is designed to convey accurate time of day information over a network despite variable network delays. I've had a quick look and it seems that most NTP clients for iOS are the simple ones, not the full NTP that measures and tunes out network delays, clock drifts, etc.

GPS

Alternatively GPS is also a very good source of time information. Again I don't know if iPhones can or do use GPS for setting their clock but if it could be done then that would likely be pretty good. On Solaris (and I think Linux too) the 1 pulse per second that most GPS chips generate from the GPS signal can be used to condition the internal OS clock, making it very accurate indeed (sub microsecond accuracy).

I fear that iPhones don't do either of these things natively; both involve using a fair bit of electricity, so I wouldn't be surprised if they did something else less sophisticated.

Cell Time Service

Some Cell networks provide a time service too, but I don't think it's designed for accurate time setting. Also it tends not to be available everywhere. You often find it at major airports so that recent arrivals get their phones set to something close to local time.

Play at time X

So if one of those could be used to ensure that all the iPhones are set to exactly the same time of day then all you have to do is write your software to start playing at a specific time. That will probably involve polling the clock in a very tight loop waiting for it to tick over; most OSes don't provide a means of sleeping until a specific time. They do at least allow for sleeping for a period of time, which can be used to sleep until close to the appointed time. You'd then start polling the clock until the right time is reached.

Delay Measurement and Standard Deviation

Your first method is doomed I think. You might be able to measure average delays and so forth but that doesn't mean that every message has exactly the same latency. The standard deviation in the latency will tell you what you can expect to achieve, and I don't think that's going to be particularly small. If so then the message has got to include a timestamp.

NTP can work because it's only interested in the average delay measured over a period of time (hours sometimes), whereas you're interested in instantaneous delay.

Streaming with RTP

Your second method may work if you can time sync the devices as discussed above. The RTP protocol was designed for use in these circumstances; it doesn't help with achieving sync, but it does help a lot with the streaming. It tells you whereabouts in the stream any one piece of received data fits, allowing you to play it at the right time.

Clock Drift

Another problem to deal with is how long you're playing for. If it's a long time then you may discover that the 44kHz (or whatever) audio clock rate on each device isn't quite the same. So, whilst you might find a way of starting to play all at the same time, the separate devices will then start diverging ever so slightly. Over a long period of time they may be noticeably out.

BlueTooth

It might be possible to do something with BlueTooth. It has many weird and wonderful profiles, and it might be that one of those would serve to send an accurate 'start now' message.

Audio Trigger

You might also use sound as a means of conveying a start signal. One device can play a particular sound whilst your software in the others is listening with the mic. When a particular feature is detected in the sound, that's the time for everyone to start playing. Sort of a computerised "1, 2, a 1 2 3 4".

Camera Flash

Should be easy to spot in software...