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Hey all, I've got a method of recording that writes the notes that a user plays to an array in real time. The only problem is that there is a slight delay and each sequence is noticeably slowed down when playing back. I upped the speed of playback by about 6 miliseconds, and it sounds right, but I was wondering if the delay would vary on other devices?

I've tested on an ipod touch 2nd gen, how would that preform on 3rd, and 4th as well as iphones? do I need to test on all of them and find the optimal delay variation?

Any Ideas?

More Info: I use two NSThreads instead of timers, and fill an array with blank spots where no notes should play (I use integers, -1 is a blank). Every 0.03 seconds it adds a blank when recording. Every time the user hits a note, the most recent blank is replaced by a number 0-7. When playing back, the second thread is used, (2 threads because the second one has a shorter time interval) that has a time of 0.024. The 6 millisecond difference compensates for the delay between the recording and playback.

I assume that either the recording or playing of notes takes longer than the other, and thus creates the delay.

What I want to know is if the delay will be different on other devices, and how I should compensate for it.

Exact Solution

I may not have explained it fully, that's why this solution wasn't provided, but for anyone with a similar problem...

I played each beat similar to a midi file like so:

while playing:

do stuff to play beat

new date xyz seconds from now
new date now

while now is not > date xyz seconds from now wait.

The obvious thing that I was missing was to create the two dates BEFORE playing the beat...

D'OH!

Daniel G. Wilson
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  • "Hey all, I've got a method of recording that writes the notes that a user plays to an array in real time." You're going to have to explain that a little bit more clearly. – Joshua Nozzi Nov 24 '10 at 16:50
  • Sorry, the main point was that I want to know whether the delay will be different on multiple devices. I'll stick some more info in now. – Daniel G. Wilson Nov 24 '10 at 16:51
  • What mechanism are you using to generate your "Every 0.03 seconds"? You say it's not a timer, so how do you determine that the appropriate amount of time has passed (spin-loop checking the clock?) – David Gelhar Nov 24 '10 at 17:14
  • It's almost exactly what apple does with its metronome example. – Daniel G. Wilson Nov 24 '10 at 17:59
  • NSDate *curtainTime = date + xyz seconds, then NSDate *currentTime = date. Then sleep for xyz seconds until currentTime is > than curtain – Daniel G. Wilson Nov 24 '10 at 18:00

1 Answers1

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It seems more likely to me that the additional delay is caused by the playback of the note, or other compute overhead in the second thread. Grab the wallclock time in the second thread before playing each note, and check the time difference from the last one. You will need to reduce your following delay by any excess (likely 0.006 seconds!).

The delay will be different on different generations of the iphone, but by adapting to it dynamically like this, you will be safe as long as the processing overhead is less than 0.03 seconds.

You should do the same thing in the first thread as well.

Getting high resolution timestamps - there's a a discussion on apple forums here, or this stackoverflow question.

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Alex Brown
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