how to use playAtTime 

i need play 2 beat with different time ,first on the 1s, tow with 3s .
how to use playAtTime.

i try use  
```

 [loop1 playAtTime:AETimeStampNone];

 AudioTimeStamp time = AETimeStampWithSamples(5 *self.saveRenderer.sampleRate);

[loop2 playAtTime:time];

```
loop1 work done!
but loop2  it not work!