What is the lowest round-trip audio latency achievable with iPhone 4?

What is the lowest input->output audio passthru latency possible with iPhone 4 / iOS 4.2? I just want to take input from the mic and 开发者_如何转开发play it over the headphones with the smallest delay. I'd like to know the theoretical minimum and minimum actually observed, if possible.

An app can usually configure Audio Unit RemoteIO input/record and output/play buffers of length 256 at a 44.1k sample rate. Thus 6 to 12 mS is probably a lowest bound, just from the minimum iOS API buffer filling/handling delay (and not including OS, driver, DAC, amp, speaker, and speed-of-sound-in-air time-of-flight delays).