Background playback

The playback latency may increase during background audio playback, even when using SonicSYNC.
The reason is that iOS itself can change the frequency of the audio buffer updates.
This is true in particular when an application plays in the background.

Known issues / phenomena

It has been reported that sound stuttering occurs when using SonicSYNC for playback during Xcode debugging on iPhone 7 or earlier models.
It is presumably due to the extra CPU load coming from the attached debugger, and not enough threads being available on the device.
It has been confirmed that the playback is uninterrupted when no debugger is attached.

Therefore, during development, it is recommended to use a relatively high-performance device such as an iPhone 8 or a more recent model.

Performance Details

Audio latency for each device

Model	iOS version	IOBufferDuration (in seconds)	Using SonicSYNC (o/x)	Minimum latency time (in milliseconds)	Maximum latency time (in milliseconds)	Average latency time (in milliseconds)
iPhone 7	14.4	0.003	o	38	70	51.80
		0.025	x	142	176	164.25
iPhone 12 Pro	14.5	0.003	o	37	72	54.70
		0.025	x	152	185	168.50

Remarks:

The following delays are added to the "latency" described in this section:

• Input latency of the Touch device (depends on the device)
• Playback request delay (depends on Atom's server frequency)
• Audio device's buffer update delay (depends on the device)
• Delay due to the output to the physical speakers (depends on the device)

The Atom server frequency is set to 60 when measuring the performance results.
These results are based on 20 times of testing on each device.

For the specifications of the devices used for the tests, please refer to the websites of their respective manufacturers.