Latency Compensation

Though it happens very, very quickly, it takes a certain amount of time after the sound is 'picked up' by the microphone for it to pass through the device's analog-to-digital converter and be stored in its memory. Likewise, it takes time between the app sending a chunk of audio data to the audio system and it eventually being heard. This time delay is referred to as latency and can result in tracks being out of sync. 

Imagine, for example, that you have recorded a guitar on track 1. Now, you want to record a vocal onto track 2. You put headphones on to allow you to hear the guitar track as you record your vocal without the guitar track being picked up by the microphone, and sing along in time to the guitar track while recording the vocal to track 2. However, although you sang along in time to the guitar track, the time it takes for the sound to be processed - the latency - means that the vocal track appears very slightly after the guitar track on the timeline and the performances on the two tracks are no longer in sync. This is where latency compensation comes in.  

Latency compensation works by measuring the amount of latency for your particular device and shifting all audio recordings made after the first one backwards on the timeline by that latency amount. This means that the recordings will all be in sync during playback.

Audio Evolution Mobile provides an automatic procedure for measuring latency and applying it as latency compensation, whilst also allowing you to apply manual latency compensation should you need to. When using a USB audio interface with the eXtream USB audio driver however, the app can determine the latency pretty accurately in most cases and there is no need to run this automatic procedure or do manual latency compensation.

Automatically determining latency

The process of measuring the latency for latency compensation can be automated in Audio Evolution Mobile and does not apply to the situation where you use a USB audio interface with the eXtream USB audio driver. If you have already played around with the app, you will have been prompted to do this the first time you pressed Record to record a second audio track into your project (as described above, latency correction is used to keep all additional tracks in sync with each other, hence why the prompt comes when you are about to record the second track). If this has not yet happened, or if you want to carry it out again, you can do so by selecting Determine latency via the More button options on the Arranger Screen. You will then be guided through the process by the onscreen instructions - it's all automatic.

The Determine latency function should be run again if you have changed the sample rate you're using since last running it and if you change the audio system (Oboe or AudioTrack) you're using.

Manual latency compensation

If you still experience that tracks do not line up, even after performing the automatic latency detection, you can try to apply an additional manual latency compensation. However, it should be noted that on some (mostly older) Android devices, latency just varies too much with every take to compensate reliably for.

First we have to measure the track to track latency. The easiest way to set this up is to have a 'tick' kind of short sound on the first track, record it on the second track and measure the time between the recorded tick and the tick on the first track. You could create a drum pattern track as the first track and add a hi-hat or snare hit as the 'tick' sound. Don't put it at the start, but at an offset like 1 second. Then, without any headphones attached, increase the volume of the Android device and press record. Because the volume is high, the audio being played on the first track (the hi-hat/snare sound) will be recorded on the second (audio) track. After it has been recorded, stop and zoom in on the area where you see the tick on the first as well on the second track. The time between these ticks is the latency we want to correct for. To measure this time period, please select the Range button, tap at the time location where the drum hit starts on the first track and drag the blue range to the start of the recorded tick on the second track. Then, with sufficient zoom level, tap once on the middle of the blue range and select 'Use as latency correction'. The latency in audio frames will be saved in the app's settings.