If your solo tracks aren't in sync after recording then you are experiencing what is known as audio drift. Some minor drift can be expected due to slightly different speeds in the sound cards of each participant. However, this is barely perceptible.
What about more severe cases of audio drift?
Recent changes in the way browsers prioritize processes has introduced some audio performance issues, leading to drift severe enough that a Host and their Guests appear to be speaking over one another.
This syncing issue is related to CPU usage on one or more of the participating computers and is most prevalent in older or slower computers. Running lots of programs in the background can hog enough CPU that there isn't enough processing power left over to record seamlessly.
Tiny bits of audio will drop out throughout the recording, adding up over time, leaving one track shorter than the other and the two tracks out of sync. The effect becomes more and more noticeable and is most pronounced at the end of the recording.
What is Zencastr doing to remedy the issue?
Recent Chrome updates have included incremental fixes for this issue. Zencastr has also released updates to reduce the amount of CPU our app demands. These updates have improved or solved this issue for most users, but there are still sporadic reports of folks running into audio drift under certain conditions. Most remaining cases occur when someone on the recording is:
- running a 48000 sample rate
- using an older machine
- running an unsupported operating system
- otherwise recording while the CPU load is high enough to trigger drift.
What can users do to prevent audio drift?
Since audio drift is related to how the browser manages CPU and since there is a threshold at which this kind of syncing issue will be triggered, most users have been able to mitigate the issue by using the most modern, powerful computer they have access to and taking the following steps to minimize CPU demand:
- Close any unnecessary programs while recording and ask your guests to do the same. This would include lots of unnecessary program running in the background. https://www.youtube.com/watch?v=Uusm3HGSo60&t=6s
- Make sure both your input and output devices are set to a sample rate of 44100, as using another sample rate will require us to resample the audio and that is taxing in terms of CPU. See how here: https://zencastr.zendesk.com/hc/en-us/articles/115002733754-How-to-change-the-sample-rate-of-your-audio-device-microphone-
- While you're setting your sample rate, you'll see an option to select Buffer size as well. Set this as high as possible to conserve processing power. This will increase latency if you're monitoring your own voice while you record, so it would be best to skip monitoring.
- As, always, make sure everyone's browser is up to date prior to recording. https://www.whatismybrowser.com/
How can I correct a track that's already out of sync?
The files can be edited to bring them back into sync using an audio editor to copy and paste bits of silence into a few key spots to line things up.
For example, if you can see that the Guest is coming in 1.25 seconds before the Host stops speaking, you can copy 1.5 seconds of silence from the Guest track and paste it back into the Guest track just before they start speaking. This will adjust everything from that point forward and reduce the amount of drift present at the end of the recording by 1.5 seconds.
You'll typically need to repeat this step a few times down the line until you reach the end of the recording where everything will now be lined up. See an example of what this looks like here:
Will Automatic Postproduction correct audio drift?
While Automatic Postproduction does balance out volume levels and reduce hum, running the tracks through Postproduction won't correct audio drift. The tracks will simply be combined and lined up based on the 0m 0s time of each track. So, the drift will still be present.
That said, you can run the Postproduction with the advanced option for "Separate Wav Tracks" selected. This will run the tracks through Postproduction and give you a mix (which may be out of sync if drift is present) and it will also give you separate tracks in a zip file sent to your Dropbox account. This will allow you to sync the tracks after the Postproduction has been run with all the levelling etc.
As always, if you still need assistance please contact our support team at firstname.lastname@example.org