I’m having some latency issues in Audacity. I record using Audacity 2.0 on multiple machines (A Windows 7 box, a Windows XP box, a Windows XP Virtual box running on a Windows 7 box, and a Mac OS X 10.6.8 box) so I can have multiple microphone inputs to record my drums. After recording, I save all of the audacity files onto my flash drive and copy them to my Mac to be compiled, edited, etc. (which is why this is posted on both the Windows and Macintosh boards). I’ve noticed that with recordings that last longer than about 5 minutes, the tracks start to get out of sync — not in a uniform fashion either; certain tracks appear to play back slower than others and the out-of-sync audio becomes apparent at different times per track. I’ve tried to fix this using the “Change Tempo” feature, but it has been very difficult to estimate how much I should speed up or slow down the track.
Is there any reason for this latency? Perhaps it is a bug that can be addressed in a future update.
Also, this is not new to Audacity 2.0. I’ve had the same issue in previous versions of Audacity.
And you’ll get the same problem with any other recording software.
Sound cards measure the analogue input signal and output the level of that signal as a number (the sample value). They do this thousands of times per second (the sample rate). In order to output the correct number of samples per second, they have a “clock” that tells the sound card when to take the next measurement.
The problem is that sound card clocks, like all clocks, are not exact. Expensive, high quality sound cards usually have very accurate clocks, but they are never “exact”. This means that if you have recordings from two different sound cards, it is very likely that over time the audio data from them will gradually drift out of synch.
The way that professional sound cards get round this problem is by having one external master clock for all the digital devices in the system. This “master” clock is called a “word clock” (http://en.wikipedia.org/wiki/Word_clock) and they tend to be very expensive (e.g. http://www.thomann.de/gb/apogee_big_ben_studio_clock.htm )
Without a clock signal that is common to all of the sound cards, it is inevitable that they will gradually drift out of time with each other. How fast they drift depends on how accurate their clock signals are.