Talk:Tutorial - Multi-track Overdubbing

Archived section on Latency - we will link to Latency Test instead

Setting the Recording Latency

Click "Home" then click "Record" and you will get a new recording underneath the first one. Sing or perform in time to the first track; when finished click on "Stop" then "Home".

The show will have two tracks, one from each performance, but it may be seriously out of time or rhythm -- even though you were in perfect time when you recorded it. This is recording latency and you can adjust it to zero using Audacity's latency tools; done properly, both the live recording session and the later playback will be in perfect time.

• Choose File > Close click "Don't Save".

• Choose File > New.

• Choose Generate > Click Track click "OK".

• Audacity Preferences > Recording set "Latency Correction" to [ 0 ] milliseconds, click "OK".

• Unplug the headphones. Turn the UCA202 headphone volume about 2/3 of the way up. Switch the Monitor to OFF.

• Plug an audio adapter cable between the Headphone jack of the UCA202 and its Line-In jack:

• Click "Record". Track one's click track is now being recorded onto track two through the adapter cable -- good fidelity or volume here is irrelevant.

• Do that for five or ten seconds and click Stop.

• Reduce the volume a bit and plug the headphones back in and put them on.

• Select the new track and choose Effect > Amplify... (accepting the defaults) click "OK".

• Click "Play", both tracks will probably play out of step.

• Magnify the Timeline around one of the pair of clicks (drag-select and <CTRL+E> or CMD+E>).

• Drag-Select the distance between the start of the click on the top track and the start of the same click on the bottom track.
  

That's how much the rhythm misses and that's the latency. Keep magnifying until you can get a good shot at accuracy. Use CTRL+3 or CMD+3 to back out slightly if you magnify too much by accident.

• At the bottom of the Audacity window in the Selection Toolbar set the middle time control to "Length" (one of the two radio buttons) then change the format using the dropdown menu to:

hh:mm:ss: + milliseconds.

You're mostly interested in the milliseconds -- the last numbers on the right. The reading in the example above is 209 msec.

• Audacity Preferences > Recording... set Latency Correction to the negative of the number in the Length dispaly - in this case -209 milliseconds - then click "OK".

• Go through the whole process again; plug the adapter cable back in and so on.

This time the two click tracks should look perfectly on (or very close to it) and sound perfectly in time. If not, zoom in, measure the new difference and add that number to the latency value.

In this example, the tracks align to within 7 samples, which is about 0.15 milliseconds - much less than the smallest correction you can make in the Latency Correction. This is as good as it gets.

Before you get too obsessive about this, an orchestral musician once said that the chances of any two instruments in the orchestra starting the same note at the same time is zero, so you don't need to adjust things down to the digital sample level. The latency values on home computers can wander in normal use.

• Remove the audio adapter cable from between the Headphone jack of the UCA202 and its Line-In jack; plug the headphones back in.