[audacity4blind] still wrestling with latency and correcting it, help please

  • From: Steve Schnelle <papaschnelle@xxxxxxxxxxx>
  • To: "audacity" <audacity4blind@xxxxxxxxxxxxx>
  • Date: Mon, 24 Mar 2014 13:36:03 -0500


I have some time off of work and promised myself that I am GOING to get this 
latency adjustment thing worked out with Audacity. I can't be that stupid, and 
just think I'm missing one little thing that is screwing me up. This will be a 
rather long email as I'm going to include the parts of the Audacity manual and 
the guide with Jaws as I am a screenreader user. I can't thank you all enough 
in advance for your help and patience and just have to say that I am missing 
the simplicity of my 4 track reel to reel at this time. If anyone takes on this 
endeavor, and feels that they want to take the time, they are welcomed to call 
me by phone or email me privately. Just respond to papaschnelle@xxxxxxxxxxx 
privately. Once again thanks for any help and here goes.


For starters I am running windows XP and Jaws 13 along with Audacity 2.4 at 
this time. Here are the steppes plus comments that I took to adjust latency.

In my preferences menu under recording, play other tracks while recording new 
was checked and the next checkbox about listening while recording was 
unchecked. The manual states to set the buffer for latency at around 100 so 
that's where its set and my correction is set at 0. This is what it shows even 
after generating click and recording the click on track 2. 

From Audacity with Jaws

Generate a click track
  1.. Choose Click Track from the Generate menu.
  2.. The second control is the Tempo [beats per minute] edit box, and its 
default value is 120. Type 60, and then press Tab twice to move to the next 
edit box.
  3.. The Beats per measure [bar] edit box has a default value of 4. Type in 
the number 1, and then press Enter to press the default OK button.
  4.. The click track is created. The track is initially selected, and a time 
range is selected which includes all the audio.
(okay, I did that successfully)

Record the click track
To record the click track being played back through your headphones, you'll 
obviously need to take them off temporarily. If you're using a separate 
microphone, rather than a headset microphone, then position it close to the 
headphones so that it can pick up the clicks. Press R to start the recording, 
and then press Spacebar to stop the recording after a handful of clicks.

(I recorded the click by holding a microphone up to my speaker)

With the first track still being the focus, press Shift + U to mute it, and 
then playback the recorded track. If the clicks are very quiet compared to 
Jaws, then amplify the track:

(had no problem with this, it was loud enough)

Find the position of a click in the recorded track
The following instructions describe how to find the position of the recorded 
click which corresponds to the click which occurs at 1 second in the generated 
track, and then update Audacity's latency correction. It's assumed that the 
original track is still muted from when you checked the level of the recorded 
track above.

(should either track be selected at this time other than track 1 being muted?)

  1.. Press Home to set both the selection start and selection end/length spin 
boxes in the selection bar to zero.
  2.. Press Ctrl + F6 to move to the selection bar. Tab to the End/Length radio 
buttons, and make sure that the Length option is selected.
 (it is and, snap to checkbox is unchecked)

  3.. Then Tab to the Selection Start spin box, and change the format to 
hh:mm:ss + milliseconds using its context menu.

  4.. To set the selection start to 1 second, press End to move to the last 
digit (milliseconds), press Left Arrow three to move to the seconds digit, and 
then press Up Arrow to increment it to one.
  5.. Press Spacebar to listen to the first few clicks. There are two cases: if 
the recorded click is later than the original click at 1 second, then time to 
the first click is much less than the one second spacing of the remaining 
clicks; if the recorded click is earlier than original click, then the time to 
the first click is roughly the same as the one second spacing between the 
remaining clicks. Finding the position of the recorded click, and modifying 
Audacity's latency correction is described for these two cases in the following 
(I get no sound when using entries under the M but when using entries under the 
S I get the first beat.)

  6.. To find the position of the recorded click when it's later than the 
original click, go round the loop of incrementing the selection start by 10 
milliseconds and then pressing Spacebar to check the time to the first click. 
When you've just gone past the click, it will suddenly change to roughly a 
second. Decrement the selection start by 10 milliseconds so that you can still 
hear the first click almost immediately, and then press Insert + Up Arrow to 
read the number of milliseconds. Adjust the latency correction by subtracting 
this number from it. For example, if the initial correction was -130ms, and the 
recorded click was 40ms late, then the latency correction should be changed to 

(I'm sorry, but I am a bit confused in this part because my correction in 
preferences is set to zero at this time and buffer at 100)

  7.. To find the position of the recorded click when it's earlier than the 
original click, go round the loop of decrementing the selection start by 10 
milliseconds, and then pressing Spacebar to check the time to the first click. 
When you move to just before the click, or during it, the click will suddenly 
sound immediately. Press Up Arrow, followed by Down Arrow to leave the time 
unchanged, but to get Jaws to read the number of milliseconds. Subtract this 
number from 1000 to get the number of milliseconds by which the recorded click 
was early. Then adjust the latency correction by adding this number to it. For 
example, if the initial correction was -130ms and the recorded click was 20ms 
early, then the latency correction should be changed to -110ms.
(I am still confused because I can't tell how one can identify whether the 
recorded click is before or after the generated click if the generated click is 
muted. As I move up the amount of digits in the entries in selection start 
under S, it just seems to decrease the count of clicks heard. For example I 
might hear 12 clicks at the (00s) setting and then as I move up to (09S) I 
might only hear 4 or 5 clicks before the sound stops. I can't get any negative 
numbers, it only goes back to zero when using the up and down arrow keys.

Also, if my correction is set to 0 in the preferences menu do you use the 
formula discussed above as far as adding or subtracting numbers in the latency 
correction tab. I know you can enter negative numbers in this area.

Also I don't see anything after this section that tells how to check your work 
and adjust settings to correct for latency.

I also tried the method from the manual using a loop-back cable but that was 
unsuccessful as well. Here's what I did. 

Set the Latency Correction to 0 (zero). 

  a.. Leave the Audio to Buffer amount at its default value. (that was 100
  b.. Click OK. 
The Audio to Buffer amount will affect latency, but the effect will vary 
between systems. 100 milliseconds is a safe value that will not put too much 
load on the CPU. Too low a value may put too much load on the CPU causing 
dropouts in the recording. If you are feeling adventurous you can repeat this 
test with different values for Audio to Buffer. But in the end you should 
settle on a value for both Audio to Buffer and Latency Correction and leave 
them there.

Again Snap to was unchecked and I had the combo box set to length, all settings 
the same as in previous trial.
Zoom in so you can see one of the clicks in the top track and its delayed 
version on the bottom track. Using the Selection Tool drag a selection starting 
at the start of the click in the top track and ending at the start of the 
delayed click in the second track. You should see something like this (your 
numbers will likely be different): 

(don't know how the zoom works for this exercise.) It tells you that you will 
see numbers in the boxes under selection start but mine just show H00M00S00)

Checking the result
Delete the second track by clicking the close box X in its Track Control Panel. 
Click the Zoom to Fit button to see the entire click track. Click in the Track 
Control Panel of the remaining track to select it, then click the Record 
button. After the recording is finished Audacity will apply the latency 
Correction by pushing the newly-recorded track back by the Latency Correction 
amount. Note the two arrows at the left end of the second track - they are 
warning you that latency correction has been applied and part of the recorded 
track has been pushed left so that it starts before 0 seconds. This is OK, it 
is what we wanted to happen. The arrows are there to remind you that latency 
correction has been applied, just in case that's not what you wanted. 

(I know this is a visual test for latency correction but seemed to have some 
good explanation.)

 I hope you can understand what I've tried to say in this email. Thanks again, 

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