I’m working on a project that requires me to listen to two hours of radio, take notes of the times when people start and stop speaking and then put it all in an excel file afterwards.I took all my notes in hours:minutes:seconds based on the time shown on the computer and for a backup recorded the entire broadcast in Audacity as I listened to it using Windows WASAP as a host. I’m using Audacity 2.0.5 which I believe is the latest version and my laptop is running Windows 8.1.
Here’s my problem. I wanted to sync the beginning of my Audacity recording with the time on my clock, so if someone starting speaking at 1:00:32 in my notes, I could put that into Audacity and get straight onto when they started speaking. I got it synced at the start but throughout the recording the time goes off by several seconds until at the very end my notes are a whole 9 seconds off the Audacity time.
So for example: Time in my notes: 00:00:10 is synced with Audacity time 00:00:10 but by the time it gets to 02:00:00 in my notes the Audacity time is on 01:59:51. This progressively happened when I compare my notes to Audacity so basically the timing I was using gradually got faster than the timing on Audacity. Is my computer clock going fast by enough milliseconds to cause a gap of 9 seconds over two hours? I checked and my laptop time is set to automatically sync with time.windows.com. If it is wrong how do I fix that? Or could it be measured incorrectly on Audacity?
Also, there’s a team of researchers working on this project. If computer clocks can be faulty what should we be using instead? I ask because I just assumed the clock on my computer would be measuring things adequately.
Feeling very confused! Any help would be greatly appreciated.
The clock on your computer probably keeps good time, and as you say, will regularly be synchronised to an on-line atomic clock.
The clock on your sound card is probably drifting. This is common on the (very cheap) audio devices that are included on motherboards.
To check your computer clock, disconnect from the Internet and check the time at hourly intervals against a known-to-be-accurate clock (such as “peeps” on the radio).
If the time on the computer drifts, it is a hardware fault and nothing you can do about it apart from change the motherboard (very unlikely).
If the time on the computer is running slow, there may be a “coin cell” battery on the motherboard that needs replacing (unlikely unless your computer is very old).
Assuming that the computer clock is accurate (probably is), then you can test the sound card clock by recording for exactly one hour (as indicated by a known-to-be-accurate clock, such as the computer clock if you tested it as above). Does the time shown in Audacity indicate exactly one hour? Then you need to test playback. Play a 1 hour recording and note the start and end times on a known-to-be-accurate clock. Does 1 hour in Audacity play back in exactly 1 hour?
I expect that you will find that recording, or playback or both, are slightly out compared with “real” time.
The worst case is if the speed “drifts” (the speed varies over time). In this case, for time critical work you would need to add a better sound card to your computer.
If the recording is too fast or too slow, but the speed is uniform, then you can “stretch” the recording so that it indicates the correct time in Audacity.
To stretch the audio precisely, you will need to know the length that it should be (in seconds).
Taking your example: “by the time it gets to 02:00:00 in my notes the Audacity time is on 01:59:51”
02:00:00 = 7200 seconds (target-duration)
Use that time in the first line of this code:
(setq target-duration 7200)
(let ((ratio (/ target-duration (get-duration 1))))
(force-srate (* ratio *sound-srate*) s))
To apply the code, select the entire track (from 00:00:00 to 01:59:51), then open the “Nyquist Prompt” effect and copy/paste the code into the Nyquist Prompt text box.
Are you making these notes in real-time, while recording?
Once the file is recorded, the “program time” shown in Audacity or an audio player should be exactly “locked-into” the audio, even if the actual speed is off. Or of course, if you pause the playback.
For example, if someone says “Hello” at the 1 hour marker, you can play-back at half-speed and “Hello” will still occur at the 1 hour marker, even though it’s actually been 2 hours.
No clock is perfect and no two clocks are the same. But if you need a more accurate “soundcard”, an [u]audio interface[/u] should give you better results than a consumer soundcard. And, they shouldn’t be anywhere near 9 seconds off after two hours!
Professionals use a very accurate master clock along with interfaces that have a master clock input to keep all of the hardware exactly in sync. (That’s not necessary for what you are doing.)
I was convinced that it my computer clock and then that it was speed drift from the soundcard. However, I went through and checked that my project rate matched the soundcard recording and playback settings. I didn’t even think I’d changed anything but it appears to be working perfectly on new recordings. I suspect the first recording I did was just at the wrong rate.
Thanks for your help.