-
Root cause of 99.9% audio sync drift problem?
I’ve encountered the “99.9%” problem whereby 2nd system audio drifts slightly slower than 29.97 fps video on long takes. Amount of drift: video is shorter by 99.9%, alternatively audio is longer than video by by 100.1%
In this situation video was 720×480 29.97 fps from an old JVC GZ-MG155, audio from a Korg MR1000, 16bit 44.1kHz. CS 5.03. Project was DV 41.1Khz, 30 fps drop frame.
There’s lots of discussion about the 99.9% sync drift problem, but no definitive explanation of the root cause (that I understand). Of course I’d like the recommended workflow to fix it, but am also interested in understanding the underlying technical cause.
E.g, if video both records and plays back at 29.97, whether drop frame time code or not, 1 hr of recorded video = 1 hr of elapsed time. E.g, 1 hr of 60 fps video if played back at 60 fps takes 1 hr elapsed time.
Likewise whether audio sample rate is 44.1Khz or 48Khz, provided it plays back at the recorded rate it will stay in sync with elapsed wall clock time (plus or minus drift precision).
This situation isn’t drift from lack of timebase precision — audio drifts slower than video by exactly 100.1%. This is reproducible and consistent. The amount of drift is the exact proportion of 29.97 to 30.00 so this implies the cause, but I don’t understand why.
Could someone help explain the root cause of this problem?
-
4 Replies
-
Hi Joe,
Video timing must be absolutely correct – if it were off even a tiny bit, the picture would be messed up! Thus, more precise (expensive) timing components are used in camcorders. With digital audio recorders, no one would ever notice if the speed were off by 1%…unless they are trying to sync it to their video, like you and I are!
I have some old iRiver MP3 recorders, which I dearly love for miking multiple sources at weddings because of their tiny size, but they have pretty serious drift issues which require me to re-sync to the video timeline at least every 10 minutes. I also have a more recent Tascam recorder, and while the sync is much tighter than the iRiver units, still needs to be tweaked as I move down the timeline.
Don’t know that there is any way around it.
Jeff Pulera
Safe Harbor -
[Jeff Pulera] “more precise (expensive) timing components are used in camcorders. With digital audio recorders, no one would ever notice if the speed were off by 1%”
Jeff, the audio in this case was recorded by a Korg MR-1000 professional audio recorder: https://www.bhphotovideo.com/c/product/485879-REG/Korg_MR1000_MR_1000_1_Bit_Professional.html
The 99.9% audio sync drift problem when using 2nd source audio is frequently blamed on a mis-match between 29.97 and 30 fps. This cannot be the case since video recorded at 29.97 is played back at 29.97. Whether video is recorded at 24, 29.97, 60 or 120 fps, if it’s played back at the same speed, the playback rate matches real time. Likewise whether audio is sampled at 44.1 or 48Khz if it’s played back at the same rate it also matches real time.
The video and 2nd system audio should only drift by the differences in their time bases. IOW you could hit PLAY on the camcorder and audio recorder, and they’d only drift based on their differences in time base precision, the same as two quartz watches drift apart in time.
That is not the case here, when the video and 2nd system audio are played back in Premier Pro. Audio is drifting slower at *exactly* 100.1% of the video rate — the exact proportion of 29.97 to 30.00.
It’s almost like Premiere Pro is erroneously applying some NTSC timing alteration to the audio or video tracks but not both.
Of course I can fix it with a rate strech, but I’m trying to understand the underlying cause of the problem.
-
OK after re-checking this I found my above statement about 99.9% drift is in error. I thought I’d measured that amount, but it’s actually much less, about 0.16 sec after 58 min, or 99.995% drift. This is within the expected timebase variance of the camcorder and audio recorder. The solution here is just use rate stretch to fix it, use more precise devices, or use common timecode. Sorry about the incorrect statement.
-
I had a problem with a comedy special I was editing. It started out in sync and over time drifted. I realized that the video was VFR or Variable Frame Rate and I changed it to Constant Frame Rate and it solved my sync issues.
Reply to this Discussion! Login or Sign Up