1/2 frame difference audio

[David Slingerland]

hello everyone,

I used my 5D and put on a rode videomic, I have been using a transmitter as a second source. I used a clap as reference. In fcp7 i tried to put the audio together, but there seems to be a half frame difference. Does any body see a solution? do I need to get plural eyes?

[Josh Bass]

hi. i have experienced this (but more like 1-8 frames off) also asked a number of the pro sound guys i work with if theyve heard about it. consensus is...cam's own audio is out of sync with picture. pluraleyes wont help because it relies on secondary source (usually audio recorded straight to cam via internal mic or external) to sync to. if that secondary source is out of sync, pluraleyes wont know that and will still sync to it. never found a real solution, just line up audio with video manually. use visual of clapper to tell where audio SHOULD be and move it over. or look at actual lip movements to do the same.

[Jeff Pulera]

I don't know if Final Cut offers anything like this, but with Adobe Premiere, the timeline mode can be changed from frames, offering 1/30 of a second accuracy basically, to "audio units", which works at the audio sample resolution, being 1/48000. So when I try to sync two waveforms and the closest I can get is that "half-frame" spot between frames, I switch to audio units and zoom in and can exactly match the two waveforms perfectly. Then switch back to frame mode and continue editing. Note that the audio is being slid over independent of the video, which remains at 1/30 resolution.

Thanks

[Roger Gunkel]

Dave, if all else fails and you are desperate to get it right, you could re-record the audio through an audiio delay unit, either software or hardware, setting the delay to 1/50th sec for PALl or 1/60th for NTSC. Just use the delay output which will then give you audio shifted by the correct amount.

Roger

[David Slingerland]

Josh, I cant move the audio over it just doesnt line up. I have been shooting in 25P mode maybe I should switch to 50I ? Or could the audo recorder be to blame (the one that is connected to the wireless) Or better I should altogether avoid using audio from the cam.

[Josh Bass]

Sorry, not sure what you mean about not being able to move audio over? are you saying the difference of sync is so slight that moving one frame in either direction still yields the incorrect result?

In that case, like the above poster said, you need to be able to move the audio at the sub-frame level. FCP is SUPPOSED to have a function like this (it's in my v6 book, you have v7). When I tried it it worked less than ideally.

I would recommend taking it into an audio software program like Soundtrack Pro (if you have that, you should) or Logic,or Protools. Something that lets you work with video, and sync it there.

Most folks DO advocate not using audio from the actual camera where DSLRs are concerned as it's generally considered poor quality due to the camera's electronics/preamps. However, if you have to, for some reason, then you'll need to find a way to sync it.

[Roger Gunkel]

As I said in post 4 if you use audio delay, (a software processor will do it), you will be able to delay the audio by 1/2 a frame which will then put the new version exactly 1 frame behind. You then simply drag it forward a frame on the time line, and mute the previous out of sync audio.

I have used this method many times over the the last 30 years.

Roger

[William Hohauser]

From the description, it seems that the wireless transmitter is somehow introducing a 1/2 frame delay although it's not clear how David is integrating the audio with the 5D footage. I am assuming that the Rode mike on the camera is properly synced. If you are dropping in the transmitter audio on a 25p timeline and it consistently is a 1/2 frame off, you could work in 50i or 50p to nudge it in sync. You can also clip the audio files in Soundtrack Pro. Another option is to experiment with FCPX which has sub-frame audio nudging in the timeline.

[David Slingerland]

Thanks everyone, I will look in to your solutions. I just didnt have the time as i had to deliver the films today. I opted for choosing one of the sources. Now I will have a look at it properly.

[Josh Bass]

Looks like FCP7 has access to the AU Delay (look under Audio Filters/Apple). Maybe that will work. I'm surprised you can SEE a half frame delay in sync. Sometimes I can't tell for sure even when it's 1 or 2!

[David Slingerland]

William, "although it's not clear how David is integrating the audio with the 5D footage" I use a clap during recording and then try to put all audiotrack over each other, the audio from the videomic, is different then the audio from the wireless connected to the H4N, about a half frame off.

[Jeff Pulera]

Here's the issue, and this will be the same problem whether syncing two cameras, or an external audio recorder to a camera. Video editing software measure time in frames of video per second, for instance 25 or 30 (okay, 29.97 to be precise) to match the video format, so that is usually the most precise edit shift you can make on the NLE timeline, just 30 increments per second. Shift audio left or right one frame, and it is 1/30 of a second. That's as accurate as it gets. If it often impossible to sync audio this way for a simple reason.

Let's change frames to SECONDS for simplicity. Assume you are watching a clock face and you hit REC on Cam 1 exactly when the second hand jumps ahead. You later start Cam 2 (or audio recorder), again starting EXACTLY on a new second. Assuming the smallest increment on the timeline is again one SECOND, you can move the secondary audio clip over a certain number of seconds and make it sync perfectly to the Cam 1 audio, because both recordings started EXACTLY on a new second, on the same clock, therefore the two signals CAN be synced by moving the proper amount of timing offset in full seconds. Both recordings were synced to a master clock, though starting at different times, both are dancing to the same beat, each recording starting exactly on a new second.

However, let's assume the first camera was NOT started exactly on the second, and the second recorder was also started at random. The chances of the start times being an EXACT number of seconds apart is not good. Maybe 8.2 seconds apart, or 47.8 seconds apart. On a timeline where seconds is the smallest movement to shift the clips, you'll never get the audio to match exactly. They will always be some part of a second off no matter what (maximum half a frame..if more, then shift over a frame one way or the other to make it the lesser half of being off, 3/4 of a second late becomes 1/4 second early when shifted left a frame).

So it is the same thing with frames. 30x smaller increment, but same thing! If the two audio clips are a half-frame apart, you will hear some kind of "reverb" effect, just doesn't sound right. You can shift one of the audio clips left or right a frame, but sync will be early or late, never "right on". The sync lies somewhere in between, at a resolution of less than 1/30 of a second.

With Premiere, you can edit audio at its native resolution of 1/48000 of a second and therefore slide clips over in very very tiny increments and match them exactly to one another (independent of the associated video clip which MUST remain synced at the frame level at best). Zoom in to view the detail and line up the samples exactly, peaks and valleys matching precisely down to the single sample.

If in FCP you feel that the secondary audio is close to 1/2 frame off, assuming 29.97 frame rate, take the audio clip into an audio editor and cut off an appropriate amount of audio to match a half frame. I've never had to do the math because I just line the waveforms up in Premiere, no math involved. If some kind of "audio delay" effect is available in FCP as the other poster mentioned, perhaps that is the answer?

Let's try here, no guarantee of accuracy. 48000 audio samples per second, divided by 30 frames (rounding), equals 1600 audio samples per frame, and we need a half frame, so in the audio editor, trim 800 samples and that should roughly equal the half frame. Save the new, shortened audio clip and import into video editor. Not sure how the numbering works in your sound editing software, so just calling it "800 samples", might need to be converted to decimal part of a second, .0166 second roughly? I knew high school would do me some good...3 decades later ;-)

So given the clock analogy of the recording starts, why does video line up from two cameras and not the audio? Well, it doesn't - it can also be up to a half frame off, but you're not going to visually notice the difference when cutting between frames from two video sources, as the 1/30 second accuracy is good enough. However, the out-of-sync audio can exhibit a buzzing or ringing or whatever you want to call it, which can effect the quality to a noticeable degree.

I'm not sure how PluralEyes works, but unless it can shift things at the sub-frame audio sample level, it could be limited to frame-accurate audio matching, I don't know that. If matching MANY clips, then the above may not be a practical manual workflow. I'm assuming a long event like a wedding cereomony, just one clip to match and then good for the duration.

Long-winded I know, but seemed to be some confusion maybe about where the "lag" comes from when really not lag at all in most cases, just the nature of things.

[David Slingerland]

Josh, I cant move the audio over it just doesnt line up. I have been shooting in 25P mode maybe I should switch to 50I ? Or could the audo recorder be to blame (the one that is connected to the wireless) Or better I should altogether avoid using audio from the cam.

[William Hohauser]

The 1/2 frame difference is curious if it is consistent with multiple takes otherwise it should float a little bit from clip to clip. I usually kill the camera audio after I use it to line up the tracks. There always seems to be a slight delay effect unless I can get the sound right on the money which is hard in FCP7.

You can experiment with sending the H4N output to the camera while you shoot and see if the delay exists still. There may be a slight delay on the wireless system you are using.

[Brian Drysdale]

Just a question having done 16mm film editing, do you actaully notice something being a 1/2 frame out of sync? Just because you can see a waveform showing it being out, is it of any consequence in practise?