Toby Alington: Sync and the Frame Edge
A little more of a technical article this month, prompted by me receiving some audio files which were very loosely in sync – or in other words “out of sync” – with the accompanying pictures. I keep seeing tales of woe on forums about synchronising audio and video, and so having the benefit of thirty years’ synchronising the two, with the transition from pilot tone and sprockets to timecode and digits, I thought I’d try and help with some of the common problems.
Firstly, a little bit about the Frame Edge. This frame edge in audio terms is the exact point in time where the picture frame starts, and to synchronise pictures and audio you need to know where this point is and maintain it throughout the production process. “Never lose the frame edge” is a Golden Rule, but this frame edge does seem to sometimes disappear through necessity or operator error. Once it’s gone, it’s a real pig to get it back accurately, with hours spent staring at the screen seeing if the audio is in sync or a quarter of a frame out.
Take a location documentary shoot by way of example. Either the audio is going straight into the camera and can be loaded into a workstation using the timecode from the video tape as master reference, or the audio was originated separately. If it was a separate recorder, it should have been receiving timecode from the camera. If not, then on the shoot they should have used the good old clapper board for you to align the audio back to the frame edge of the picture. There’s nothing bizarre about using a clapperboard on a digital shoot – it’s the perfect solution for the miniature audio toys we now have which enable us to record multiple tracks on a little box which doesn’t understand timecode, rather than having to mix to stereo on location. We can then sync these to the rushes in postproduction. Always stick to BWAV format when you’re loading audio, then you not only have a reference back to the source timecode, but also to the frame edge. Your audio can also be loaded into any video editing system, and it will understand the timecode stamp.
On a music shoot, we’re always on “station sync” which means that all the cameras, VTR tapes and our multitracks are receiving the same video sync and timecode. This source reference remains all the way through to the final edit, and the frame edge won’t have moved. An EDL (Edit Decision List) can automatically relate our source-code to the edited master with little effort. Each edit will always be an exact number of frames, as they can’t do anything else in picture editing. Even if a grading process shifts the video by one or two frames, remember that it will always be an exact number of frames and our frame edges are in the same place on the audio.
So how does the modern audio workstation know where each frame is starting? There are various ways, depending on whether you are synchronising your audio to a video machine (VTR), or whether you’re slaving the VTR to the audio workstation. Let’s look at the first one, which is a bit simpler. All you need is timecode from the video machine to drive your workstation, presuming you’re able to read external LTC (Longitudinal Timecode) either directly or via an LTC to MTC (Midi Timecode) converter. The audio workstation will align itself to the incoming frame edge, and play pretty close to sample-accurate in the right place. This is great for starting the audio in sync, but you also need to ensure the machines are running at the same speed, otherwise they will eventually drift and the audio workstation will have to jump to re-synchronise with the video machine. The only way to ensure they are running at the same speed is to use a clock source, and the best way of doing this is to have a master SPG (Sync Pulse Generator) which provides video sync to the VTR and word-clock to the workstation, but if you’re using a digital VTR you can clock your workstation off one of the digital audio outputs of the video machine.
Using things the other way around, slaving the VTR via 9-pin control to the audio workstation demands that both systems are on video sync, and this often requires some additional hardware associated with the audio workstation. Once they’re both reading video sync, they both know where the frame edge is, and the audio workstation will make sure it plays out the audio with the frame edge aligned to where the VTR is going to be playing. You see, a VTR can tell you which frame it’s on via 9-pin, but it’s very sloppy at saying exactly where that frame starts, so the workstation needs to have its own reference to the video frame edge via video sync. In any case, they both need the same speed reference of external sync.
The third way is to control the video machine via 9-pin, but for the workstation to follow the LTC. Unfortunately, very few workstations are capable of this. SADiE, by way of example, handles all of the above methods effortlessly.
Of course, we’re now moving into tapeless everything, including video. This doesn’t change how meticulous you need to be with keeping the frame edge of audio in line with the video frame edge. If you’re shunting audio around, make sure you’re using a 1-frame grid, and it goes without saying that you need to meticulously set up the options on your workstation to ensure that the frame rates and sync options are all set correctly. You should also experiment with the available video pre-delay options, and using material you know is bang in sync set your system up so it appears perfectly synchronised. Always ask for picture files to have BITC (Burned In TimeCode) so you can align them to the right frame on your timeline.
I still keep a CRT monitor in the studio, as all the affordable flat screens have some element of picture delay. So whether we’re using video files or tapes, this is our final reference for fine sync.
However clever or expensive your post-production toys and facilities are, the frame edge is a magical and totally defined point in audio. Lose it at your peril, and remember that everything in video world is edited in whole frames, so do the same with your precious sync sound!
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