Pro Tools Pointers #5
Video Sync
by Dave Whittaker
This week's
exciting episode: a primer on important video sync
stuff.
Excitement for the whole family!
SECTIONS:
Warning Will Robinson! Proceed with caution!
The "speed" at which Pro Tools plays and records is governed by the frequency of the "Word Clock" which regulates the sampling rate frequency. This word clock is available to Pro Tools from both internal (built into the Audio Interface) and external sources. Digital audio has a "Sync Word" in the digital bitstream and Pro Tools will derive its own over-sampled word clock from this sync word in the digital audio signal. A typical digital audio source is the digital output of a DAT from which you are recording into ProTools through the digital inputs. The sync word of that digital deck is "seen" by Pro Tools at its digital input and Pro Tools will need to slave to it when recording digital to digital; that is why Pro Tools will switch to "Digital Sync" when you switch to "Digital Input" in your Hardware Setup.
However, it is not wise to leave the sync set to "Digital" when you are done recording and wish to edit. Pro Tools' word clock should be tied to the source deck's sync word only when recording from it digitally. Switch the sync setting to "Internal" for all editing (as well as recording from analog sources). If you don't, you run the risk of discovering later on that edits are a bit off when the session is played elsewhere, such as on the stage or in transfer (where it's likely a different sync word/word clock source is being employed). Read on for why . . . .
One of the key technical issues that seems to be causing confusion among Pro Tools users who are working on their own or in a facility that is new to the brave new world of workstations is the usefulness of a "Video Black" sync reference signal generator (often called "House Sync").
All video gear is regulated by a video sync pulse that controls the rate at which the picture is "scanned" across the screen. Because video black generators operate at an extremely high frequency, which is then divided down to the 59.94Hz NTSC video field rate, they produce an extremely accurate pulse. This pulse can be up to 10x more accurate than the built-in sync reference in your video deck which resolves the speed at which the deck plays.
Once Pro Tools locks to the incoming timecode off your videotape, audio playback does not actually lock to the time code. The only thing "chasing" your videotape's timecode is the "address" information - which is what the Pro Tools insertion point cursor represents (the workstation equivalent of a playback head).
The key here is that once an audio file starts playing, its speed is not tied to the timecode from your video deck at all. The speed is regulated by the frequency of the word clock. When Pro Tools is chasing video picture this is generated by the Video Slave Driver (or whatever device you're using, such as a MicroLynx with ACG card) in response to any video sync that appears at its input. This sync source should be video black, not composite video from your deck.
Pro Tools will default to using externally generated word clock if it is available; in the absence of external word clock, Pro Tools is rolling along regulated only by its own internal word clock. This might sound OK, but the Pro Tools internal clock, while fairly accurate, is not necessarily going to be as critically accurate as the clock generated by a Video Slave Driver-type device in response to high-quality video black sync. In playing a long audio file (like a music cue or a monologue) you run the risk of Pro Tools subtly drifting away from relative sync with your video picture if Pro-Tools is running along on its own clock while meantime your video deck is locking to its own internal sync.
Here's where video black steps in to save the day. When you feed video black to the Video Slave Driver (or equivalent) and to the "Sync In" on your video deck, all parties are now really locked together, because the video deck will ignore its internal sync reference and resolve to the incoming video black.
There are Pro Tools systems around that are running successfully without video black tying them all together, but this seems due largely (as far as I can determine) to the good luck of having fairly accurate internal sync references.
The Motion Picture Editors Guild Newsletter
Vol. 17, No. 2 - March/April 1996.