Module 57 Addendum

      

Updated: 06/29/2006

 

Video Editing, 

Time-Code Addendum

 

 

User Bits

There is room within the time-code signal that will accommodate extra data. Specifically, there are 32 bits of space called user bits where a small bit of information can be entered.

This can be used to trigger equipment or simply to record abbreviated user data , such as reel numbers, recording dates, camera information, etc.

User bits are limited to four letters or eight digits, which, admittedly, isn't much in the way of potential information. Not all equipment will record or read user bits, so you need to check out this feature if you want to use it.

 

Adding Time-code

Time-code is not an inherent part of the video signal; it must be recorded on the videotape or video disk as the production is being shot, or, in some cases, when it's being reviewed. The same goes for user bits, of course, which are a part of time-code.

Time-code numbers can be used to organize and specify segments on a videotape or hard drive, as well as to calculate total times.  Editing equipment will also use the numbers to correctly cue and edit the segments you've selected, and, if needed, to recall the segments at a later time.

 

Approaches to Recording Time-code

Time-code can be recorded on videotape in two ways: as part of an audio signal, or as part of the video signal.

 

Longitudinal Time-code (LTC)

Time-code consists of 2,400 bits of information per second in the NTSC system and 2,000 bits of information in the PAL and SECAM systems.

Although it's digital information, it can still be recorded in analog on either audio track #2, an address (time-code) track, or a cue track.

When it's recorded in this way it's referred to as longitudinal time-code (LTC ).

The digital pulses are converted into an audio signal, much like a modem converts a computer's digital pulses into sound for transmission over telephone lines.

Although the longitudinal system of time-code has been greatly improved in recent years and it's the easiest to record, it can have three major weaknesses.

First, if you are using videotape as a medium, time-code can only be reliably read off the tape on most VCRs when the tape is moving. This can be a problem when you are constantly starting and stopping the tape during the editing process.

Second, when a videotape is copied, the audio track can suffer a loss of signal quality that will make the high speed digital pulses unreliable, especially when the videotape is shuttled back and forth at various speeds during editing.

To solve this problem a jam sync process is used to regenerate the longitudinal time-code as a new copy of the tape is being dubbed (copied).

Finally, if the time-code is recorded on an audio track at too high a level, it can cause crosstalk, between the audio tracks, which results in a obnoxious "whine" in the program audio.

Longitudinal time-code requires a VTR with well-aligned recording heads, excellent audio capabilities (wide band amplifiers and broadband reproduce heads), and well-aligned tape guides, not to mention a near perfect time-code signal to start with. Otherwise, when the tape is shuttled at high speeds, the machine will lose count of the pulses.

With digital recordings made on hard disks or with solid-state memory these tape-related problems don't exist.

VITC Time-Code

VITC(vertical-interval time-code) and other video-tape systems that record time-code with the video heads have many advantages over the longitudinal system.

Since VITC is recorded by the video heads, it's less likely to be affected by the variety of technical problems that can plague LTC. VITC time-code is also always visible, even when the tape is in the pause mode.

VITC is recorded within the video signal in the vertical blanking interval between video lines 10 and 20. This means that the signal is recorded four times per frame, a bit of redundancy that lowers the chance of things getting messed up due to dropouts, or whatever.

And, if that isn't enough, there is built-in error checking (CRC error checking code) that ends up making the process pretty much fail-safe.

When VITC time-code is used, it should be recorded as the video is shot. Otherwise, you will have to copy the entire tape over in order to add the time-code. You may lose significant video quality in this process unless you are doing the copying from uncompressed digital video.

 

Jam-Sync in VITC Editing

Of course, during the editing process you are bringing together a variety of segments, all of which have different time-codes. If these are all just transferred onto the edited master, you would end up with a hodgepodge of time codes on your final tape.

In particular you need to make sure that the sequence of time-codes goes forward in time. If one or more segments jump back in time or have duplicate time intervals, it will, as they say in computer lingo, "produce unpredictable results," meaning that you will probably end up with some unappreciated segment surprises. 

The solution is jam sync, where the time-code is started fresh and is completely regenerated during the editing process on all of the segments In the words of one editor, "Jam-sync with VITC isn't optional, it's mandatory."

One last caveat.

With any videotape segment involving time-code, you need to have enough time-coded footage preceding the actual segment you want to use so that equipment will have an opportunity to "pick up the count."

Systems vary, but some editors suggest as much as 20 seconds. This isn't a lot when the editing machine is speeding backwards to find a time-code number to cue up a segment.