One of the wonderful things about professional video equipment is that every field of every frame on a videotape has a unique address. That's 60 addresses for each second of video signal. The address is recorded on a special track using SMPTE Timecode. This timecode track is in addition to the CTL (control) track, linear audio tracks, and helical-scan video track. The address is displayed in decimal format as HH:MM:SS;FF.
The consequences of every frame being permanently labeled are enormous. It makes it possible to eject a tape from it's player and reload it later, and still be able to find exactly the same frame as before. Compare this to the situation with consumer ("home") VHS and cassette machines, where you have to zero the tape counter when you are "roughly" at the beginning of the tape.
Having the timecode "permanently" associated with the video means that frame-accurate "cue sheets" can be drawn up, so that the director or editor can find important points in the program just by seeking to a specified timecode number. Timecode thus allows editing sessions to be spread out over days or even weeks, with perfect confidence that any edit point can be precisely re-visited at any time.
When a "real-time" recording is being made, the recorder simply starts at some arbitrary timecode value and counts smoothly up from there. Thus, recording directly from the Abekas video disk or from the screen of an SGI requires little more than routing the appropriate video signal to the "program video" bus and pressing the RECORD and PLAY buttons on the recorder.
Note however that it is often convenient if the timecode starts at the beginning of such tapes at 00:00:00;00. This is easily arranged using exactly the same procedure as described later on; the only difference is that program video and/or audio is routed to the recorder in place of black(video) and silence(audio).