| Time Code: Introduction | |
| Time code, sometimes known as SMPTE time code or SMPTE code, is an electronic signal which is used to identify a precise location on time-based media such as audio or video tape or in digital systems. |
| Essentially a string of 80 pulses or bits, containing information pertaining to the hour, minute, second and frame, the type of time code (nondrop or drop frame) and 32 user-definable bits. | Time code was developed in the early sixties to provide a kind of "virtual sprocket" for videotape. It is used extensively as a synchronization signal or reference throughout audio and video production and post production. |
| In analog media time code usually takes the form of an audio signal which is recorded on one of the tracks of the tape. Referred to as LTC or longitudinal time code, this signal comes in several different formats relating to the number of frames per second and, in the case of colour video (29.97 f/s), whether it is "non-drop" or "drop" frame. | In video production, the time code may be recorded as part of the video signal (in an unused line which is part of the vertical interval). Called VITC ("vitsee" or Vertical Interval Time Code) it has the advantage of being readable when the playback video deck is paused: since LTC is an audio signal, it is silent if the tape is not moving. |
| Time code systems generally have a single Master or time code source with all of the other devices referred to as "slaves. |
| (a) read the time code recorded on their tracks and adjust playback speed to keep locked to the master (continuous resynchronization) or | (b) read the time code and start playing when a specified time is reached (trigger sync). |
| In either case, the slave is usually connected to (or has built in) a synchronizer. |
| This device reads the incoming code and compares it to the code written on the slave"s tracks. | The synchronizer searches for the correct spot (with fast forward or rewind) and then plays with variable speed until the slave code equals the master code. This is the moment when the two have "locked" and there is usually some visual indication of this status on the slave or synchronizer. Synchronizers allow the user to program an arbitrary "offset" into the system so that the slave code need not bear any resemblance to the master code: | for
example you might want the slave to sync its location
01:20:39:15 to the master"s 01:22:39:15, etc.
It is important to note that most systems will not tolerate differences in time code type: always insure that all tapes are "striped" or recorded with exactly the same frame rate and nondrop or drop type! |
| The act of recording or poor tape quality , etc. may degrade the time code signal and make it difficult for the synchronizer to differentiate pulses. |
| A time code refresher is sometimes used to amplify and alter the on/off slope of the pulses to reduce the amount of errors. | If the code disappears entirely the slave will stop unless the system provides for "jam sync". Jam sync refers to the synchronizer's ability to take over generating code when the source stops. | Of course the accuracy of the synchronization is in jeopardy because the new source is "freewheeling" or independent of the actual code recorded - however if the drop out is for short period, jam syncing will work. |
| The master time code should be a stable signal. In professional studios there is usually a "house sync" or master time code available to all synchronization systems. In addition to the hour/minute/second/frame information, it also contains a steady signal referred to as video black which serves as a time base for all of the devices: thus the video signal synchronizes and the time code locates. | Computers (usually functioning as sequencers or Digital Audio Workstations) can synchronize to time code although most will only read or generate MIDI time code. Therefore a card or external device is needed to (a) read MIDI and (b) convert SMPTE to MIDI and vice versa. | dropframe |