Time code

Introduction

In the mid-1950's, the first practical videotape recorders were introduced. This shortly gave rise to the need for editing content, but the nature of those machines was such that if tape movement were stopped, a picture was not and could not be displayed. This led to the invention of a time code address that was recorded on the second audio track of the tape. Times were of the form: hh:mm:ss:ff, where hh means hours, mm means minutes, ss means seconds, and ff means frames. In the days of monochrome television, there were 30 frames per second; color television in the United States modified this to 29.97 frames per second (fps). Correction of the resulting error with respect to real time was accomplished through the addition of a format called drop-frame, in which two frame addresses were omitted at the first frame of the first second of each minute not evenly divisible by 10. Outside North America, other frame rates were used, such as 25 fps in Europe. Time code has also been used at 24 fps for application to film.

Variations

In addition to time code on an audio track, which is generally referred to as longitudinal time code, other approaches have been employed, including the embedding of time code data in the television vertical interval and even the embedding of a single data bit in each of the first 80 horizontal sync pulses. Some videotape recorders were designed that provided special tracks for the recording of time code. In the move to digital television, time code has been designed into the data definitions from the start, so the notion of a time code track is moot.

Standards

The definition of this time code was formalized by the SMPTE ([Society of Motion Picture and Television Engineers]) in the SMPTE 12M standards document (SMPTE Standards documents are not available for free download.)

External Links

[SMPTE: Society of Motion Picture and Television Engineers]