Synchronization - The SMPTE timecode

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This is an analogue time code, in that the time code's signal gets recorded on an analogue support system as if it were an audio signal. Its recording can take place in two ways, depending on the kind of sound support system being used:

  • VITC (Vertical Interval Time Code): in this case a support system with rotating heads is used [Rotating head media ] and the time code is memorized on appropriate tracks parallel to the audio tracks. In some machines such as Alesis ADAT, this cannot take place, so the time code is therefore recorded onto one of the audio tracks. The advantage of this time code is that it can be read even when the tape is not in motion, due to the heads' continuous rotation.

  • LTC (Longitudinal Time Code): a fixed head support is used in this case (like for example a normal reel-multi-track recorder). The time code gets recorded onto one of the audio tracks along the tape's longitude. Its advantage is that it can be read even at high tape speeds (for example during a fast forward).

The SMPTE time code divides the temporal axis according to the following format:

hours : minutes : seconds : frame : sub-frame (optional)

As well as being referred to in terms of time, as we can clearly see the seconds are also subdivided into frames[24 ] (which in turn are divided into sub-frames). This mode refers to video timing, where frames correspond to the number of photo-frames per second. Clearly photo-frames are not present in audio, but nonetheless it is useful to employ this timing system. This is because before audio time codes came into use, video time codes already existed and they worked out to be ideal for audio scopes too. Furthermore with this kind of timing, synchronization of audio and video occurs naturally, seeing that they use the very same time code. The number of frames varies depending on the video format being used. The following table illustrates the different kinds of formats:

Table 17.1. Video formats 

UseFrames per second (fps)
Film24
TV - Europe25
TV Black and White - America30
TV-Colour - America29.97

Time codes are none other than an uninterrupted sequence of 1 and 0 which moment by moment indicate the present instant. To indicate a 0 or a 1, the SMPTE uses a code named Manchester Bi-Phase Modulation.



[24 ] Photo-frames








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