A single master reference for time of day and timecode provides advantages for logging and filing material, and as an accurate reference for station automation systems. There are, however, a few important considerations you should bear in mind when designing or specifying such a system...
 

It is a frequent requirement to lock a television station's master timecode generator to its master clock system, providing accurate time of day information in the timecode signal for video recorders, station automation systems and other broadcast equipment.

The 482D Master Clock has an optional EBU timecode output module which can be used to lock ITC Intelligent Timecode Clocks, SI-100 Source Identifiers, and as a timecode signal source for burning time / date information into video. Whilst this output is precisely locked to the time reference, it is not correctly locked to the master SPG video reference, which may make it unsuitable for some applications where a guaranteed timecode / video phase relationship is required. There are several ways to lock timecode to both video and the master clock, but in general there will need to be some compromise in absolute timecode / time accuracy of 1 or 2 frames.

When an atomic reference transmits the time as 00:00:00:01 (the first frame after midnight), the master clock will output this timecode data. The Relationship between timecode and video dictates that the sync word from the timecode is within 1 bit of video frame sync and that the SPG is producing a video frame 1 (fields 1 and 2 of the 8-field sequence). In order for both these criteria to be fully met, both the SPG and the master clock must be referenced to the same oscillator (in this case the atomic reference), and the SPG must be adjusted to output the correct frame sequence.

Whilst this is an expensive option incorporating high-stability disciplined master oscillators, it still does not provide a 100% solution, as leap-second adjustments to the time of day are introduced on an irregular basis by the International Earth Rotation Service - and one second (25 frames) does not constitute a whole number of 8-field video sequences.

When the master clock and master SPG are both referenced to the same traceable frequency/time source, it is still better to let the downstream timecode generator adjust the frame count by an offset (which will be constant in this case) to match the video sequence, rather than adjust the SPG, as corrections in timecode data are less disruptive than discontinuities in the sync reference. This timecode correction takes place only on initial power-up and when there are leap-second corrections.

A simple, reliable solution is to operate the master clock and master SPG systems independently, referencing the station timecode generator to the SPG (and thus ensuring its correct video phase relationship), and updating it from the master clock on a periodic basis. Correct timecode/video relationship  can be easily verified using the HETL-2 8-field timecode / ScH phase monitor.

The least expensive way to accomplish this uses the mater clock's voltage free contact closure at midnight to reset the timecode generator to zero, from which it free-runs locked only to the video SPG. The timecode generator does not need jam-sync capability, but must be capable of reset / run as a single operation, triggered by a contact closure. Furthermore, the master clock does not require the EBU timecode driver module. The disadvantages are that the timecode generator can only be updated at midnight (00:00:00:00) and its timecode output will only contain time information, without the date.

Probably the best solution is to jam-sync a downstream timecode reader/generator to the master clock, which in this case is equipped with an EBU timecode output module. At a pre-determined time, the timecode generator can automatically jam-sync to the time of day indicated by the timecode received from the master clock, subsequently relying on the SPG for its reference. Manual jam-sync is possible at any time of day. This solution still allows for the two systems to drift apart, but in practice this is very slow as both the master clock and the SPG use accurate oscillators.

When the master clock is synchronised to a local time reference (e.g. MSF in the UK or DCF in continental Europe), then adjustments to the time of day will happen automatically in this solution, and will be updated in the station's timecode the first time a jam-sync is requested after the time change.
 

Last Updated