1 Topic by cindy

  • cindy
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  • From: Ensemble Designs
  • Registered: 2009-03-06
  • Posts: 34

Topic: How is ATC different from VITC? Time code details on Ensemble TSGs

Time code is carried in HD as Ancilliary Time Code or ATC. VITC (Vertical Interval Time Code) is time code for standard definition (SD) signals. 

In SD, it matters which line is used to carry time code because it will actually consume the entire active portion of the line.  And if other services were expecting to use that line there would be a conflict that could not be resolved without moving one of the them.  These sorts of services will only be found by downstream devices if they are located where the device is going to look.  Position is important.

But in HD, where the time code is carried as a data packet, it can be placed most anywhere in the vertical interval.  These data packets do not need to be tied to a specific line.  They are short bursts of data (like embedded audio, but not nearly as many of them), with each burst (a packet) completely identified by the header (label) it carries with it.  In general, these packets are well-behaved and when a device wants to insert one, it will be a good citizen and not put it 'on top of' an existing packet.

Ensemble's test pattern generators (also referred to a test signal generators or TSGs,) such as the 7400 and 9400,  always put the ATC (Ancilliary Time Code) packet in the same place in the vertical interval.

Having said that, why do we still have a line selection control on our time code generator when we are operating in HD?  Because the ATC standard requires that the packet include an identifier as to which line this time code should appear on if the signal is downconverted to SD. 

One would not see the actual physical location of the ATC packet change when adjusting the line select control in the time code menu.  But if you looked inside the ATC packet you would see that we're changing they 'SD equivalent' line marking.  Also, since the 7400 (and 9400) can generate HD SDI on the digital output while simultaneously generating analog NTSC black on the reference output, we still need to be able to move that time code around to suit the needs of the user.