HOLLYWOOD: Monitoring signal integrity within a facility is one thing. Catching a glimpse of it in the air is another. David Wood of Ensemble Designs considered monitoring a signal from the studio to the transmitter relay sites. He used KJLA-TV in Los Angeles as an example. Its studio is 29 miles from the Mt. Wilson transmitter site, with a 6 Gb digital STL and an 8 Mb data link.
“We’re taking a transport stream and calculating a checksum across a group of data packets,” he said, crediting Bob Castleberry.
They started by taking all of the data that fell between two program clock reference packets. That method was ultimately abandoned, but the idea was to take all the data packets in a chunk of clock time and run them through a checksum by taking a null packet out of the stream and replacing it with a checksum packet. This packet doesn’t reference any higher level structures in the stream and does not adhere to the SMPTE standard, but is eventually taken out of the stream, he said.
They hit upon blocking at a 10 Hz rate. Across the course of a second, they borrowed 10 null packets to carry checksum information. At the other end, the checksum packets are collected and run through the calculation to determine the data integrity. Either there is or there isn’t integrity, he said.
“There’s no gray area here. It works no matter what the content is, he said, “and it works very quickly.”
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