Originally Posted by levibluewa
My ? is how can 2 different channels occupy the same sub-channel 12.1 & 12.2 ????
They don't. KBTC's PBS and MHz Worldview feeds are transmitted on 12-1 and 12-2 without PSIP mappings.
KVOS's Me-TV and THECOOLTV feeds are transmited on 117-1021 and 97-4, PSIP mapped to 12-1 and 12-2.
This is somewhat analagous to most OTA viewers seeing multiple 7-1s and 7-2s from KIRO and its translators, although in that case, they are all PSIP mapped to 7-1 and 7-2.
In a perfect world, every receiver would know how to deal with this: display two 12-1s and two 12-2s.
In the real world, every receiver reacts differently to inconsistencies in any of the nine tables that make up the PSIP stream, and every receiver reacts differently to conflicts between two or more streams.
I have an old LG set-top that handles this admirably: it doesn't attempt to decode QAM-delivered PSIP. On that tuner, everything tunes reliably by transport channel.