Originally Posted by bdfox18doe
An excellent explanation there blackcat.
Now, not to add to some of the uniformed speculation that is offered here by others, based on working closely with the FOX lab on the splicer since day one, here is what I suspect you will see:
In the past FOX engineering has believed that ATSC using high efficiency encoders can support 720p with a maximum of 2 SD subchannels and maintain acceptable quality. The network satellite feeds are originated @ TOC in LA using some of the best encoders available, and are on the bird at about 18 mb/s.
If the local affiliate has no subchannels, then the video and audio elementary streams are "spliced" (think film as a rough analogy) into the local signal with local PSIP without any modification.
IF the local has subchannels, the splicer re-quantizes the network feed to fit the data rate needed by the local affiliate. Based on the math and processing power of the splicer, this is limited to a lowest rate of about 12 mb/s or so without introducing considerable artifacts in the HD signal. In the past FOX
has been resistant to do this. At times, in network programming such as House and Bones, I have seen the rate around 9 mb or so..but this is from the encoders in LA, and not altered locally so the quality is excellent. The splicer also gives priority to the network HD and will pull bits from the local subchannels
"if" if feels necessary to maintain network quality in times of high demand programming such as Nascar. This is why MeTv has looked a bit rough during some network periods.
So, when watching a network program, you are seeing the encoders in LA and not the local encoder. When a local break or program is airing, the network feed is spliced out and you are seeing the local encoder. There is no local HD output from FOX here, master control actually switches from an analog proxy.
The "splicer" is a highly modified Cisco DCM, with the 4 receivers and FOX News server takes up a full rack. Time Warner here uses the same basic Cisco DCM for its channels.