Originally Posted by markofmayhem
From what I've seen with Quantum HITS, the "receiving head-end" only has to modulate to 256QAM with no encoding/decoding needed, just modulation (from Q to 256). [see above posting for the rest of the posting]
The modulation schemes that I know of for satellites are QPSK, 8PSK and 16QAM. But the 16QAM is not the same as the 256QAM used on cable. In the case of these HITS feeds, I believe the only modulation available for DCII is QPSK. I've not seen 8PSK DCII satellite feeds.
That said, by sending the MPEG-2 stream at the bitrate needed for squeezing 3 HD channels into a single QAM channel, no recoding is required of the MPEG-2 video. Just demod it from the receiver and feed the MPEG-2 directly into the QAM modulator.
It is the same kind of scheme that FOX does with their splicer system at their affiliates. They encode the MPEG-2 in LA, send it out over a bird, when then goes through the splicer box, to get joined up with the audio and all of the other ATSC PSIP data bits and sent to the transmitter. No re-encoding by the local affiliate.
There is FEC with the sat feed. It is a must. In this case it is 3/4. The FEC that is in the DCII sat feed is not the FEC used for the QAM channel
The reason that ABC, CBS, NBC and The CW have bitrates up into the 35 Mbps+ range is to satisfy a kind of rule regarding reencoding of MPEG-2. As a general rule, the incoming bitrate should be at least twice as high as the final encoded rate. So, for ATSC, the final rate maxes out at about 18 Mbps. So the sat feeds are 36 Mbps, or better. It seems that there can be all kinds of funny MPEG-2 encoding artifacts when the source is MPEG-2. The closer the source MPEG-2 bitrate is to the final bitrate will result in the errors being worse.
In the case of these HITS sat feeds, no reencoding of the MPEG-2 is taking place, so those encoding problems don't appear.
With the total payload of the QAM channel being limited to 38.8 Mbps, so no way in Hell should they be shoving three 1080i HD video streams into that limited space. As was seen, the video can look like crap.