Not sure how to ask this question...On OTA Hd signals the local station could use 18Mbps or so for one channel with no subs...Maybe not 18mbps but something close. What is the typical rate for a signal on FTA S2? or is it that easy to figure out?. I imagine there is quite a varience on FTA HD. I thought I saw an old post on this somewhere but couldn't find it. I know for example Ion Hd and the PBS HD on 125W seems to be even better quality than the HD I get OTA...which to me is quite good already. Hope what I'm asking makes sense? Thanks Blind
Right.
A typical OTA channel has the capability for approximately 19.5 mbps bitrate if they just put 1 HD channel, it could occupy nearly that whole space. Where I live, they put 1 HD channel and one low quality weather channel on 2 of the 3 channels, and the other has a SD version of the programming instead of the weather channel. The main HD channel is about 15.5 mbps for the video, and about 0.4 mbps for the audio. Of course, if they tried squeezing more SD channels in there, they would have to compress the main HD channel into a lower bitrate.
A 1920x1080i OTA HD channel would require over 30 mbps even if it were just white/black with no gray scales or colors, and would be MUCH higher than that with gray scales and colors if not for the MPEG2 techniques and compression. I have no idea of what bitrate completely uncompressed video with complete color info would be, perhaps someone else knows, but the MPEG2 certainly does a good job of getting a lot of info into a reasonable bitrate.
I have read that the big 3 networks had a policy of starting with at least twice the bitrate of the OTA signal, which is why in the past, CBS network video was approximately 40 mbps for the raw HD, which is more than twice the 19 mbps possible on an OTA station. The AMC21 PBS feeds are about 17.5 mbps, so they apparently don't go by that factor of 2 rule.
NOW, CBS uses slightly less than the 40 mbps that they used to use {I don't have that feed up right now, but I think it's about 33 mbps), however now they are using 4.2.2, which has more color information, so they must feel that they are making up for the bitrate losses with a higher quality image color wise, I don't know.
Now, that HD signal can realistically be sent at any bitrate, it's just that as you lower the bitrate, you need to have more MPEG compression, and the quality of the signal gets worse and worse. If they use MPEG4 instead of MPEG2, the same quality video requires approximately 1/2 the bitrate.
Last time I checked, ABC MPEG4 was around 23 mbps, HDNET was around 18 mbps, Nebraska PBS was around 12 mbps {although when they have the one HD channel using that Huskervision transponder they have over 39 mbps, and I have no idea of whether they have some other source of that, or if they just put AMC21 17 mbps signal out over a 39 mbps channel???}. Dishnet HD in MPEG4 tends to be in the 4 to 6 mbps range, but I've seen it up to 9 at times. I have also seen a full blown MPEG4 1080i HD channel with a bitrate of about 1 mbps, although it was pretty poor quality. So any bitrate is possible.
Now, re your question, and I don't completely understand what you're trying to ask.... Ie DVB S2 doesn't really have anything to do with the bitrate of the video so much as it gives the uplinkers more room to put a higher bitrate signal there, however generally they end up putting more channels on the transponder rather than higher bitrate on the same channel. DVB S2 allows them to have approximately 1.5 times the bitrate within the same SR transponder. Actually DVB-S2 doesn't allow the 1.5 factor, it's really the 8PSK that's part of the DVB S2 standard that allows it. 8PSK basically allows 2^3=8 bits per symbol vs the 2^2=4 bits per symbol that QPSK allows, minus the overhead requirred for the packaging of the info, thus the 3/2=1.5 factor. (I don't think I worded that quite right, but with 8 on off states, you get 3 bits vs 2 bits with 4 on off states, vs 1 bit with 2 on/off states of BPSK.
But with the CBS example, switching to DVB S2 8PSK they can fit about 1.5 the info into the same signal, so they have something in the range of 60 mbps available using MPEG2, so they can fit 2 33 mbps signals into the same transponder albeit a bit lower SR rate. The old 30000 SR transponders really had space for nearly 50 mbps, and the new S2 29860 8PSK transponders have room for about 74 mbps, but rather than using all that on one channel they use the space for the 2 33 mbps channels.
Anyway, other than that, I'm not completely sure what the question is. S2 just gives them 1.5 times as much room for bitrate, and MPEG4 gives them twice the quality for the same bitrate, and the actual bitrate of an HD signal can be virtually anything between 1 mbps to up in the hundreds of mbps without compression.
