Many people involved in the DVB world (S/T/C) hear about the new HDTV (
High Definition TeleVision), more and more channels appears across Europe and United States that provides HDTV, mostly operated by the big providers, so yes the HDTV is here, but to stay ? Are the providers in disposition of reduce their number of channels to get a new HDTV one ?
Most people is not aware about the cost of this “new” channels (cost for the provider, of course). Using the current wide available
DVB-S (DVB by satellite) transmission system on a typical
transponder with a
symbol rate of 27500 Ms/s (Megasymbols per second) you receive 33.5 Mb/s (Megabits per second), how the provider distributes this information affects the quality of the image and sound that you receive, its a shame that most providers give less than 3 Mb/s for each channel when a ratio of 4 Mb/s is the recommended value. So, take up our calculator and use “real” values, given 3 Mbit/s for video stream, and 192 and 128 Kbit/s (Kilobits per second) for two audio streams, adding the SI (Service Information) proportional from the total to that service/channel and to get a bit more simple results I will think that each service/channel eats around 3.5 Mbit/s, so:
33.5 / 3.5 =
9.5 SDTV (Standard Definition TeleVision) by transponder.
Well, its fine 10 channels by transponder, nice for they accounts. Now use the same transponder to transmit HDTV channels, using for this “test” channel a “low HDTV” channel of 1280x720i (i means interlaced, in PAL this means 50 fields = 25 full frames per second) the “equivalent” bitrate per image point is a bit more than the double of the regular SDTV channel (720x576i), so we need a “minimum” to 7-7.5 Mbit/s, so taking the calculator again:
33,5 / 7 =
4,7 HDTV by transponder.
Well, that's “exactly” the half of the channels that you can transmit in SDTV resolution. But as you get into the HDTV format things goes wrong, and wrong for the provider:
HDTV 1280x720p (p means progressive, in PAL means 50 full frames per second), so bit rate is “exactly” the double again needed for each channel, or 4 times a SDTV one:
33,5 / (3.5*4) =
2.39 HDTV by transponder.
And here the problems starts, you can not broadcast 2 channels and a bit of other one, moreover you can not transmit 3 channels!!! or the average bitrate per channel will be much less than the “minimun”. 33.5 Mb/s / 3 Mb/c = 11.1 Mb/s per channel! When “minimum”is aroun 14 Mb/s. So the provider is forced to transmit only 2 with a bit better quality or add one SDTV channel to the stream.
HDTV 1920x1080i is the near top HDTV quality, it eats compared to SDTV channel 5 times the SDTV bitrate, so 3.5 * 5 = 17.5 Mb/sec:
33.5 / 17.5 =
1.9 HDTV by transponder.
This is even worst in the HDTV field, you are in the “limit” to reach 2 channels per transponder, I think that the providers will reduce a bit the “quality” in this streams mostly because as the image gets bigger the information per pixel needed is a bit lower. In the other side, if you offer a 1920x1080i channel is to get a “perfect” image, not to see
macroblocks from time to time (specially watchable in live concerts due the flashing lights). The channels that I know that offers 1920x1080i are being broadcasted 1 per transponder rating around 19 Mb/s.
HDTV 1920x1080p is the “top gun” of the HDTV, 50 full frames per second, perfect motion at high resolution, eating the double of previous channel:
33.5 / 35 Mb/s =
0.9 HDTV by transponder
It does not feet 1 channel per transponder, but it is factible reaching all the limits and lowering a bit the quality.
As a conclusion HDTV better than 1280x720i will not be widelly available in major providers meanwhile
DVB-S2 and
MPEG4/AVC is not the standard for this channels. This two concepts will be the focus of upcoming articles.
PS: Yes, I'm aware the
D+ and
some Cryptoworks providers are now “insecure” but this is not the main purpose of this blog ;) so please, enter some comments about this article in order to focus the next ones.
