BBC HD to switch to DVB-S2 on 6th June 2011
Brian Butterworth published on UK Free TV The high definition (HD) television channels on satellite require a lot more bandwidth (number of bits per second) than the standard definition (SD) ones, as you might expect.
However the additional capacity required is reduced in two ways. First the MPEG-4 compression system is used (rather than MPEG-2) and generally the DVB-S2 modulation system increases the bandwidth of each satellite transponder.
Generally speaking, around nine SD television channels can be carried on a transponder in DVB-S mode, and four HD channels on one in DVB-S2 mode.
For historical reasons, the BBC's Astra 2D transponder 50 has been providing BBC One HD and BBC HD (and also ITV1 HD Merdian South) in the DVB-S mode, and this has used all the capacity on the transponder, because of the lower bitrate.
The changes
One 6th June 2011, the BBC will switch the transponder to DVB-S2. If you watch using Sky+HD, Virgin Media or Freeview HD you will not have to do anything.If you use a branded Freesat box (or TV set), you may need to put it into stand-by mode for 30 seconds on 6th June. If that doesn't work, perform a 'first time installation'.
Update: Some Panasonic Freesat boxes are unable to deal with the change - see comments below
If you are using a unbranded Freesat box, you may find that you also have to do a 'first time installation' to get the channels back.
Some older satellite equipment will not support DVB-S2. If you can't watch Channel 4 HD, then you will no longer be able to watch the BBC HD channels.
Sky HD, Freesat HD EPG issues
Currently BBC One HD cannot automatically replace BBC One in the Sky EPG at position 101, because there are 15 regional versions of BBC One, and only a single BBC One HD 'stream' on the satellite.BBC Two carries four regions for England, Scotland, Wales and Northern Ireland. For a BBC TWO HD to appear in the Sky EPG at position 102, the BBC would need to provide four versions of BBC TWO HD.
All the other BBC channels are national, and have no regional components. BBC Three timeshares with CBBC, BBC FOUR timeshares with CBeebies, plus there is BBC News, BBC Parliament and BBC Alba.
The extra capacity found by moving to DVB-S2 could allow the BBC to provide BBC Three, BBC Four, CBeebies and CBBC channel in HD, and these could be provided in the "usual" EPG slots for Sky HD and Freesat HD users, which would put them on the lower numbered channels nearer the top of the guide.
Help with Free satellite?
In this section
Monday, 6 June 2011
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MarcSimms11:34 AM
For info (gathering all inof in one post!!) Astra2D transponder 50 (containing BBC One HD & BBC HD) details are now as follows:
Frequency. 10847
Polarity. V
Symbol Rate. 23000
Modulation. QPSK
Mode. DVB-S2
FEC: 8/9
Good luck!
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NottsUK12:29 PM
MarcSimms: Unfortunately good luck will not work. Either the BBC change the Symbol Rate to one that my Freesat receiver will recognize, (22000, 27500 or 29500) or Panasonic quickly bring out an update. At least Channel 4 had the sense to use a Symbol Rate of 27500 on their DVB-S2 HD channel.
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NottsUK: The symbol rate depends on the "width" of the transponder. The larger the frequency difference between adjacent transponders, the more carriers you can squeeze in.
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NottsUK1:40 PM
Briantist: Why didn't the BBC leave the Symbol Rate at 22000 (22.0), then there wouldn't have been a problem. I have read on a internet board that BBC HD and BBC One HD have disappeared from the SKY HD receivers add channels option. I assume this is because SKY HD receivers add channels, only offer 22.0, 27.5 and 29.5 Symbol Rates.
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NottsUK: The BBC is using the appropriate parameters to make the best use of the available bandwidth, to provide the highest bitrate.
I'm not sure why "add channels" is something you would want to do as BBC HD and BBC One HD are in the Sky EPG?
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Mike Dimmick3:11 PM
NottsUK: The Symbol Rate, or baud rate, is the fundamental speed at which it changes from one 'symbol' to the next. The symbol is the combination of bits currently being conveyed: in QPSK, two bits are conveyed by each symbol. The symbol rate is the same as the bandwidth occupied by the signal, less a roll-off factor. DVB-S mandates a 0.35 roll-off factor, DVB-S2 allows 0.25 and 0.20 to cram more symbol transitions into the same space.
To avoid hitting the signal from the transponders above and below, the transmission bandwidth must be limited to about 30 MHz. Using the 0.35 roll-off at 22.0 Mbaud, you get 29.7 MHz. 23.0 Mbaud would be a roll-off of around 0.29. 27.5 would be a roll-off of only 0.08, far too low and damaging the transmissions from tp48 (various BBC services) and tp52 (a Sky-operated multiplex).
The higher symbol rates are only available in the upper band, from Eurobird 1, Astra 2A and 2B, because the transponders are spaced further apart.
There's nothing in the DVB-S or DVB-S2 specifications limiting broadcasters to using specific symbol rates. Annex C of the DVB-S spec gives 8 different rates for 8 different transponder bandwidths - none of which match any settings that have been used 'in the wild'. The User Guidelines for DVB-S2 document refers to many different symbol rates, again, not really matching anything used in the wild.
The DVB-SI (Service Information) specification's 'Satellite delivery system descriptor', which I believe forms the basis of the EPG information telling the box where to find the channels, uses 7 numeric digits to encode the rate with a resolution of 0.1 kbaud, maximum 100 Mbaud.
Panasonic really should not have restricted their equipment to only work with three fixed rates.
DVB-S does not even carry any information on how the data was encoded - the receiver has to figure that out for itself. DVB-S2 tells the receiver using binary phase shift keying (one bit per symbol, just for the information period) which error-correction mode is in use and how many bits are carried per symbol.
Briantist: DVB-S and DVB-S2 use one carrier modulated very quickly, it is not an OFDM system with lots of carriers modulated slowly, unlike DVB-T and -T2.
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Paul3:29 PM
Does that mean my panasonic freesat recorder is useless for bbc hd now?
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Peter Challis: SD transmissions will not be moved to DVB-S2 as there are 10 millions SD receivers in use that do not support it.
However, all official HD receivers support DVB-S2.
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Paul: The problem seems to be a software one, that should be fixable via and update. The question is probably how quickly Panasonic can provide one and deliver it via the "over the satellite" software update channel.
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