Freeview modes
Brian Butterworth published on UK Free TV Two of the Freeview multiplexes (collections of channels) broadcast in one mode known as "64QAM mode, FEC=2/3,", and the other three broadcast in another mode called "64QAM mode, FEC=3/4".
The " FEC=2/3" mode provide a more reliable signal than a "FEC=3/4" mode signal at the same power level, but there is a trade-off that means less digital data can be carried.
This diagram shows the current channels carried on the Freeview system. The wider the box, the more bandwidth is used to carry the pictures. The more bandwidth used for a service, the better the picture quality.
Boxes split horizontally denote channels that occupy the bandwidth at different times of the day.
The third line shows the configuration of the BBC "B" multiplex switchover which is used for HD and is in another mode DVB-T2 256QAM.
64 QAM (3/4) mode
In this mode, the digital TV signal is transmitted in groups of 64 symbols. The "forward error correction" is 2/3, and this results in a bitrate of 24M.1b/s (megabits per second). This can be used to carry up to eight simultaneous television channels.64 QAM (2/3) mode
In this mode, the digital TV signal is transmitted in groups of 16 symbols. The "forward error correction" is 3/4, and this results in a bitrate of 27.1Mb/s. This can be used to carry ten simultaneous television channels.Help with Television sets?
Friday, 19 August 2011
Tim
5:18 PM
5:18 PM
HA! It knows the aerial man is coming. Signal strength is now up to a steady 8! Neighbours are unaffected. Picture is now good, sound is good!
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J
jb386:12 PM
Tim: Before you call anyone in, getting back to scratch, although you have said that the aerial was well aligned you did not actually confirm that it was vertically positioned, is it? Another possible factor exists and that concerns the booster, have you tried by-passing it? as although used indoors they are very reliable its not impossible for one to develop a fault.
And to narrow the problem down further, you should really try to determine if you are the only one experiencing difficulties by making enquiries locally, as if they weren't then that would be a positive indicator as to the possibility of some other fault applying like cabling etc.
A final puzzle I have concerning something you said on the 15th @ 09.12am, that being your comment that your recorder was tuned to another transmitter and as such is less affected, what transmitter?
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Tim
7:12 PM
7:12 PM
Aerial is vertically oriented.
No neighbours have the problem.
Recorder is tuned to Beacon hill, yes, even on the imperfect aerial! The one is almost on a back nearing of the other, and we may get reflection/scatter of signal.
Changed the fly cables, no effect.
Checked connections in booster box, no effect.
Moved aerial in a 40 degree arc, no effect on signal strength on set.
Banged head on protruding nail in loft. Swore a lot.
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J
jb388:57 PM
Tim: Sorry to keep on about it, but it wasn't if you had checked the booster connectors I enquired about, it was asking if you had tried by-passing it? meaning to remove the in / out plugs and join them together.
The reason for requesting this check is that if a fault developed in the booster it would be like fitted a large value attenuator in line with the aerial input.
However, if the plug joining exercise is too inconvenient to try, a simpler way of testing the booster is to simply switch off its mains power whilst watching the TV, because if the picture rapidly vanishes then its OK, but if it makes little difference then that suggests that it isn't!
For accuracy, this test should really be done whilst the picture is bad, this just in case its failing later on in the day due to thermal reasons.
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Tim
9:32 PM
9:32 PM
Switched off, instant death to signal. On again, instant on again. PIcture was middling bad at the time.
Plugs are screw in type, not plug in type
Went through a good patch this afternoon, now this evening is unwatchable. Loft is hot hot hot, less so at night
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Saturday, 20 August 2011
J
jb389:04 AM
Tim: Thanks for your update, and so long as the test was carried out whilst the picture was bad (very important!) then the booster can be excluded, and just like Briantist has previously mentioned, I also will be most interested in the diagnosis received from anyone you call in, as it really seems to be one of these type of problems that will only be rectified (hopefully!) by a site visit, as I suspect that other aspects exist of a nature which might appear as insignificant, but not! might also be applying.
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J
jb389:56 AM
Tim: Should have really have suggested this earlier, but if your source of interference is NOT of a UHF RF nature, then a simple highly effective test can be carried out using a small portable radio that has a LW band on it.
This is because long wave radios can act as great electrical interference source sniffers, this achieved by tuning it to a blank spot on the scale where no stations are being received, (e.g: Radio 4LW) then turning up the volume slightly.
You can then try it sitting close to a mains power socket prior to the time that the problem with reception starts, making sure that no excessive buzzing noises etc are heard, then noting if this situation has changed after the picture problem has started, this indicating that the source of the problem is mains borne.
Don't position it anywhere near to the TV though, as some of these can radiate all sorts of interference up to short distances away.
You can also use this type of device to check on faulty water tank thermostats etc, or anything that involves electrical contacts.
Mains sourced interference may be nothing to do with your problem, but thought it worth mentioning as a final suggestion.
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J
jb3811:11 AM
Tim: Just to clarify any possible ambiguity, when I referred to Radio 4LW I was really meaning NOT anywhere near to it, as anywhere near it would kill the receivers sensitivity for sniffing out interference.
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