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As mentioned in How digital television worksFreeview signals can be broadcast in a number of different "modes", which are a combination of five parameters. Some of these have changed over the years because the increase in computing power has made more efficient methods of broadcasting cheap enough to fit in a set-top box - it is now possible to broadcast high definition television or more standard definition services on the same physical wavelength.
As some of the ideas that are implemented would fill a text book by themselves, I have presented the concepts in a simplified way.
Each Freeview multiplex is broadcast within an 8MHz channel that was originally allocated to analogue television. Rather than being a single broadcast, the channel is split into a number of different signals that sit next to each other.
The original digital terrestrial broadcasts (from the launch of ONdigital) used 1,705 sub signals (referred to as 2k mode) about 4,600 Hz apart. After switchover 6,817 subcarriers (8k mode) means the signals are 1,170 Hz apart, with the high definition service using 27,841 (32ke) subcarriers just 280Hz apart.
It is possible to space carriers so close together because they are carrying digital information, in the analogue domain the problem with sidebands would render a similar system useless.
For reasons detailed below, the information is not transmitted in single bits on each carrier, but as groups of bits which are referred to as "symbols". Each symbol is held in the transmission system for a given duration, 1024 us. The longer this time is, the less information is carried, but too short a duration for each symbol would not allow the receiver to detect the information correctly.
Each subcarrier also uses a guard interval, which turns off each carrier as part of the transmission cycle. With SD broadcasts, this is 1/32th of the symbol time (32us), on HD it is 1/128th (8us). The guard intervals start each symbol and exist to deal with the problems of multipath - where the signal is being received directly and also by reflections.
Forward error correction
In non-synchronous digital transmission systems, such as those used for the internet, the presence of data corruption between the sender and receiver can be signalled back to the source and the data resent.
However, a digital broadcast is a one-to-many unidirectional system, with no ability to ask for data to be resent.
For this reason, the transmissions use "forward error correction", which is a system that allows a certain level of errors to be detected.
The most basic form of a forward error correction (FEC) would be to transmit each bit twice - as long as each bit was sent along a separate subcarrier, then a single bit loss on one carrier could corrected by using the one sent on the other carrier.
However, this is very inefficient, as it reduces the useful capacity of the system by half.
So, instead the DVB-T system uses a Punctured Convolution coding system. Instead of sending each bit twice, bits are grouped and then a code that describes the bits in a way that a single bit error can be detected and corrected most of the time.
Quadrature amplitude modulation
In the above descriptions, the word "symbol" was used to describe what is transmitted. You may have thought that a digital system would transmit information on the subcarriers as bits. However, doing this in a broadcast system is actually unproductive.
For this reason, quadrature amplitude modulation is used. This takes the incoming bits and encodes them in groups. The most basic form encodes bits two at a time, using sine (Q) and cosine (I) functions that are then added to the main carrier. In effect "00" is encoded as -Q-I, "11" as +Q+I, with "01" and "10" being +Q-I and -Q+I.
At the next level, as used for DVB-T, the bits are encoded in groups of four bits (16QAM) or eight bits (64QAM), with DVB-T2 being groups of 16 bits (256QAM).
UK Freeview modes
There are 10 modes defined for use in the UK, these are:
As Dave Lindsay suggests for Belmont you may have to replace your old group A (red tipped) aerial for a group W wideband (black tipped) aerial,or for extra gain on the lower frequencies (group A ones) either retain your old group A aerial or buy a new group A aerial,then diplex with a group W wideband aerial.
However the 2020 scenario for Belmont is still opaque at present for if the 700mhz clearance does take effect then,the ArqA on frequency 53 and ArqB on frequency 60 from Belmont will have to relocate elsewhere.There is a possible rumour if the 2020 scenario did happen that Belmont by then could revert back to an all group A transmitter so group W wideband aerials will obviously at a future date be replaced due to their poor performance on group A frequencies,especially on an all group A transmitter in a poor reception or marginal reception area on that particular mast concerned.Either that or Belmont could possibly instead become a group K transmitter (utilising grey tipped aerials),or even but most unlikely a group B transmitter (using yellow tipped aerials).
Although not confirmed as such there is a remote likelyhood that Belmont if the 2020 scenario did occur could switch either to dual horizontal and vertical polarity,or even to vertical polarisation only.But i will stress this is only a remote possibility itself !
At the moment I receive my Freeview signals from Blueberry Hill in North Kent, my postcode is SS74AX.The problem I have is that after retuning I now do not receive Meridian local news whereas before I did.I have discovered just recently that if I go to "Channel 816" the local Meridian news is there. Will this ever be retuned back to "ITV1" conventenial location ??. Thanking You.
Mr Ronald Dry: This could be caused by your TV or box having picked up the lower channel numbers being used by Crystal Palace thereby pushing the channel you require up to the 800 ranges, and although you could possibly (dependant on TV or box) manually tune in Bluebell Hill try this easier method first.
Remove the aerial connection on your receiver and carry out a re-scan without it being connected as this will blank out all the channels that are stored, then whilst holding the aerial plug in your hand carry out a second re-scan whilst observing the progress indicator, then as soon as its seen to pass Ch35 or so plug the aerial back in again and that will then start to store the channels from Bluebell Hill.
If you cant get this method to work, then the only other option you have is to manually tune in Bluebell Hills channels after having deleted anything stored. Bluebell Hill using 46(BBC) - 43(ITV1) - 40(HD) - 45 - 39 - 54
Stan: This is correct; Alexandra Palace only carries Public Service Broadcaster (PSB) multiplexes. It does not carry the Commercial (COM) ones which carry Russia Today, Al Jazeera, ITV3, Pick TV, Dave, Film 4 and others.
For a full list of Freeviewservices, see here (those with a bullet in the "E"/England column apply):
There are three PSB multiplexes and three COM multiplexes. Each multiplex is carried as a single signal. PSB3 is the HD one.
Your current aerial may not be the best for Alexandra Palace, as it may be a Group A design (red tip). If it is a wideband, then it will be suitable. A wideband isn't needed for Ally Pally; a C/D aerial will work.
If you are considering realigning your aerial because you're having reception issues from Crystal Palace, then you may now be considering replacement of the aerial (still on Crystal Palace so as to get all channels). See this page for guidance and information about aerials for Crystal Palace:
In the last 2 weeks I have lost Mux2 (ITV1 Ch4/5 etc). I see the tranmission level is reported as low. All other mux are there. Will mux2 return once the power level goes back up amd when mightthis be?
We live on the outskirts of Clacton on Sea and thanks to a short sighted Planning Inspector now have a wind farm consisting of five 125 metre high turbines between us and the Sudbury transmitter. They are not yet operational. My question is "Will they interfere with the signal to our home"? At present I have one aerial on my bungalow roof which supplies perfect reception to three TV,s via amplifier boosters.
david durney: I will be very surprised if they dont! and that is irrespective of where (distance from wise) they are located between Sudbury and yourself, as in some areas where Freeview reception has been ruined by these grossly inefficient abominations the energy company involved has provided Freesat installations to the viewers affected at no cost to themselves, not of course that I consider that adequate compensation for the loss of such a flexible system as Freeview where the one aerial can feed a number of devices, unlike Freesat that requires a separate dish feed for each location.
david durney: Your original posting was made on August 11th @ 10.07am and which I replied to at 3.29pm, a copy of pasted below.
>> I will be very surprised if they dont! and that is irrespective of where (distance from wise) they are located between Sudbury and yourself, as in some areas where Freeview reception has been ruined by these grossly inefficient abominations the energy company involved has provided Freesat installations to the viewers affected at no cost to themselves, not of course that I consider that adequate compensation for the loss of such a flexible system as Freeview where the one aerial can feed a number of devices, unlike Freesat that requires a separate dish feed for each location. <<
The only qualification I will put on what I said is, that it depends up to a point on whether or not the signal you receive crosses the turbine blades in a diagonal fashion, as if it does then obviously the chopping effect of the blades would be of a higher intensity.
Chris Calver Monday 10 September 2012 10:05PM London
I have 2 Windows media centres one an old XP based one and a new Windows 7 one, both have SD freeview tuners. For some reason the Windows 7 box doesnt like channels like Pick TV on ArqA/Com5 ( crystal palace )where I often get momentary freezes. The other XP box fed from same aerial is ok so it isnt a simple signal issue. Other multiplexes are fine on the Windows 7 box, so not a faulty machine. Signal seems to be a good one. So wondering if there is something atypical about ArqA/Com5 from crystal palace that is upsetting the tuner in the windows 7 machine
jb38 Thank You for your replies to my queries asking if a new wind farm with five 125 metre high turbines will interfere with the Sudburysignal to my home in Clacton on Sea. At present we are getting decent HD pictures on our three TV,s. As soon as these monstrosities become operational I will let you know the result, as I,m sure this must be affecting a lot of viewers all over the country as these things are appearing everywhere.
david durney: Yes! please do, although the main problem with wind farm blade interference is the totally erratic element attached to it whereby any disruption to service that they might cause can vary from day to day, this not only because of changes in the blades rotational speed but also the fact of the blade positions on two or more turbines being synchronised with each other at certain times and not others, and of course the other factor of the situation being different again if it rains.
However only time will tell as to whether or not they will affect your particular location, as some people for some magical reason seemingly escape relatively unscathed from the effects created by these inefficient money wasting menaces.
What is going on with the Tacolneston transmitter. Perhaps I have a fault but for the first time since the switch over I am having some problems with pixillation at times, and "weak signal". When checking engineering work at Tacolneston it says possible week signal, is this my problem, is anyone else having the same problem? If so why is it going on for so long?
We have two freeview recorders running from the same Labgear powered widebandaerial. One of the boxes, a Humax HDR Fox T2 has been going through periods when the picture pixelates. This has only started happening since the last retune. We are tuned to the wrekin transmitter and I have tried a reboot and restore, and also tried manual retune to no avail. I am at a loss as to what the problem is. Anyone got any ideas?
Phil: Its not really easy for anyone to offer advice on your problem without knowledge of your location, this in the form of a post code or at least one from nearby, e.g: a shop, because if its only the Humax that is causing problems then this could possibly suggest that the signal received might be slightly too strong, as Humax devices (all types) have one of the best tuners around thats known "not" to glitch in situations where others will.
I would normally have suggested to try a test by-passing the aerial amplifier, but "if" you are using the type of aerial with the amplifier as part of it (albeit an excellent device) then this test is not really possible.
Further info can be supplied once your location is known, as this will enable access to the reception predictor.
anyone else receiving terrible signals from ALL BBC channels off Winter Hill Tx in Bolton?
Mine has gone terrible, been dropping out for the last month or so, all other channels are between 98-100% signal and 100% Quality, ALL BBC channels are 10%-68% Signal and 40%-85% Quality, varying constantly on all Tv's in the house.
Aerial was replaced about 2 years ago, new co-ax, good quality, very well shielded with copper foil and braid, fitted at same time.
Lead from aerial goes straight down the side of chimney, (8 Feet) into loft via waterproof insert, then into quality distribution amp, via N connectors. Each TV has it's own co-ax cable from the amplifier directly via a belling and lee plug.
I have suspicions of the 4G phone transmitter, which is line of sight from my aerial, towards Winter Hill, and is about 350 Meters away..
Any one got any thoughts?, help would be appreciated.
Thanks in anticipation..
Fred Britton Sunday 6 January 2013 12:19PM Stockton-on-tees
My mother has a freeview TV which is fine during the day However at 4am the reception for BBC channels in particular is non existent. Is the signal strength reduced during early morning transmissions? Her arial is on Bilsdale Transmitter
I have just bought a new tv and channel 83 Al-jazeera and 85 Russia today are breaking up. I live in Edmonton, London N18 and when i press the info button on these channels it says that they are on CH28. Is this the correct channel for these programs? Itv1 and Channel 4 were breaking up after having first tuned in the tv and i realised that the tv kept incorrectly picking up ch56 so i manually retuned them and they are now fine. It is just al-jazeera and russia today that i have problems with.
Sally Ahmed: When using the automatic tuning you should be able to avoid other transmitters, such as Edmonton's ITV/C4 etc which is on C57, by having the aerial lead unplugged after the first 30% of the scan.
C28 is correct for Crystal Palace. Judging by the prediction from Digital UK, you could perhaps potentially be getting a signal going into your tuner which is on the high or perhaps excessive side.
Sally Ahmed: I have retuned and the picture is now not breaking. However the signal strength on all channels is 100% and the quality on all channels except C28 are also 100%. However the quality on c28 is only around 40-45%. Not sure why this is or how i can improve this. Thanks again
Sally Ahmed: What you are describing could be as a result of the signal being too high. The effects of this appear the same as a signal that is not strong enough and/or not of sufficient quality due to the fact that the receiver gives false readings because its circuitry is being overwhelmed by signal.
Signal strengths vary a bit over time and it could be that it has relented a little. The trick is to ensure that natural variances in signal strength does not push it OTT.
Remove any booster and if there is no booster install some attenuation. I referred you to a page with some more information on this (see above).
Sally Ahmed: Ok. Yesterday all channels on C28 (and only this channel)were freezing and breaking up. After having done a re-tune it corrected the problem. But my question is why would it just suddenly out of the blue need retuning, no-one had touched it or fiddled with it? This is the second time that this has happened. Could it be a fault with the tv even though it is only 2 weeks old?
Sally Ahmed: It could be the negative offset that this mux has, resulting in the frequency reverting to C28 and therefore off-tune on the occasions you have mentioned. The other standard definition muxes use exact UHF channel frequencies.
Sally Ahmed: Further to the post of jb38 and myself above, when doing a signal check, observe the frequency that has been stored for the errant mux. If your receiver is one that does automatic re-tunes at set intervals it is possible that C48 from Sandy Heath is being selected, with resulting poor reception as mentioned. If this is found to be the case this facility should be turned off if possible.
Free view: we've had a retune on 27/03/13 in the Bristol area. Since them whenever my freeviewbox turns off (either manually or after recording a programme) I lose the signal on all BBC channels, so I don't get to record or watch my programmes. I've tetuned more times than I care to mention. Ut don't know what else to do. I've a Sony hard drive recorder about 4 years old; it's been retuned out of necessary before but at the moment it needs retuning every time I want to watch or record BBC!
Emma: If the programme EPG is still being seen even although that selecting BBC1 results in a blank screen, then rather than carrying out another auto-tune, which by the way should never be necessary anyway unless having been previously announced, just go into the signal check screen whilst on BBC1 (albeit blank) and carry out a signal strength test, as in most cases the signal is still there but is sitting at just under the level necessary to resolve a picture.
If though nothing is seen, then go into the boxes "manual tune" facility (should it allow) and enter mux Ch49 (BBC Mendip) but do NOT press search or scan, because the level that the signal is being received at should be indicated, giving an update on results.
Another point being, that before carrying out an auto-tune on a box or TV its always advisable to carry out a complete "reset" or "default setting" first to blank out anything already stored, if though the box has a "first time installation" facility then that should be suffice.
@Jb38: thank you for the advice. I've just tried doing this and the signal strength shows 'good' by putting it to ch49 but still no picture comes on. If I come out of that screen and go back in it has reverted to ch61. I think this is the only place I can do a manual tune. We have a freeviewbox upstairs (no recorder) which uses a plug in aerial and that is fine, but it picks up the Wales signal. Is there anything else you can suggest or do you think it could be a problem with the box?
Emma: I should really have requested the model number of the box that you are referring to, but though should it offer one of the reset options I mentioned in my previous reply then carry this out to make sure that nothing is stored in the tuners memory, then either (1) check to make sure that no EPG listings are seen (should maybe say "no signal or "no channels stored") and if not carry out a manual tune on Ch49, storing the results then checking that selecting BBC1 results in a picture, or (2) just simply carry out another auto tune then check BBC1 again.
Emma: Just as an after thought, as by not having indicated your post code I was unable to check on this, but the advice given was based on the assumption that you are receiving Freeview from Mendip and not the Bristol Kings Westontransmitter, because if you are using the latter then BBC is on Ch43.
@jb38 thanks again for all your advice. The device is a Sony RDR-HXD870. I've tried doing the easy set up which seems to be the closest to restoring to factory settings, I retuned without the aerial in to see if that cleared the memory.
I've tried this morning doing the manual retune to ch43 (yes on the kingsweston relay) but again it says good strength but the image doesn't come up! I'm concerned if I buy a replacement machine that I may have the same problems .... Thank you
If it is tuned to a particular UHF channel and that channel is changed on the signal strength screen (as you are doing), it reverts back or otherwise doesn't display. I tried this at switchover (when channels changed), before having to do a full retune.
You need to go to the menu: Initial Setup > Digital Tuner > Replace Channels. This (apparently) wipes the memory and starts again.
If it's reverting to C61 for BBC, then this implies Mendip was tuned before as that it what channel it used prior to Wednesday.
@dave Lindsay. Thank you. Yes have done the retune which brings up the channels but whenever it switches off (ie after it records) it untuned it. I've tried to see if there is a way for it not to switch off after recording but haven't found one yet! Any ideas?
Emma: See page 158 of the manual. It says that to reset it, turned it on and remove any disc that may be in the drive. Then hold down the Stop button on the recorder and press the power/standby button. Then power on and it should be reset.