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Freeview modes - a simplified explanation

How is it possible to broadcast high definition television or more standard definition services on the same physical wavelength?

How is it possible to broadcast high definition television or m
published on UK Free TV

As mentioned in How digital television works Freeview 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.

Sub-carriers

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.

Freeview subcarriers

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.

Symbol duration

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.

Guard interval

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.

Freeview guard intervals

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.

Freeview FEC

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.

Freevoew Quadrature amplitude modulation

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:

  • Mode 1: DVB-T 1705 (2K) carriers, 64QAM mode, FEC=2/3, 1/32 guard = 24.13Mbps
  • Mode 2: DVB-T 1705 (2K) carriers, 16QAM mode, FEC=3/4, 1/32 guard = 18.1Mbps
  • Mode 3: DVB-T 6817 (8K) carriers, 64QAM mode, FEC=2/3, 1/32 guard = 24.1Mbps
  • Mode 4: DVB-T2 6913 (8KE) carriers, 64QAM mode, FEC=4/5, 1/32 guard = 34.7Mbps unused
  • Mode 5: DVB-T2 27841 (32KE) carriers, 256QAM mode, FEC=3/5, 1/128 guard = 36.1Mbps unused
  • Mode 6: DVB-T2 27841 (32KE) carriers, 256QAM mode, FEC=2/3, 1/128 guard = 40.2Mbps
  • Mode 7: DVB-T 6817 (8K) carriers, QSPK mode, FEC=1/2, 1/32 guard = 6.0Mbps
  • Mode 8: DVB-T 6817 (8K) carriers, 64QAM mode, FEC=3/4, 1/32 guard = 27.1Mbps
    "high capacity commercial multiplex mode".
  • Mode 9: DVB-T2 27265 (32KN) carriers, 256QAM mode, FEC=3/5, 1/128 guard = 35.2Mbps unused
  • Mode 10: DVB-T2 27265 (32KN) carriers, 256QAM mode, FEC=2/3, 1/128 guard = 39.2Mbps
  • Mode 11: "Northern Ireland mode" details TBC


Diagram showing capacity for each mode:

UK Freeview modes

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Comments
Friday, 29 June 2012
M
Mark Fletcher
sentiment_satisfiedGold

2:01 PM
Lytham St. Annes

Mike B,Hull.
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 !

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Mark Fletcher's 673 posts GB
Wednesday, 4 July 2012
M
Mr Ronald Dry
7:49 PM

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.

link to this comment
Mr Ronald Dry's 1 post GB
J
jb38
sentiment_very_satisfiedPlatinum

9:15 PM

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

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jb38's 7,179 posts GB
Thursday, 5 July 2012
S
Stan
11:06 AM
London

I would like to redirect my antenna to receive tv and radio transmission from Alexandra Palace relay transmitter, but I am worried that I may not get all tv channels I am currently getting directly from Crystal Palace (ie. RT, Al Jazeera etc.)Please adbvise.

link to this comment
Stan's 1 post GB
Dave Lindsay
sentiment_very_satisfiedPlatinum

11:29 AM

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 Freeview services, see here (those with a bullet in the "E"/England column apply):

DMOL Post-DSO Multiplex Channel Allocations

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.

For information about aerial groups, see:

Aerials, TV Aerial and Digital Aerial


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:

Crystal Palace Transmitter

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Dave Lindsay's 5,724 posts GB
Tuesday, 7 August 2012
J
JohnD
12:18 PM
Hexham

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?
Thanks

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JohnD's 1 post GB
Saturday, 11 August 2012
D
david durney
10:07 AM

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.

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david durney's 10 posts GB
J
jb38
sentiment_very_satisfiedPlatinum

3:29 PM

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.

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jb38's 7,179 posts GB
Saturday, 8 September 2012
J
jb38
sentiment_very_satisfiedPlatinum

1:26 PM

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.

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jb38's 7,179 posts GB
Monday, 10 September 2012
C
Chris Calver
10:05 PM
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

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Chris Calver's 1 post GB
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