Jim's Aerials - Channels & MUXs

Channels & MUXs

Analogue & Digital Signals

UHF TV channels occupy specific 8MHz wide frequency slots, with each one having a designated channel number. Grouped aerials can only receive a limited number of channels, so the TV aerial has to be matched to the channel frequencies in use - see the Aerials & Coax page for more information.

One analogue TV programme fills one 8MHz channel, and the four analogue channels have to be separated so that no two channels are adjacent, or are separated from each other by a gap of 5 or 9 channels. This is essential to avoid interference effects.

Digital TV programmes are quite different - several programmes can be combined and transmitted in a cluster, know as a "multiplex" (or MUX for short). One MUX can contain eight TV programmes, enabling digital TV to provide many more programmes. Each digital MUX also occupies one 8MHz channel slot, but MUXs can operate using adjacent channels and channel separations of 5 and 9 due to the transmission format (COFDM). This allows the digital MUXs to be "slotted in" around the analogue transmissions.

The following plot from a spectrum analyser shows the actual signal received from the Beacon Hill transmitter and illustrates some of the differences between an analogue signal and a digital MUX:

Spectrum Analyser - Beacon Hill analogue & digital TV

Each analogue signal has a large "spike" for the vision signal (i.e. picture) and a smaller "spike" for the audio signal (i.e. sound) which is 6MHz higher in frequency. The four analogue signals have been labelled corresponding to their programmes as seen at the TV. Prior to digital transmission, only these "spikes" would have been observed.

Each digital MUX has the appearance of a roughly rectangular "block" of signal, with a fuzzy top. At Beacon Hill, six digital MUXs are transmitted (since it's one of the main transmitters), but at reduced power to avoid interference (hence the MUX "height" is much lower than the analogue signals).

Each MUX has been identified with its current label. These are being changed after switchover - the following table gives the old and new labels, and shows the MUX classification; PSB (Public Service Broadcast) or COM (Commercial):

PSB
MUX 1	MUX 2	MUX B
BBC A	D3&4	BBC B

COM
MUX A	MUX C	MUX D
SDN	NGW A	NGW B

Only the 3 PSB MUXs will be transmitted from local relays after the digital switchover. The 3 COM MUXs (as well as the 3 PSB MUXs) will only be available from the main Beacon Hill transmitter.

Beacon Hill Channels & MUXs

The following table shows the channel line-up on Beacon Hill currently:

Channel	42	43	44	45	46	47	48	49	50	51	52	53	54	55	56	57	58	59	60	61	62	63	64	65	66	67	68	Aerial Group	Pol.
Content											MUX 1	CH 4	MUX B		MUX C	BBC 1	MUX A		ITV	MUX 2		BBC 2	MUX D					C/D	H
Power(kW)											1	100	1		1	100	1		100	1		100	1

The analogue channels are shown with a grey background. The four analogue channels and the six digital MUXs are all within the "green" channel numbers prior to switchover. Channels 48 to 68 match a group C/D aerial, so one standard aerial works for all these channels.

Post switchover, the Beacon Hill channels will be as follows:

Channel	42	43	44	45	46	47	48	49	50	51	52	53	54	55	56	57	58	59	60	61	62	63	64	65	66	67	68	Aerial Group	Pol.
Content	SDN			NGW A						NGW B		D3&4				BBC B			BBC A									E	H
Power(kW)	10			10						10		20				20			20

All six digital MUXs have been moved onto new channel numbers. This is to allow channels 63 to 68 to be "sold off". Two of the COM MUXs will move to channels 42 to 47 (shown with a yellow background), which are outside the specified channel range for a standard C/D aerial, so reception at these lower frequencies could be compromised. Group E aerials are designed to cover channels 35 to 68, and group W aerials will cover the whole range (channel 21 to 68). In practice, most existing group C/D aerials are likely to give an adequate signal, since channels 42 and 45 are relatively close to group C/D.

Programmes on MUXs

The following tables show the programme line-up for each MUX:

MUX	QAM	Channel Name	Provider	Service	Freeview
1	16	BBC ONE	BBC	TV	1
FFT:	2k	BBC TWO	BBC	TV	2
CH:	52	BBC THREE CBBC Channel	BBC BBC	TV	7 70
Class:	PSB	BBC NEWS	BBC	TV	80
		BBC Red Button	BBC	I/active	105

MUX	QAM	Channel Name	Provider	Service	Freeview
2	64	ITV1	ITV	TV	3
FFT:	2k	ITV2	ITV	TV	6
CH:	61	ITV3	ITV	TV	10
Class:	PSB	ITV4	ITV	TV	28
		Channel 4	Channel 4 TV	TV	4
		Channel 4+1	Channel 4 TV	TV	13
		E4	Channel 4 TV	TV	29
		More 4	Channel 4 TV	TV	14
		Heart	Chrysalis Radio	Radio	728
		Teletext	Teletext Ltd.	Teletext	100
		Rabbit	Teletext Ltd.	Teletext	102

MUX	QAM	Channel Name	Provider	Service	Freeview
B	16	BBC FOUR CBeebies	BBC BBC	TV	9 71
Class:	PSB	BBC Parliament	BBC	TV	81
FFT:	2k	301	BBC	I/active	301
CH:	54	Community 302	Community Channel BBC	I/active	87 302
		303	BBC	I/active	303
		305	BBC	I/active	305
		BBC Radio 1	BBC	Radio	700
		BBC 1Xtra	BBC	Radio	701
		BBC Radio 2	BBC	Radio	702
		BBC Radio 3	BBC	Radio	703
		BBC Radio 4	BBC	Radio	704
		BBC R5L	BBC	Radio	705
		BBC R5SX	BBC	Radio	706
		BBC 6 Music	BBC	Radio	707
		BBC Radio 7	BBC	Radio	708
		BBC Asian Net.	BBC	Radio	709
		BBC World Sv.	BBC	Radio	710

MUX	QAM	Channel Name	Provider	Service	Freeview
A	64	five	five	TV	5
FFT:	2k	Five US The Jewellery Channel	five Jewellery Chan Ltd.	TV	35 43
CH:	58	FIVER TVX Red Hot	five	TV	36 97
Class:	COM	G.O.L.D. UKTV Style TopUp Anytime 3 Teachers TV	UKTV UKTV TopUp TV Teachers TV	TV	17 26 40 88
		QVC	QVC	TV	16
		bid TV	Sit-Up Ltd	TV	23
		price-drop tv	Sit-Up Ltd	TV	24
		smileTV 2	SDN	TV	46
		SETANTA SPORTS TopUp Anytime 1	ITV TopUp TV	TV	34 38
		TopUp Anytime 2	TopUp TV	TV	39
		Nuts TV Gems TV1 CITV	Turner Entertainment Jewellery Chan Ltd. ITV	TV	42 44 75
		Smash Hits!	EMAP	Radio	712
		MOJO	EMAP	Radio	721
		Ttext Holidays	Teletext Limited	Teletext	101
		Teletext Casino	Teletext Limited	Teletext	103

MUX	QAM	Channel Name	Provider	Service	Freeview
C	16	Sky News	Sky	TV	82
FFT:	2k	Sky Spts News	Sky	TV	83
CH:	56	Sky THREE	Sky	TV	11
Class:	COM	Dave Smile TV	UKTV Cellcast Group	TV	19 37
		E4+1	Channel 4 TV	TV	30
		talkSPORT	talk Sport	Radio	723
		Premier radio	London Christian Radio	Radio	725
		Absolute Radio	Absolute Radio	Radio	727
		Sky Text	Sky	Teletext	108

MUX	QAM	Channel Name	Provider	Service	Freeview
D	16	ITV2+1	ITV	TV	31
FFT:	2k	Film4 TopUp Anytime 4 Gems TV Lottery Xtra	Channel 4 TV TopUp TV Gems TV Camelot Group	TV	32 41 43 45
CH:	64	4Music	EMAP	TV	18
Class:	COM	TMF	MTV Europe	TV	21
		UKTV History Virgin1	UKTV Flextech	TV	12 20
		Ideal World	CCI	TV	22
		The Hits Radio	EMAP	Radio	711
		Kiss	EMAP	Radio	713
		heat	EMAP	Radio	714
		Magic	EMAP	Radio	715
		Q	EMAP	Radio	716
		SMOOTH RADIO	GMG	Radio	718
		Kerrang!	EMAP	Radio	722

Programmes shown with a yellow highlight are subscription services (i.e. they aren't "free").

Some programmes share the same transmission frequency with other programmes, hence BBC THREE doesn't appear on the telly until CBBC has ended - the different Freeview programme numbers conceal this detail.

Digital TV uses software compression to squeeze several programmes into one channel space - the main benefit for viewers being the increased choice of programmes. The number of programmes in a single MUX depends on the modulation format (indicated by the number in the QAM (quadrature amplitude modulation) column for each MUX. A higher QAM number can carry more data (i.e. programmes), but reliable decoding of the digital signal becomes more error prone. Although four out of the six MUXs are QAM16 at present, they will all become QAM64 after switchover. Most digiboxes can work with both QAM16 & QAM64, but will need a re-scan when the modulation format is changed. Some digiboxes will rescan automatically - for the non-automatic ones, a manual rescan will recover the "missing" channels if they suddenly disappear for no apparent reason. Re-scanning digiboxes tends to be required on a fairly regular basis in any case, as Freeview programmes change, or get shuffled onto different numbers. Old OnDigital digiboxes unfortunately won't be able to receive the QAM64 programmes.

The QAM transmission rate only matters from the viewer's perspective when digital reception is marginal. In that scenario, the QAM64 programmes are more likely to break up than the programmes in a QAM16 MUX. After switchover, this comparison won't be available as everything will be changed to QAM64.

Having discovered what programmes are available on the six digital multiplexes, viewers receiving their TV from a local relay transmitter may be disappointed when they discover that they will only receive three MUXs. The local relay transmitters are all configured to transmit four analogue TV channels only. Transmitting six digital MUXs would require additional hardware (for two additional channels) at each relay site, which would be a substantial expense for over 1000 relay transmitters in the UK - this has not been factored in to the budget for the digital switchover process, and so only the 3 PSB MUXs will be provided.

Local Relay Channels & MUXs

The following list shows the existing analogue channel numbers for Beacon Hill and its local relays. The list also shows the aerial group & polarisation (see Aerials & Coax for more information), the transmitted power and the mean height of the transmitting antenna. The three relays shown with a red highlight on the aerial group are the ones likely to require a new aerial due to the channel changes for the digital switchover.

Site Name	Grid Ref.	Aerial Group	BBC 1	BBC 2	ITV	CH4	Power	Aerial Height
Beacon Hill	SX857619	C/D H	57	63	60	53	100kW	285m
Ashburton	SX758687	A V	21	27	24	31	3W	186m
Bovey Tracey	SX818787	B V	39	45	49	42	8W	112m
Brixham	SX921562	B V	40	46	43	50	18W	69m
Buckfastleigh	SX742664	B V	51	44	41	47	10W	117m
Chudleigh	SX876789	B V	51	44	41	47	8W	144m
Clennon Valley	SX885596	B V	39	45	49	42	4W	74m
Coombe	SX928736	A V	21	27	24	31	7W	101m
Dartmouth	SX875511	B V	51	44	41	47	10W	109m
Edginswell	SX886658	E V	39	49	45	67	4W	88m
Halwell	SX781528	B V	51	44	41	47	2W	180m
Harbertonford	SX780559	B H	39	45	49	42	2W	87m
Hele	SX912657	B H	40	46	43	50	7.5W	113m
Kingskerswell	SX873681	C/D V	55	68	48	65	4W	84m
Liverton	SX811734	C/D V	55	68	48	65	12.5W	169m
Newton Abbot HP	SX851713	B H	40	46	43	50	1W	70m
Newton Abbot VP	SX851713	B V	40	46	43	50	3W	70m
Occombe Valley	SX886625	A V	21	27	24	31	1W	112m
Sidmouth	SY136879	E V	39	49	45	67	20W	139m
South Brent HP	SX690607	B H	40	46	43	50	0.6W	212m
South Brent VP	SX690607	B V	40	46	43	50	4W	212m
Teignmouth	SX936735	E V	39	49	45	67	25W	100m
Torquay Town	SX915637	B V	51	44	41	47	45W	95m
Totnes	SX805594	A V	21	27	24	31	3W	115m

Post switchover, the following relays will have digital MUXs on the same channel numbers that were used for analogue, and hence shouldn't require a new aerial:

	Channel	Channel	Channel
Site Name	PSB 1	PSB 2	PSB 3	Power	Digital Aerial Group	Pol.
	BBC A	D3&4	BBC B
Ashburton	24	27	21	2W	A	V
Brixham	43	50	46	2W	B	V
Buckfastleigh	41	47	44	2W	B	V
Chudleigh	41	47	44	2W	B	V
Coombe	24	27	21	2W	A	V
Dartmouth	41	47	44	2W	B	V
Halwell	47	41	44	2W	B	V
Hele	43	50	46	2W	B	H
Newton Abbot HP	43	50	46	0.66W	B	H
Newton Abbot VP	43	50	46	2W	B	V
Occombe Valley	24	27	21	20W	A	V
South Brent HP	43	50	46	0.3W	B	H
South Brent VP	43	50	46	2W	B	V
Torquay Town	41	47	44	40W	B	V
Totnes	24	27	21	2W	A	V

The next table shows the Beacon Hill relays where new channel numbers / frequencies have been allocated for the digital TV signals, but these have been kept within the original aerial group, so these also shouldn't require a new aerial:

	Channel	Channel	Channel
Site Name	PSB 1	PSB 2	PSB 3	Power	Digital Aerial Group	Pol.
	BBC A	D3&4	BBC B
Edginswell	52	48	56	2W	C/D	V
Kingskerswell	62	59	55	2W	C/D	V
Liverton	62	59	55	2.5W	C/D	V
Sidmouth	52	48	56	6W	C/D	V
Teignmouth	52	48	56	40W	C/D	V

N.B. Edginswell, Sidmouth and Teignmouth have gone from a Group E aerial to a Group C/D aerial. This looks like an aerial change, but a Group E aerial covers all of Group C/D and more, hence the existing Group E aerials should work fine.

The following table contains the main Beacon Hill transmitter and three relays where the new channel numbers / frequencies have been allocated which lie outside of the existing aerial group:

	Channel	Channel	Channel
Site Name	PSB 1	PSB 2	PSB 3	Power	Digital Aerial Group	Pol.
	BBC A	D3&4	BBC B
Beacon Hill	60	53	57	20kW	E	H
Bovey Tracey	52	48	56	3.6W	C/D	V
Clennon Valley	52	48	56	2W	C/D	V
Harbertonford	52	48	56	2W	C/D	H

	Channel	Channel	Channel
Site Name	COM 4	COM 5	COM 6	Power	Digital Aerial Group	Pol.
	SDN	NGW A	NGW B
Beacon Hill	42	45	51	10kW	E	H

Beacon Hill has two channels moving to lower frequencies, which changes the aerial group from C/D to E. Since a typical aerial response drops more slowly for lower frequencies, most existing group C/D aerials should still work.

Bovey Tracey, Clennon and Harbertonford all have their channels moving from a group B aerial to a group C/D (i.e. higher frequency). These locations are more likely to require a new aerial for digital reception.

Switchover Sequence

The switchover process happens in two stages, which normally lasts approximately one month for each transmitter or relay. For the Beacon Hill and Stockland Hill transmitters, however, this period will last just two weeks.

The first step is typically to switch off the BBC 2 analogue signal, move the ITV analogue signal onto what was BBC2, and then put the BBC A MUX (carrying BBC1, BBC2, BBC3 / CBBC and BBC News 24) onto the channel that was carrying the ITV analgue signal.

Viewers getting their signal from Beacon Hill will need to re-scan any existing digiboxes to find the BBC A MUX in its new location.

For local relay viewers, there will be two immediate issues:

1) When programme "2" is selected on an analogue TV, it will display ITV (which can be resolved by delving into the setup menu on the TV set - but it's hardly worth the effort for just one month).

2) To receive BBC2, a working digibox will be required straight away - but only for the four BBC programmes. Watching ITV or Channel 4 will still require an analogue TV.

Recording programmes on a VCR during the one month transition will also require appropriate selection of either the VCR's own analogue tuner (normal operation) or its SCART input (for recording from the digibox).

For the next 2 weeks (or month for other transmitters) nothing happens, other than the remaining analogue TV programmes will carry a banner message informing viewers that they will need to prepare for the the completion of the switchover process. As the deadline approaches, the messages get bigger and appear more frequently.

At the end of this period, the remaining three analogue programmes will be removed, and replaced with the appropriate digital MUXs.

This just leaves one final question: "What is the MUX power during the switchover process"?
Despite concerns about interference, it appears that the BBC A MUX will transmitted at full power at the start of the switchover process. At the end of the one month period when the other MUXs are added (or moved for Beacon Hill), there will be no more analogue programmes, so all MUXs should be at full power.

Channel Interference

Interference can occur between TV channels, which can produce annoying picture effects such as horizontal lines, or possibly make the picture unwatchable. Analogue TV channel separations are particularly sensitive to this, and need to be selected carefully. The rules for analogue TV transmission are as follows:

No adjacent channels

No channels separated by ±5

No channels separated by ±9

Taking Beacon Hill (with analogue channel numbers of 53, 57, 60 and 63), there are no adjacent channels and no channel differences of either 5 or 9 (the difference between any two are either 3, 4, 6, 7 or 10 channels). To find out what's "special" about channel differences of 5 and 9, we have to examine the spectral content of an analogue TV signal, and take account of the the way a domestic TV tuner works.

A few years back, prior to any digital transmissions, the RF spectrum of a single 8MHz analogue TV signal looked like this:-

The edges of the 8MHz "channel" are indicated by the black dashed lines.

The vision carrier is located 1.25MHz above the channel edge. The vision signal (luminance) is amplitude modulated, which would normally generate equal sidebands extending to 5.5MHz on either side of the main carrier. This would give inefficient use of the available spectrum, since only one sideband is required to recover the modulated information. Since ideal single sideband transmission would be expensive and difficult to achieve, a compromise using a "vestigial" sideband for the lower frequency has been adopted. This gives an upper sideband of 5.5MHz and a lower sideband of 1.25MHz, and TV tuners contain a shaping filter to accommodate this frequency distribution. The blue luminance curve above indicated the frequency distribution for peak sync (so normal picture content will have a lower amplitude in the sidebands).

The chrominance (colour) carrier is located 4.43MHz above the vision carrier (shown by the pink dotted line). The upper sideband extends by 1.07MHz and the lower sideband by 1.3MHz, hence a small proportion of the chrominance information can also appear in the vestigial sideband.

The sound carrier is 6MHz up from the vision carrier and provides frequency modulated mono (i.e. L + R) sound, with a peak frequency deviation of %#177;50kHz, indicated by the yellow curve above.

The NICAM (Near Instantaneous Companded Audio Multiplex) stereo signal (cyan curve above) is added into the analogue channel space to carry separate L and R sound that has been digitised into a 728kbit/sec data stream. Even though the rest of the TV channel is analogue, the NICAM stereo sound has always been digital.

The dominant factor that affects adjacent channel transmission for analogue TV is the vision carrier's vestigial sideband which extends into the adjacent lower channel space, which would give interference. This is why adjacent analogue channels are not transmitted.

When digital transmissions were introduced, they had to be fitted in between existing analogue transmissions, so the adjacent channel issue had to be addressed. Current analogue transmissions are filtered to truncate both the "out of channel" low frequency content from the vision vestigial sideband and the "out of channel" high frequency content from the NICAM stereo, so that the signal powers in the "edges" are significantly below the level of the digital MUX content. In addition, each digital MUX fills 7.61MHz out of the available 8MHz window, leaving a small "guard band".
N.B. When digital transmission power is increased after switchover, adjacent digital channels will still be avoided to prevent interference. The current arrangement with digital channels adjacent to other digital and analogue channels was implemented only as a compromise.

Taking a wider view of the frequency range, across a number of channels:

Analogue TV channel with L.O. and image channel

A single analogue channel being transmitted is depicted on the left (shown with just the vision peak sync envelope and the sound carrier). To "tune" this channel on a domestic TV set, a local oscillator inside the TV tuner is set to a frequency that is 39.5MHz above the vision carrier (indicated by the pink dotted line). By mixing the received signal with the local oscillator, sum and difference frequencies are produced, and the difference frequency (39.5MHz, the "intermediate frequency") is processed inside the TV to extract the picture and sound information.

Looking at the chart above, the local oscillator can be seen to be in the channel space that is 5 above the wanted analogue signal. This is why a channel separation of 5 is avoided - any signals here, other than the TV set's local oscillator will give severe interference. The local oscillator signal also comes in handy for the TV license detector van!

The problem with using a local oscillator and intermediate frequency system is that a 39.5MHz signal can also be generated by a TV channel that is 39.5MHz above the local oscillator frequency. This is known as an "image" channel, indicated by the blue dotted curve on the right in the chart above. RF filtering can be used to reduce the impact of this higher frequency, but since analogue TVs are very sensitive, even with substantial filtering it is still necessary to avoid having transmissions in this space. The diagram shows that most of the interference "bandwidth" will occur 9 channels above the wanted analogue TV channel.

UHF Modulators

UHF modulators are used in VCRs and SKY satellite receivers, and can be used to distribute TV signals around the house using coax cable (see the TV Distribution section).
Domestic UHF modulators have two important limitations:

Mono sound only

Double sideband modulation

Mono sound isn't a great limitation, but should be borne in mind when configuring e.g. a satellite receiver setup. A Sky box connected to an expensive TV set via the RF output will work (and seem easy to use), but improved picture quality and stereo sound will be available using the SCART interface.

Double sideband modulation is more of a limitation, which makes the task of searching for an "interference free" UHF channel on the TV set more difficult. As described above, a transmitted analogue TV channel has a vestigial sideband at frequencies below the vision carrier. Domestic UHF modulators are small, low cost items which have no finesse like this - they use double sideband modulation, and as a consequency occupy two adjacent 8MHz channels - the wanted one and the adjacent lower one. This gives twice the scope for interference, and requires the adjacent channel upwards and the two adjacent channels down in frequency (from the notional modulator channel) to be avoided.

Avoiding the ±5 channel spacing now requires avoiding channels at +4 and +5 and at -5 and -6

Avoiding the ±9 channel spacing similarly requires avoiding channels at +8 and +9 and at -9 and -10.

All of which is a bit more complicated, especially when one factors in the other analogue and digital channels, all of which must be avoided similarly. In general, the easiest method is to choose a channel number for the modulator that is more than 10 above or below the main transmission grouping. The only complication then is to avoid other weak / distant transmissions that may be received at a low level from the side or back of the TV aerial.