Finding a Free CH Number

Rick_

Here's one for you mathmeticians...

I have made a list of all the available channels to me, their respective CH frequencies and transmitter names. I just want to check that I have it right as I understand for tuning in an analogue CH frequency you can't use a CH number that is +/-3 or +/-9 from another number. So, going by my chart, it would seem that only CH 41, CH 44 and CH 64 are suitable for using as a spare UHF frequency for say the RF output of a Sky box so it can be watched in another room. I understand that the interference does not apply as much for digital (hence why some are on consecutive CH numbers) but I have included them on the list to show which channels they occupy and which they would interfere with. I have put the numbers beside each frequency for the channels that can't be used in relation to it, could someone take a peek and see if I am right? This is based on +/-3 and 9, but if these numbers are wrong, please correct me!

CH - SERVICE (TRANSMITTER) CH'S NOT AVAILABLE
21 - CH 4 (DIVIS) 24, 30
23 - UK MUX (DIVIS) 26, 32
24 - UTV (DIVIS) 21, 27, 33
26 - UK MUX (DIVIS) 23, 29, 35
27 - BBC 2 (DIVIS) 24, 30, 36
29 - UK MUX (DIVIS) 26, 32, 38
31 - BBC 1 (DIVIS) 22, 28, 34, 40
33 - UK MUX (DIVIS) 24, 30, 36, 42
34 - UK MUX (DIVIS) 25, 31, 37, 43
37 - CH 5 (BLACK MOUNTAIN) 29, 34, 40, 46
39 - BBC 1 (ARMAGH) 30, 36, 42, 48
42 - CH 4 (ARMAGH) 33, 39, 45, 51
45 - BBC 2 (ARMAGH) 36, 42, 48, 54
48 - UK MUX (DIVIS) 39, 45, 51, 57
49 - UTV (ARMAGH) 40, 46, 52, 58
52 - RTÉ ONE (CLERMONT) 43, 49, 55, 61
53 - IRE MUX (CLERMONT) 44, 50, 56, 62
56 - RTÉ TWO (CLERMONT) 47, 53, 59, 65
57 - IRE MUX 48, 54, 60, 66
60 - IRE MUX (CLERMONT) 51, 57, 63
63 - IRE MUX (CLERMONT) 54, 60, 66
66 - TV 3 (CLERMONT) 57, 63
68 - TG 4 (CLERMONT) 59, 65
CHANNELS NOT MENTIONED IN TABLE 41, 44, 64

What's interesting / confusing to me is that given the +/- rule, there are some analogue channels that exist on these slots and there is no interference, and some do have them. For example, BBC 1, Channel 4 and BBC 2 from Armagh are all +3 from each other, yet no interference. But CH 57 seems to be interfering with CH 66 (+9 away). Perhaps someone could explain it to this dum-dum about it so I can get my system set up with better pictures. Thanks.

Antenna

Paddy C wrote: »

you can't use a CH number that is +/-3 or +/-9

who says that a 3 channel gap is bad? It isnt.

Regarding 9 channel gap, a stronger signal 9 channels above a weaker signal can lead to problems, but not the other way round, unless your TV tuner is designed differently to the usual.

watty

All the oringinal rules assume a particular kind of Tuner with a particular L.O. to RF relationship and 39MHz approx vision IF. Not all tuners work this way anymore.

Some tuners are dual conversion with a 1.2GHz 1st I.F. Such tuners cope with any channel spacing or none.

Digital systems may use a Zero IF concept with negative frequencies mathematically by using I & Q (in phase and 90 degree signals via RF conversion of two multipliers fed by 0 and 90 L.O.). It's possible to do Analogue reception also with suitable DSP as well as DVB-s, DVB-c, DVB-t etc. For such tuners the original issues don't exist.

9 x 8 = 72 MHz, approximately twice the IF, thus at the vulnerable "image" frequency. Depends on quality of design if this is an issue or not. With Dual IF conversion it's irrelevent.

Replacing poor pre-amps, ditching rubbish amplified aerials and even fitting an attenuator can all reduce inter-channel interference and cross-modulation. Too much signal or poor amplifier is worst scenarios.

Antenna

Paddy,
you list NI channels coming from two different sources. I assume you only use one.
Obviously you could use a channel adjacent to, or +9 above a channel you do not actually need to receive (the weaker of the two sources) for your modulator, since it is not going to matter if a channel you do not use suffers interference.

Rick_

I don't use the Armagh signals, I use Divis as I also get Freeview, but because the Armagh transmitter is so close I still pick them up quite strongly, even though the aerial is a different polarity and pointed in another direction!

So, taking +9 from all the available signals, it seems that only 22, 25, 28, 41, 44, 47, 50, 55, 59, 64, 67 are free to use then, but with analogue signals being beside 22, 25, 28, 42, 44, 47, 50, 55 and 67 it leaves only 59 or 64 as potential places for a spare UHF slot? Have I got that right?

The Cush

Paddy C this link might be of interest to you
http://www.kat5.tv/taboo.html

Antenna

That site makes the taboo channel issue to be more serious than it actually is.

If you do not watch the 'Armagh' signals, using a channel +/- 1 from these should not be a problem
What about channel 41 or 44? try one of those for a modulator and see how you get on


BTW its a pity that Modulators in Skyboxes etc aren't also designed to cover some of the 'Hyperband' Cable channels around 300MHz (as well UHF)!

In areas with a very occupied UHF, it would be another option, provided the TVs have Hyperband tuners

Rick_

I tried 35 last night and the picture came through fine, but a little grainy, but I just realised that the TV the signal will be sent to will actually be using the Armagh channels (doh!) and the Clermont channels so it seems anything from 21-37 should be fine for it. As it will be a stronger signal than the ones being received from Belfast, then there shouldn't be any interference problems. I will have to get another fiddle tonight and see if anything in the 20's gives a clearer picture than 35 does. Thanks for the replies guys!

watty

The Skydigibox modulator is noisy and causes a lot of interference. I find it better at a high channel than a low one, as at ANY low one it degrades 39, 42, 46 & 49 (the Woodcock Analogue Group).

Also built in Modulators use part of the next lower channel as they are full double sideband, not Vistigial Sideband like true Broadcast Analogue. You can get Helical filters where each one only allows a specific channel. They are then all combined to feed the distribution system with a much cleaner result on all the TVs. In such a case I don't loop the RF through a Sky box.

You might find not looping RF through a box but using an external Y adaptor to combine a modulator to aerial signal works cleaner.

lawhec

The convention for "taboo" channels on UHF is + or - 5 or 9, not 3. Otherwise the way the channels are broadcast currently from most transmitters would interfere with each other.

In any case, this convention was thrown out the window when Channel 5 started in the UK when the likes of Black Hill and Preseli used channel 37 even though channel 46 is also used from the same site. Another one is the use of channel 35 from The Wrekin (England) and Darvel (Scotland) where channel 26 is also broadcast.

The use of DTT multiplexes in most cases using adjacent channels to current analogue broadcasts in the same aerial groups has thrown this convention completely out the window. Nevertheless, some old and newer (cheapish) makes of TV's might still suffer problems.

One way to tackle this problem is through using a filtered combiner often used for combining the signals from two aerials that receive different services in two different aerial groups (classic case of receiving BBC/UTV/Channel 4 from one aerial and RTÉ/TV3/TG4 from another). In this case one aerial, if all signals fit within a certain group of channels, is sent to one input while the other is not connected to an aerial but rather has its input terminated either with a resistor or (if it uses an F connector) using a terminator. For example if all the wanted signals were in the group A UHF frequencies, the aerial would be fed into the input covering channels 21-37, while the other input would be properly terminated, leaving the upper end of the band clear to use for modulated signals. However for a Divis/Clermont Carn combo this would prove difficult as ideally the Divis aerial will be a group K for all services and Clermont will be group C/D. A wide-ish notch filter from channels 40-45 (to give enough elbow room to cause little/no effect to channels 37 & 48) could be one solution, but in reality that is like trying to crack a nut with a sledgehammer for the effort to be put in.

These days, where possible I try to use direct connections where possible but with the folks wanting Sky pumped around the house, the modulator for that is set to channel 67.