Domestic Multi Channel Tv

8/17/2019 Domestic Multi Channel Tv

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IntroductionHow to combine and distribute aerial signals,satellite receiver outputs, VCR outputs, and othersources so that they can be distributed to anumber of locations.

1995 was a long time before the advent of digital television, and even analogue Channel 5was a nightmare yet to come. The basicprinciples outlined in the article still applythough, and it still has relevance to the modern

situation.

Incidentally, the article was written as Bill slowlyrecovered from a severe back injury. Heremembers, ‘I was immobile, in severe pain, anddrugged up to the eyeballs. The keyboard wastaped to a gantry above my chest, made fromaerial masts. The painkillers caused memoryloss, and when the article came out I had only aslight recollection of writing it.’

Many private houses, especially the larger andmore affluent ones, often have a surprisinglylarge number of tv sets in use. As well as theobvious locations we now find sets in Dad’sstudy or workshop (bolthole?), the gazebo, thegranny flat, and even places where only a fewyears ago tv watching would be regarded ashighly eccentric. Even some small properties,especially those where the owners are blessedwith a host of adult children, their live-in-loversand the fruit of the latters’ loins, seem to haveastonishingly large quantities of tv sets tuckedinto every possible nook and cranny. I have acustomer whose originally tiny terraced househas been enlarged over the years in everypossible direction in order to accommodate his

ever-increasing extended family. At the lastcount the house contained nine tv sets and fourVCRs.

Our customers range from the nouveau riche kidin his six bedroom, triple garage monument tobad taste, set in five acres with Olympic-sizepool, to the hard-pressed grandfather of 45 whofinds that his house seems to have turned into amulti-occupancy doss house and crèche. Thesepeople, and lots of others in between, have oneproblem in common that you can solve forthem. How to get good reception, includingsatellite and VCR playback, on all their tellys.

It may be that you are initially asked to fit asimple aerial for the tv set in the bedroom of,say, child number three. This can be regarded asno more than evidence of the customer’s lack of imagination. There is an obvious opening herefor the sale of a distribution system. This is notunscrupulous salesmanship. These people havegot about £3000 worth of consumer electronics,and none of it is working to anything like its fullpotential, just for the lack of a few hundred

pounds worth of tv distribution system. Theseinstallations, carried out properly, generate a lotof customer satisfaction, and frequently lead torecommendations. The existing hotchpotch of aerials, cheapo amplifiers, splitters, and loosecables probably has considerable irritant value.It might not have occurred to the customer thatthe VCR and satellite outputs could be piped allround the house, or even that everyone couldactually enjoy good terrestrial reception. Pointout that a proper distribution system wouldmean an end to interminable ear-splitting Mtv in

the living room, and to add extra psychologicalpressure make sure that the family teenagers arepresent during the discussion!

Domestic multi-channel TV distribution systems

Bill Wright from Television, October and November 1995

TV distribution systems in private houses are becoming ever more elaborate, and trial-and-error installation methods are not adequate. Some of the techniques used in major commercial installationsare now appropriate for the domestic environment. In this article Bill Wright considers the fundamental

principles of system design and looks at the use of channel pass filters and other components.


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Another possibility is that the house has hadsome sort of cobbled-up distribution systeminstalled, so the customer is aware of theconcept and the possibilities, and would be veryhappy if the system could be made to workreasonably well. Very often the opportunity for asale arises when such a customer moves house,and is determined to have the thing working

properly at his new address. Where the newhouse is being built to the customer’sspecification the opportunity arises to install areally excellent system, with all cables hiddenand ducting installed for future expansion. Getin at the planning stage: if possible use the siteelectricians to install the cables, following yourplans, as part of their first fix.

Before we discuss the design of good systemsthat obey all the rules of correct RF distributionpractice, let’s look at a representative example of

a cheap and cheerful system. Later in the article,I’ll describe how this system was redesigned. I’mnot necessarily saying that these cheap andcheerful methods are wrong, by the way. I spenttoo many years of my life installing fixed-priceaerials for national rental chains to take thathighfalutin’ attitude. Often the customer’s pricelimit or scale of priorities precludes anythingother than the most basic installation. After all, if the sole requirement is VCR reception on themonochrome portable in the kitchen so thatGrandad’s sudden craving for a bacon sandwich

doesn’t cause Grandma to miss the long-awaitedexhumation of Mr Jordache from under thepatio, then there’s nothing wrong with a splitter

in the back of the VCR, 5 metres of coax, a holein the plasterboard wall with a screwdriver, andoff to the next call sharpish. This article,however, is primarily concerned with the otherend of the market: the man who wants it all, andwants it all to work well. This is the section of the market which is rapidly expandingdownwards to take in the middle ground.

The system that had slowly evolved

Fig. 1 shows a system we found recently at thehome of Mr and Mrs Clutter. Mr Clutter makes avery decent living as a ‘general dealer’, but hisweakness is that he can’t bear to part withcertain items, so the house is, well, rather over-stocked. To make things worse, the Clutters arewell-off technofreaks who acquire gadgets of allkinds, never quite master their use, and thenleave them lying about gathering dust. Hugepiles of all sorts of things stand on everyhorizontal surface. Mrs Clutter has long agogiven up all pretence of domestic tidiness, andthe general impression is that the only clearspaces are those just wide enough for a humanbody of Mr Clutter’s generous proportions tomake the journeys essential to reach thearmchair, the fridge, and the lavatory. I wasnever able to work out quite who was whoamongst the other inhabitants of the house, butthere seemed to be plenty of them.

Initial impressions were that this was not apromising call. The feeling was strengthened

early Amstrad

satellite receiverRF in

decoder

resistive splitterand high gain

masthead amplifieron aerial mast

inductive Ysplitters

living roomTV set power supply

for masthead

amplifierRF out

1

inductive Y

splitter

3 outputamplifier

2 3

4 6 video recorderRF in

RF out

5

set topaerial

roof aerials

AB

Fig.1. Mr Clutter’s original distribution system


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when I saw the densely-packed, dust-coveredpile of assorted consumer electronics in thecorner of the living room. A massive tv set, aprehistoric Amstrad satellite receiver withoutboard decoder, two VCRs, assorted bits of hi-fi, and other apparently disused items, all gavethe impression that making even the simplestconnection at the rear would need help from

Pickfords. A badly faded press cutting in a framesurmounted the heap. It showed a remarkablyslim Mr Clutter in 1970s attire with a large silvercup and two prize greyhounds. Despite myinitial impressions I began to warm to Mr Clutterwhen he interrupted my nervous preamble—‘Your television system is a load of old rubbishand we’ll have to start from scratch…’ with anairy wave of his lager can, a scratch of hisimpressive abdomen, and words to the effectthat he was more than ready to pay out someserious money to get the tv system sorted. He’d

had bad service from the satellite installer, thelocal tv shop, and the local aerial rigger; theproblems had been going on for years, and hewas mighty fed up. I didn’t know it at the timebut I’d come highly recommended. Nor did Iknow that the local rigger’s last visit hadculminated in his being pursued from thepremises by Mr Clutter’s well-trained tyre-bitingpit bull. I arranged to commence work on thefollowing Thursday, by which time Mr Clutterwould have ‘sorted out the telly and that’—presumably with a JCB—and would even have

moved enough stuff out of the loft so that Iwould be able to squeeze in there.

Mr Clutter’s tv system was by no means aparticularly hair-raising example; it was typicalof a set-up that had not so much been installedas had evolved, over a period of about twentyyears. As requirements had changed, little bitshad been added piecemeal. There’s some out-and-out botchery, certainly; but mainly the faultswere just the result of ignorance of the basicprinciples of RF distribution. I’ll list these basicprinciples, and then we’ll look at how thissystem cheerfully disregards them. Then we’lllook at the alterations which were madesubsequently.

First, though, I must say a word about testequipment. It is not possible to do this jobprofessionally without the right gear. A reallygood signal strength meter is the minimum. Aspectrum analyser is highly desirable. Ananalyser is not a luxury: it will pay for itself overand over again.

UHF tv distribution systems:the basic principles

The off-air signals, both satellite and terrestrial,should be as strong, clean, and consistent aspossible.

Each signal source (tv channel) should, if necessary, pass though suf ficient filters before itis combined with other signals, so that itsaccompanying noise cannot interfere with othersignals.

Signals should pass through active devices at alevel within the correct operating range of thedevice.

The signal levels of each channel should beequal, or sloped to allow for differential cableloss if appropriate.

Channel allocation must obey the following rules:No two signals on the same channel (!)No two signals on adjacent channelsNo two signals 5 channels apart if possibleNo two signals 9 channels apart if possible

If for some reason strong local tv broadcasts arenot carried on the system, the relevant channelsshould not be used on the system.

Very strong unwanted signals such as local UHF

police repeaters should be filtered out at allinputs.

If the system has to carry VHF-FM radio, theVHF aerial input should pass though a bandpassfilter, and the filter output should be closelyexamined for high level (possibly intermittent)signals.

Off-air reception

At Mr Clutter’s location reception of the nearbyhigh-powered transmitter was badly affected byscreening and ghosting, and the only way to gethalf-decent reception was to use a distanttransmitter which gave rather weak signals, andTyne Tees ITV instead of Yorkshire. Hence thetwo aerials, connected together with a splitter.Since the received channels were 23 to 33 and41 to 51 respectively, a diplexer would havebeen far better than a splitter, since the aerialintended for reception of the distant transmitterwas able to contribute a lot of ghosting to

reception of the local transmissions. Theamplifier following the splitter was having ahard time. Input levels from the distant


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transmitter were about -14dB/mV on three of thefour channels, which is a reasonable level tofeed a two-stage masthead amplifier. Because of the cheap aerial used, channel 33 was at about–18dB/mV: badly designed aerials usually havevery poor gain on the top few channels of thegroup. Also entering the amplifier, however, wasthe output from the other aerial. Because of the

reception conditions from the local transmitterthe signal levels on each channel varied widelyand unpredictably. At times when any channelexceeded about +12dB/mV the cross-modulation from the masthead amp. waswondrous to behold, especially on the channelscoming in at about -12dB/mV, which of coursewere the ones without the ghosting…

The masthead amplifier was one of the verycheap ones with no input filtering of any kind—all signals from long wave to cellphones

cheerfully accepted. I didn’t know it at the timebut about 75m from the house was a policetransmitter using the 450MHz band, which Ilater found was putting about 20dB/mV for afew seconds every so often into the unfortunatemasthead amplifier. All this adds up to a reallyhorrible RF mess. No distribution system canwork from an aerial input like this.

The other off-air reception, of course, was fromAstra via an Amstrad dish with the old bluecapLNB. I’m sure it’s unnecessary for me to describe

the resulting reception quality. The aerial waslooped through the Amstrad satellite receiver. Asis customary, the outboard decoder was addingslight patterning to the lower Group A channels.Mr Clutter also showed me his ‘calmer-downer’.This turned out to be a 12dB attenuator whichsome previous expert had advised should befitted at the input to the Amstrad ‘when theweather was bad’. Following the Amstrad was amodern VCR. The output from this entered a Ysplitter . The other leg of the splitter introducedthe output of a Betamax VCR into the system.This machine had long ago lost the ability torecord, and so thankfully had no aerial input.

At the output of that splitter, marked ‘A’ on fig 1,all the various signals are together on the samepiece of coax for the first time. This is the RFspectrum which is fed into the amplifier, andwhich eventually finds its way to the tv sets. Let’slook in some detail at the output of splitter ‘A’.

Channel ‘planning’

Firstly, the question of channel allocations. TheRF output frequencies of the three applianceswith modulators would have been tweaked inorder to minimise mutual interference.‘Minimise’ is the word: not ‘eliminate’. Thefamily knew, and accepted without question,that if the satellite was on there would be wavylines on BBC2, and that if Mr Clutter felt theurge to relive the glory of his greyhound racingdays his Betamax playback would snarl up both

the satellite and the BBC1. These things hadbeen accepted in the Clutter household like therest of us accept supermarket queues andcontraflow.

Noise

Every active device adds thermal noise. Noiseon a system is additive: the more sources of noise there are the higher is the noise floor. Thehigher the noise floor, the harder it is to achieve

a signal-to-noise ratio which gives apparentlynoise free pictures. It doesn’t need much noiseto degrade a tv picture: thermal noise 30dBbelow signal has a visible effect. You can seethis clearly on a spectrum analyser, or you canset up a little experiment which makes the point.Connect up the arrangement shown in fig 2.With the power to the masthead amp turned off,adjust the variable attenuator until the picture is just grain-free. Turning on the masthead ampwill introduce noise: the effect is virtuallyindistinguishable from reducing the attenuator

setting. The signal to noise ratio has beenreduced by adding noise from the amplifierrather than by reducing the signal level. Theresult is the same. The designers of good qualitymasthead amplifiers strive to reduce the noisefigures of their products: for a more dramaticdemonstration precede the masthead amp withsomething noisy, such as a cheapo distributionamplifier. This is pure thermal noise with nomodulation. It makes tv pictures look ‘snowy’ or‘grainy’.

Even more pernicious is noise in the sense of ‘unwanted signals’, such as co-channelinterference or cross-modulation. Quite tiny

Ω

Test set-up to show that lifting the noise floorhas the same effect on the picture as reducingthe signal level

Fig. 2.Test set-up to show that lifting the noisefloor has the same effect on the picture asreducing the signal level


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signals can have a disastrous effect. A co-channeltv signal at –46dB will produce noticeablepatterning.

The point of all this is that the noise, boththermal and in the form of modulation, from twoVCRs, a satellite receiver, and a mastheadamplifier, were all present at splitter A. More

noise would be added at the distributionamplifier. If all the signals were well above thisrather elevated noise floor no great harm wouldbe done. But because nothing had been done toset the signal levels the signal to noise ratio of each channel was a matter of chance. Of course, the practice of cascading, or ‘daisy-chaining’ a satellite receiver and a VCR is almostuniversal, and usually no harm is done. Theproblems start when more and more equipmentis allowed to add noise, and where signal levelsare incorrect.

The amplifier

Signal levels entering the distribution amplifierwere very low. Splitter B was a Y splitter of theinductive type. Unlike the resistive version, it isnecessary to differentiate between the input andthe two output ports on these items. Very littlesignal passes from one output port to the other.Unfortunately, the fact that the output ports arefemale and the input port is male had led MrClutter to fit the input port directly into the

aerial socket of the living room tv set. Thisseverely attenuated the signal feed to thedistribution amplifier. Since the amplifier hadvery high gain at each output, measured signalstrength at each outlet was quite acceptable.Picture quality was very poor, however, due tothe poor signal to noise ratio. Just because themeter reading is OK, don’t assume that thepicture will be noise-free! The best terrestrialreception in the house was on the tv set fedfrom a set-top aerial!

The design and installationof the new system

Obtaining good off-air signals

Not wanting to take anything for granted, thefirst job was to check that the local transmittercould not be made to yield good reception. Testsshowed that this was in fact the case; a greatpity because the use of strong local signals

including the correct ITV region would havesimplified matters greatly. The use of a thirdpossible transmitter, Belmont, was considered,

but was ruled out because field strengths werelittle better than Bilsdale, and Belmont receptionis all too susceptible to co-channel interferencefrom Europe in this area, since an aerial whichis pointing at Belmont is also pointing atHolland and Belgium.

A good quality high gain aerial was duly

installed, pointing due north to Bilsdale, some60 miles away. This yielded –9dB/mV to–11dB/mV, which made me happy. BecauseBilsdale is north of here, we find that we canuse it without fear of interference from Europe,even where the field strength is low, so–11dB/mV is OK. A standard Band IV notchfilter was used to notch out the policetransmissions, 450MHz being just within itstuning range. Because the police transmissionswere of short duration and not very frequent itwas not possible to tune the notch filter directly

to the transmissions. Instead, a spectrumanalyser was used to determine the exactfrequency. The filter was then connectedbetween a broadband noise source and theanalyser and the filter tuned for a notch at theappropriate frequency. It was, of course,necessary to fit the filter in front of the mastheadamplifier. Through loss on the notch filter wasabout 1dB on ch23.

Discussions with the Clutter clan revealed thatthey absolutely had to have Yorkshire TV, but

they seemed oblivious to the regionaldifferences on BBC1 and BBC2. I decidedtherefore to put Yorkshire ITV only on thesystem, removing the other three channels. Thiswas done at the channel pass filter stage of thesystem—see later, and fig 3. There were anumber of advantages. The fewer channels thereare on a system the better, because the cross-modulation ceiling lifts 3dB every time thenumber of channels is halved. In this case, theEmley Moor signal levels varied widely, soreducing them to one channel greatly reducedthe potential for intermittent cross-modulation.Furthermore, if I had left dodgy versions of BBC1, BBC2, and Channel 4 available, theClutters would have undoubtedly tuned in tothem and then moaned. As it was, I stressed thatYTV should be regarded as a secondary service,only to be used when there was a differentprogramme on Tyne-Tees.

Careful positioning of the Emley Moor aerialyielded 17db/mV on Ytv. This was about 12dBbelow the line-of-site figure. The aerial wasaligned with reference to YTV reception only:there was noticeable ghosting on YTV, but twoof the other channels were very poor and onewas virtually unwatchable.


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Fig. 3. Mr Clutter’s new system


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Channel pass filters

In the case of a ‘dif ficult’ system like MrClutter’s, where the available off-air signals arenot good and where a variety of modulator-derived signals are involved, each and everysignal input must be filtered immediately beforeit is combined with the others. Since we are

dealing with tv channels 8MHz wide, the basicfilter used should handle one such channel, andshould have bandwidth just suf ficient to handleone channel without distorting the signal.

Channel pass filters are available from a numberof manufacturers, but the types with which I ammost familiar are imported by Taylor Bros of Oldham, and sold as the TCFL series. The filterscome in units containing one, two, four, or sixseparate channel pass filters. Each channelpasses through three tuned stages and a variable

attenuator. Through loss is usually 2 or 3dB. Atypical frequency response for a four channelunit is shown in fig 4. In all units the outputs of all channels are common; that is they all appearon the same socket; but any input configurationis possible. For instance, in the case of the fourchannel unit you can have four separate inputs,two inputs each of two channels, or one input of one channel and one input of three channels.This input configuration is specified whenordering, but it’s easy enough to alter. Likewisethe channels should be specified, but can be

altered afterwards within a limit of about ±7channels. Further retuning is possible, butthrough-losses may become excessive. It isimpossible to stock every conceivable filter, soaertain amount of re-configuring and retuning isoften necessary. Accurate retuning, however, isnot possible without a spectrum analyser and anoise generator. Simply retuning for maximumoutput on a signal strength meter will give verypoor results, as the tuned stages must be alignedfor correct bandwidth.

Planning signal levels

It is important that signals pass through anamplifier or other active device at a level whichis within the device’s correct operating range. Adomestic system will inevitably use a singlebroadband amplifier to bring the signals up to

the correct level for distribution. If levels are toolow, excessive noise will be introduced; if theyare too high cross-modulation and otherundesirable effects will appear. As a rule of thumb, run the amplifier at a maximum of about10dB below the level where cross-modulation isvisible.

Usually the amplifier will be the multi-outputtype, where separate cables are taken back tothe amplifier from each outlet socket. I am inthe habit of planning my signal levels

backwards: that is I start at the outlets andcalculate back towards the signal sources. Thismakes sense, sort of, because if there’s anysignal shortfall the compensating amplifiershould be in front of the channel pass filter, notafter it. At no point should the level drop belowabout +3dB/mV, or noise will be introduced. Toallow levels to get really low somewhere alongthe system and then to increase them byadditional amplification is no good at all. Inorder to calculate the input signal required forthe amplifier take into account the loss on the

longest downlead and the amplifier gain to eachoutput. Although it is supposed to be acceptableto provide a tv set with 0dB/mV, I always planfor no less than 8dB/mV at each outlet. Therequired amplifier input plus the filtering andcombining losses gives the signal level requiredfrom each signal source. In the case of MrClutter’s system the Amstrad satellite receiverproduced only 5dB/mV, which was notsuf ficient. A little set-back ‘booster’ amplifierwas used to correct this. It is most important thatany such amplification is fitted before the

channel pass filters. In cases where severalinputs to one filter unit are low it is verytempting to fit one amplifier after the filter unitrather than several before it, but this negates thewhole point of using filters, which is to keepeach channel clean and uncontaminated bynoise and interference from other sources.

The variable attenuators in the filter blocks areused for final adjustment of signal levels.Normally all channels will be set level, but if cable runs are long and channels at both ends of

the band are used, a little bit of slope might beapplied; say 4dB across the whole band. In thecase of Mr Clutter’s system, I was concerned thatthe ch47 Emley Moor signal might rise

nominal video frequencies(standard four channel group)

Fig. 4. Frequency response of a four channelpass filter. One channel has been set at -10dB


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suf ficiently to affect reception of other channels.Careful alignment of the aerial had produced asignal 12dB below the line-of-sight figure, so Iwas confident that in practice the level wasunlikely to rise more than about 6dB.Accordingly I set the ch47 level at –4dB relativeto the other channels. What if the off-air leveldrops dramatically? Well, the Clutters don’t get

to watch YTV, unless they want to pay me tomove the aerial to suit the changed receptionconditions. When the incoming signal isunreliable, make the customer understand thatyou cannot guarantee reception.

If your signal level planning is about right it islikely that you will drop each channel about4dB at the variable attenuator. If you have toleave any channel full up you have obviouslypushed your luck. If you have to drop anypreviously amplified channel much more than

about 10dB then you are using unnecessaryamplification, which adds noise, gives thepotential for instability, and is a waste of money.Always use just enough amplification. Forinstance, do not use a two stage mastheadamplifier only to discover that a 12dB attenuatoris needed at the distribution amplifier input:plan your signal levels and fit a single stagemasthead amplifier.

Combining signals

Because each channel strip within a block of channel pass filters consists of a series of tunedfilters, it is possible for the outputs to be simplyconnected together inside the unit withoutimpedance mismatch. In this way up to sixchannels can be combined with virtually noloss. Compare this with the alternative of combining using a six way inductive splitter,with losses of about 10dB.

In many cases all channels can be passed

through one block of filters, so the filter blockoutput can go straight into the amplifier. In thecase of Mr Clutter’s system two filter blocks wereused, and so the outputs were combined withan inductive splitter. This splitter should be a topquality item in a metal case. The ones that use‘f ’ sockets are ideal. There is no point in using adiplexer, even where this is possible. Three, four,six, or eight-way splitters can be used wherenecessary, but the lowest signal losses will beachieved by using the smallest possible numberof channel pass units, each carrying the

maximum number of channels.

Signal input for the VCR

If a VCR is feeding into a distribution system, adif ficulty arises if it uses a system output for itsaerial input, as would normally be the case. Asignal loop will exist, and if the overall gain of the loop is unity or more, the system willoscillate. For this reason a notch filter, tuned to

the VCR output channel, should be fitted on theamplifier output which feeds the VCR. Theunsuppressed lower sideband of the VCRmodulator output needs to be notched out, eventhough it will have been somewhat attenuatedby the channel pass filter. For this reason adouble notch is best, with the tuning slightlystaggered, as shown in fig 5. As an additionalprecaution ensure that signal levels at the VCR

input are not unnecessarily high. Care must betaken to ensure that the notch filter does notaffect the VCR’s reception of the channels justabove and below the VCR output channel. Thisis easy with a spectrum analyser; not so easywithout. If channel space allows it is helpful tohave two unused channels on each side of theVCR output rather than one. This makes theadjustment of the notch filter less critical. The RF

output of the VCR also includes all otherchannels on the system, but these are removedby the channel-pass filter, which is of coursetuned to the VCR output channel.

If a system is carrying nothing but off-airterrestrial channels all derived from one aerial,there is no need for the VCR to be fed from theamplifier output. It can be fed direct from theaerial, via a splitter. This avoids all thecomplications described above.

0dB

-5dB

-10dB

-15dB

-20dB

-25dB

-Fig. 5. Frequency response of a double notchfilter, with the tuning staggered slightly. Adeeper and narrower notch is obtained if the

two stages are tuned to precisely the samefrequency.


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Channel planning

There are 47 channels in the UHF tv band. Itsounds a lot, if your system only needs six oreight active channels. So what’s the problem?Well, there are constraints which must beconsidered if various forms of interference are tobe avoided. It is surprisingly easy to ‘run out of

channels’, in fact.

It may seem that I am stating the obvious, but notwo signals should occupy the same channel.Yes, I have seen it done, and more than once.Actually, I feel a funny story coming on, but I’llrestrain myself.

Adjacent channels should not be used. It ispossible to get the impression that you can getaway with adjacent channel working, but in thisdomestic context don’t do it.

As far as possible avoid five channel or ninechannel spacing. Some tv sets are verysusceptible to image interference and otherspurious responses, and local oscillatorinterference can also be a problem, though thisis less common. In practice it is not necessary toconsider the finer technical details. Simply avoidn±5 and n±9 channel spacing and you will keepout of trouble for 99% of the time. If there is areal shortage of channels, risk five channelspacing. It is less likely to cause problems than

nine channel spacing.

All signals should be exactly on the nominatedchannel. When setting the output frequency of the modulator in a VCR or whatever it isnecessary to tune it to a known referencefrequency. This usually means using thefrequency synthesised tuner in a spectrumanalyser.

The frequencies we can’t normally alter are theterrestrial broadcasts, and these form the fixed

points of the eventual channel plan. In anextreme case it might be necessary to translate achannel using a ‘channel changer’. This iscommon practice on commercial systems but isbest avoided in the domestic environment, if only on grounds of cost.

It is my practice when planning a system todraw up a chart of the UHF band (see fig 6). Thefirst things to mark up are the terrestrial channelsto be carried, with all their n±5 and n±9relations. Any strong local signals which are not

to be carried should be marked, together withany other channels where there is an obviouspossibility of interference. A scan with thespectrum analyser will ofteneveal these. The

channels derived from modulators can then beslotted in, making sure that all the rules areobeyed. VCRs and satellite receivers atsubscriber outlets should be connected to the tvset using a SCART lead if at all possible, and theRF output tuned to somewhere where it will dono harm. In the case of Mr Clutter’s system thechart showed that theoretically there was no

channel available for the bedroom VCR,although if it had not been possible to SCART itinto the tv set I would have used ch 31 or ch 35,checking for n±5 or 9 problems on channels 26and 40 on the associated tv set.

It is unfortunate that the modulators in mostVCRs and satellite receivers will only tune alimited range of channels in the middle of theband—roughly from ch30 to ch40. The result of this is that it is often dif ficult to find suitablechannels in the middle of the band, while vast

tracts of unused space lie at the top or bottomends. Mr C.’s system is a good example of this,with nothing above channel 47, but with aproblem finding a spot for the second VCRwithin the tuning range of its modulator. Thechannel chart showed up this problemimmediately. I envisaged having to put theBetamax output on ch31, which would consitutean n±5 clash with BBC2 Bilsdale and an n±9clash with the satellite output on ch40. I found,to my surprise and delight, that the Betamaxwould tune up to ch43— just. If that had not

been the case I would have had a problem.

It is a great advantage of Pace satellite receiversthat the modulator output can be set to anychannel in the range 21 to 68. About a monthafter Mr Clutter’s system had been installed herung up to say that the satellite reception wasvery poor. During the whole installation period Ihad deliberately avoided discussion of the meritsor otherwise of the aged Amstrad satellite set-up.Mr Clutter seemed genuine enough, but I feltthat I would like to be paid for the distributionsystem before embarking on anything else. I wasfairly sure that a satellite sale would materialiseeventually, but I preferred that it should be‘phase 2’; not an addition to ‘phase 1’. I dulycalled round to ‘have a look’. Reception on theterrestrial channels was extremely satisfactory,and this meant that the Clutters now knew forthe first time what good reception looked like.This was the main reason for the dissatisfactionwith the satellite reception, together with a weekof very rainy weather. Mr Clutter had been toldby the original installers that satellite receptionwould always be poor compared to ordinary tvbecause the satellite was millions of miles away,whereas ordinary tv only came from about tenmiles away, and he had accepted this argument


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completely because of its obvious logic andcommon sense. Mr Clutter was not short of money, and he liked his telly. Selling him a newsatellite installation was just a case of convincing him that satellite reception could beas good as terrestrial. This proved to be analmost insurmountable intellectual hurdle, butwe got there in the end. The possibility thus

arose of installing a Pace with its RF output on achannel at the top end of the band, but in theevent I kept to channel 40. This had beenentirely satisfactory, and I didn’t feel like fitting anew channel pass filter. Had a Pace receiverbeen installed at the start, however, I wouldcertainly have put it up on channel 58 orthereabouts.

Another cause of channel congestion is thebroadcasters’ use of the standard four-channelgroupings n, n+3, n+6, n+10, and n, n+3, n+7,

n+10. This came about because it was felt at thetime that ‘adjacent plus one’ channel spacingwould cause problems. The next option was n,n+3, n+6, n+9, but this was rejected because of the n+9 clash. The groupings now in use take upten channels. If ‘adjacent plus one’ spacing hadbeen used the four transmissions could havebeen accomodated within a bandwidth of sixchannels. This would have greatly simplified thechannel planning of multi-channel distributionsystems using fully-tunable modulators because,in cases where only one grouping of the four

terrestrial channels was to be carried, the‘adjacent plus one’ sequence could be extendedacross the full band. With the standardgroupings now in use a sequence of ‘adjacentplus one’ channels can be added, but channelshave to be missed out because of n±5 or 9clashes with the transmitted channels. Anotheradvantage of ‘adjacent plus one’ spacing wouldhave been the fact that the standard channelgroups could have been much narrower, with aconsequent improvement in aerial performance.

The standard groupings allow the insertion of one channel within the group: for instance inthe grouping 21, 24, 27, 31 channel 29 can beadded, and in the grouping 23, 26, 30, 33channel 28 can be added. This will, however,always result in an n±5 clash.

In conclusion

The complexity and cost of domestic distributionsystems varies enormously. At one end of thescale is the simple multi-outlet amplifier in

the loft supplying terrestrial signals to three orfour tv sets. At the other end are systemscarrying the outputs from perhaps three VCRs,three satellite receivers, and a couple of

surveillance cameras (via modulators), as well asa variety of terrestrial channels, to a largenumber of outlets. Most jobs fall somewhere inbetween these extremes. The first part of theinstallation process is to find out just what thecustomer needs and requires; to make himaware of the possibilities, if necessary; and tofind out how much he is prepared to spend.

Because of the many permutations which arepossible, it’s a good idea to have a separaterough price in mind for each part of aninstallation. This way you can build up a totalprice quite easily: simply add together the costof the aerial, the basic distribution amplifier, theadditional cost of adding each VCR or satelliteoutput, the cost of each outlet and feeder cable,and so on. There’s a good profit to be made fromdomestic tv distribution. A decent job can beworth as much as ten or more simple aerialrigging jobs. In these days, when the heyday of

aerial rigging is long gone, and everyone in thedomestic field is—let’s be honest—a bit short of work most of the time, domestic systeminstallation is a valuable source of extra income.If you go about the job methodically, plan inadvance exactly what you’re going to do, anduse good test equipment, each job can betrouble-free and profitable. And I know of noother aspect of our trade that leads so readily torecommendations and further work.


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Fig. 6. Channel planning on Mr Clutter’s system

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Domestic Multi Channel Tv