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How to connect two TV sets to one VCR In September, we ran a project that could extend a video recorder's IR remote control to allow operation from another room. It was a great little project but it didn't explain just how the remote TV set was also connected to the VCR. Here's how to do it. By JIM LAWLER So how does the VCR in the lounge room feed its picture to a second TV set in the bedroom? Well, it's quite easy to arrange a simple two-TV set-up in any home of average size. The first thing to understand is that the RF signals at the output of your VCR are at a very high frequency and very low voltage. The signals are measured in microvolts which means that special care must be taken with distribution if they are not .going to be lost along the way. First and foremost, it is necessary to understand that high fre- quency transmission lines are not like ordinary power cables. They have to be properly matched to the VCR and the TV if the signal is not to be dissipated in the wiring. Second, if a cable is not properly terminated, it can set up reflections along the line which show up as ghosts on the picture. To avoid signal losses and ghosts, it is important to match the impedance of the transmission line to the output of the VCR and to the input of the TV. For convenience, this impedance has been standardised at 75 ohms and all domestic equipment is designed for this value. So, provided that you select cable and hardware suited to this application, no problems will be encountered. To avoid complex mathematics in signal level calculations, engineers and technicians have adopted the use of decibels as a convenient measure of signal level. For TV signals, most antenna installers use dBµV, or decibels above 1 microvolt. By using dBµ V values, it is only necessary to add together all the losses in the system and subtract them from the signal level available to arrive at the level that will be available at the TV. Or working the other way, one adds the calculated losses to the required signal level at the TV, to arrive at the necessary input level from the VCR or other source (eg, an antenna). If that sounds confusing, don't worry. We'll give you a few examples to show how it's done, For the rest of this article, we will talk about dBµV on the understanding that older TVs need 60dB of signal to give a good picture. Modern sets need only around These TV signal splitters are made by HPM Industries Pty Ltd and are available from large hardware stores (eg, BBC Hardware & Mitre-10). The unit at left is a 2-way splitter box (Cat. 406/2S/TV) while at right is boosted 4-way splitter (Cat. 406/B4S/TV) which has an overall gain of 8dB at each outlet. 6 SILICON CHIP L.....______.1 VCR VCR TWO-WAY SPLITTER i----- □ LOCAL TV (b) ------.0 Dc::::::::::l REMOTE TV VCR Ct:::=::J LOCAL TV (c) 1 2 3 TO REMOTE RECEIVERS Fig.1: here's how to split the output from your VCR to feed two or four TV sets. In the arrangement shown in (b), a simple 2-way splitter is used to split the output from the VCR, while at (c) the output from the VCR is amplified before being fed to a 4-way splitter. The amplifier & splitter may be separate units or combined in a single box. 55dB for the same result but it's wise to use the higher level to leave a good margin of safety. For difficult or critical applications, it is often necessary to measure the available signal with a signal strength meter. However, for domestic applications, an approximation of signal strength will usually give satisfactory results. Most video recorders have an output level of about 65dB. This represents the level of signal that would reach a TV set fed from an average antenna in an average reception area. Thus, if the VCR output is fed to a single TV at this level, the picture should be just about perfect. Even allowing for poor contacts in the plugs and sockets, enough signal should still be left over to give an excellent picture. Splitting the signal If one wants to split the signal between two TV sets, certain precautions must be taken. As already mentioned, the impedance must be matched and maintained throughout the system. This means that the leads to the TV sets cannot simply be cut and twisted together, as this would result in a gross mismatch of impedances. If this were done, you would get ghosts and a very noisy (or snowy) picture. Instead, a splitter umt must be used which allows accurate matching and a minimum of signal loss at the split. Fortunately, splitters are readily available from electronics retailers and even from most hardware stores. Splitters come in many styles, some square and boxy and others similar in appearance to the common mains power outlet. Depending on your application, you can get 2-way splitters (to feed two TV sets), 3-way splitters and 4-way splitters. In-line splitters that connect to the cable with common coax plugs are also available but these are not the best choice because the plugs and sockets can work loose after awhile. The wall mounted, screw fixing type are much more reliable. The main necessity is that the splitter be designed for 75-ohm systems. It must also be a wideband type if it is to handle UHF signals. In the normal domestic VCR installation, a lead is taken straight from the VCR's "RF Out" socket to the TV's antenna socket (Fig.la). For a two-TV system, we need to split the single output cable from the video using a 2-way splitter. One output from the splitter is then connected to the local TV while the other is connected to the remote TV in the other room (Fig.lb). The cable to the remote TV should be as short as possible otherwise cable losses may become a problem. If the cable length is 10-20 metres or so, the losses can usually be ignored (except in difficult cases). But 40 or 50 metres of cable will cause a loss that must be allowed for in most installations. The need to keep the lead short means that it is best run under the floor. This way, the cable can be run in a straight line from a point near the VCR to a spot immediately under the outlet in the other room. The best procedure is to pull the carpet back, drill a 10mm hole for the cable hard up against the skirting board, and then fit the splitter (or outlet socket) just above the hole. When the carpet is pushed back, the cable will be almost invisible and the whole assembly is neat and inconspicuous. Calculating signal levels The best place to start this kind of installation is to fit an outlet socket at the remote TV end of the cable. To get a good picture, you will need 60dB of signal at this point. The next step is to measure the cable length back to the VCR and add the loss represented by this length of cable. Let's say it was 2dB, which is an average figure for a bout 15 metres of domestic grade coax. This means that there must be at least 62dB of signal fed into the cable. We now need to take into account the losses in the splitter. A NOVEMBER 1990 7 4-way splitter. The losses in these items can amount to 5, 7 or even 9dB, so they become quite important in the calculations. Note that you can use a 4-way splitter to feed three TV sets, although it will have somewhat higher losses than a 3-way type. If you do use this kind of setup, the unused output should have a 75-ohm resistor connected across it to ensure correct matching. Do not leave the unused output open circuit. Distribution amplifiers These splitter units are sold by Dick Smith Electronics but similar units are also available from other retailers. The units shown are a 2-way splitter, a 4-way splitter & a 2-way plug-type splitter (bottom). This photo shows the 2-way & 4-way splitters with their covers removed. All you have to do is connect the lead from your VCR to the input terminal, then run coaxial cable from the outputs to your TV sets. good quality 2-way splitter will have an insertion loss of 3dB on each leg. This means that the input to the splitter must be + 3dB above its calculated output, or 65dB. Notice how the losses are added all the way from the output to the input. In this exercise, we have calculated that we want 65db into the splitter and this is just about what 8 SILICON CHIP is supplied by the average VCR. If the cable loss was higher (as it might be for a large house or if the cable could not be run via a direct route), then there could be too little signal at the remote TV. In this case, some kind of amplifier would be needed. Also, if you want to send your VCR signals to more than two TV sets, you will need to use a 3 or For anything but the most compact systems, you will almost certainly need a small distribution amplifier to compensate for the higher losses in the multi-way splitter. Various manufacturers have recognised this need and have provided a series of small distribution amplifiers for domestic use. These give from 10 to 25dB signal gain and can compensate for the losses in any reasonable home situation. Mostly, they are powered from the mains, usually via a 6-12 volt plugpack supply. The amplifier can be mounted close to the back of the VCR or on the skirting board close to the power point. The "RF Out" from the VCR goes to the amplifier input, while the amplifier output is fed to the splitter (Fig.le). Do not install the amplifier after the splitter. (Some years ago I made my own distribution amplifier from a disused masthead amplifier. I modified the input to suit 75-ohm coax and arranged a 12-volt power feed from within the VCR itself. This amplifier provided 26dB of gain following the VCR output and allowed me to reticulate signal to the TV set in the lounge, as well as to the kitchen, a bedroom and to four outlets in my workshop downstairs!) The simple distribution system described here will suit up to about four TV sets. If more sets are needed on the system, or if long ea ble runs are necessary, then another system of distribution will be needed. This is the "Main Trunk" system which is simple enough in principle but somewhat beyond the scope of this article. Maybe another time. ~