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Clean up your video signals with this: Video-audio booster for home theatre If your home theatre setup involves sending video sign­als over fairly long cables, you’ll really appreciate this project. It’s a wideband amplifier that can boost both composite and S-video signals, or even component video signals with the right cables. And it handles stereo audio signals as well. By JIM ROWE W HEN SETTING UP a home theatre, there’s often a need to run fairly long video cables between your signal sources (DVD player, VCR and/ or laserdisc player) and your big screen display. The reason for this is simple it isn’t always convenient to have the signal sources and the display at the 18  Silicon Chip same end of the room. Of course, there’s no great problem feeding audio signals over long cables, provided that the cables are of reasonable quality. However, that’s not the case with video signals due to their much greater bandwidth. Video signal frequencies can range up to 5MHz or more (as against just 20kHz for audio) and can suffer quite noticeable degradation when fed through cables longer than about five metres. This signal degradation is due mainly to cable capacitance. This causes high-frequency losses and occurs even when you use high-quality coaxial cable that has been correctly terminated at each end. The resulting pictures lack contrast and colour satura­tion, and also become noticeably “softer” (ie, lacking in fine detail) due to the lower bandwidth. Video booster The best way to tackle this kind of problem is to use a video “booster” every five metres or so. Basically, you take a 5-metre cable run and plug it into the booster – essentially a wideband video amplifier. The booster siliconchip.com.au Fig.1: this diagram shows how the video booster is connected for composite video signals (top), S-video signals centre and component video signals (bottom). It’s basically a matter of buying (or making up) the necessary cables. restores the incoming signal so that it is close to original before feeding it to the next 5-metre cable run and so on. A booster for conventional “composite” video signals needs just a single wideband video amplifier channel. However, if you want to take advantage of the higher quality available from the “S-video” output of your DVD player, the booster needs two chan­ nels. That’s because, in S-video, the luminance (“Y”) or black-and-white picture information is not combined with the chromi­nance (“C”) or colour information. Instead, the two signals are fed along separate cables to prevent them interacting – see Fig.1(b). The video booster described here can handle either com­posite or S-video signals as required, because it uses an IC which actually contains four wideband amplifier channels. This siliconchip.com.au allows us to devote one channel to the composite video input and output, while two more are dedicated to the S-video input and output sockets. This means that there’s no switching and the composite video and S-video channels can even be used at the same time; eg, to pipe composite signals to another room while you’re watching S-video signals to your home theatre display. The fourth channel is spare and can only be accessed inter­nally. What about handling the even higher quality “component video” signal outputs? With this type of signal, as well as the luminance (Y) being kept separate, the two “colour difference” signals (R-Y or “Cr” and B-Y or “Cb”) are also kept separate – ie, instead of being combined as the chrominance (C) signal. If your DVD player provides these outputs and your display can also handle them, the video booster can help here too. All you need to do is buy or make up some adaptor cables, so that the three component video signals can be fed through the three main booster channels – see Fig.1(c). Audio amplifier As well as the video amplifier channels, the booster also includes a pair of low-noise audio line amplifiers. This means that it can also be used to handle any stereo audio signals which accompany the video, so these too will reach the far end of the cables in good condition. Probably the main use for the audio channels will be where you’re feeding the video and audio to a different room. They’ll also come in handy if December 2005  19 Parts List 1 PC board, code 02104031, 117 x 102mm 1 plastic instrument case, 140 x 110 x 35mm 2 PC-mount 4-pin mini-DIN sockets 6 PC-mount RCA sockets 1 PC-mount 2.5mm concentric male “DC” connector 1 9V AC plugpack (500mA) with 2.5mm female connector Semiconductors 1 MAX497 quad video amplifier (IC1) 1 LM833 dual op amp (IC2) 1 LM7809 +9V regulator (REG1) 1 LM7909 -9V regulator (REG2) 1 LM7805 +5V regulator (REG3) 1 LM7905 -5V regulator (REG4) 1 3mm green or red LED (LED1) 2 1N4004 1A diodes (D1,D2) Capacitors 2 2200µF 16V RB electrolytic 2 100µF 16V RB electrolytic 2 10µF 10V RB electrolytic or tantalum 2 2.2µF 35V TAG tantalum 2 1.0µF MKT 2 220nF MKT 4 100nF monolithic ceramic Resistors (0.25W, 1%) 4 100kΩ 8 75Ω 2 47kΩ 2 10Ω 3 1kΩ Where to buy a kit The design copyright for this project is owned by Jaycar Electronics. Complete kits will be available from Jaycar Electronics by the time this article appears in print. you need to send one or more of the signals in a 5.1, 6.1 or 7.1-channel surround sound system to remote power amplifiers; eg, you might want to send the SB (surround back) signals from your 6.1/7.1-channel decoder to the rear of your home theatre room, to drive a power amplifier for the rear centre speaker. Alternatively, you might want to drive an active subwoofer with the LFE (low frequency effects) channel signals. Presentation As you can see from the photos, the 20  Silicon Chip The A/V output sockets are all accessible from the rear of the unit. They include a 4-pin mini-DIN socket for the S-video signals, plus three RCA sockets for the composite video and left & right channel audio output sockets. The socket at far right is the DC power connector. booster is very com­pact. Everything fits in a small ABS instrument case measuring just 140 x 110 x 35mm. Power comes from a 9V AC plug­pack. Incidentally, Jaycar Electronics will be making a complete kit for the booster available, so you should be able to build it up very easily and at an attractive price. How it works The booster’s video amplifier channels are all provided by IC1, a Maxim MAX497. This high-performance device is designed expressly for hand­ling video signals. It includes four closedloop buffer amplifiers, each operating with a fixed voltage gain of 2.0. Other features of the MAX497 include a full-power -3dB bandwidth of over 200MHz, exceptional gain flatness (±0.1dB up to 120MHz), low distortion, very low differential phase/gain error between the four channels and the ability to drive four back-terminated 75Ω (or 50Ω) output cables simultaneously. As shown in Fig.1, we’re using one amplifier for the com­ posite video channel and another two amplifiers for the Y and C channels for S-video. Each amplifier has a 75Ω resistor across its input and these ensure correct termination of the cables from the video source. In addition, 75Ω resistors are used in series with each output to give correct “back termination” of the output cables. As mentioned, the amplifiers in the MAX497 have a feedback-controlled gain of exactly two. This compensates for the attenua­tion produced by the interaction between the back termination resistors and the termination resistors at the far end of the output cables. In effect, the Video Booster “restores” the incoming signal before feeding it to the next cable segment. The input and output connections to the composite video amp channel are made via RCA sockets, as these are now standard for domestic equipment. Similarly, the connections for the S-video channels are made via 4-pin “mini DIN” sockets, as these too are the accepted standard for S-video. Note that the fourth “spare” amplifier in the MAX497 is also provided with input and output termination resistors. This is done to ensure that it doesn’t interact with the three active channels. The resistors will also make it easy to use the spare channel if you ever need it. The two audio line amplifier channels are provided by the two halves of an LM833 dual low-noise op amp (IC2). As shown, these two stages are identically connected as non-inverting buffers, with the 100kΩ resistors providing negative feedback for a gain of two. The performance of these audio buffers is quite respect­able. They have a frequency response from 30Hz to 120kHz at the -1dB points, a THD (total harmonic distortion) below .006% for 2V RMS output, a signal-to-noise ratio of better than 91dB relative to 2V RMS output, and an output clipping level of just on 14V peak-to-peak (5V RMS). siliconchip.com.au The audio buffers operate with a gain of two to ensure sufficient signal to drive your remote power amplifiers, etc. However there may be cases where even this small amount of gain could cause problems - producing distortion due to input stage overloading, for example. If that turns out to be the case with your particular application, there’s a simple modification which can be done to solve the problem. All you need do is remove the 100kΩ resistors connecting pins 2 and 6 of IC2 to ground. This turns the buffers into unity-gain voltage followers, increasing the overload margin by 6dB. Power supply The power supply section is quite straightforward, even though the video and audio amplifiers require four separate DC supply rails. The MAX497 requires ±5V supply rails, while the LM833 require ±9V rails. Because the overall current drain is quite low (about 100mA total), two simple half-wave rectifier circuits (D1 & D2) are used to derive nominal ±12.8V DC rails from 9V AC plugpack. These rails are filtered using two 2200µF capacitors and then fed to 3-terminal regulators REG1 and REG2. REG1 and REG2 produce the +9V and -9V rails respectively. They also drive 3-terminal regulators REG3 and REG4 which pro­duce the ±5V rails. LED1 is driven from the +9V rail via a 1kΩ current-limiting resistor and provides power on/off indication. The associated 100µF and 10µF capacitors are used to filter the regulator outputs. The ±9V supply rails are then further decoupled using 10Ω resistors and 2.2µF capacitors before being fed to IC2. Four 100nF capacitors provide additional filtering for the ±5V rails to IC1. Construction All the parts are mounted directly on a small PC board, so the unit is easy to build. This includes all the connec­ tors, so there’s no off-board wiring at all inside the booster box. The PC board measures 117 x 102mm and is coded 02104031. It’s double sided, with copper tracks on both top and bottom, but the top pattern is mainly an earthed ground plane. Only a handful of component leads are soldered to this top pattern, so we don’t need a board with expensiliconchip.com.au Fig.2: the booster circuit is based on a Maxim MAX497 quad video buffer IC (IC1). One amplifier in IC1 is used for the com­posite video channel, while another two are used for the Y and C channels for S-video. Op amps IC2a & IC2b (LM833) boost the left and right channel audio signals. December 2005  21 This photo shows how the power indicator LED is mounted on the PC board and pushed through a matching hole in the front panel. Left: inside the completed booster unit. Keep all component leads as short as possible and be sure to solder the leads to both sides of the board where necessary, as indicated by the red dots on Fig.3. sive plated-through holes. Fig.3 shows the assembly details. Begin by fitting all the input and output connectors, as they often need a small amount of juggling and pin straightening before they’ll mount without stress. Make sure that they’re pushed down hard against the board, while you make the solder connections underneath. Next, fit the two PC board terminal pins (for the input and output of the spare video channel), followed by the resistors and the diodes D1 and D2. Be sure to fit each diode the correct way around as shown on Fig.3. Note that some of the resistors have their “earthy” ends soldered to the top copper pattern as well as to the pad under­neath. The leads concerned are shown with a red dot on the board overlay diagram. The four voltage regulators can go in next. These are all TO-220 packages and are mounted horizontally on the top of the board. Be sure to fit each one in the correct position, as all four are different and mixing them up could result in component damage when you apply power. All regulator leads are bent downwards 6mm from the package body. This allows you to mount them by pushing the leads down through the mating holes and then fastening their tabs down against the copper using 6mm x M3 machine screws and nuts. The leads are then soldered to the pads underneath and, in some cases, to the top pads as well - see Fig.3. The two 2200µF capacitors and the two 100µF capacitors adjacent to REG1 and REG2 can go in next. Make sure you fit all of these polarised parts the correct way around, as shown in Fig.3. LED1 is fitted with its “flat” cathode side to the left (ie, furthest away from CON4). To install it, first bend both its leads bent down 90°, 6mm away from the LED body. That done, it can be soldered into place with its axis exactly 8mm above the PC board. Power supply checks At this stage, it’s a good idea to check all of the power supply wiring by plugging the lead from your 9V AC plugpack into CON9 and turning on the power. LED1 should immediately light and you can now check the regulator outputs. You should get +9V from REG1, -9V from REG2, +5V from REG3 and -5V from REG4. These voltages are all measured relative to earth and at the righthand pin of each regulator, as indicated on Fig.3. Table 1: Resistor Colour Codes o o o o o o No. 4 2 3 8 2 22  Silicon Chip Value 100kΩ 47kΩ 1kΩ 75Ω 10Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown brown black red brown violet green black brown brown black black brown 5-Band Code (1%) brown black black orange brown yellow violet black red brown brown black black brown brown violet green black gold brown brown black black gold brown siliconchip.com.au Table 2: Capacitor Codes o o o o Value IEC Code EIA Code 1.0µF  1u0  105 220nF  220n  224 100nF  100n  104 If everything is correct, you can switch off and continue fitting the remaining parts to the PC board. Conversely, if one or more of the regulator outputs is incorrect, switch off imme­diately and check for wiring errors. Most likely, you’ll have made a mistake fitting D1 or D2, one of the electrolytic capaci­tors or one of the regulators. With a bit of luck, you’ll be able to fix the problem and not have to replace any parts. Completing the PC board The remaining parts can now be fitted to the board, start­ing with the MKT audio coupling capacitors, the 2.2µF tantalum bypass capacitors and the 10µF electrolytic capacitors. The two ICs can then be installed, taking care that you fit each one the correct way around. Note that the pins for IC2 (the LM833) are only soldered to the copper pads underneath, while some of the pins for IC1 (the MAX497) are soldered to the top copper pattern as well. This applies to pins 1, 3, 5, 7, 9, 11 & 13. The next components to fit are the two 100nF bypass capaci­tors, which are at each end of IC1. These mount with their “earthy” leads soldered to the top copper pattern as well as the pads underneath. That done, fit the two remaining 100nF bypass capacitors for IC1 and the remaining 10µF electrolytic capacitor for the -5V rail. As before their leads are soldered to pads on the top of the board, with their “earthy” leads soldered to the bottom pads as well. Final assembly All that remains now is to fit the booster board to the case. First, you have to fit the front and rear panels over their respective RCA connectors, before lowering the three items to­gether into the bottom of the case. That done, LED1 can be pushed into its 3mm mating hole on the front panel and the board secured to integral pillars in the bottom of the case using eight 6mm self-tapping screws. siliconchip.com.au Fig.3: install the parts on the double-sided PC board as shown here. The red dots indicate where component leads must be soldered to the copper tracks on the top of the board (and usually underneath as well). Be sure to use all eight screws to secure the PC board. These are necessary to give the board added support in the vicinity of the various input and output connectors. The final step of all is to fit the top of the case, using the two long countersink-head self tappers provided. Don’t lose these screws by the way, because they’re a special size and surprisingly hard to get. Your Video & Audio Booster is now be finished and ready for use. Component video cables Before we end up, let’s take a look at the adaptor cables required if you want to use the booster for component video signals. There’s nothing terribly complicated about this. All you need to do is buy or make up four cables - two for the luminance (Y) signals and two for the chrominance (Cb and Cr) signals. The cables for the Y signals each consist of single lengths of coax with an RCA plug at each end. These connect to the boost­er’s composite video channel, as shown in Fig.1(c). The other two cables are each of double coax, with a mini-DIN plug connected at one end and a pair of RCA plugs at the other. They are used to carry the Cb and Cr chrominance signals and are connected to the booster’s S-video channels. Note that both RCA-RCA and 2 x RCA-miniDIN video cables are available from many suppliers. However, you may want to make up your own using high quality coaxial cable, to ensure lower signal degradation - especially if you have fairly long cable runs. Some prewired cables leave a bit to be desired in this respect. By using the correct adaptor cables, the booster will operate just as effectively with component video as it does with composite video or S-video. Happy home theatre viewing! SC December 2005  23