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The unit is easy to build, with all parts installed on a double-sided PC board to eliminate internal wiring. It simply connects between the video source (eg, a set-top box) and your TV set or video projector. By JIM ROWE RGB to Component Video Converter OK, YOU’VE JUST landed home with your new widescreen TV set and tried to hook it up. But there’s a problem – your new set has Y/Cb/Cr component video inputs while your digital set-top box only provides high-quality signals in RGB format. You’ve got three choices – chuck a wobbly, use the composite video output (but at the expense of picture quality) or build this low-cost “RGB to Component Video Converter”. 36  Silicon Chip siliconchip.com.au I F YOU LIVE in an area where either pay-TV or digital FTA (free-to-air) TV signals are available, it’s well worth investing in one and/or the other service because of their better picture and sound quality. However, to achieve the best possible picture quality, you have to use the component video signals from the pay-TV or DTV set-top box and feed these into the matching inputs of your TV set or video projector. The big catch here is that some settop boxes of European origin only provide RGB video signals, with separated red, blue and green outputs. In most cases, these signals are made available via one of the large 20-pin SCART sockets or Euroconnectors. This doesn’t suit most of the latest large-screen (and widescreen) TVs and video projectors sold in Australia. These are usually designed to accept Y/Cb/Cr (or Y/B-Y/R-Y) component video, the same format as provided by the latest DVD players. Unfortunately, you can’t feed RGB signals directly into these sets or projectors. But you can convert the RGB signals into Y/B-Y/R-Y form, using the simple converter unit described here. It simply connects between your set-top box and your TV set or projector. As shown in the photos, the complete converter fits in a small instrument box. It runs from a 9V AC plugpack supply, drawing less than 50mA – ie, less than half a watt of power. Fig.1: the RGB signals are added in the correct proportions in op amp IC1a to produce a -Y (inverted luminance) signal. This is then fed to IC1b & IC2b to produce the R-Y and B-Y colour difference signals, while inverter IC2a produces the Y signal. fier. This stage is used to combine the three input signals in the right proportions, as determined by the three input resistor values. Because IC1a is connected as an inverting amplifier, the signal at its output is an inverted version of the Y signal (ie, -Y). This -Y signal is then added to the R signal in IC1b to derive the R-Y colour difference signal. In fact, IC1b operates with a gain of two (as set by the R1 resistor values), so its output signal corresponds to 2(RY). This is done to compensate for the voltage division that occurs when the converter’s R-Y output is connected to the R-Y input of a TV set or video projector – ie, due to the effect of the converter’s 75Ω “back termination” output resistor and the set’s 75Ω input resistor. Exactly the same arrangement is How it works The operation of the converter is quite straightforward, because it simply duplicates the kind of matrixing used to produce the luminance (Y) and colour difference (R-Y and B-Y) signals from the original colour camera signals. To do this, it first creates the Y signal by combining the R, G and B signals in the correct proportions; ie: Y = 0.3R + 0.59G + 0.11B That done, it subtracts this Y signal from the R and B signals, to create the colour difference signals. Fig.1 shows how this is done. The Y signal is produced by the mixer/adder stage based on IC1a which (like all of the other op amps used) is one half of a MAX4451ESA dual wideband amplisiliconchip.com.au Above: the rear panel provides access to the three component video RCA output sockets and the power socket. October 2004  37 Fig.2: the complete circuit for the RGB To Component Video Converter. Op amps IC1a, IC2a & IC2b all operate with a gain of two, to compensate for the signal losses that occur due to the 75W “back termination” output resistors and the set’s 75W input resistors. used to produce a 2(B-Y) colour difference signal, using adder stage IC2b. In this case, we simply add the -Y signal to the B signal and again amplify their sum by two. The centre output buffer stage using IC2a operates as an inverting amplifier with a gain of two and converts the -Y (luminance) signal from IC1a into an output signal of 2Y. As before, this stage operates with a gain of two to compensate for the inevitable voltage division due to the 75Ω back termina38  Silicon Chip tion and input resistors. Now take a look at Fig.2 which shows the full circuit details. As shown, all the resistors shown as R1 in Fig.1 actually have a value of 510Ω. These resistors are in the feedback networks and at the inputs to IC1b, IC2a & IC2b. By contrast, the various parallel resistor combinations between the three video inputs and IC1a’s inverting input (pin 2) are chosen to give the correct mixing proportions. For example, the 2.2kΩ and 7.5kΩ resistors from CON1 give a value of 1701Ω, which is very close to the correct figure for the R component (ie, 510/0.3 = 1700Ω) Similarly, the 1kΩ and 6.2kΩ resistors give a value of 861.1Ω, which is very close to the correct figure for the G component (510/0.59 = 864.4Ω). And finally, and the 5.1kΩ and 51kΩ resistors give 4636Ω, exactly the right figure for the B component (510/0.11 = 4636Ω). The 91Ω and 82Ω resistors across the three video inputs ensure that each has siliconchip.com.au the correct 75Ω input resistance. Note that these resistors are all somewhat higher than 75Ω, to compensate for the effects of the various mixing resistors connected to them. This impedance matching is necessary to ensure that the input cables from your set-top box or other RGB video source are correctly terminated, to prevent ringing. Power supply The converter’s power supply is simple, as the MAX4451 devices operate from ±5V supply rails and draw quite low current. Power is derived from a 9VAC plugpack and this feeds half-wave rectifiers D1 and D2. These produce +13V and -13V rails which are filtered using two 2200µF electrolytic capacitors and fed to 3-terminal regulators REG1 and REG2. The +5V and -5V regulator outputs are then filtered using 100µF capacitors and fed to the op amps. LED1 provides power indication. It is simply connected across the +5V rail in series with a 470Ω current-limiting resistor. Construction All of the converter circuitry is built on a double-sided PC board coded 02110041 and measuring 117 x 102mm. This in turn is housed in a standard instrument case measuring 140 x 110 x 35mm, to produce a very compact and neat unit. There’s no off-board wiring at all – all the RCA input and output connectors are mounted directly on the PC board along the front and rear edges. These are all accessed through holes in the front and rear panels when the case is assembled. Fig.3: install the parts on the top of the PC board as shown here. The red dots indicate where component leads (and the single “via” above left from CON1) are soldered to both sides, if the board doesn’t have plated-through holes. At least one kit supplier has indicated that they intend supplying PC boards with plated-through holes for this design. However, if your board is not plated through, you will have to solder some of the component leads on both sides of the board. You’ll also need to solder a short length of tinned copper wire (such as a resistor lead offcut) through one “via” hole, to make the connection between top and bottom tracks. To make it easy, these points are all indicated on the PC board overlay diagram (Fig.3) with red dots. Most of the components fit on the top of the board in the usual way. The only exceptions are the two MAX4451ESA Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o siliconchip.com.au No.   1   1   1   1   1   1   1   2 5   1   2   2   1   3 Value 51kΩ 7.5kΩ 6.2kΩ 5.1kΩ 2.2kΩ 1.5kΩ 1kΩ 820Ω 510Ω 470Ω 270Ω 91Ω 82Ω 75Ω 4-Band Code (1%) green brown orange brown violet green red brown blue red red brown green brown red brown red red red brown brown green red brown brown black red brown grey red brown brown green brown brown brown yellow violet brown brown red violet brown brown white brown black brown grey red black brown violet green black brown 5-Band Code (1%) green brown black red brown violet green black brown brown blue red black brown brown green brown black brown brown red red black brown brown brown green black brown brown brown black black brown brown grey red black black brown green brown black black brown yellow violet black black brown red violet black black brown white brown black gold brown grey red black gold brown violet green black gold brown October 2004  39 The assembly is straightforward but be sure to install all polarised parts with the correct orientation. These include the diodes, 3-terminal regulators, the LED and the two op amps. surface-mount SOIC packages, which are mounted on the bottom of the PC board (more on this later). Begin the board assembly by fitting the short wire link which forms a “via” between the top and bottom copper tracks of the -5V supply rail. It’s located near the front of the board, about 17mm to the right of the 470Ω resistor just behind LED1. Fitting this link first will make sure you don’t forget it. Next fit the resistors, making sure Fig.4: these full-size artworks can be used as drilling templates for the front and rear panels. 40  Silicon Chip you solder their “earthy” leads to both sides the board where indicated. Table 1 shows the resistor colour codes but we advise checking each value on a multimeter before it is fitted, just to make sure. That done, install the RCA sockets and the 9V AC power socket, using a small drill to enlarge their mounting holes if necessary. The three small 100nF monolithic capacitors can be fitted next, again taking care to solder their leads to both sides of the board where indicated. That done, fit the two 10µF tantalum capacitors and the larger electrolytics, making sure each of these polarised components is orientated correctly. The earthy lead of both tantalum capacitors is soldered to the top copper as well, as shown in Fig.3. Next fit the two diodes (D1 & D2) in the power supply, again watching their polarity. Follow with the two regulators, making sure that you fit each one in the correct position. REG2 (the 7905) goes on the lefthand side, while REG1 (the 7805) mounts to the right of siliconchip.com.au Parts List Mounting the SOIC-8 Devices 1 PC board, code 02110041, 117 x 102mm (double sided) 1 plastic instrument case, 140 x 110 x 35mm 6 RCA sockets, PC-mount (2 x red, 2 x blue, 1 x green, 1 x yellow) 1 2.5mm concentric LV power connector (CON7) 2 M3 x 6mm machine screws with M3 nuts 6 4G x 6mm self-tapping screws, pan head Semiconductors 2 MAX4451ESA dual wideband op amps (IC1,IC2) 1 7805 +5V regulator (REG1) 1 7905 -5V regulator (REG2) 1 3mm green LED (LED1) 2 1N4004 1A diode (D1,D2) Fig.5: the two MAX4451ESA dual op amps are mounted on the underside of the PC board, as shown here. Be sure to install them the right way around. REG2. Don’t get them mixed up! Each regulator is mounted horizontally, with its three leads bent downwards 5mm from the device body so that they pass through the holes in the PC board. They are both secured using 6mm x M3 machine screws and nuts and this should be done before soldering their leads. Note that REG1’s centre lead is soldered on both sides of the board, as are two leads for REG2. Surface mount ICs Once the regulators are in, you are ready to fit the two surface-mount ICs (IC1 & IC2). These are 8-lead SOIC packages and mount on the underside of the board – see Fig.5. They have a 1.25mm lead spacing, so they’re not You will need a fine-tipped soldering iron to install IC1 & IC2. Make sure that you don’t overheat them or leave solder bridges between their pins. siliconchip.com.au too small for manual handling and soldering, providing you’re careful and use a soldering iron with a finetipped bit. To fit these ICs, invert the board and locate their mounting positions – you’ll find the two sets of four small rectangular pads in each position. That done, remove the devices from their packaging and examine each one with a magnifying glass to identify the small chamfer along one side (ie, adjacent to pins 1-4 of the device). Both devices are mounted on the board with this chamfered side towards the front – ie, downwards in Fig.5. Be sure to use a fine-tipped soldering iron for this job and be careful not to overheat them or leave solder bridges between their pins. The best way to install them is to hold each device in place with a vacuum pick-up tool or a toothpick while you press down gently on one of its leads with the tip of the soldering iron. This will usually make a weak solder joint between the lead and the tinning on the board copper – enough to hold the device in place while you solder the remaining leads to their pads. That done, you can then go back and solder the first lead properly, to complete the job. Capacitors 2 2200µF 16V RB electrolytic 2 100µF 16V RB electrolytic 2 10µF 25V tantalum 4 100nF multilayer monolithic (code 100n or 104) Resistors (0.25W 1%) 1 51kΩ 2 1kΩ 1 7.5kΩ 2 820Ω 5 510Ω 1 6.2kΩ 1 470Ω 2 270Ω 1 5.1kΩ 2 91Ω 1 2.2kΩ 1 82Ω 1 1.5kΩ 3 75Ω The final component to fit is LED1 (the power LED). This in installed on the top of the board, with its longer anode lead towards the right (ie, towards CON1). It should be mounted with its body about 17mm above the top of the board (a strip of cardboard between the leads makes a handy spacer). After mounting, bend its leads down together at right angles at a point 9mm above the board. This ensures that it will later protrude through its matching hole in the front panel when the board is installed in its case. Drilling the panels The next step in the construction is to prepare the front and rear panels of the case. This involves drilling and reaming a small number of holes for the various connectors and the power indicator LED, using photocopies of the panel artworks as templates. October 2004  41 Finally, any excess tape is trimmed off and the holes cut out using a sharp hobby knife. Of course, if you buy a complete kit for the converter, you won’t have to do any of this. Instead, the panels will be supplied pre-punched and with silk-screened lettering for a really professional finish. Final assembly Now for the final assembly. This is done by first fitting the panels over the connectors on each side of the board (and also over the LED in the case of the front panel). That done, lower the assembly into the bottom half of the case, sliding each panel into its mating slot. It’s then simply a matter of fitting eight 6mm-long self-tapping screws (four along the front and four along the rear) to hold the PC board in place. Finally, the top half of the case can be fitted and secured from the bottom using the two long countersink-head self-tappers provided. Your RGB to Component Video Converter is now complete and ready for use. There are no adjustments to make – all that’s needed is to connect a suitable 9V AC plugpack and it should spring to life. Troubleshooting Fig.5: here are the full size top and bottom etching patterns for the PC board. Once that’s done, additional photocopies of the artworks can attached to the outside of each panel for a professional finish. The way to do this is to first make a copy of each artwork on adhesive-backed A4 label sheet paper. 42  Silicon Chip The labels are then trimmed, peeled off the backing and attached to the panels. That done, a length of clear packaging tape (ie, wide adhesive tape) is applied over each panel to protect it from dirt and finger grease. In the unlikely event that it doesn’t work, the first step is to go back over your work and carefully check that all components are correctly positioned and orientated. Check also for missed solder joints, especially where leads have to be soldered on both sides of the PC board. Next, check the power supply rails with your multimeter. There should be +5V at the output of REG1 and -5V at the output of REG2. If you don’t get these voltages, check the two regulators and diodes D1 and D2, plus the polarity of all electrolytic capacitors. You should also be able to measure +5V (with respect to board earth) on pin 8 of each of the two surface-mount ICs. Similarly, pin 4 of each device should be at -5V but be careful not to short out adjacent pins with the meter probe when making these measurements. Finally, if LED1 fails to light even though the +5V rail is correct, check that the LED has been installed correctly. Check also that its 470Ω resistor SC is correct. siliconchip.com.au