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by JIM ROWE S-video to composite video converter Some digital TV set-top boxes provide only S-video and component video outputs, which can pose a problem if your TV set only has a composite video input – or you’ve used up the S-video and component video inputs. The same can happen with video tuner cards for PCs. Here’s an easy-to-build adapter to get you out of trouble. Y OU CAN BUY CHEAP S-video to composite video adapters in bargain stores but the unit described here will do a much better job. Although those really cheap bargain-store adapters do work, if you examine the pictures critically, you’ll find that their quality leaves quite a bit to be desired. In particular, you’ll find that wherever the image has large areas of fine detail – like a shirt with a fine striped or check pattern, or an exterior panning shot of a multi-storey Fig.1: adding a low-value capacitor in series with the chrominance signal at the S-video input can reduce cross-colour interference but also softens the picture. Fig.2 (right): the unit described here uses an LC circuit to notch out a narrow band of frequencies centred on the 4.433MHz colour subcarrier frequency. This reduces cross-colour interference while leaving a sharp picture. 84  Silicon Chip siliconchip.com.au Low-cost “bargain-store” adapters commonly produce pictures that suffer from cross-colour interference, as shown in the photograph at left. By contrast, the SILICON CHIP converter dramatically reduces cross-colour interference – see photo at right. building – then you’ll see a very obvious coloured Moire interference pattern, usually in shades of yellow and purple. This effect is called “cross-colour interference” and it’s caused by heterodyne beats between the higher frequencies in the luminance (Y) signal and the chrominance (C) subcarrier in the receiver’s decoder. In effect, the higher luminance frequencies tend to behave as if they were part of the chrominance signal and as a result, produce fake colour patterns. This happens when the two signals are simply mixed together in the video adaptor – which is what commonly happens in the bargain store units. This interference pattern can’t happen when the Y and C signals are kept separate, which is why S-video produces much better image quality. Reducing the interference Some of the better low-cost adaptors try to reduce this cross-colour interference by adding a small capacitor in series with the chrominance input signal, as shown in Fig.1. The capacitor’s value is chosen so that it passes most of the chrominance (C) information (it’s in a band about 2.5MHz wide, centred on 4.43361875MHz) while at the same time attenuating the higher frequency luminance signals – ie, by shunting the luminance output into the chrominance output of the S-video signal source. This reduces the cross-colour interference although it also removes some of the fine detail from the images, so siliconchip.com.au Fig.3: the circuit is based on a MAX4451 dual-wideband video amplifier, with each stage wired as a non-inverting amplifier with a gain of 2.0. RFC1 and VC1 provide the 4.43361875MHz notch in the luminance signal. September 2006  85 Par t s Lis t 1 PC board, code 02109061, 76 x 46mm 1 UB5 jiffy box, 83 x 53 x 31mm 1 68mH RF choke (RFC1) 1 mini 4-pin DIN socket, PC-mount (CON1) 1 RCA socket, yellow, PC-mount (CON2) 1 2.5mm concentric power connector, PC-mount (CON3) 4 M3 x 10mm machine screws, countersink head 4 M3 star lockwashers 10 M3 nuts 2 M3 x 6mm machine screws, round/pan head This screen shot shows the Snell & Wilcox moving plate test pattern as fed through an “el-cheapo” S-video to composite video converter. Note the obvious colour Moire patterns on the moving plate section and in other areas where there are fine lines. Semiconductors 1 MAX4451ESA dual video op amp (IC1) 1 7805 +5V regulator (REG1) 1 7905 -5V regulator (REG2) 2 1N4004 diodes (D1,D2) Capacitors 2 470mF 16V RB electrolytic 1 220nF MKT metallised polyester 2 100nF multilayer monolithic 1 6-30pF trimcap, green (VC1) Resistors (0.25W 1%) 1 680W 4 510W 4 75W This is the same test pattern as above but this time fed through the SILICON CHIP converter. As can be seen, the cross-colour interference effects have been dramatically reduced. they become softer – ie, the chrominance output is also shunted to some extent. By contrast, the unit described here does a better job of reducing cross-colour interference without sacrificing the higher frequencies in the Y signal nearly as much. As a result, the images stay reasonably sharp. It’s admittedly a bit more complex than the “el-cheapo” adaptors but it’s still low in cost and very easy to build and get going. How it works By way of comparison, this is the direct S-video signal. It’s completely clear of cross-colour interference effects and clearly demonstrates the advantages of S-video. 86  Silicon Chip The approach taken here to reduce cross-colour interference is to use a simple LC trap circuit to notch out a fairly narrow range of frequencies in the incoming luminance (Y) signal, centred on the 4.43361875MHz colour subcarrier frequency. This removes most of the higher luminance frequencies that cause obvious cross-colour patterning, while leaving the luminance frequencies below about 3.5MHz and above 5.4MHz untouched. You can see the resulting luminance response in Fig.2. By contrast, the incoming chrominance signal passes through its channel largely untouched, so there’s no degradation of colour detail. As a result, the image quality of the composite video output signal is quite good. Of course, it’s not as good as watching S-video directly but it’s noticeably better than you get with an “el-cheapo” adaptor. siliconchip.com.au Table 1: Capacitor Codes Value μF Code EIA Code IEC Code 220nF 0.22µF 224 220n 100nF 0.1µF 104 100n Now let’s take a look at the circuit diagram – see Fig.3. As shown, the incoming S-video (Y/C) signals come in via CON1, a standard 4-pin mini-DIN socket. The C signal is then fed through a 220nF coupling capacitor and is terminated by a 75W resistor to prevent ringing due to cable reflections. From there, the signal is fed to pin 5 of IC1b which is one half of a MAX4451 dual-wideband video amplifier wired here as a non-inverting amplifier with a gain of 2.0. This gain is necessary to allow for mixing and output cable back-termination losses. The incoming Y signal is treated a little more harshly. After being terminated in the correct 75W impedance, it’s then passed through the “notch” circuit. This consists of a series 680W resistor and a series LC tuned circuit formed by a 68mH RF choke (RFC1) and a 6-30pF trimmer capacitor (VC1). When VC1 is adjusted to resonate with RFC1 at 4.43361875MHz, this LC circuit forms a low-impedance path to earth at that frequency. This acts together with the 680W series resistor to produce the desired notch in the response, as shown in Fig.2. From there, the rest of the Y signal is passed through IC1a, the other half of the MAX4451 device which is also wired as a non-inverting amplifier with a gain of 2.0. The outputs of both IC1b and IC1a are then mixed using the two 75W output back-terminating resistors, to produce the final composite video output signal at output connector CON2. Fig.4: install the parts on the PC board as shown here, making sure that all polarised parts are correctly orientated. The leads designated with a red dot must be soldered to both sides of the board but only if your board doesn’t have plated-through holes. Fig.5: the MAX4451 comes in an SOIC-8 package and is mounted on the underside of the PC board as shown above and in the photo below right. Be sure to mount it with its chamfer side towards the bottom and use a soldering iron with a very fine chisel-tip to solder its leads. Power supply The MAX4451 IC needs a DC supply of ±5V and this is provided using the simple power supply shown at the bottom of Fig.3. This is the fully-assembled PC board, mounted on the lid of the case. Power comes from an external 9V AC plugpack. Table 2: Resistor Colour Codes o o o o siliconchip.com.au No.   1   4   4 Value 680W 510W 75W 4-Band Code (1%) blue grey brown brown green brown brown brown violet green black brown 5-Band Code (1%) blue grey black black brown green brown black black brown violet green black gold brown September 2006  87 Fig.6: follow this diagram to mark out the holes to be drilled in the ends of the box and the box lid. Alternatively, you can scan this diagram, print it out and use it as a drilling template. The larger holes are best made by drilling a small hole first and then enlarging them using a tapered reamer. Power comes from a 9V AC plugpack supply and this feeds two halfwave rectifiers based on diodes D1 and D2. Their outputs are filtered using two 470mF capacitors and fed to positive and negative 3-terminal regulators REG1 & REG2. REG1 then provides the +5V rail while REG2 provides the -5V rail. Construction All of the parts for the converter fit on a small double-sided PC board measuring 76 x 46mm and coded 02109061. This board has rounded cutouts in each corner, so it will fit snugly in one of the small plastic UB5 jiffy boxes. The S-video input socket Fig.7: the PC board is mounted on the lid using M3 x 10mm machine screws, nuts and lockwashers. 88  Silicon Chip CON1 is at one end of the board and box, while the composite video output and power sockets (CON2 and CON3) are at the other end. Fig.4 shows the assembly details. Begin by checking the hole sizes for the three connectors and enlarge these if necessary. That done, start the assembly by installing the resistors and the capacitors. Note that the two 470mF electros are polarised and must be fitted with their positive leads towards the left, as shown in the overlay diagram. By the way, although the board is double sided, it may not be supplied with plated-though holes. In that case, you must solder the component leads to both sides of the PC board in those locations marked with a red dot on Fig.4. That way, the component leads themselves make the necessary connections between the two sides of the board. Trimmer capacitor VC1 can go in next, noting that its flat side goes towards the bottom of the board. Follow this with the 68mH RF choke (RFC1) – the PC board can accept either an axial-lead or “single-ended” choke, so use whichever set of holes is the most convenient for the part supplied. Next, fit diodes D1 and D2, followed by regulators REG1 and REG2. The latter are both installed with their leads bent downwards by 90° about 6mm from their bodies. Slip them into the positions indicated, then fasten their metal tabs to the PC board using M3 x 6mm machine screws and nuts before soldering their leads. It’s important not solder the leads before the metal tabs are secured. If you do, the solder joints could fracture (or the copper tracks could lift) as the nuts are tightened. Take care also to use the correct regulator type in each position (REG1 is a 7805 type while REG2 is a 7905). Now for the dual video op amp (IC1). This comes in a very small SOIC-8 surface-mount package and is mounted on the underside of the board – see Fig.5. Note also that it’s mounted with its chamfer and notch side towards the bottom edge of the board. Because its leads are spaced just 1.25mm apart, you need to take great care when soldering them to the copper pads. Be sure to use a soldering iron with a very fine chisel-tip and make sure the tip is very clean. Soldering this type of device in place is also much easier if all its board pads are lightly tinned first. That done, place the device and its leads carefully over the pads and hold it in place with a toothpick (or “crossover” tweezers) while you just touch the tip of the iron to one lead for a second or two, to melt the solder underneath. This should then be sufficient to hold the device in place while you solder all the other leads to complete the job. The board assembly can now be completed by turning it back over and fitting the three connectors (CON1CON3). Final assembly The PC board fits neatly into a UB5size plastic box but first you have to drill the various holes in the box and its lid. Fig.6 shows the locations and sizes of these holes. There are only seven holes in all: four in the lid for the PC board mounting screws and three larger holes in the box ends for the connectors. Once these holes have been drilled, mount the PC board on the inside of siliconchip.com.au the lid using four M3 x 10mm machine screws with countersink heads – see Fig.7. Note that each screw has an M3 star lockwasher fitted to it first, after which a nut is fitted and tightened to secure it in position. In practice, these nuts act as spacers which raise the PC board about 3mm from the lid (which is used here as the base). Once all the screws are in position, slip the PC board into place and secure it with the four remaining nuts. Fig.8: this full-size artwork can be cut out and attached to the lid of the case. A single layer of clear, wide adhesive tape will protect it from damage. Alternatively, you can download both this and the PC board artwork (in PDF format) from the SILICON CHIP website. Notch adjustment The luminance notch trap can either be set visually or you can use an RF signal generator and an oscilloscope. If you have access to the required test instruments, simply set the RF generator to 4.43361875MHz (use a frequency counter to do this if necessary) and feed its output into the Y signal input – ie, pin 3 of CON1 (or the junction of the 75W and 680W resistors). That done, use your scope to monitor the signal level at the composite video output of the converter (the centre pin of CON2) and adjust trimcap VC1 carefully until you see the signal level 0 00 $10 I Z E P R OL! PO drop down into a sharp null. The correct setting for VC1 is right at the bottom of that null. Of course, you won’t be able to set the notch frequency this accurately if you don’t have access to test instruments. In that case, you’ll have to set it visually, with the converter operating on a suitable S-video signal from your set-top box or a DVD player. Try to pick a scene where there is some cross-colour patterning visible in the images. It’s then just a matter of slowly and carefully adjusting VC1 with an alignment tool or jeweller’s screwdriver until the cross-colour “nasties” disappear. Note that you may need to repeat this procedure a few times, until you’re confident that you’ve found the correct setting. Finally, the box can be slipped over the lid assembly and secured using the small self-tapping screws provided. That’s it – you’re now ready to connect your set-top box or DVD player, etc to your TV’s composite video input via SC your new adapter. 2006 SILICON CHIP Excellence in Education Technology Awards CLosing Soon SILICON CHIP’S Excellence in Education Technology awards carry a prize pool of $10,000. Separate awards will be made to students of secondary schools throughout Australia and to students of universities and TAFE colleges throughout Australia. The secondary school awards have three categories: AWARD FOR EXCELLENCE (a) Best final year assignment of an individual student involving electronics technology. (b) An award to the school sponsoring the winning individual student. (c) Best school project involving electronics technology. The university and TAFE college awards have three categories: (a) Best project from a student as part completion of a degree, diploma or certificate in electronics or a related field (ie, mechatronics). (b) Best research project from a post-graduate student working in an area of applied electronics. (c) An award to the university faculty or school sponsoring the best research project. Entries and judging The awards will be judged by the editorial staff of SILICON CHIP, convened as a judges panel. The decisions of the judges will be final. Entry requirements are as follows: (1) A description of the project in no more than 1000 words. (2) Full circuit and wiring diagrams, performance plots, etc. (3) Good quality photographs to show all visual aspects of the project. (4) Details of software. Entries for the 2006 awards close on October 16th, 2006. All submissions will be confidential, until the winners are announced, in the December 2006 issue of SILICON CHIP. Each award will take the form of a cash prize and a commemorative plaque. All enquiries about these awards should be directed to the editor via email to: awards<at>siliconchip.com.au siliconchip.com.au September 2006  89