Fullscreen HTML5Med Res (??MB)HTML4

There are not many video test pattern generators on the market and the ones that are cost big dollars. This kit is a fraction of the cost of commercially available units, it’s portable and it has audio outputs as well! by Mick Gergos H ow often have you found yourself touching the end of an audio cable and listening for the 50Hz buzz, or shorting the ends of a video cable so that you can meter the other end to confirm its destination? How often have you needed a video source then reached for the VCR in the corner only to find that you don’t have a power lead or a tape with anything on it? No matter which facet of the electronic industry you work in, once you’ve experienced the convenience of this portable audio/video test signal generator, you’ll wonder how you ever lived without it. It is an essential piece of test gear for any techo’s toolkit. Whether you’re in CCTV, broadcast, TV/VCR repair, AV installation or just a devoted hobbyist, you’ll find that this will become one of the most useful and best value-formoney products in your arsenal. Features The Pocket AV Signal Generator literally is pocket-sized at just 123 x 80 x 25mm, including signal output sockets. An RCA socket provides the composite video, with nine fields (screens) to choose from, cycled through with the touch of a pushbutton. In addition, two more RCA sockets provide stereo channels of approximately 1.5kHz audio, with the right channel Colour Bars 28  Silicon Chip White clearly identifiable to assist in cable troubleshooting. And to make it completely portable, it operates from either a 9V battery or a 9V DC plugpack. Operation Operating the Pocket AV Signal Generator could not be easier. Simply insert a 9V battery for up to 10 hrs of operation or connect a 9V plugpack for continuous operation. Connect the RCA cables, flick the power switch, push the button, select your required test pattern and away you go. The Pocket AV Generator has a professional look and feel thanks to the deluxe Hammond case from Altronics. At the business end you’ll find the video output, along with the L & R audio outputs on low profile RCA sockets. You will also find the 9VDC socket (centre positive) and a toggle power switch, recessed to avoid accidental bumps. Further down the case you’ll find the pattern select push button and LED power indicator. With energy conservation in mind, the power indication LED flashes with an 8% duty cycle, which also serves to attract more attention. Using the video output The default test signal at power up is 100% saturated Green Red Blue siliconchip.com.au Specifications Composite video output Video output source impedance Frame rate Vertical synchronisation Horizontal synchronisation Composite synchronisation Chroma sub-carrier frequency Colour system Sub-carrier to horizontal phase Patterns (shown below) 1 V p-p (Pulse & Bar terminated into 75W) 75W 25Hz 50Hz 15.625kHz As per Australian standards 4.43361875MHz PAL Non-synchronous 100% Colour bars Flat Fields; white, red, green, blue & black Crosshatch 20 x 15; vertical line width 0.2mS Dot 20 x 15; dot width 0.2mS Pluge with 2T Pulse & bar 550mV p-p (-12dBm with RCH ID) <600W <450mW (<50mA <at> 9V) Audio output Audio output source impedance Power consumption colour bars. Other than confirming correct operation of the device or cable under test, colour bars are of limited use to the average user. Their primary role is in the television broadcast industry where they are used as a reference, aiding the interchange of recorded material and checking the quality of ‘video bearers’. Colour bars provide a reference for black, white and sync level, burst to chroma phase & colour saturation. Pressing the button cycles through the various patterns – eight in all. Five flat fields are available: white, green, red, blue & black. Excluding black, these rasters are used primarily for purity adjustments on colour TV sets or video monitors. The black output can be used for a variety of applications where a composite sync source is required. Be aware, however, that the subcarrier is not locked to the horizontal sync (no 8-field sequence). This makes the device unsuitable as a master genlock source in any live switched colour video system such as a linear edit suite, where the subcarrier-to-horizontal phase is critical. Next are the crosshatch and dot patterns, used to check and adjust raster centering, geometry and convergence. The lines and dots are perfectly centred, resulting Black siliconchip.com.au Crosshatch in an array of 20 x 15 perfect squares corresponding to a 4 x 3 aspect ratio. The final test pattern combines a couple of lesser known but very useful signals: Pluge and the 2T Pulse & Bar. The latter (2T Pulse & Bar) is the more visible signal which may be used for transient analysis of video processing systems while the Dot Pluge/2T Pulse & Bar June 2006  29 Fig.1: the generator is based around two ICs – a PIC16F84A-20P which sets up all the timing and waveforms; and an AD724JR which converts these to composite (PAL) video. Pluge is there simply for brightness adjustment. What’s a Pluge? If you wind up the brightness you will vaguely see a bar that appears lighter than black level, followed by a bar that is slightly darker than the black level. The idea is that you gradually adjust the brightness so that the lighter bar can barely be seen but the blacker one cannot be seen. This enables you to accurately set the brightness of your display, subject to the ambient lighting conditions. This is very handy for AV installations such as board rooms or home theatre systems where the preset brightness position doesn’t suit. If you view the video output on an oscilloscope equipped with TV field triggering, it is interesting to note that when changing patterns, the synchronisation is not interrupted. Also note that you can trigger to either field 1 or field 2. It is the inclusion 30  Silicon Chip of the serration pulses in the vertical block that makes this possible. Many of the cheaper test signal generators and even some PC video cards omit this important feature defined by Australian standards. Using the audio outputs There are two audio outputs; left and right. Both outputs are fixed in phase, frequency (1.5625kHz) and level (550mVp-p), corresponding to -12dBu. There is however, one important and useful difference: the right channel breaks briefly every few seconds to identify it from the left. This is extremely useful when looking for left right swaps in cabling or patching. The audio signals produced from the RCA outputs are not perfect sinewaves but they are not far from it – you will be able to clearly identify level and clipping with certainty. As a field technician, I felt that phase, level, channel identification, low cost and circuit simplicity took precedence over the ability to take distortion measurements. Circuit description Fig.1 shows the complete circuit diagram which employs only two integrated circuits, a PIC16F84A-20/P microprocessor and an AD724 RGB to PAL/NTSC encoder. The circuit is powered from a standard 9V alkaline battery or a 9V DC plugpack. Using typical 500mAh alkaline batteries, you should get around 10 hours of continuous operation and of course significantly longer intermittent use. The PC-mount DC socket is switched (break before make) so you need not worry about paralleling the power sources. Diode D1 provides reverse polarity protection while the 7805 (REG1) regulates the incoming supply to 5V. siliconchip.com.au Fig.2: here’s a representation of the Pocket AV Generator’s sync waveform, showing how the odd and even fields are time shifted with respect to one another. Note also the serration pulses during the vertical blanking interval – the start of the odd field is identified by an extra pulse. with serration pulses for field identification. At this point, all of the signals remain in the digital domain; ie, they are either 0V or 5V. The intelligence is contained within the timing relationship of the signals. RGB signal conditioning Note that the various sections of the PC board are laid out in a star pattern from the regulator to isolate the digital from the analog circuitry. Inductor L1 and associated capacitors provide decoupling for the PIC processor while inductor L2 and its capacitors decouple the rails to the PAL encoder IC. RGB, sync & audio generation The pre-programmed PIC16F84A20/P microprocessor generates all the signals required to produce the test patterns and audio and it monitors the pushbutton for a pattern change request on pin 13. Pins 17 & 18 provide a 1.5625kHz square waveform to the audio filters while pins 7, 8 & 6 provide the raw R, G & B pulses respectively. The pluge pattern requires some additional signals from pins 1 & 2. Lastly and most important is the composite synchronisation signal from pin 10. This signal contains all the horizontal and vertical synchronisation, along siliconchip.com.au A resistor divider network attenuates the raw RGB (red, green, blue) signals from the PIC to an appropriate level, while three 10pF capacitors provide noise filtering before the signals are coupled via 100nF capacitors to the inputs of the AD724 RGB to PAL/ NTSC encoder. The 10pF capacitors form a single pole filter with a -3dB point around 2.8MHz. This may seem a little low but keep in mind that the maximum output frequency of the PIC is only 2.5MHz. The 10pF capacitors are tied to the positive supply line for IC2. This ensures that the RGB signals are filtered with respect to the device for which they are intended (ie, the AD724). The additional signals being bled into the RGB input lines via the 330kW resistors are for the Pluge pattern and are only active during the time that this pattern is selected by the pushbutton. Scanning begins at the top of the picture and moves from left to right across the screen. The brightness of the electron beam varies in intensity as it scans. Upon encountering a horizontal sync pulse, the beam is blanked and quickly retraces to the left side of the screen before scanning the next line. The beam scans 312.5 lines before encountering a vertical sync pulse, which once again blanks the beam and retraces, this time to the top of the screen ready for the beginning of the next field. To interlace the two fields, the second field must be shifted down slightly with respect to the first. This is done by offsetting the horizontal lines with respect to the vertical sync pulse as shown in Fig.2. Note the serration pulses during the vertical interval. These assist the TV’s horizontal oscillator to maintain lock during the vertical interval and also provide a means of identifying each field. The inner workings of the AD724 are typical of most PAL encoders. We’ll briefly touch on the basics of PAL encoding without fully analysing the PAL encoding Before discussing any PAL theory we should review the basics of television. A standard Australian television picture is made up of 625 horizontal lines that are refreshed 25 times per second. To avoid flicker of the picture, the 625 lines are divided into two interlaced fields consisting of 312.5 lines each, effectively doubling the refresh rate. The frequency of the vertical sync is therefore 50Hz (20ms) and the horizontal sync 15,625Hz (64ms). The A-V generator can be powered by an external 9V DC plugpack or by its own internal 9V battery, as seen here. June 2006  31 Fig.3 (left): raw RGB information from the PIC is fed into the AD724, where it’s weighted and added together. Horizontal and vertical sync pulses are added to the mixture to create the luminance (Y) signal. Fig.4 (below): the two colour difference signals (R-Y & B-Y) are modulated with a reference frequency and a burst reference signal added, after which they’re added together to form the ‘chrominance’ signal. Following a short delay, luminace is added to chrominance to create the final PAL composite signal. gizzards of the AD724. Initially, the RGB signals are attenuated according to their weightings then summed to make the luminance or Y signal. This Y signal is then subtracted from the R and B signals to produce the R-Y and B-Y signals respectively. Sync is then added to the luminance signal to create the monochrome video signal (often still referred to as the Y signal). The monochrome video component contains all the detail, brightness and contrast information within the picture. In addition, the luminance signal carries the horizontal and vertical synchronisation pulses. The R-Y and B-Y signals are often referred to as the colour difference components and contain only the colour information. Note that when there is no colour information in the picture, the R-Y and B-Y signals remain at 0V. The diagram of Fig.3 illustrates the process. Once the Y, R-Y and B-Y signals have been produced we quadrature modulate the R-Y and B-Y signals along with the ‘burst reference signal’ onto a 4.43361875MHz suppressed carrier to make what is called the ‘chrominance signal’. Finally the luminance signal is delayed to compensate for the delays caused by chrominance processing. Together, the luminance and chrominance signals are referred to as ‘S-video’ or separate video signals. Adding the luminance and chrominance signals produces the complete PAL composite video signal (see Fig.4). The term ‘composite’ refers to a collection of signals, in this case the red, green, blue, horizontal sync, vertical sync and serration pulses. One feature not shown in the dia32  Silicon Chip grams is the PAL (phase alternate line) switching. In summary, the phase of the carrier fed to the R-Y modulator is inverted every second line. At the receiver, any phase errors caused by the transmission path are cancelled out by vectorial subtraction with the previous line via a one-line delay. The process of PAL encoding may seem a little complicated however there were several prerequisites when it was initially proposed. Firstly, it had to be backward compatible with B&W television sets, which is why the RGB matrix produces the Y signal. Secondly, the frequency for the colour sub-carrier had to carefully chosen so it did not produce a significant visual effect in the picture. Thirdly, it had to be better than the American (NTSC) system, thus PAL switching was implemented, eliminating the need for tint control at the receiver. The AD724 can accept separate horizontal and vertical sync but in this case we tie the VSYNC input low and feed a composite sync source to pin 16, via the 1kW resistor from pin 10 of the PIC. Crystal X2 and trimmer VC1 form an adjustable, parallel resonant circuit that is the reference for the internal 4FSC clocks. The AD724 quadruples the reference frequency at pin 3 to 4FSC (4 times the subcarrier frequency). This assists the internal generation of the required phase shifts for the R-Y balanced modulator; ie, 90° and 270°. The 2Vp-p composite video output appears at pin 10 of IC2 and is fed via a 75W resistor which thereby sets the source impedance. Thus when used with a good quality 75W cable such as RG59 and a suitable termination, considerable distances can be achieved without serious degradation of the signal. A 220mF capacitor couples the signal to the video output connector. Capacitive coupling the video results in some distortion; however this is not considered a problem as the black level clamps (otherwise known as DC restoration) at the TV set or monitor take care of this. Audio signal conditioning The audio circuitry is a very siliconchip.com.au straightforward arrangement that was chosen for its simplicity and cost effectiveness. Two square waves generated by the microprocessor at 1/10th of the line rate (ie, 1.5625kHz) pass through the low pass filters, each consisting of three 1.5kW resistors and three 100nF capacitors. These 3-pole low pass filters attenuate the signal to an appropriate level while removing the upper harmonics to give the signal a more sinewave characteristic. Polyester capacitors with a 5% tolerance have been used to eliminate the need for level adjustments. The audio signals are buffered via emitter-follower output stages consisting of transistors Q1 & Q2 then coupled to the RCA connectors via 220mF capacitors. The 100nF capacitors connected between base and collector of Q1 & Q2 serve to increase the stability of the output stages and reduce the risk of oscillation. Breaking the right channel audio for identification purposes is achieved by increasing the frequency of the right channel square wave to 7.8125 kHz for the period of the break. This results in the signal being severely attenuated by the 3-pole low pass filters. This technique ensures that the average DC level is unchanged at the output, thus eliminating any thuds that would be heard if the output from the microprocessor switched off altogether. Construction All components for the Pocket AV Test Signal Generator mount directly on the PC board so there is no external wiring, with the exception of the 9V battery snap. Case preparation Before starting construction, use the PC board as a template to drill the hole for the “pattern select” pushbutton switch. Place the PC board copper side up into the top half of the case then mark the exact point for the button using a 1mm drill in a pin-vice. Gradually increase the size of the hole using bigger drills until you get to 8.5mm. You can also mark out the position of the power LED by marking the case with the position of the two legs of the LED. Next, centre your pin-vice between these two marks and gradually drill out to 3mm. Using the template as a guide, you should now drill out A pinvice is one of the handiest tools you can have in your workshop – here one is being used with a 1mm drill bit to accurately mark the two hole positions in the case lid. And here’s what you’re trying to achieve: the pushbutton switch (left) advances the pattern while the LED blinks to show the device is on. Fig.5: component overlays for both sides of the PC board. Start with the copper side and IC2, the AD724JR surface-mount chip, then turn the board over and mount the other components in the conventional way. The 9V battery snap can go on at the end, otherwise it could get in the way. Note that much of the testing is done before you insert and solder IC1. Compare these diagrams with the completed project photo overleaf. siliconchip.com.au June 2006  33 If you don’t have a suitable hand drill, simply use a power drill that is not plugged in. The chuck can still be easily turned, giving you greater control of the cut. Start with smaller holes, then gradually work your way up to the required size. PC board construction The AD724JR surface-mount IC (ringed here in red) mounts on the copper side of the PC board. You’ll need a fine iron and a steady hand – and for heaven’s sake, don’t put it in the wrong way around. The battery snap wires also solder to this side of the PC board after passing through the tension hole at the opposite end. the holes in the front panel. There are three 9 mm holes for the low-profile RCA connectors, a 7mm hole for the DC socket and a 2.5 x 6.5mm elongated hole for the recessed power switch. To reduce the risk of breakage, the holes should be drilled out by hand. You’ll find that the front panel will be ripped from your clutches if using a power drill or drill press. Check to make sure that the PC board fits in to the case. If it needs filing, it’s easier to do it now than later. It’s also a good chance to check your drilling. It is worthwhile temporarily fitting the RCA sockets to make sure that the plastic posts overhang the PC board. If not, file the PC board to that they just overhang. The sockets should be firm to insert into the PC board. This assists the solder joints of the RCA socket to cope with the stress of plug insertion and removal. Contrary to our normal practice, which is to leave semicondutors until last, we are going to start by soldering in the surface-mount IC2 (AD724JR) which is placed on the rear (copper side) of the PC board. You should be at least somewhat competent at surface mount soldering to do this; fortunately the pitch on this IC is not too fine. Use a fine tipped iron – if you are having trouble, try using a good quality flux to assist with the flow. Be careful not to overheat the device and be sure to get it the right way around! Pin 1 is clearly marked on the PC board for your convenience. Begin loading the through-hole components starting with the reverse Here’s what the composite output from the generator looks like when the colour bar pattern is selected. Compare this to the diagram in Fig.4. Note the spot-on signal amplitude of 1Vp-p. 34  Silicon Chip protection diode D1. Keep the cut leads from D1 and use them for TP_GND and the earth link for IC2. Solder in the resistors noting the orientation of the uprights. Try to copy the prototype in the photos. Use a the resistor colour code table and/or a digital multimeter if there is any doubt. Now solder in the non-polarised capacitors and inductors. That done, mount some of the miscellaneous components such as the IC socket, Q1, Q2, VC1, X1 and X2 making sure that X2 is a low profile device. Mount the tantalum capacitors noting the polarity and laying them over as per the photograph. Special care should be taken to ensure that the electrolytic capacitors are pressed firmly against the PC board (see photo). Failing this, the top of the case will not go on properly. REG1 needs to be bolted to the PC board as it provides a link between earths. Be sure to use a shake-proof washer with the nut. Mount the remaining hardware such as the power switch, DC socket and low-profile RCA sockets, making sure that the plastic posts on the RCA sockets overhang the front edge of the PC board as discussed earlier. Mounting the pushbutton switch seems straightforward; however make sure that the orientation is correct. If it is wrong, you will find that the pattern changes continuously. There are four pins on this switch but only two poles! The push button switch should be mounted flush against the PC board. By sheer fluke, The serration pulses are clearly visible in the output, with the even field shown here in the upper trace, odd field in the lower. siliconchip.com.au this puts the top of the switch exactly flush with the face of the Hammond case. The last PC board-mounted component is the power LED. Again, note the orientation as this component is polarised. The height should be set the same as that of the pushbutton switch. Finally, feed the cables of the 9V battery snap through the hole in the lower left corner of the PC board. This will remove any stresses on the solder joints of the 9V battery snap. Solder the ends to the pads provided on the copper side of the board, noting the polarity. Once again this is clearly marked on the PC board for your convenience. You should now have an attractive package that fits neatly into the case, ready for setup. The last component to fit is the pre-programmed microprocessor PIC16F84A20P but it is best to wait until after the setup and test procedures before inserting this component. Expect a few thousand ohms. If all is well, connect the battery (with IC1 still removed), switch on the power and immediately check the voltage at pin 3 of REG1. It should be very close to 5V; if not check the path from the 9V snap, through the DC socket and diode D1 to the input of IC3. If you’re happy that REG1 is Setup and test First things first: check and recheck the orientation of IC2 and all other polarised components, including the electrolytic capacitors (don’t forget the tantalums!). Make sure that REG1 is bolted down. Check resistor values and placement. When you are satisfied that all is well and there are no solder bridges, do a quick ohms check between pin 3 of REG1 and TP GND. The reading may take a while to settle due to the decoupling capacitors however there shouldn’t be any shorts to TP GND. regulating, check the voltage between ground (battery snap black wire) and both pin 14 of IC1’s socket and pin 14 of IC2. Both should be 5V. If you’re satisfied that none of the components are operating in their Chernobyl mode, disconnect power, insert the pre-programmed microprocessor ensuring correct orientation and re-power the device. Close-up of a horizontal sync pulse, highlighting the 10-cycle colour burst signal which follows it. The onscreen measurement shows that the burst occurs 5.6ms after the falling edge of the sync pulse, as required by the PAL standard. siliconchip.com.au You should immediately notice the illumination of the power LED, shortly followed by an intermittent flashing. This means the processor is functioning. If this does not happen, remove power and start checking component orientation again. Unfortunately, if you are experiencing difficulties, you’ll need a ’scope to track down the fault. Check that the This shot highlights the break that is inserted in the audio tone of the right channel. The 7.8125kHz signal is attenuated nicely by the RC filter, with about 20mVp-p evident here. Recovery to the normal frequency (nominally 1.5625kHz, here measured at 1.402kHz) is clean and ‘popless’. June 2006  35 RGB signals at the input to IC2 are about 700-750mV p-p. Check the SYNC signal is getting to pin 16 of IC2. Also check for oscillation of X2. Use the theory of operation described earlier as a guide to finding the source of your fault. If all appears well, connect the video output to the video input of a television. If you are not immediately rewarded with colour bars, try adjusting VC1 with a suitable non-metallic tweaker. If you find that the generator is cycling through patterns continuously, check the orientation of the pushbutton switch. Check the audio outputs and note that the tone is present on both channels but breaks on the right channel intermittently. If you want to accurately set VC1 you will need access to a spectrum analyser. For the rest of us, simply adjust VC1 so that you get colour lock every time you flick the power switch. Try it on various TVs as some sets will have a tighter capture window than others. Scope shots Finally, you should check the video levels on an oscilloscope or waveform monitor. The waveforms shown here were captured using a short length of RG59 terminated into a T-piece at the input of the oscilloscope, as seen below. Parts list – Pocket AV Generator 1 PC board, 63 x 77mm, coded AV Sig Gen 1 deluxe Hammond case with 9V battery snap 1 pushbutton switch, DPST, PC-mount (S1) 1 miniature toggle switch, SPDT, PC-mount (S2) 3 low-profile RCA sockets, PC-mount (CON1-3) 1 DC socket, PC-mount (CON4) 4 x M2.5 self-tapping screws 1 6mm M3 screw, nut and shakeproof washer Semiconductors 1 PIC 16F84A-20/P, pre-programmed (IC1) 1 AD724JR PAL encoder (IC2) [Alternate AD722] 1 78L05 regulator (REG1) 1 1N4004 diode (D1) 1 3mm LED (green) (LED1) 2 BC547 transistors (Q1, Q2) 1 20MHz 3-pin ceramic resonator (X1) 1 4.43361875MHz low-profile crystal (X2) Capacitors 3 220mF 25V electrolytic 3 47mF 10V tantalum 1 470nF polyester (code 474 or 470nF) 6 100nF polyester 63V 5% (code 104 or 100nF) 5 100nF monolithic (code 104 or 100nF) 1 10nF monolithic (code 103 or 10nF) 1 150pF ceramic (code 150 or 150p) 3 10pF ceramic (code 10 or 10p) 1 5-30pF variable capacitor (yellow) (VC1) Inductors 2 47mH inductors (L1, L2) Resistors (0.25W, 5%) 6 330kW 4 5.6kW 6 1.5kW If your level is not close to 1Vp-p or your TV is having trouble synchronising, it’s likely that you have some resistors in the wrong place. Measurement tips; White level should be 1V (±5%) with respect to sync tip. There are no adjustments for the video levels as they rely purely on the tight tolerances of the metal film resistors. • Use field triggering and time delay to capture one line of video. This will display the waveform more clearly as the non-synchronous SC-H phase and capacitively coupled video output make the signal appear noisy if viewed using line triggering. • Ensure that VC1 is set correctly as it can cause weird effects. SC 6 1kW 1 150W 1 75W Where from, how much A complete kit of parts (cat K-2725) will be available from Altronics Distributors retail stores in Perth, Sydney and Melbourne, from selected dealers and also via the Altronics online store at www. altronics.com.au, for $129.00 plus packing and postage. Firmware is not available separately, nor will it be altered to facilitate NTSC or on-screen text capabilities. Resistor Colour Codes o o o o o o No.   6   4   6 6   1   1 36  Silicon Chip Value 330kW 5.6kW 1.5kW 1kW 150W 75W 4-Band Code (1%) orange orange yellow brown green blue red brown brown green red brown brown black red brown brown green brown brown violet green black brown 5-Band Code (1%) orange orange black orange brown green blue black brown brown brown green black brown brown brown black black brown brown brown green black black brown violet green black gold brown siliconchip.com.au