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This handy test generator produces a standard monochrome video signal with a 4-step greyscale pattern, as well as a 500Hz audio tone. It’s just the shot for testing VCRs, video monitors and the continuity of video cables. An Audio-Video Test Generator By LEON WILLIAMS W HEN INSTALLING or repairing video equipment or systems, a test pattern generator is a must. However, for this type of work you don’t need an expensive colour pattern generator with a myriad of options and settings. What is required is a simple signal source that allows a go/no-go indication. While the specifications for this project don’t put it in the professional instrument class, it is light and rugged, can be carried in a toolbox and has the distinct advantage of being cheap. It also uses common components, is easy to build and should work first time. There is no setting up to do and there are no controls to fiddle with. The circuit is powered from a 9V DC supply, which would normally be a 9V plugpack. It produces standard 38  Silicon Chip non-interlaced monochrome video and audio signals (see specifications) that are compatible with just about all TV sets that have a A/V inputs, VCRs and video monitors. Note, however, that this device is not suitable for testing most computer monitors. Circuit details Fig.1 shows the full circuit details of the Audio-Video Generator. The circuit operation may not be obvious at first glance, mostly because the way in which the video signals are generated is a bit more complicated than normal. In addition, some circuit simplification and trickery has been applied to reduce the component count and keep costs down. Clock signals for the circuit are derived from a 4MHz crystal oscillator formed around NOR gate IC1a. The 10MΩ resistor places the gate into linear mode and feedback is accomplished with the 4MHz crystal (XTAL1) and the two 22pF capacitors. Because we are using a crystal, the resulting clock signal is accurate and stable. Inverter stage IC1b buffers the oscillator output which then clocks pin 1 of IC2, a 74HC393 dual 4-bit binary counter. In this case, IC2 has been cascaded to form a single 8-bit divider. Three of its outputs (pins 8, 9 & 10) are used for video timing, while a fourth output at pin 5 (500kHz) is fed to a divider circuit to derive a 500Hz audio signal. The signal at pin 5 is also divided down to produce a 50Hz vertical sync signal. Greyscale generation Pins 8 and 10 of IC2 provide two January 2000  39 Fig.1: clock signals for the circuit are provided by a 4MHz oscillator based on IC1a and these are divided down by dual 4-bit binary counter IC2 to produce most of the video timing signals. Dual decade counters IC3 & IC4 further divide the 500kHz output from IC2 to produce the vertical sync and audio output signals. a result, a stream of low-going 5µs pulses appear on pin 5 of IC5d and this provides the horizontal sync signal (see Fig.3). Diode D3 limits the voltage on pin 5 when pin 8 switches high again, by clamping it to the +5V supply rail. This is done to protect the IC from possible damage due to voltage spikes. Note that because the width of the sync pulses is determined by a simple capacitor/resistor combination and the switching threshold of the exclusive-OR gate, they may not be exactly 5µs. However, this shouldn’t cause any problems in practice. Vertical sync The Audio-Video Generator produces a 4-step greyscale pattern surrounded by a black border, as shown here. Note that the on-screen pattern is off-centre due to design limitations. square waves at 15.625kHz and 62.5kHz, respectively. These two waveforms are the input signals for the greyscale generator. This generator is a simple 2-bit (4-level) D-to-A converter consisting of three 3kΩ resistors and the 1kΩ resistor to ground. If both inputs are low, there is no voltage at the output of this divider network. However, as each input is taken high, a progressive voltage is built up until the voltage is at maximum when both inputs are high. So this simple but effective circuit provides four voltage steps. Diode D1 level shifts the greyscale video waveform generated by the D-to-A converter by 0.6V. We’ll look more closely at this when we discuss the following output buffer stage (Q2 & Q3) later on. Horizontal sync As mentioned above, the output on pin 8 of IC2 is a square wave with a frequency of 15,625Hz. This has a period of 64µs and is exactly the length of a line of video (as used in Australia). The horizontal sync pulses are derived by feeding this 15,625Hz signal to pin 10 of exclusive-OR gate IC5b. IC5b’s other input, pin 9, is connected to ground and so this stage simply functions as a buffer, the signal on 40  Silicon Chip pin 8 following the signal applied to pin 10. Each time pin 8 of IC5b switches low, pin 5 of IC5d is also pulled low via the .001µF capacitor. The .001µF capacitor then charges via the associated 4.7kΩ resistor, so that pin 5 switches high again after 5µs. As Specifications Power Supply Supply voltage ..........9-20V DC Current drain ............15mA <at> 9V Video Output connector ......RCA female Output level ..............1V peak-topeak into 75Ω; 2.4V peak-to-peak unloaded Pattern ......................4-step greyscale Horizontal sync .........5µs negative sync every 64µs Vertical sync .............500µs negative sync every 20ms Audio Output connector ......RCA female Output level ..............840mV RMS; 2.35V peak-to-peak unloaded Output frequency ......500Hz The vertical sync signal is derived by first using the 500kHz signal from IC2 to clock IC3. This stage is a dual decade counter which is wired to divide by 100. The resulting 5kHz signal appears on pin 14 and in turn clocks IC4, another dual decade counter stage. This produces a 50Hz signal on pin 14 of IC4, which is the video frame rate. IC5c buffers this square wave signal and the .001µF capacitor and a 470kΩ resistor on pin 3 generate 500µs vertical sync pulses on pin 4 of IC5d in the same way as for the horizontal sync pulses. The frame period is the inverse of the frame frequency; ie, 20ms. Given that the line period is 64µs, this means that there are 312.5 lines per frame. This figure may seem rather odd but is quite normal. In fact, the picture we see on our television screens is constructed of two frames of 312.5 lines each, to give a total of 625 lines. The half line length allows the two frames to be interlaced, or placed on top of each other, so that the lines of one frame fit between the lines of the other frame, to form one complete picture. The horizontal and vertical sync pulses are combined using IC5d to form a single composite sync signal. An exclusive-OR gate is used here for a special reason. During the vertical sync period, the horizontal sync pulses remain active and this creates what is referred to as “serrated sync”. Normally, in the absence of sync pulses, both inputs to IC5d are high. Because IC5d is an exclusive-OR gate, this means that pin 6 of IC5d will be low. The rule is that an exclusive-OR gate only switches its output high when its inputs are at different logic Fig.2: this scope shot shows the 500Hz audio waveform generated by the unit. The waveform is quite clean and has a level of about 2.35V peak-to-peak or about 840mV RMS. levels (ie, one high and one low). If the vertical sync signal is not active (ie, pin 4 of IC5d is high), a (low-going) horizontal sync pulse applied to pin 5 thus causes the output (pin 6) to go high. This turns on Q1 and pulls Q3’s base to ground (ie, the sync voltage is equal to 0V). Conversely, when the vertical sync is active, pin 4 of IC5d is low for 500µs and a number of horizontal sync pulses also occur during this period. As a result, Q1 cycles on and off at the line frequency and so the vertical sync pulse on pin 6 of IC5d appears to be “serrated”. Blanking To enhance the appearance of the on-screen display, a black border is placed around the greyscale pattern. This black border is generated by the blanking circuitry. Note that the voltage level of this blanking is less than the video black level and so it is often referred to as “blacker than black”. There are two forms of blanking: (1) horizontal blanking on the sides of the screen; and (2) vertical blanking at the top and bottom of the screen. The horizontal blanking signal essentially blanks the video at the beginning and end of each line and it does this by pulling the video signal on Q3’s base to ground (or close to it). It is derived by feeding the outputs from pins 8 and 9 of IC2 into exclusive OR gate IC5a. The output of IC5a switches low when both inputs are the same (ie, at the beginning and end of each line) Fig.3: this composite video waveform clearly shows the horizontal sync pulses, the horizontal blanking signals and the 4-step greyscale signal. Note that the blanking signals before and after each sync pulse differ in length and this is why the on-screen display is off-centre. and this pulls the base of Q3 low via diode D2. Similarly, the vertical blanking signal pulls the video to ground at the beginning and end of each frame. This signal is derived by feeding the outputs from pins 12 and 13 of IC4 into NOR gate IC1c. This gate switches its pin 4 output high only when both inputs are low. IC1d inverts the output from IC1c and pulls the video signal down to 0.6V via diode D5 during the blanking period. Because we are producing a monochrome test pattern, there is no need to generate a colour burst signal. This is a burst of approximately 10 cycles of 4.433MHz which is normally placed on the blanking line (porch) just after the horizontal sync pulse, to allow the receiver to correctly decode the colour information. If the colour burst is absent (as in this case), a colour TV set simply dis- Parts List 1 PC board, code 04101001, 120mm x 80mm 1 plastic case, 158 x 95 x 53mm 1 panel-mount DC connector to suit plugpack 2 panel-mount RCA sockets 6 PC board stakes 1 4MHz crystal Semiconductors 1 74HC02 quad NOR gate (IC1) 1 74HC393 dual 4-bit binary counter (IC2) 2 4518 dual BCD counters (IC3, IC4) 1 74HC86 quad exclusive-OR gate (IC5) 1 LM358 dual op amp (IC6) 1 7805 5V positive voltage regulator (REG1) 2 BC548 NPN transistors (Q1, Q2) 1 BC558 PNP transistor (Q3) 7 1N4148 signal diodes (D1-D7) 1 1N4004 silicon diode (D8) Capacitors 1 470µF 25VW PC electrolytic 2 100µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 6 0.1µF MKT polyester 1 .01µF MKT polyester 2 .001µF MKT polyester 2 22pF ceramic Resistors (0.25W, 1%) 1 10MΩ 3 3kΩ 1 470kΩ 1 1kΩ 4 22kΩ 1 560Ω 3 10kΩ 1 470Ω 1 6.8kΩ 1 75Ω 1 4.7kΩ Miscellaneous Tinned copper wire for links, light-duty hook-up wire, 3mm machine screws and nuts. January 2000  41 Fig.4: install the parts on the PC board and complete the external wiring as shown here. Make sure that all polarised parts are correctly oriented and that the correct part is used in each location. plays a monochrome picture (the PAL colour television system is designed to be compatible with monochrome signals). Output buffer The waveform at Q3’s base thus consists of the 4-step greyscale signal, the horizontal and vertical sync signals, and the horizontal and vertical blanking pulses. Together, these signals make up the composite video signal. However, this signal needs to be buffered before it can be connected to a 75Ω load. Transistors Q2 & Q3 form the buffer stage and are connected in similar fashion to a class-B audio amplifier. These two transistors are wired as complementary emitter followers, with forward bias provided by diodes D6 and D7 to minimise crossover distortion. In operation, D6 & D7 maintain a constant 1.2V between the two transistor bases. A 75Ω resistor sets the output impedance, while the associated 100µF capacitor provides AC coupling to the video output socket. Note that when there is no blanking, no horizontal or vertical sync and the video is black, the video level will be slightly higher than 0.6V (the blanking level). This voltage is developed across D1 and the resistors in the D-to-A converter, due to the current that flows via the output buffer bias circuit (ie, through the 10kΩ resistors Table 2: Capacitor Codes  Value IEC Code EIA Code  0.1µF  100n 104  .01µF   10n 103  .001µF    1n 102  22pF   22p  22 and diodes D6 & D7). Audio generator Pin 5 of IC4 produces a 500Hz square wave and although its duty cycle is not exactly 1:1, this is of no concern in this application. This square wave is applied to a 500Hz bandpass Table 1: Resistor Colour Codes  No.   1   1   4   3   1   1   3   1   1   1   1 42  Silicon Chip Value 10MΩ 470kΩ 22kΩ 10kΩ 6.8kΩ 4.7kΩ 3kΩ 1kΩ 560Ω 470Ω 75Ω 4-Band Code (1%) brown black blue brown yellow violet yellow brown red red orange brown brown black orange brown blue grey red brown yellow violet red brown orange black red brown brown black red brown green blue brown brown yellow violet brown brown violet green black brown 5-Band Code (1%) brown black black green brown yellow violet black orange brown red red black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown orange black black brown brown brown black black brown brown green blue black black brown yellow violet black black brown violet green black gold brown The completed PC board is secured to the bottom of the case using machine screws and nuts, with additional nuts used as spacers. Twist the output leads together as shown, to minimise noise pickup. filter based on IC6a, part of a LM358 dual op amp IC. A bandpass filter is used here rather than a low pass filter because it has a much greater filter slope than a low pass filter with the same number of components. The output from IC6a appears at pin 1 and is a 500Hz sinewave of reasonable quality. This signal is then buffered by IC6b, with the .01µF capacitor across the 22kΩ feedback resistor providing additional low-pass filtering. The output from this stage appears at pin 7 and is coupled to the audio output socket via a 560Ω resistor and 10µF capacitor. The 560Ω resistor provides short circuit protection for the op amp and sets the output impedance at about 600Ω. Note that the non-inverting inputs (pins 3 & 5) of IC6a & IC6b are biased to about 1.8V by a common divider network consisting of 10kΩ and 6.8kΩ resistors. A 10µF capacitor provides filtering for this bias voltage. Power for the circuit is derived from a 9V DC plugpack. This is fed to 3-terminal regulator REG1 via diode D8 which provides reverse polarity protection. A 470µF electrolytic ca- pacitor filters the input to REG1 and the regulated 5V output is decoupled using a 100µF electrolytic capacitor and a number of 0.1µF MKT polyester capacitors scattered around the circuit. Construction Construction is straightforward because all the parts are mounted on a PC board, the only exceptions being the DC supply socket and audio/ video output sockets. This PC board is coded 04101001 and measures 120 x 80mm. Fig.4 shows the parts layout. Start by checking the PC board for faults, as it is much easier to spot these now than when it is covered in solder and flux. This done, straighten some tinned copper wire by stretching it slightly. You can do this by clamping one end in a vyce and pulling on the other end with a pair of pliers. This wire can now be used for the five wire links. Install these first, then fit the resistors and six PC stakes at the external wiring points. Next come the diodes and the capacitors but double-check these to ensure correct polarity. The transistors (Q1-Q3) and voltage regulator REG1 can go in next. The transistors all look the same so make sure that you install Q3 (BC558) in the correct position. The 7805 voltage regulator (REG1) mounts with its metal tab facing towards the centre of the PC board. Finally, install the 4MHz crystal and the ICs. Remember that some of the ICs are CMOS types, so take the usual precautions against static discharge; ie, earth yourself before touching them and solder the supply pins first. Be sure to use the correct IC in each position and note that they all face in the same direction. Before mounting the completed PC board in the case, it’s a good idea to check that it is operating correctly. This will make it easier to do any fault-finding if necessary. First, connect a suitable 9V DC supply to the relevant PC stakes and use your multimeter to check the output voltage of REG1. If it is within 0.25V either way of 5V, you can proceed. If the output voltage is incorrect, switch off and check for construction errors. A low output voltage probably means that the regulator has a short on it’s output. Check for short circuits or components in the wrong way around. January 2000  43 Fig.5: this is the full-size artwork for the front panel. It can be cut out and used directly if desired. Note that the greyscale pattern will not be positioned in the centre of the screen. This is due in part to the simple circuit employed and will also depend to some extent on the characteristics of the video monitor. The reason for this is shown in the scope photograph of Fig.3. As can be seen here, the horizontal blanking signal immediately following the negative-going sync pulse (ie, just before the 4-step greyscale signal) is much shorter than the blanking signal that precedes the sync pulse. Final assembly Fig.6: check your PC board for defects before installing any of the parts by comparing it with this full-size artwork. If you cannot measure any output voltage, leave the power on and check that there is power at the regulator input. If there is no power here, D8 may be reversed or the power supply may be connected with reversed polarity. Once everything is OK, you can have a look at the video output with a CRO. If you don’t have a CRO, the best way to test the unit is to simply connect it to a video monitor using a patch cable. The screen should show a 4-level grey scale pattern surrounded by a black border (see photo). The 44  Silicon Chip lefthand bar should be black, the righthand bar white and two bars with shades of grey in between. You can also check the audio output at this stage using either a CRO or by feeding it into an audio amplifier. You should hear a clean 500Hz tone with good volume. If there is no output, you will probably need a CRO to trace the waveforms around the circuit. In particular, check the video timing signals at the various IC outputs. When all these tests are positive, you can finish the construction. The case has two RCA connectors at one end, one for the video output and one for the audio output. These can be purchased with different colour inserts. The standard is yellow for the video output and red for the audio output. The DC connector is a single hole type and is mounted at the other end of the case. The PC board is secured to the bottom of the case using 3mm screws and nuts. Place an extra nut between the case and the PC board on each screw to act as a spacer. This done, wire the connectors to the PC stakes using light-duty hookup wire, twisting each pair of wires together. Finally, screw on the lid and your audio-video test generator is complete. We’re sure that you will find it SC a handy test instrument.