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By DARREN & GARY YATES Low-cost colour video fader Forget those messy edits on your VCR! This project uses readily available components & smoothly fades any composite PAL video signal to black level to enhance your home movies. It can also wipe left or right across the screen for special effects. Let’s face it – editing home movies is not easy. Have you ever tried to get your VCR to do a clean edit from one scene to the next? It’s virtually impossible unless you have one of the top-of-the-line models. Often, the only result you get is the brief flash of a “snowy” screen as you cut from one scene to the next. It not only looks unprofessional and messy but is annoying to look at as well. This Colour Video Fader solves that problem. It accepts any colour or black-and-white composite PAL video 18  Silicon Chip signal and can fade it smooth­ly down to a black screen and back up again. This makes it easy to fade one scene to black, set your cam­corder for the next scene and then bring that scene up from black to full brightness again. Result – a profession­al transi­tion from one scene to the next. Since no movie is complete these days without some special FX (movie lingo for “effects”), we’ve also added a screen wipe facility. This allows you to wipe the screen to black from leftto-right or from right-to-left. Again, once the screen is black, you can make your cut, move on to the next scene and wipe the video back on again by turning the control in the other direc­ tion. What could be easier? In addition to these features, the Colour Video Fader also features an external control input. This input is for a future project that will allow you to wipe a scene to black in over 50 different ways; eg, from top to bottom, diagonally, disappearing squares, and so on. This optional add-on project will be de­scribed in SILICON CHIP in a few month’s time. By now you’re probably thinking, “OK, so what expensive chips have been used?” Well, we haven’t used any. The circuit uses just two common CMOS ICs, a few transistors and a handful of other components. Fading video signals Before we dive straight into the circuitry, let’s take a look at a typical Colour burst signal Immediately following the sync pulse is the colour burst signal, which is nominally 10 cycles of 4.43MHz sinewave. This signal provides a phase reference so that your TV can decode the colour (or “chrominance”) information in the video signal. (Note: this signal does not appear in blackand-white TV systems). Both of these signals are vital to your TV set. If the horizontal sync pulse disappears, your TV picture will start to tear horizontally and the picture will break up. And if the colour burst signal disappears, then so will the colour from your TV screen. No amount of knob-twiddling on the front of your TV will help if either of these two signals has disappeared. One signal that doesn’t appear in Fig.1 but which is also vitally important is the field (or vertical) sync pulse. This occurs once every 20ms and has a 250µs duration. It synchronises the TV to the field rate – each time a vertical sync pulse is received, the set begins BLUE RED MAGENTA GREEN CYAN WHITE 100% YELLOW video signal – one that we would like to fade down to black. To fade down the audio level, you simply reduce the amplitude of the audio signal and that’s about it. But that’s not the case for a composite video signal. Fig.1 shows a typical PAL composite video signal from a colour bar pattern generator. This waveform represents just one of the 625 horizontal lines on the TV screen and has three main features: (1) a line sync pulse; (2) a colour burst signal; and (3) the picture information (luminance & chrominance). The line sync (synchronisation) pulse is used to signal the start of a new line on the screen. This pulse lasts for 5µs and occurs once for every line. Since there are 625 lines on the screen and they are updated 25 times per second, the sync pulse frequency is 15.625kHz (more commonly called the horizontal line frequency). In a colour TV receiver, the 625 lines are interlaced into two groups or fields, each containing 312.5 lines. Thus, one field consists of lines 1, 3, 5, 7, etc, while the other field consists of lines 2, 4, 6, 8, etc. Each field is displayed alter­nately at a 50Hz rate and this virtually eliminates the flicker that would otherwise be apparent at a 25Hz rate. 30% BLACK VIDEO SIGNAL 10-CYCLE COLOUR BURST 0% LINE SYNC PULSE Fig.1: a typical PAL composite video signal from a colour bar pattern generator. This waveform represents just one of the 625 horizontal lines on the TV screen and has three main features: (1) a line sync pulse; (2) a 10-cycle colour burst signal; and (3) the picture (or video) information. scanning a new field. The video component of the waveform follows the colour burst signal and it is this that determines what appears on the screen. In the case of the waveform shown in Fig.1, the result will be a set of vertical colour bars, starting with white on the left­hand side of the screen and going through yellow, cyan, green, magenta, red, blue and finally black on the right­hand side. The sync pulses ensure that all the horizontal lines match up so that the bars are vertically aligned. If we reduce the video section of the signal in amplitude, we reduce the “brightness” of the display and we can fade all the way to black. However, we must leave the sync pulse and colour burst signals at their original amplitude otherwise the picture will lose sync and colour during the fade. In practice, what we have to do is reduce the amplitude of one part of the waveform (the video information) and keep the rest the same (sync pulse and colour burst). This may sound difficult but in the end it is fairly simple due to the repeti­tive nature of a composite video waveform. The trick is to first extract the sync pulse and colour burst signals from the waveform, play around with the video information that remains, and then mix the sync pulse and colour burst signals back in. Block diagram Block diagram Fig.2 shows the bas­ics of the circuit. As shown, the incoming video signals are fed into a The Colour Video Fader can smoothly fade a video signal from full brightness to black & back up again, or can wipe left-to-right or right-to-left across the screen as shown on the facing page. It uses only low cost parts. August 1993  19 INPUT VIDE0 + SYNC MIXER Q2,Q3 Q1 SYNC + COLOUR BURST ENABLE IC1c DC CLAMPING VIDEO ENABLE FADER IC2a,IC1a IC1b VR1 VIDEO BUFFER VIDEO AMPLIFIER SYNC + CB ONLY +10dB Q4,Q5 VIDEO ONLY SYNC + WIPE GENERATOR IC2b,IC2d,IC1d D1,D2 buffer stage, after which the signal is fed three ways: (1). to a DC clamping stage (IC2a & IC1a). This clamps the bottom of the video signal to a steady DC voltage regardless of the video amplitude. In this case, it’s the bottom of the sync puls­es; ie, the level corresponding to 0% in Fig.1. (2). to the sync and colour burst enable circuitry (IC1c); and (3). to the video enable circuitry. The job of the sync and colour burst enable circuitry is to allow just the sync and colour burst signals to pass through to the final mixer stage. It blocks out all other video signals. Conversely, the video enable circuit only allows the pic­ture information to pass through and rejects the sync and colour burst signals. Once the composite video signal has had the sync and colour burst components stripped from it, it can be manipulated in the fader stage (ie, faded up or down). The signal is then fed to the mixer stage which mixes the sync and colour burst signals back in to produce the modified composite PAL signal. This signal is then fed to your VCR. At this stage, we haven’t mentioned the sync and wipe generator circuit. This part of the circuit is a bit more SYNC + COLOUR BURST ENABLE IC1c DC CLAMPING IC2a,IC1a TO MIXER SYNC SEPARATOR TO MIXER VIA FADER VIDEO ENABLE IC2a IC1b D1,D2 IC1d COLOUR BURST PULSE GENERATOR IC2b VARIABLE WIPE PULSE MONOSTABLE IC2d EXTERNAL INPUT DRIVE Fig.3: this expanded block diagram shows the sync & wipe generator circuitry in greater detail. The output from the colour burst gating pulse generator (IC2b) is used to trigger a variable wipe pulse monostable (IC2d). Its output is ANDed with the pulses to the sync & colour burst enable circuitry via diodes D1 and D2 & fed to the video enable switch. 20  Silicon Chip OUTPUT Fig.2: block diagram of the Colour Video Fader. The incoming composite video signal is stripped of sync & colour burst signals before being applied to the fader section (VR1). After fading, the sync & colour burst signals are mixed back in & the resulting signal amplified to make up for losses in the circuit chain. in­ volved and needs another block diagram to explain fully – see Fig.3. The first thing to notice is that the incoming signal from the video buffer doesn’t go directly to the DC clamp but via a sync separator. This separ­ates out the horizontal and vertical sync pulses and generates positive-going pulses which switch in the DC clamping circuit. The output of the sync separator is also fed to a colour burst gating pulse generator (IC2b). This produces negative-going pulses about 7µs in length, which cover the length of the colour burst. The sync separator and colour burst pulse generator outputs are then ORed together and the resulting signal fed to the sync and colour burst enable circuitry. Thus, the sync and colour burst enable circuit allows only the colour burst and the sync signals to pass through to the mixer. The output from the colour burst gating pulse generator is also used to trigger a variable wipe pulse monostable (IC2d), which produces variable-length pulses. These pulses are then ANDed with the pulses to the sync and colour burst enable circuitry via diodes D1 and D2 and fed to the video enable switch. The outputs of both enable circuits are then fed into the video mixer as before. Circuit diagram Let’s now take a look at the complete circuit – see Fig.4. All the major circuit elements depicted in the two block diagrams can be directly related to this diagram. As shown, the incoming video signal is AC-coupled to the base of tran­sistor Q1 via a 0.1µF capacitor. The 82Ω resistor connecting the input +5V Q2 BC548 B 10k VIDEO IN Q1 BC558 0.1 C E B C 82  IC1b 4066 E 1.2k 1.2k 10k C B 14 3 Q4 BC548 4 470  1.5k 5 Q5 BC548 B E 1.2k FADE VR1 1.5k 10k 0.1 Q6 BC558 B 6.8k 2.2k C E 100  680  IC1a E C 100 16VW 2.2k 150  VIDEO OUT 470  1 13 100 16VW +5V 1M 2 VR3 20k 1.2k 10 16VW +5V IC2a 4070 1 3 IC1c 1k 10 Q3 BC548 6.8k B 11 2 47pF 12  C E 10k 470  +5V 4.7k 75k 100pF 13 5 IC2b 6 6 4 270pF EXTERNAL INPUT +5V L TO R EXTERNAL WIPE S1 IC2c D1 1N914 B 4.7k 11 E C VIEWED FROM BELOW 7 I GO 7 9 4.7k 100k 12 IC1d 8 14 SET BLACK LEVEL MAY NEED ADJUSTMENT 8 WIPE VR2 500k LIN D2 1N914 IC2d 10 D3 1N4004 9 9VDC 300mA PLUG-PACK 100 16VW IN 7805 GND OUT 100 16VW +5V 0.1 0.1 5.6k R TO L COLOUR VIDEO FADER Fig.4: the various elements in the circuit diagram can be directly related to the two block diagrams. IC2a functions as a sync separator, its output switching high for the duration of each sync pulse. The video signal (minus the sync & colour burst signals) passes through IC1b & is faded by VR1. The signal is then buffered by Q4 & mixed with the sync & colour burst signals from IC1c & Q3. to ground provides the correct terminating impedance so that “ghost” or reflected signals do not occur. Transistors Q1 & Q2 form the buffer stage. Because a PNP/NPN arrangement is used, the required level of input im­pedance has been achieved with negligible voltage difference between the base of Q1 and the emitter of Q2. This is important for the correct functioning of the DC clamping circuitry. From the emitter of Q2, the signal path is split three ways, as mentioned before. First, it goes to IC2a via a lowpass filter consisting of a 1kΩ resistor and a 47pF capacitor. This reduces the amplitude of the colour burst signal so that it doesn’t cause IC2a to false trigger. IC2a is an exclusive-OR gate and is used here as a very high gain ampli­ fier/comparator. By tying one input to the supply rail, we have also made it work as an inverter. IC2a and CMOS analog switch IC1a together form the DC clamping circuit. Q2’s emitter is set to +2.7V by virtue of the bias voltage applied to the base of Q1. A video signal applied to the base of Q1 will swing high and low but each time a sync pulse arrives it will cause IC2a to switch its output high. This will cause CMOS switch IC1a to close and thus “clamp” the bottom of the sync pulse to +2.7V. This happens for every sync pulse that arrives at the base of Q1. Thus, the incoming video signal at the base of Q1 (and therefore at the emitter of Q2) can only swing between +2.7V and +3.7V (approx.). At the same time, IC2a effectively August 1993  21 10-CYCLE COLOUR BURST BLUE RED LINE SYNC PULSE MAGENTA GREEN CYAN YELLOW WHITE 100% 30% BLACK 0% +5V PIN 3 IC2a 0V PIN 4 IC2b produce the wipe pulse. IC2d triggers on the rising edge of the pulse from IC2b, as shown in Fig.5, and its output pulse length is set by VR2. By varying VR2, we can vary the pulse length from almost zero to 64µs (ie, the length of a screen line). Because IC2d is triggered once for every line, we can thus create the effect of a wipe from one side of the screen to the other. In order to eliminate sync and colour burst signals from the video signal we wish to modify, IC2c is used to invert the signal at pin 8 of IC1d. Its output at pin 11 is then ANDed with the wipe control signal at pin 10 of IC2d, using diodes D1 and D2, and the resulting output applied to the control input of IC1b. Typical waveforms PIN 8 IC1d PIN 10 IC2d WITH PIN 8 = GND PIN 11 IC2c PIN 5 IC1b Fig.5: this diagram shows the waveforms produced at various points in the sync & wipe control circuitry. The width of the screen wipe is controlled by the pulse width on pin 10 of IC2d & this in turn is set by VR2. functions as a sync separator, its output switching high for the duration of each sync pulse. These sync pulses are used to trigger the colour burst monostable, made from IC2b. IC2b is triggered by the falling edge of the sync pulse appearing at pin 3 of IC2a, so that the colour burst pulse fol­lows the sync pulse. This monostable produces a brief negative-going pulse about 7µs long, as set by the RC time constant on its pin 5 input – see Fig.5. The colour burst pulses are applied to the control input (pin 6) of IC1d, while the sync pulses are applied to pin 9. Its output (pin 8) is high for the combined duration of the sync and colour burst pulses, and is low while ever video infor­mation is present. 22  Silicon Chip The output at pin 8 is used to control IC1c which is anoth­er CMOS switch. Thus, by feeding in the control signal from IC1d, only the sync and colour burst signals pass through IC1c, while the picture information is eliminated (ie, the video is blanked). The output from the colour burst gating pulse monostable (IC2b) is also used to trigger monostable IC2d to CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ Value 0.1µF 220pF 100pF 47pF IEC Code 100n 220p 100p 47p EIA Code 104 221 101 47 Fig.5 shows the results of these machinations. The waveform applied to pin 5 of IC1b begins with a low-going pulse that covers the sync pulse and colour burst signals. This is then followed by a variable length positive-going pulse that is con­trolled by VR2. As a result, IC1b blanks out all of the sync pulse and colour burst signals and only passes video information while the output of the diode AND gate is high. Thus, if VR2 is set so that each positive pulse covers only half the line length, then only that half of the picture will be shown while the other half of the screen will be blacked out. In other words, the amount of picture shown is determined by the length of the positive pulse and this can be continuously varied using VR2. Switch S1 controls the wipe direction. If pin 8 of IC2d is pulled high, then the black is wiped from left to right (L-R). Conversely, if pin 8 is pulled low, the black is wiped from right to left (R-L). S1 also makes another interesting effect possible. If the WIPE potent­i­ ometer (VR2) is turned fully in one direction, the picture can be instantaneously flicked on or off using S1. This facility is much more versatile than it may first appear at first sight, as it allows us to create a myriad of wipes including diamonds, centre-splits, diagonal wipes and more using a plug-in external controller. At this stage, we have produced the wipe function by modifying the control signal to pin 5 of IC1b. What POWER SOCKET EXTERNAL INPUT VIDEO IN VIDEO OUT 11 10 4.7k 1.5k 1.2k 0.1 10k 6.8k 5.6k 7 6 100uF 680  Q6 150  9 10 4 100uF 470 5 2 3 ▼ S1 remains of the video signal is now fed to a resistive divider network that includes 10kΩ potentiometer VR1. This is the FADE control and it allows the picture to be smoothly varied from full brightness at one extreme to full Q3 8  SEE TEXT Above: view inside the completed prototype. Keep the wiring neat & tidy & use PC stakes to terminate all wiring connections to the PC board. Fig.6 (right): be careful when installing the transistors on the PC board, as both NPN & PNP types are used. The 7805 3-terminal regulator is mounted with its metal tab towards the adjacent 10µF capacitor.  470W 100uF 0.1 100uF 5 270pF 75k 6 7 8 Q5 2.2k 10uF 7805 D3 4 1 4.7k 0.1 VR3 IC2 4070 100pF 10k 100  1k 2.2k Q4 6.8k IC1 4066 1 47pF 11 1.2k 1M 82  470  Q2 D1 1.5k 1.2k 0.1 1 2 100k 3 D2 10k 4.7k 1k Q1 9 black at the other extreme. The video signal from VR1’s wiper is fed to buffer stage Q4, after which it is mixed with the sync and colour burst infor­mation coming from IC1c and buffer stage Q3. The combined 1 VR1 VR2 com­posite video signal is then fed to transistors Q5 and Q6 which together act as a wide bandwidth amplifier with a gain of about 3.2. This gain compensates for any losses in the buffer stages and CMOS switches and RESISTOR COLOUR CODES ❏ No. ❏   1 ❏   1 ❏   1 ❏   1 ❏   1 ❏   4 ❏   2 ❏   1 ❏   3 ❏   1 ❏   2 ❏   2 ❏   3 ❏   1 ❏   3 ❏   1 ❏   1 ❏   1 Value 1MΩ 100kΩ 75kΩ 22kΩ 12kΩ 10kΩ 6.8kΩ 5.6kΩ 4.7kΩ 2.2kΩ 1.5kΩ 1.2kΩ 1kΩ 680Ω 470Ω 220Ω 150Ω 82Ω 4-Band Code (1%) brown black green brown brown black yellow brown violet green orange brown red red orange brown brown red orange brown brown black orange brown blue grey red brown green blue red brown yellow violet red brown red red red brown brown green red brown brown red red brown brown black red brown blue grey brown brown yellow violet brown brown red red brown brown brown green brown brown grey red black brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown violet green black red brown red red black red brown brown red black red brown brown black black red brown blue grey black brown brown green blue black brown brown yellow violet black brown brown red red black brown brown brown green black brown brown brown red black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown red red black black brown brown green black black brown grey red black gold brown August 1993  23 is set by the 2.2kΩ and 680Ω resistors. The result is that the overall peak amplitude of the video signal is the same at the output as at the input (provided that the signal has not been faded). This video output can then be fed into your VCR which provides the output termination. Power for the Colour Video Fader is derived from a 9V DC plugpack supply. PARTS LIST 1 PC board, code 02107931, 103 x 57mm 1 plastic case, 130 x 68 x 41mm 2 self-adhesive labels 1 single-pole 3-position toggle switch 3 panel-mount RCA sockets 1 2.5mm chassis-mount DC power socket 1 10kΩ linear pot. (VR1) 1 500kΩ linear pot. (VR2) 2 knobs to suit 15 PC stakes 4 rubber feet Semiconductors 1 4066 quad analog switch (IC1) 1 4070 quad 2-input OR gate (IC2) 2 BC558 PNP transistors (Q1,Q6) 4 BC548 NPN transistors (Q2-Q5) 2 1N914 signal diodes (D1,D2) 1 1N4004 silicon diode (D3) 1 7805 3-terminal regulator Capacitors 4 100µF PC-mount electrolytic 1 10µF PC-mount electrolytic 4 0.1µF MKT polyester 1 220pF MKT polyester 1 100pF ceramic 1 47pF ceramic Make sure that all polarised parts are correctly oriented when installing them on the PC board. Pin 1 of each IC is adjacent to a notch or dot at one end of the plastic body. Diode D3 provides reverse polarity protection, while a 7805 3-terminal regulator is used to derive a regulated +5V supply rail for the circuit. The 100µF and 0.1µF capacitors fitted to the input and output terminal of the regulator provide filtering and supply decoupling. Construction If you’ve had trouble following the circuit, don’t worry – construction is a cinch. That’s because most of the parts are mounted on a single PC board (code 02107931) and the external wiring is straightforward. Fig.6 shows the parts layout on the PC board. Begin the assembly by installing PC stakes at all external wiring points, then install the wire links and resistors. The accompanying table shows the resistor colour codes but it’s also a good idea to check them on a digital multimeter just to make sure (the colours on some resistors can be difficult to decipher). Note that two of the resistors are installed end-on (near IC2) to conserve board space. Once the resistors are in, the remaining parts can be in­stalled on the board. These include the transistors, diodes, capacitors, the two ICs and the 3-terminal regulator. Be sure to install the correct part at each location and make sure that all polarised parts are correctly oriented. In particular, make sure that you don’t get the transistors mixed up, as both NPN and PNP types are used in the circuit. Take care also with the orientation of the 3-terminal regu­lar. It should be installed with its metal tab towards the centre of the board. This done, the completed board should be carefully checked for missed solder joints and solder splashes. A little time spent Resistors (0.25W, 1%) 1 1MΩ 1 2.2kΩ 1 100kΩ 2 1.5kΩ 1 75kΩ 2 1.2kΩ 1 22kΩ 3 1kΩ 1 12kΩ 1 680Ω 4 10kΩ 3 470Ω 2 6.8kΩ 1 220Ω 1 5.6kΩ 1 150Ω 3 4.7kΩ 1 82Ω Miscellaneous Light-duty hook-up wire, tinned copper wire for links, machine screws & nuts. 24  Silicon Chip Fig.7: check your etched PC board against this full-size artwork before mounting any of the parts. COLOUR VIDEO FADER L➙R EXT. R➙L WIPE DIRECTION FADE WIPE + VIDEO OUT VIDEO IN EXTERNAL INPUT 9VDC IN Fig.8: these full-size artworks can be used as drilling templates for the case. checking at this stage can save a lot of frustration later on. The assembled PC board is housed inside a standard plastic case measuring 130 x 68 x 41mm. This will have to be drilled to accept the PC board and the various hardware items. Before drill­ing, attach the front and side panel labels to the case so that they can be used as drilling templates. The various hardware items can now be installed in the case and the wiring completed as shown in Fig.6. The connections to the front panel components are run using light-duty hook-up wire, while the power and RCA socket connections are run using light-duty figure-8 cable. Once all of the connections have been made, the PC board can be secured to the bottom of the case using machine screws and nuts, with additional nuts used as spacers. Complete the con­struction by fitting four self-adhesive rubber feet to the bottom of the case. is correct, check the current consumption by connecting your multimeter in series with the positive supply rail between the DC power socket and the PC board – you should get a reading of about 50-60mA. Now for the big test – you’ll need either two VCRs or a camcorder and a VCR. The Colour Video Fader is wired into circuit as follows: (1). Connect the VIDEO OUT from the camcorder (or one of the VCRs) to the VIDEO IN of the Colour Video Fader. (2). Connect a lead from the VIDEO OUT of the Colour Video Fader to the VIDEO IN on your VCR (note: if you are using two VCRs, this connection goes to the VIDEO IN of the second machine). (3). Set your camcorder or first VCR to either view a scene or replay an existing tape. The second VCR should be set to AUX IN or AU and the TV connected to its RF OUT socket. (4). Rotate the FADE and WIPE controls fully clockwise, and set the WIPE DIRECTION switch to R-L. This sets the fader to maximum luminance and the wipe function to show a full picture (note: if the screen is black, flick the switch to the L-R position. If the picture now appears, the switch is upside down). The FADE and WIPE controls can now be varied to check that they operate correctly. If everything is OK, set the FADE control to full brightness and set the WIPE control for half-picture/ half black. This done, flick the switch to its alternate setting and check that the picture and black areas of the screen are imme­diately transposed. Finally, most camcorders and VCRs have RCA sockets for their video and audio outputs but some older model VCRs have a BNC socket for their VIDEO OUT connection. If your VCR has a BNC socket, then it’s simply a matter of purchasing a BNC plug-toSC RCA socket converter. Testing To test the unit, first apply power and check for +5V on pin 14 of both ICs. If the reading is 0V, check the plugpack polarity – it’s probably reversed. Assuming that the supply rail The external input accepts signals from an external control unit to create a myriad of fancy wipes. This external controller will appear in a future issue. August 1993  25