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Send video signals over twisted pair cable Use this Video Transmitter and Video Receiver to wire your home or business with remote video for entertainment or for CCTV security systems. By JOHN CLARKE What’s a twisted pair cable? For all intents and purposes, it is equivalent to a pair of telephone wires and you can’t send video over telephone wires, can you? Well, with this new chipset you could, although that is not why we are presenting this arti­cle. With the recent introduction of 12  Silicon Chip low-cost monochrome video cameras, (less than $200 retail) and this project, you can now easily monitor your front door, your swimming pool or any other part of your home that needs watching from a central location. Video signals are normally sent through 75Ω coax cable and for short runs over several tens of metres there is very little loss in signal level. But over long distances the losses in the cable reduce the signal to an unacceptably low level for the receiving equipment. One way of overcoming the signal loss is to modulate it onto a carrier in the UHF or VHF range. Any signal loss in the cable can then be made good by wideband distribution amplifiers. However, at the receiving end, the signal must be demodulated before it can be displayed on a monitor. This approach is well-proven but coax cable and distribution amplifiers are expensive. Wouldn’t it be nice to be able to send video over ordinary twist­ ed wires? Using the video transmitter Fig.1: the general arrangement of the MAX435 & MAX436 ICs. The MAX435 has a differential output while the MAX436 only has a single ended output. These are transconductance amplifiers so the outputs produce a current that’s proportional to the applied differential input voltage. Features • • 1.5km range (expected) Video transmitted over low cost twisted pair • Audio transmitted in stereo over 50m using op amp transmitter • Up to 1.5km range for mono audio using video transmitter and receiver described here, video can be sent over distances up to 1.5km. Transconductance amplifiers The heart of this project is a pair of ICs made by Maxim Integrated Products, the MAX435 and MAX436. These two ICs are classified as high speed, wideband transconductance amplifiers (WTAs) with true differential, high impedance inputs. The unique architecture of these amplifiers provides accurate gain without negative feedback. Without the feedback, the possibility of spurious oscillation is virtually eliminated. Fig.1 shows the general arrangement of the ICs. The MAX435 has a differential output while the MAX436 only has a single ended output. The outputs produce a current that’s proportional to the applied differential input voltage, providing inherent short circuit protection for the outputs. The circuit gain is set by the ratio of the output impedance “RL”, the user connected transconductance network “ZT” and an internally set current gain factor, K. In the case of the MAX­435, the current gain is nomi­ nally 4 (±2.5) and for the MAX436 this figure is 8 (±2.5). Inside the video transmitter. The top view shows the audio board “hinged” back to reveal the video board, which sits on the bottom of the case. The view above shows the audio board in place. The MAX435 has a 275MHz bandwidth and 800V/µs slew rate, while the MAX436 has a 200MHz bandwidth and 850V/µs slew rate. The common mode rejection ratio for both is -53dB at 10MHz and -90dB at DC. While MAX ICs could also be used to transmit audio signals, we have taken a cheaper approach and used dual op amps to produce a balanced October 1996  13 Fig.2: the transmitter circuit takes composite video and provides a balanced output to the twisted pair. The unbalanced audio signals are converted to balanced outputs by the LM833 op amps. audio transmitter and receiver. These produce high quality stereo results over short runs of less than 50m. This will allow you to pipe composite video and stereo audio signals around your house. For many applications though, we envisage that the video transmitter and receiver boards will be all that are required. Transmitter circuit Fig.2 shows the transmitter circuit. Unbalanced video signal is applied to the IN+ input of the MAX435. The inverting input IN- is connected to ground. The transconductance element impedance between pins 3 and 14  Silicon Chip 5 is set at 220Ω, while the output impedance is a nominal 50Ω. The 4.7kΩ resistor sets the supply current for the IC. Supply decoupling for the ±5V rails, pro­vided by the 0.1µF capacitors, is necessary for best performance at the high frequencies involved. Power is derived from a 12VAC plugpack. This is rectified with halfwave rectifier diodes D1 and D2 to supply the ± rails before regulation. The 470µF capacitors filter the raw DC to produce a relatively smooth voltage. REG1 and REG2 regulate the supplies down to ±5V for IC1. Audio input is applied to a single ended to balanced output amplifier comprising IC2a and IC2b for the left channel and IC3a and IC3b for the right. IC2a is a unity gain buffer which is non-inverting. The output at pin 1 is therefore the + output which drives the positive twisted pair line via a 680Ω resistor. IC2b is connected as an inverting amplifier and the resulting output at pin 7 drives the negative twisted pair line via its 680Ω resistor. The 22pF capacitor across the feedback resistor of IC2b prevents high frequency oscillation. The right channel audio amplifier operates similarly to the left channel circuit. Receiver circuit Fig.3 shows the receiver circuit. IC4 is a MAX436 which accepts the Fig.3: the receiver circuit uses a MAX436 to convert the balanced input from the twisted pair to an unbalanced video output. Simi­larly, the balanced audio signals are converted to single-ended signals by op amps IC5 and IC6. balanced input from the twisted pair and produces an unbalanced output. The 51Ω resistors at the IN+ and INinputs at pins 2 and 6 provide the correct loading for the twisted pair line. A 100Ω resistor in series with trimpot VR1 is connected between pins 3 and 2. VR1 sets the gain of IC4, to compensate for losses in the twisted pair line. A second 100Ω resistor from pin 3 is connected in series with a 56pF capacitor and VC1. The capacitance corrects for the loss of high frequency signal through the line. In prac­tice the capacitance is adjusted until the colour burst signal is at its correct level. The 4.7kΩ resistor at pin 11 sets the current for IC4. The power supply circuit is iden- Specifications Video transmitter/receiver pair Frequency response ����������������� typically -3dB at 200MHz Common mode rejection ����������� typically -53dB at 10MHz; -90dB at DC Audio transmitter/receiver pair Signal to noise ratio ������������������ -102dB unweighted (20Hz to 20kHz) with respect to 1V RMS Common mode rejection ����������� -62dB at 50Hz and 1kHz Harmonic distortion ������������������� less than .016% from 20Hz to 20kHz Frequency response ������������������ -0.25dB at 20Hz and 20kHz Clipping level ���������������������������� 1.7V RMS at input Crosstalk ����������������������������������� -80dB (20Hz to 20kHz) with 20m twisted pair along­side video pair October 1996  15 Fig.4: the component overlays and wiring details for the trans­mitter boards. tical to that used in the transmitter. The audio signal is converted from the balanced twisted pair signal to an unbalanced output using op amps IC5a & IC5b for the left channel and IC6a & IC6b for the right channel. The balanced signal is applied to the non-inverting inputs of IC5a and IC5b. The 330Ω resistors tie the inputs to ground and provide a load for the twisted pair line. A .001µF capacitor is included across the input terminals to remove high frequency noise from 16  Silicon Chip the line. Both IC5a and IC5b are set for a gain of two due to the 1kΩ feedback resistors. Signals which are common to each input are rejected at the output and this is due to the feedback for IC5a being connected to the output of IC5b. Difference signals are amplified at the pin 1 output of IC5a. The 100Ω output resistor prevents oscillation in IC5a due to capacitive loading. The right channel audio amplifier operates in exactly the same manner as the left channel audio amplifier. Construction The Video Transmitter and Video Receiver are housed in separate plastic cases measuring 130 x 68 x 42mm. The video transmitter (using the MAX­435) is built onto a PC board measuring 60 x 102mm (coded 023­06961). The audio transmitter PC board (coded 023069623) is piggy-backed onto the video transmitter. The two receiver PC boards (coded PARTS LIST Video transmitter board 1 PC board, code 02306961, 60 x 102mm 1 plastic utility case, 130 x 68 x 42mm 1 self-adhesive front-panel label, 62 x 126mm 1 12VAC 500mA plugpack 1 SPDT toggle switch (S1) 2 3mm screws and nuts 1 DC socket 2 panel-mount RCA sockets 8 PC stakes 1 40mm length of 0.8mm tinned copper wire Semiconductors 1 MAX435CPD high-speed transconductance amplifier (IC1) 1 7805 3-terminal regulator (REG1) 1 7905 3-terminal regulator (REG2) 2 1N4004 1A silicon diodes (D1,D2) Video Receiver board 1 PC board, code 02306962, 60 x 102mm 1 plastic case, 130 x 68 x 42mm 1 front panel label, 62 x 126mm 1 12VAC 500mA plugpack 1 SPDT toggle switch (S2) 2 3mm screws and nuts 1 DC socket 2 panel-mount RCA sockets 8 PC stakes 1 40mm length of 0.8mm tinned copper wire 1 500Ω horizontal trimpot (VR1) Semiconductors 1 MAX436CPD high-speed transconductance amplifier (IC4) 1 7805 3-terminal regulator (REG3) 1 7905 3-terminal regulator (REG4) 2 1N4004 1A diodes (D3,D4) Capacitors 2 470µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 3 0.1µF ceramic Capacitors 2 470µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 3 0.1µF ceramic 1 56pF ceramic 1 3-60pF trimmer (optional) Resistors (0.25W 1%) 1 4.7kΩ 1 75Ω 1 220Ω 2 51Ω Resistors (0.25W, 1%) 1 4.7kΩ 1 75Ω 2 100Ω 2 51Ω Audio transmitter board Audio receiver board 1 PC board, code 02306964, 60 x 102mm 4 12mm spacers 4 6mm spacers 4 20mm x 3mm screws 4 3mm nuts 11 PC stakes 1 20mm length of 0.8mm tinned copper wire 1 PC board, code 02306963, 60 x 102mm 4 12mm spacers 4 6mm spacers 4 20mm x 3mm screws 4 3mm nuts 11 PC stakes 1 20mm length of 0.8mm tinned copper wire Semiconductors 2 LM833 op amps (IC2,IC3) Semiconductors 2 TL072 op amps (IC5,IC6) Capacitors 4 10µF 16VW PC electrolytic 2 22pF ceramic Capacitors 4 10µF 16VW PC electrolytic 1 .001mF MKT polyester Resistors (0.25W, 1%) 6 x 10kΩ 2 330Ω 4 680Ω Resistors (0.25W, 1%) 8 x 10kΩ 2 100Ω 4 330Ω YOU CAN AFFORD AN INTERNATIONAL SATELLITE TV SYSTEM SATELLITE ENTHUSIASTS STARTER KIT YOUR OWN INTERNATIONAL SYSTEM FROM ONLY: FREE RECEPTION FROM Asiasat II, Gorizont, Palapa, Panamsat, Intelsat HERE'S WHAT YOU GET: ● ● ● ● ● ● 400 channel dual input receiver preprogrammed for all viewable satellites 1.8m solid ground mount dish 20°K LNBF 25m coaxial cable easy set up instructions regular customer newsletters BEWARE OF IMITATORS Direct Importer: AV-COMM PTY. LTD. PO BOX 225, Balgowlah NSW 2093 Tel: (02) 9949 7417 / 9948 2667 Fax: (02) 9949 7095 VISIT OUR INTERNET SITE http://www.avcomm.com.au YES GARRY, please send me more information on international band satellite systems. Name: __________________________________ Address: ________________________________ ____________________P'code: __________ Phone: (_______) ________________________ ACN 002 174 478 October 1996  17 Fig.5: the component overlays and wiring details for the receiver boards. TABLE 1: RESISTOR COLOUR CODES ❏ No. ❏  6 ❏  2 ❏  8 ❏  4 ❏  6 ❏  2 ❏  2 ❏  4 18  Silicon Chip Value 10kΩ 4.7kΩ 1kΩ 680Ω 330Ω 100Ω 75Ω 51Ω 4-Band Code (1%) brown black orange brown yellow violet red brown brown black red brown blue grey brown brown orange orange brown brown brown black brown brown violet green black brown green brown black brown 5-Band Code (1%) brown black black red brown yellow violet black brown brown brown black black brown brown blue grey black black brown orange orange black black brown brown black black black brown violet green black gold brown green brown black gold brown 02306962 and 02306964) are mounted in a similar fashion to the transmitter. On both cases, RCA sockets for video input and output are mounted at the sides of the box while the audio sockets are at one end of the box. The DC socket is mounted at the opposite end while the power switch is attached to the lid. Fig.4 shows the component overlay and wiring details for the transmitter boards while Fig.5 shows the wiring details for the receiver boards. Begin construction by checking each PC board for breaks or shorts in the copper pattern or any undrilled holes. Fix any defects before proceeding further, then insert all the PC stakes. These are located at the signal input and output wiring points and for power supply. Next, solder in all the links and resistors. Table 1 shows the resistor colour codes and it is a good idea to check each resistor value with your multimeter before soldering it into the board. The capacitors can be mounted next, noting that the electrolytic types must be oriented with the correct polarity as shown. Finally, insert the ICs, making sure that each one has the correct orientation. If you do not intend to use stereo audio channels, IC3 on the audio transmitter board and IC6 on the receiver board can be omitted. The voltage regulators are each mounted horizontally on the PC board and secured with a screw and nut. Bend the leads for each component before insertion into the PC board holes. Take care to orient the diodes correctly. Drill holes in the plastic cases for the RCA and DC sockets using the front panel as a guide to their location. A hole is also required in the lid for the power switch on each box. Wire up the PC boards as shown in Fig.4 and Fig.5. In each case, the audio board is stacked on top of the video board and the two are separated by metal spacers. The integral side pillars in each box will need to be removed so that the PC board assembly can fit comfortably within the case. Affix the label to each lid and attach the power switches. Testing The completed units are now ready for testing. Apply power to the transmitter PC boards and check voltages. These two photographs show the completed boards inside the receiver case. The top view shows the video receiver board, while above is the audio board. Fig.6: the top trace of the oscilloscope display shows a PAL colour bar as the video source signal. Note the colour burst at the far lefthand side of the trace. The lower trace shows the received signal after transmission over 20m of twisted pair. The gain has been compensated for signal loss and for video colour burst level. Note the faithful reproduction of the signal. October 1996  19 TRANSFORMERS • TOROIDAL • CONVENTIONAL • POWER • OUTPUT • CURRENT • INVERTER • PLUGPACKS • CHOKES STOCK RANGE TOROIDALS BEST PRICES APPROVED TO AS 3108-1994 SPECIALS DESIGNED & MADE 15VA to 7.5kVA Tortech Pty Ltd These beautifully-made binders will protect your copies of SILICON CHIP. They are made from a dis­tinctive 2-tone green vinyl & will look great on your bookshelf. Price: $A11.95 plus $3 p&p each (NZ $8 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. 20  Silicon Chip ON + OFF VIDEO TRANSMITTER L R AUDIO IN BALANCED VIDEO OUT Silicon Chip Binders L R BALANCED AUDIO OUT 24/31 Wentworth St, Greenacre 2190 Phone (02) 642 6003 Fax (02) 642 6127 VIDEO IN Fig.7: actual size front panel & PC board artworks for the transmitter. Check your PC boards carefully before installing any of the parts. Connect the nega­tive multimeter lead to GND and touch the positive lead on pins 1, 12 & 14 of IC1 where +5V should be present in each case. Similarly, there should be -5V on pins 7, 8 & 10 of IC1. IC2 and IC3 should have +5V on pin 8 and -5V on pin 4. For the receiver PC boards, there should be +5V on pins 1, 12 & 14 of IC4 and -5V on pins 7, 8 & 10. IC5 and IC6 should have +5V on pin 8 and -5V on pin 4. To transmit video you will require one twisted pair while each audio channel will require a separate twisted pair. Use 75Ω coax cable from your video source to the transmitter and from the receiver to the video input of Are you frustrated using DOS or non-compliant Windows software? If so then you may be interested in the following schematic design software trade-in offer from OrCAD. Here are 7 good reasons to trade-in your old schematic software tool to ON + OFF VIDEO RECEIVER L R BALANCED AUDIO IN VIDEO OUT L R AUDIO OUT OrCAD Capture for Windows… BALANCED VIDEO IN Fig.8: actual size front panel & PC board artworks for the receiver. All PC boards measure 60 x 102mm. your monitor or VCR. Audio connec­ tions should be made using standard shielded audio cable. VR1 on the receiver is adjusted to obtain best black and white levels as seen on the monitor. The 56pF capacitor on IC4 should be satisfactory for twisted pair up to 50m. A larger capacitor value can be used if the colour burst signal is marginal. This can be adjusted until the monitor provides a solid colour pic­ture. Lack of picture sync (rolling) means that there is 50Hz present on the video signal. Either re-route the twisted pair away from mains power wiring or use shielded pair earthed to the GND SC terminals. ❶ De-facto standard schematic capture software. OrCAD is the best-selling package with over 180,000 licensed users worldwide. ❷ Easy to use and learn. Capture has an online tutorial and hypertext ‘Help’. ❸ Works on Windows 3.x, Windows 95 and Windows NT. Support for all platforms provided in one box. ❹ True 32-bit application. Faster processing on 32-bit platforms. ❺ Cut, copy and paste between Capture and other Windows compliant software. Developed to comply with Microsoft Foundation Class. ❻ Supports hierarchical designs. Create complex designs in modular form. ❼ Only $799 (Trade-in offer to all registered owners of Protel schematics and selected other schematic capture software tools. Normally $2195). ✄ Please send me more information on OrCAD Capture for Windows. My details are: Name: Company: Address: Phone: Fax: I am using the following brands of software: Schematic Entry: Simulation: PCB Design: (Fax this form to EDA Solutions on 02-9413 4622 or ring and ask for Richard on 02-9413 4611) SC11/96 Level 3, South Tower 1-5 Railway Street CHATSWOOD NSW 2067 Australia Ph: +61-2-9413 4611 fax: +61-2-9413 4622 email: info<at>eda.com.au Offer for a limited time only. October 1996  21