RF Column 22 - July 1993 Copyright (c) 1993,1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: Current progress in digital video compression for satellite How to get connected to Compuserve for broadcast info. (This is old information and may not be valid. See later columns for updates) Isolating voltages for remote control connections -------------------------------------------------------------- The use of digital video compression for fiber and satellite feeds has started . By this time next year the chances are good that at least some of your station's video feeds will use compression. This month I'll pass on some tips on compression system evaluation. Read on for details on a way for you to try out the Broadcaster's forum on CompuServe free. Finally, data on Analog Devices' low cost isolation amplifiers. A recent visit to Scientific Atlanta and an earlier visit to General Instruments gave me an idea of what sort of video gives compression systems problems. Static test signals shouldn't have any problem with video compression - look at how many test signals you can cram into the ROM chips on a test generator. Video compression depends on redundancy and the way the eye perceives objects. A wide shot of a play on a football field, with the crowd in focus waving their hands in the background, is tough. A compression algorithm might give up most of the data available to the crowd, while letting the grass on the field lose resolution ("blocking") or the little football in the air disappear. I also noticed that individual events which by themselves are handled with no problem, have the potential to cause problems when combined. Most TV action moves horizontally, with detail moving across the screen rather than changing. Vertical motion, like the movie promo clips with individual moving images in individual fake film frames moving up or down the screen, challenges compression techniques. Combine this with some random hue changes and a quick change to a detailed scene and you have the potential for problems. The test tape from Cable Labs demonstrates some of these problems. If you are testing a compression system, I strongly recommend you obtain a copy of the Cable Labs compression test tape. It is one of the best I've seen. Hewlett Packard showed a PC based test instrument for evaluating compression system performance at NAB in Las Vegas. It looked at the video before and after the compression/decompression process and noted differences in spatial resolution and image motion, among other things, and plotted them on a series of graphs. I like to have one to carry around to the system manufacturers when checking out systems! Another factor that may be important in some applications is the time it takes for a receiver/decoder to lock onto and decompress a newly acquired signal. If it takes more than a second or two, it will be tough to flip through channels on a sat receiver. General Instruments claims rapid acquisition time as one of the advantages their Digicipher system has over the MPEG-2 based systems. MPEG-2, on the other hand, will have the advantage of inter-operability with computer systems. As chip sets based on the MPEG-2 standard become available, look for a flood of computer and broadcast video products using them. G.I. says their new Digicipher 2 system, expected to be available next year, will decode both Digicipher 2 and MPEG-2 compressed video. Gather round the computer... If you're located in a major TV market, you probably have a chance to get together with other TV engineers regularly to swap ideas and collect information. Or maybe you don't - too much work! There's a gathering place where TV engineers from all over the United States the rest of the world can get together. It's the Broadcast Professional's forum on CompuServe. I've been able to obtain a special offer for readers of this column to get on CompuServe and try it out for free. Many engineers I've talked to about joining other broadcasters on CompuServe generally say its too expensive and too complicated. CompuServe has recently revised its pricing. The standard pricing plan costs $8.95 per month. While many services are included in this price - weather and airline reservations for two - forums like the Broadcast Professionals forum are not. For this and most other forums, an extra charge applies based on connection time. The rates for this vary depending on the speed of the connection. As I write this, the current rate for 2400 baud access is under $.25 a minute. If you are accessing the CompuServe network from outside the U.S., extra charges may apply, depending on the time of day and the location. In some countries CompuServe has its own networks and applies the surcharges only during prime time (during the day). Exact details on pricing are available on CompuServe by typing GO RATES or calling them at 1-800-848-8990 in the U.S. or 1-614-457-8650 in other countries. One of the things that makes CompuServe seem complicated compared to a local BBS is the huge number of messages and files available. Its not unusual for there to be several thousand messages available. CompuServe sells a slick software package called "CompuServe Information Manager" that makes navigating the various forums and information sources easy. You could say the package is free, since it includes an on-line credit equal to the cost of the package. It is a good way to get started, although not the most efficient for forum messaging. Most active forum members use some sort of automatic software to retrieve, read and reply to messages. They work off-line, so the reading and reply writing can be done without the meter running. You can find out about this software once you get on-line. What will you find on the Broadcaster's forum on CompuServe? First, you'll find all the software I've written for this column there. Because of my real job, it can take me a long time to get around to copying the disks and mailing the listings you request. Using CompuServe Mail, you can leave me questions and get answers back much faster than via USPS. Under the monthly plan, some mail credit is included in the monthly charge. My programs are all available on the forum. These include PWRCAL.EXE, a calorimetric RF power calculation and logging programming and the main and accessory programs for my cheap remote control (RMTCTL.BAS and RMTACC.BAS). You'll find a lot of other interesting software there. There are numerous programs for calculating satellite dish look angles, a satellite sun outage prediction program and toolboxes, including RFS.ARC, a collection of handy programs for engineers. More manufacturers are using CompuServe for product support. Tektronix has been active on the broadcaster's forum. When necessary, Jeff Noah, the main Tek rep on the forum, has been able to get replies from the key designers of products like the VM-700 test set. By the time you read this, Grass Valley Group should be set up in the Tektronix portion of the forum as well. Hewlett Packard was on recently to describe the correct way to make signal to noise measurements to show compliance with the new FCC cable rules. I have not seen much representation from TV transmitter manufacturers, however, there are many experienced engineers ready to help. One of the things that impresses me about the Broadcaster's forum is the amount of technical help available. Ask a question about a tower location problem, for example, and you are likely to get replies from other engineers and consultants who have solutions. If it involves the FCC, you might even get a reply from John Reiser, from the FCC. I don't have the space here to describe how to use CompuServe. The best way is to try it yourself. Thanks to John Hoffman, the chief system operator (SYSOP) on the Broadcast Professional's Forum, I can offer readers a free sign up kit with $15 of on- line time to try it out. Visit the forum, download my files, look at the other offerings - the monthly plan offers access to airline reservations, weather information (including satellite photos) and more at no extra charge. To obtain the sign up kit, call 1-800-848-8199 and ask for operator 175. (# If the 800 number does not work in your area, use the main number 1-614-457-8650. #) I should point out here that I have NO financial interest whatsoever in your joining the forum. I do think it is valuable tool for all broadcast engineers and particularly RF engineers, which you've no doubt noticed are becoming scarce. Isolating voltages... Last month I promised information on an inexpensive way to measure voltages that are not referenced to ground. Some remote control systems float both wires for analog voltage readings. Most have one side grounded. Even those system which have differential inputs have limitations. In many high voltage supplies for tube amplifiers (conventional and klystron), the low side of the supply is often elevated above ground with a resistor. This provides a convenient way to protect the transmitter from high voltage arcs. Any arcs that occur to ground will cause current to flow in this resistor, which can be sensed and used to shut off the voltage. In such circuits, the shunt resistor used for sensing the current through the tube must be not be connected to ground. Analog Devices has developed a line of low cost, miniature isolation amplifiers. These amplifiers come in various flavors. The AD202 series operates directly from a 15 volt DC supply, while the AD204 series is powered by an external, isolated clock (the AD246). If you plan to use several of these, the AD204 series offers some cost savings, lower power consumption and higher bandwidth. The AD202 series is simpler to use. That is the one I'll describe here. The isolation amplifiers work by using an input op amp to drive an amplitude modulator, which drives an isolation transformer. The secondary side of the transformer is connected to a demodulator, which provides up to +/- 5 volts peak output. The input op amp has all terminals available, so it can be used to amplify, offset and/or invert the input voltage. The AD202 has an internal oscillator which is used to supply AC power to an internal power isolation transformer. An isolated voltage is available on the other side, however, it is limited to +/- 7.5 volts at 5 milliamperes. Figure 1 shows a functional diagram of the AD202 along with the pin connections for the DIP package. Operation of the isolation amplifier should be self-evident from figure 1. Analog Devices does have a few tips for designs. Remember that voltages in and out cannot exceed +/- 5 volts. Negative voltage can cause current to flow through the input if the power is removed to the isolator. For that reason, use a 2,000 ohm resistor on the input to prevent the current from damaging the isolator or preventing the oscillator from starting. If the source can't deliver more than a few milliamperes, this resistor isn't needed. For unity gain applications, pins 3 and 38 are shorted. If gain is needed, a feedback resistor (Rf) can be inserted between pins 3 and 38 with a resistor (Rg) from pin 3 to input common (pin 2). Analog Devices says that Rf should be 20,000 ohms or more. If gain exceeds 5, add a 100 pf capacitor from pin 38 to input ground pin 2. The resistors needed for a given gain can be determined from the equation: Gain = 1 + (Rf / Rg). Other standard operational amplifier circuits can be used around the op amp in the AD202, just keep in mind that Rf must be 20,000 ohms or more and a cap may be needed. When designing circuits using the AD202, ground everything well and be sure that wiring or PC traces do not reduce the up to 2000 volt isolation. The AD202 family includes both SIP and DIP packaged units with isolation up to 2,000 volts. The AD202JY is in a SIP package with 1000 volt isolation and +/- 0.05% linearity. The AD202JN is the same unit in a DIP package. For 2000 volt isolation, replace the J with a K in the part number. The 2000 volt model has better linearity - +/-0.025%. Allied Electronics stocks the AD202JN, under their part number 630-0265. The last catalog price I saw on it was $45.00. For other distributors or distributors outside the U.S., contact Analog Devices at telephone 1-617-329-4700 or Telex 924491. The address is One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106 U.S.A. The Analog Devices data sheet on the AD202/AD204 has physical details, pin outs for all versions and application examples. A final warning! These amplifiers isolate up to 2000 volts - they will not isolate the output from the full plate voltages used in most transmitters. Take care in how they are installed and make sure that if they should fail, they do not place dangerous voltages where they could harm someone. That's it for this month. As always, I welcome your comments and questions. My schedule makes it somewhat difficult to reach me, so MAIL via CompuServe is best - my ID is 70255,460. Postal mail will also work, eventually, if sent to 2265 Westwood Blvd, Suite 553, Los Angeles, CA 90064. Phone number is 305-884-9664. Because of my travels it may take a day or more to return calls if I'm on the road. ((8/95 > UPDATE! - Use dlung@gate.net for e-mail!)) Copyright (c) 1993,1995 H. Douglas Lung ALL RIGHTS RESERVED