Digital Amateur Television (D-AN)
igital communications are as old as amateur receiver design. However, TV broadcasters are radio (Morse code is digital, after all) and under FCC pressure to convert to digital trans- DAmateur Television (ATV) is almost as old missions before the end of this decade. as image communications, with radio amateurs at Hams were there when Fast Scan TV became the forefront even before the first commercial TV available, and so it is with digital TV as well. Here broadcast. Today, like just about everything else, in the United States, few have not heard of High the world of video and TV is moving rapidly towards Definition Television, or HDTV. Some stations are more digital technology. These days you can't already broadcasting HDTV signals. Its increased avoid hearing about the latest satellite receiver or resolution-about that of a computer monitor, or high-definition television (HDTV) setup. triple what's available in analog broadcasts- This month we'll have a lookat how ATV is going combined with reduced noise and better color ren- digital, the building blocks and what they do, and dition provide for a much sharper and clearer pic- some ideas for those wanting to get involved. The ture. In fact, I read an article about some actors move to digital ATV, or D-ATV, began a few years who were complaining about HDTV, as it shows ago when some talented hams in Europe, capi- all their flaws, such as caked-on makeup or skin talizing on inexpensive satellite receivers, built a blemishes, with excellent clarity. Perhaps they're digital video transmitting system. Building a digi- afraid we'll learn they are just human like the rest tal ATV transmitter from scratch isn't for the faint of us. of heart, and detailed construction information is Of course, when there's a transmitter involved, far beyond the scope of this column. My intent is some hams will figure out a way to get involved. to tell you about how this technology works and, So it is with Digital ATV. At the close of the last cen- just maybe, get you interested enough to pursue tury, I was privileged to witness a demonstration of it further. a D-ATV setup designed and implemented by some friends in Germany. The hardware was ugly, the victim of many hand-soldered board revisions, Today, virtually all Amateur Television (ATV) but the picture was better than the TV we were transmissions, especially here in the U.S., are using could display. While that system eventually analog, using either Amplitude Modulation (AM) was demonstrated in public, if you wanted one or Frequency Modulation (FM). Regular broad- you'd have to build it yourself, since there were no cast TV uses a form of AM known as Vestigial kits or instructions available. More recently, two Sideband, or VSB, which keeps only a small part Dutch hams, Henk, PEIJOK, and Werner, of one sideband (by convention, the lower) and a PE1OBW, created a website to explain the system. complete upper sideband. This allows the receiv- As explained in the sidebar accompanyingthis arti- er to demodulate the signal more like AM than cle, at least one club has built a few board sets for Single Sideband (SSB), considerably simplifying sale, and some of the other links seem promising. DVB Choices *P.0. Box 1 14, Park Ridge, NJ 07656 A few years ago, when D-ATV was being devel- , e-mail:
70crn Fig. 1- A iypical - Analog ATV - MPEG-2 Digital ATV repeater Receiver 1 Encoder site. The users uplink their video to the repeater as a regular analog signal, and it 23 crn is repeated over a wide area as a digital signal. Each uplinked signal is carried on a different digital " channel" on the downlink, allowing 10 GHz many hams to use the repeater - Analog ATV - MPEG-2 simultaneously. Receiver 4 Encoder
Visit Our Web Site the rigors of space flight, it could be vis ited and repaired as necessary. On the other hand, quite a bit of signal pro cessing was required, and the require ments were not trivial. Let's have a brief look at the hardware used for a D-ATV repeater. Although there are at least two imple mentations of a D-ATV repeater that I am aware of (see sidebar), they both fol low the same general architecture. The subject is far too complex to discuss in detail, but this general overview should provide enough information for you to understand what's involved. Before we start, one point of clarifi cation: These systems, designed and deployed in Europe, use encoders for the PAL standard, which is incompati ble with the NTSC standard used in the United States. Although the Dutch web site lists an NTSC version as optional ly available, I've been unable to find information about one actually being built. Nevertheless, there are only minor Photo A- The Dutch D-A TV boards under test. In front of the laptop computer are technical differences between an NTSC (from left to right) the MPEG-2 encoder, the DVB-S baseband encoder, and and a PAL implementation. (toward the front) the transmitter. Note the LCD display, which is used by the DVB-S encoder to display status. (Photo courtesy of Henk, PE1JOK) The D-ATV System Video Broadcasting (DVB): DVB-C that's what the receiver thought it was The first piece in the D-ATV chain is the (Cable), DVB-T (Terrestrial), and DVB- hearing. Most of the signal processing MPEG encoder. MPEG stands for S (Satellite). The DVB for Cable expects for a satellite broadcast is done on the Motion Picture Experts Group, a con a very low noise environment, such as ground, with the satellite acting as a sortium which developed a number of would be found on a cable network, and repeater. A similar approach to D-ATV standards for the (digital) compression is poorly suited for over-the-air use. The was implemented. of motion pictures and sound. Much like DVB-T standard was created specifi Because of the relatively high com the JPEG standard for still images, cally for local terrestrial broadcast, with plexity (and cost) of the transmitter, the MPEG compression can significantly very strong resistance to multipath dis D-ATV implementation that made the reduce the amount of data required to tortion, but it requires an ultra-linear sig most sense was to allow users to uplink represent a moving image. There are a nal path, including amplifiers, making into the D-ATV repeated site with nor few MPEG standards, including ones the design of a transmitter non-trivial. mal Analog video, and have the for audio. For D-ATV, MPEG-2 encod The DVB-S standard works well with repeater transmit in Digital, as shown in ing was chosen. weak signals and places no heavy fig. 1. The DVB-S system allows for mul Essentially, the MPEG-2 encoder is a requirements on the amplifier linearity. tiple downlink channels on the same specialized, real-time digital signal It appeared that the DVB-S standard signal, so multiple uplinks can be mul processor (DSP) for video compres was the best choice for an ATV system. tiplexed onto different digital downlink sion. The German implementation uses A major advantage of DVB-S was the "channels" at the same time. a Fujitsu MB86390 MPEG-2 encoder wide availability of set-top receivers, Most of the design for a D-ATV sys along with considerable support circuit operating in the 950-2150 MHz range, tem came from off-the-shelf hardware. ry, including a micro-controller and that could be had cheap or for free. Certainly, most amateurs already memory. The Dutch implementation Testing showed that the weaker multi- equipped for ATV have what they need doesn't specify the chip set, and you path performance wasn't really a prob to feed video to the repeater site, and can't tell what it is from the photo, but it lem. Thus, with the receivers being set-top boxes, possibly equipped with a looks quite different from the Fujitsu freely available, all that was left was the frequency converter, are readily avail evaluation board. One module is used transmitter design. (Note that more able to receive digital signals from the for each video input stream. The output recently introduced satellite TV sys repeater. The receivers for the repeater of this module is a wide, fast digital data tems prevalent in the United States, are commonly available as well, either stream, to handle the relatively high such as DISH Network and DIRECTV, commercially or built from an old VCR data rate. use newer Digital Broadcast Satellite or TV. All that remained was to put a The second piece of the chain is the [DBS] standards that are incompatible.) satellite—or at least what looked like DVB-S baseband encoder. This piece one—at the repeater site. takes the MPEG-2 compressed video Satellite hardware normally isn't and audio data and converts it to a DVB- A Grounded Satellite available off the shelf, as you can imag S signal at baseband, feeding it to the The transmitter needed to replicate the ine. Of course, this wasn't really a satel Quadrature (I and Q) transmitter. The signal coming from a satellite. After all, lite; not only did it not have to withstand German implementation can handle up
62 . CQ • June 2004 Visit Our Web Site to four MPEG-2 input data streams, NTSC video. A quick search of eBay long before it makes its way to North putting each one on a different digital (www.ebay.com) found over 100 items, America. This kind of project is perfect channel. Although we're discussing most of them receivers, for under $50. for an active ATV group to implement, Digital ATV, the output of this module is You just need to be sure of what you're since it requires a bit of money and analog. After all, the real world, where buying: a DVB-S receiver (often called expertise. The end users will rejoice, transmitters live, is still analog. a Free To Air satellite receiver) with since their investment for the upgrade The last module in the chain is the NTSC video output. is minimal, and the ATV repeater's transmitter. Aside from the wide band Therefore, the real expense of a capacity will be vastly improved. width, this is an ordinary l/Q transmit Digital ATV system is the digital trans ter. In fact, the design of the German mitter system, which I estimate might be version is based on a transmitter built for about $1200. This is a perfect Summary designed for high-speed packet. The project for an active ATV group, which This month we took a look at yet anoth output power is only a watt or two at 1.2 can convert its existing ATV repeater er new digital mode for amateurs to use GHz, and even less at 2.4 GHz, but a into a digital system with multi-channel with their wonderful playground in the decent linear power amplifier is not a capabilities. Both of the websites listed aether. The past few columns have problem. With some of the coding gain in the sidebar have general information looked at digital operating modes, so of DVB-S (about +10 dB) one can ex about the design of their systems, but next time I thought we'd have a look at pect to use much less power than an I'm confident that they would be happy some hardware: Local Area Networks. FM ATV repeater for the same perfor to share their detailed knowledge with Useful for sharing files at home or man mance. However, path loss at 23 cm you, if you're serious about building a aging a serious contest effort, this how- and 13 cm is greater than at 70 cm, and D-ATV system. to column will explain the nuts and bolts so it might end up an even exchange. Digital Amateur Television is actually of assembling a computer network. being used in Europe, and it won't be Until then ... 73, Don N2IRZ Receivers Digital ATV Resources The best part of the D-ATV system is Dutch D-ATV site,