Skip, K1NKR a local friend and VHF/UHF expert and I began talking about the idea of building a Fast Scan Amateur Television (ATV) System some time ago. Our early research and the antenna equipment which we had in place at our stations led us to plan our ATV project around the 70 cm band. The 70 cm band plan in the United States has allocations for Fast Scan ATV transmissions with a bandwidth of up to 6 MHz. Our research led us to Jim Andrews, KH6HTV’s excellent website where we discovered that it was possible to build a Digital ATV station using reasonably priced commercially available DVB-T format Modulators and Demodulators. Jim’s site has a wealth of great Applications Notes on Digital ATV and its a great place to start to learn about this technology. A combination of a DVB-T Modulator and Demodulator from Hi-Des was chosen as the heart of our Digital ATV System. We also worked with Jim to secure the needed Wideband Linear Power Amplifiers for the 70 cm band. We began receiving the equipment to build our Digital ATV Stations late last year. We’ve done quite a bit of testing on the air and some custom development work which has resulted in a pair of excellent performing Digital ATV stations. The picture above shows a Digital ATV “CQ” that I sent to initiate one of our early QSOs.
Here’s a picture of Skip receiving my “CQ” at his end. The picture quality produced by the equipment that we’re using and the DVB-T format is phenomenal. The Hi-Des Modulators which we are using have a large number of parameters which can be set to determine the format and bandwidth of the signals we generate. After some experimentation, we have settled on using QPSK modulation and a 6 MHz signal bandwidth. This combination delivers excellent picture quality with more that adequate motion performance. We see very few if any picture artifacts using our current format. We’ve also done some experimentation with QPSK and a 4 MHz signal bandwidth. I plan to share more on signal formats in a future article on our blog.
We are both using HD Digital Camcorders as our primary video signal sources and 1080p monitors to display our received signals. I opted to include an HDMI Video Switch from Gefen in my setup which also allows me to send video and graphics from a variety of different sources including my PC over the air. The monitor in the picture above on the right is a touch screen display which I use to control my ATV Transceiver system.
Early on, I decided to build a Transceiver like setup. I wanted to create a unit which was simple to use just like the HF Transceivers that are available today. Some of the key capabilities that I wanted to create included:
- Real-time selection and switching between multiple HD video sources
- Transmission of PC sourced Video and Graphics over the air
- Preview and cueing of the next video transmission while receiving
- Simultaneous display of both receive and pending transmit video
- Built-in Transmit/Receive (T/R) switching with termination and protection of the Tx power stage
- Sequencing of T/R stages including my tower mounted pre-amplifier system
- Power and SWR monitoring with automatic trip on high SWR
- An internal low-noise RF preamplifier to provide additional receive signal gain if needed
- Touch screen graphical interface for configuration and operating the system
- Recording of both sides of on-air video QSOs to an attached PC
To achieve these goals, I decided to build a Raspberry Pi 2 based Linux controller of my ATV Transceiver and to package all of the ATV components and video switching/conversion gear needed in a small rack mount enclosure. Many of the components in the system communicate with each other over an ethernet LAN and the transceiver is networked to computers and other devices via an external ethernet connection. More on the details of the Transceiver design to come in a future article.
Skip and I recently produced a short video to demonstrate how Fast Scan Digital ATV works and to show the quality that these systems are capable of producing. Our project is still a work in progress and I expect that we will continue to learn as we perform more tests and continue development of our systems. I plan to post additional articles here to share the details of our designs and learning from our on-air testing as we proceed.
– Fred (AB1OC)