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Digital ATV First!

Ned Mountain, WC4X
and
Mike McCombs, KM4YW

This Announcement First Appeared 13 August 1996 on the ATV Mailing List

I thought the ATV community might be interested in what we think is a "first". On 1 August, 1996, WC4X and KM4YW achieved the first 2 way MPEG 2 digital ATV QSO on 1290.00 MHz over an obstructed 3 mile path using 32 milliwatts of power and antennas made out of coffee cans.

Editor's note: MPEG is a form of data compression applied to video signals. Forms of MPEG are used in digital video systems on computers and on "digital" direct broadcast satellite TV systems.

The video data rate was 2.5 Mbits/Sec, and the modulation was QPSK with rate 1/2 Forward Error Correction.

During the next month, we will be attempting DX digital ATV QSO's using higher power and better antennas, and higher data rates. The equipment we used is manufactured by Wegener Communications, our employer. It is designed for digital satellite news gathering and is currently used around the world by major broadcast organizations like CNN.

The bandwidth of our first transmission was 2.7 MHz. Modulation type was QPSK and rate 1/2 error correction was used. If we had more power available, we would have used higher data rates and higher bandwidth. These tests will be done next.

The equipment is too expensive for hams. Each transmit system costs about $70,000, and the receivers are $3,000. This equipment is just now making its way into professional broadcast applications. I think it will be several years before we see prices anywhere near what we need for ham use. But, we are doing this experimental work to show what can be done in the future. We are furtunate to have access to this equipment as it is being manufactured by our company. Anybody working for a television broadcast station or network is very likely to see this type of equipment showing up in the satellite news gathering trucks during the next 12 months.

Ham Radio Online Editor's note: If you don't require real-time digital video, it would be possible to conduct Amateur affordable digital TV using Macintosh and PC-based video technology. For example, at VPC we have two digital video capture boards that convert analog video signals to digital and perform MPEG data compression to capture video as data files on disk. These data files could be transmitted using a digital transmitter, then received and demodulated, and displayed as received video. Since digital video capture is built in to some new Macintosh computers, the costs of such a set up would be around $3,000 for each Macintosh, plus the cost of the digital transmitter and receiver system. We'd guess that since many hams experimenting with ATV already have a Macintosh or PC suitable for video upgrades ($200 to $800 in cost), the cost of a delayed transmission (as compared to real-time) digital TV system would be mostly for the digital data links. Amateur Digital TV (ADTV) would likely require link speeds in the 1.5 Mbps to 3.0 Mbps range.

Video quality is as good as what you get over the DSS satellite system, which transmits each channel at data rates similar to what we used. We can set the equpment for any data rate between 2.5 and 15 Mbits/sec. Most broadcasters will use data rates in the 8 mbit/sec range for high quality transmissions. With digital ATV, the picture is either perfect, or gone! In out first transmission, the picture was perfect, and we had about 3 dB of margin. At the 2.5 mbit data rate, pictures are perfect for normal ATV video. Higher data rates will insure perfect video under all motion conditions. In addition to transmitting the video, we also transmit 4 channels of 15 Khz audio and a 19.2 kbit/sec data channel.

Further experiments are planned in September using higher power and higher data rates, and real antennas between the QTH of WD4MBK in Atlanta, and the two of us from various mountain tops in Georgia and North Carolina. Using digital ATV, we enjoy approximately 15 to 20 dB advantage over conventional VSB-AM transmissions. Thus, we expect to easily communicate over paths that would be impossible using normal analog transmission. 

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