A low bandwidth, still image transmission system

In the event of a major earthquake in or near Tennessee Wing of the Civil Air Patrol (TN CAP) is tasked to perform aerial reconnaissance of Tennessee roadways and bridges. To enhance the analysis of aerial observations, TN CAP has proposed broadcasting amateur television from aircraft cockpits to ground receiving stations. However, because of the large bandwidth required for video transmission and the low transmitter power available in the small single engine CAP aircraft cockpits, the range of video transmission is very limited. As an alternative to amateur television transmission, this thesis develops a software based, near real time, continuous-tone still image transmission system.

A digitized copy of a single, color picture at television resolution contains nearly one million bytes. Digital imaging is, therefore, not often implemented due to high storage or transmission costs, even when image capture and display devices are quite affordable. However, by employing a high quality image compression technique, images may be compressed from 1/10 to greater than 1/50 [21] their uncompressed size without visibly affecting image quality. These compressed images can then be transmitted over a low bandwidth channel in a reasonable time.

The major issues in developing this system were the image compression and data transmission techniques. The selected image compression technique is based on the Joint Photographic Expert Group's Still Image Data Compression Standard, known officially as ISO DIS 10918-1 or CCITT Recommendation T.81.

The system's data link will use the Civil Air Patrol's high speed (9600 bits per second) packet radio (CCITT X.25) for air to ground broadcast. The prototype system described in this work employs a telephone modem link using various modulation techniques: 2400 bps (Quadrature Amplitude Modulation, CCITT V.22); 14,400 bps (Trellis coded Modulation/Quadrature Amplitude Modulation CCITT V.32bis); and data compression (CCITT V.42bis) and error correction (CCITT V.42).