Compression of underwater images for their transmission on a low bit rate acoustic channel

THOMSON-CSF DASM has developed industrial equipment which enables the transmission, at 20 kbits/s rate, of instantaneous TV images by means of acoustic waves. An uncompressed 150,000 bits image is therefore transmitted in 7.3 seconds. The purpose of this paper is to describe an inter-image compression which brings back the image transmission time to a second or below. The chosen compression only transmits the moving zones described by 3 × 3 pixels blocks, coded by the BTC technique with variable resolution. This scheme leads to three gains onto the number of bits used for their coding : a movement gain, a BTC gain, and a resolution gain. A compression stategy controls each of the three gains according to the quantity of motion in order to maintain the compression ratio above 7.3.