|
The fundamental assessment criterion for TV-reconnaissance images is the perceptibility of targets, e.g. vehicles or buildings. An approach to reduce jam susceptibility of the video transmission between a sensor, e.g. at a remotely piloted vehicle (RPV), and a receiving station is bandwidth reduction by data compression, either spatial (e.g. differential PCM (DPCM)) or temporal (e.g. frame rate reduction). The influence of data compression on image quality can be investigated either by objective or subjective evaluation methods. An objective approach is the measurement of the observer performance variation (detection rate, identification rate) at various coding and noise conditions. We made these measurements with two-dimensional (2-D) DPCM-coded static and dynamic aerial scenes for varying bit rates and noise simulations. It is suggested that, under the assumption of a noiseless channel, bit rate can be reduced up to 1.5 - 2 bits per picture element (pixel) without considerable loss in observer performance. In the case of a channel with constant noise power density ('white noise') one must compromise between a better image quality at lower data compression and an increase in the anti-jam capability at higher data compression. In our investigations we found hereby the observer's performance maximum at a bit rate of 1.5 - 2 bits per pixel, whereby it is remarkable that a reduction below 2 bits per pixel resulted in considerable amount of error responses in the case of dynamic scenes. The results of the objective measurements are compared with experimental results of a subjective evaluation method ('pairwise image comparison'). It is shown that the results of the subjective interrogations correspond to the data of the objective measurements. We conclude that for the purpose of decreasing jam susceptibility DPCM-coding enables reduction of the transmission bit rate up to 3 - 4 Mbit/s, relating to a compression ratio of 4 : 1 to 5 : 1, whereby image quality remains acceptable referred to objective and subjective quality criteria.
|
