Analogue and digital video signal processing using a scrambling strategy
The work described in this thesis proposes and investigates further use of scrambling in industrial analogue and digital monochrome video systems. This scrambling inevitably entails some signal modification. Providing the receiver is able to distinguish between the original and the scrambled signal, regardless of which one was transmitted, more efficient signal exploitation is possible. This more efficient signal exploitation is performed at the expense of the inherent redundancy present in the analogue and di ital signals. Analogue video signals are usually of a highly correlative nature and, this characteristic is exploited, in this thesis, by enabling them to be unwitting data carriers. The video signal is made the data carrier while the data gets a free ride. Each scan-line of the video signal is sampled, and blocks of pels are scrambled or not by modulo masking, depending on whether the data necessary for transmission is a logical 1 11 or 1 01 respectively. Prior to transmission the combined data and video sequence is converted into a continuous signal with a bandwidth that is no greater than that of the original video signal. From the knowledge of the original and the modified signal statistics, the receiver is able to perform the inverse operation of the transmitter, recovering the video signal and the data. Three novel systems are proposed for embedding data into analogue pictures. Two of these systems are capable of supporting an average of 17430 and 8713 bits per (256x256) image LV respectively, with excellent recovered picture quality. The third system produced a constant number of bits per image, with a slight degradation in the recovered picture quality but, with a capability of conveying up to about 0.5 mega bits/sec of data. The idea and technique of embedding data into analogue signals was then carried on to the digital method of coding video signals using differential pulse code modulation. However, the scrambling technique here was used to obtain a novel switched quantization scheme, with forward estimation, without the necessity of sending any side information. Scrambling was performed on the quantizer output levels by inversion. Initially, experiments were carried out using fixed length code words with one and two dimensional predictors. Blocks of quantizer output levels are scrambled, or not, depending on which quantizer was used in encoding the video signal. Hence the switching information was carried by the quantized block of error signals. This type of set-up produced only modest improvements. The quantizers were then altered by using a different number of levels and the switching information was carried one block of quantized error signals in advance. As a result, the average bit rate was reduced to about 2.7 bits/pel using a one dimensional predictor with exceptionally good subjective picture quality. When used with a two dimensional predictor, the scheme produced an average bit rate of about 1.7 bits/pel with excellent subjective picture quality.