| Summary: | The picture quality of digital HDTV in the presence of bit errors in the transmission
channel is addressed in this thesis. New coding and data synchronization schemes which
improve the performance of the picture quality are subsequently developed.
I first analyze the causes of the different kinds of digital picture impairments which may
arise when the compression schemes proposed by the present HDTV systems are used. All
these compression schemes are Discrete Cosine Transform (DCT) based. The picture quality
performance is found to suffer mostly from the proposed differential coding of the DC terms
of the DCT coefficients and from the variable length coding used. Due to the differential
coding of the DC terms a bit error in one of these terms will propagate to consecutive bits.
The use of variable length coding, i.e., coding the different DCT terms by words of different
lengths, results in the synchronization problem where the received bits do not correspond
anymore to the originally intended information.
I develop a new method which codes the actual values of the DC terms instead of
their differences. This solves the error propagation problem without altering the overall
compression ratio of the system. To solve the synchronization problem, I present a new
synchronization method which restricts the effects of any bit error to a block of the picture
whose size is much smaller than those presently proposed by the HDTV systems. By
modifying the originally proposed schemes for HDTV with my proposed DC term encoding
and synchronization schemes, the signal-to-noise ratio at which the HDTV picture suddenly
deteriorates is deferred by 2.5 to 3 dB. This has a special advantage when higher order
modulation transmission schemes are used. Using higher order modulation improves the bit
transmission rate but increases the system’s susceptibility to noise.
I then develop a novel fixed length coding method which compresses each DCT coefficient by a codeword of a fixed length. Fixed length coding methods do not suffer from
the error propagation and synchronization problems inherent in the variable length coding
methods. Thus, my method is extremely resistant to errors, produces high quality pictures
and is easier to implement. However, it does not improve the compression ratio. It is, thus,
an ideal candidate for compressing video sequences with fast motion, such as sports, where
the high compression ratios obtained by the motion compensation schemes are not attainable
due to the fast motion.
Finally, a hybrid method that modifies the originally proposed HDTV schemes using the
three above proposed methods is presented. The three modifications involve the use of the
fixed length coding method, the actual DC coding method and the synchronization scheme I
developed. These modifications significantly improve the picture performance of the system
in the presence of noise and do not alter the overall data transmission rate. === Applied Science, Faculty of === Electrical and Computer Engineering, Department of === Graduate
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