Walter Tengler

A new Hybrid Coding Technique for Videoconference Applications at 2 Mbit/s

A new hybrid coding technique is introduced which is based on the Discrete Cosine Transform, interframe DPCM, conditional replenishment, detection of significant subareas in the transform domain, adaptive quantization, adaptive Huffman coding and postbuffer control. This coder concept is the result of a comparison of several coding methods including uniform/nonuniform quantization, constant/variable word length coding and prebuffer/post-buffer control schemes. An important feature of the presented coder is the selection of transform coefficients within each block which are grouped in subareas for adaptive entropy coding. The components of the coder are described and its excellent performance is demonstrated by means of SNR-measurements and a typical videoconference sequence.

Coding scheme and hardware structure of a high-rate digital HDTV codec with partly error-free encoding

Abstract For contributing HDTV signals inside a studio and between different studios the and cability for multiple encoding and decoding of signals without visible errors is demanded. Ideally there would be no errors at all. For that reason a rather high bit-rate of 560 Mbit/s for transmission is under discussion. To start out from an active range of 1152 lines, 1920 pels/line and a field frequency of 50 Hz, a data rate of 885 Mbit/s and a compression factor of about 1.6 is needed. The paper describes an intraframe HDTV coder which meets this condition with a simple one-dimensional DPCM and a subsequent entropy coder. Only in the cases where pure entropy coding is not sufficient, a small quantization with 6 bits for the luminance component signal and 5 or 4 bits for the chrominance component is performed. The resulting quality after coding and reconstruction is excellent for different test sequences with a signal-to-noise ratio of about 60 dB. A discussion of the hardware implementation of the proposed codec structure shows that using gate arrays makes it possible to realize nearly all of the codec components in fast CMOS technology. The critical part of the scheme, the VLC decoder, will be discussed in detail. The system frequency for coding and decoding was limited to 72 MHz.

A versatile DCT coding system for HDTV with interlaced and progressive scanning

Abstract HDTV implies various new technologies with strong implications for information and imaging technology, and telecommunications as well. Various standards, image formats and scanning modes are under discussion for various applications. Until now of HDTV signals via satellite and B-ISDN a video data rate of 125 Mbit/s was considered to be necessary for distribution. But the fast development of efficient coding methods indicates that also for HDTV the trend is moving toward lower bit-rates. That could open new areas of application in the range of 50 Mbit/s. We present a DCT coding concept that is versatile with respect to scanning mode - interlaced and progressive - and data rate. It shows very good image quality also at the low data rate of 50 Mbit/s. It contains efficient mechanisms for local adaptivity which are applied for switching between different coding modes and for the control of quality parameters. For this purpose local image analysis is largely performed. Criteria and activity measures are computed in the spatial domain in a regular ways as sums of absolute spatial differences. To some extent analysis is also performed in the DCT domain for coefficient thresholding and an adaptive assignment of Huffman coding classes. An advanced computationally efficient motion estimation method is described. It improves the coding performance and open the possibility for coding with much lower bit-rates. We have performed a comparison experiment at 125 Mbit/s between interlaced and progressive scanned scenes. The most noticeable gain with the progressive scene is the absence of aliasing defects. This positive effect influences the quality more than a slightly increased coding noise and advocates the usage of the progressive scan format for high quality studio application. In a second experiment an interlaced scene is coded at 50 Mbit/s with the usage of the advanced motion estimated prediction scheme. The result is very promising with respect to a possible application for distribution.

Hybrid DCT-codec with local adaptivity for the interlaced HDTV transmission with 140 Mbit/s

With a HDTV coding method for the interlaced format, that is based on the DCT, mechanisms for local adaptivity are described. There are two alternative adaption methods regarding the special situation with the interlaced format. Further mechanisms concern quality parameters as a spatial frequency cut-off and the quantizer step size which adapt to the local image content. Results of a study which compares the different adaptive approaches show the influence on the image quality. So already a high degree of quality can be obtained. The application of a new motion compensation method between the fields adds essential further quality reserves.

Bewegungsvektorschätzung aus Bildfolgen mittels Oktavbandzerlegung

Ein Verfahren zur Ermittlung von Bewegungsvektorfeldern aus Bildsequenzen wird vorgestellt, bei dem die einzelnen Bilder durch Anwendung einer zweidimensionalen Filterbank in oktav gestufte Ortsfrequenzbereiche, sog. Teilbander, aufgespalten werden. Die Filterbank wird mittels Quadratur-Spiegel-Filtern realisiert, wodurch eine fehlerfreie Rekonstruktion der Bilder aus den Teilsignalen bei grostmoglicher Unterabtastung der Teilsignale moglich ist. Zur Bewegungsvektorschatzung werden die Teilsignale in Blocken zu 8x8 oder 16x16 Pixeln zusammengefast. Fur jeden dieser Blocke wird ein Bewegungsvektor durch Korrelation mit dem vorhergehenden Bild bestimmt. Dabei sind fur die grosen Verschiebungen die niederfrequenten Teilsignale und fur die kleinen Verschiebungen bzw. fur die Feinjustierung der Vektoren die hoherfrequenten Teilsignale masgebend. Das Verfahren ist fur die Bewegungskompensation bei der Bewegtbildcodierung mit niedrigen Ubertragungsraten vorgesehen.