Walsh-Hadamard Transform/Differential Pulse Code Modulation (WHT/DPCM) Processing Of Intraframe Color Video

AbstractHybrid processing of NTSC color images for achieving bandwidth compression is simulated. The processing involves combination of transform and predictive coding of intraframe video. Walsh-Hadamard transform (WHT) along each row followed by differential pulse code modulation (DPCM) along each column is implemented in (16 x 16) and (32 x 32) blocks. Also 2d-WHT of (4 x 4) blocks together with prediction of adjacent blocks is investigated. Based on the histogram of the difference signal, quantizers are optimized for minimum mean square error between the original and processed images. Variable bit allocation reflecting the variance of the error signal is adopted for maintaining a specified bit rate. The processing schemes are evaluated in terms of both subjective and objective criteria.IntroductionResearch in transmission of video over digital communication channels has attracted the attention of a number of researchers 1-35 in view of the rapid growth in digital electronics (memory, switching, logic etc.). Because of the large bandwidth coupled with increased costs required for digital data transmission, data compression or bandwidth reduction has been the major focus of this research. One constraint in this field is the image fidelity which is to be maintained at acceptable levels. For broadcast television (TV) this con­ straint is highly rigorous2 /19 / 29 / 31 whereas for other applications13"15/ 25,28,34 such as industrial TV, teleconference, picture phone, reconnaisance, target detection and related fields, there can be a higher trade off between image quality and bit reduction. Most network TV is in color and satellite communications have accelerated the worldwide transmission network for instant relay of significant events. Network color TV digital transmission at broadcast quality -has been a major thrust in redundancy reduction research. This was initiated through spatial and temporal decorrelation (predictive methods such as delta modulation - DM and differential pulse code modulation - DPCM) followed subsequently by transform techniques. More recently hybrid processes8 ' 9' 14' 26 involving combination of prediction and transform have also been explored. Digital^hardware for implementing these processes have also been designed and built.15' 23' 25'^8 '31-35 Some industrial companies are also marketing the codecs 31-35 for network broadcasting and conference TV.Obj ectiveThe object of this paper is to investigate hybrid (WHT-DPCM) coding of color images with a view to transmit them at reduced bit rates. Processing is carried out on intrafield basis in composite form with quantizers designed on the basis of the error statistics. The bit distribution reflects the error variance and the image quality versus bit rate is studied. Sensitivity of the coder to channel noise has not been attempted.Video dataNTSC color intraframe images in composite form are sampled at fs =10.7 MHz corresponding to three times the color subcarrier frequency (fgc =3.56 MHz) and quantized uniformly to 8 bits (256 levels). As the TV bandwidth is 4.2 MHz, this sampling rate is higher than the Nyquist frequency. Although the data is available both in aligned and unaligned form, only the latter has been used for simulation. Each frame consists of two fields and each field has 256 lines. There are 512 samples per line which represent the visible portion of the image. Present processing is limited to intrafield data. Four images have been utilized for processing (see below). As the WHT is the simplest of all discrete transforms,36 it is chosen for both horizontal decorrelation (Id-WHT) and intrablock processing (2d-WHT). This is followed by linear prediction in the vertical direction for the former case and inter­ block decorrelation for the later. Before proceeding with the simulation, correlation of the picture elements (pels or pixels) along both vertical and horizontal directions is de­ termined. Horizontally every third pel has the highest correlation as these pels have the"^This paper is based on the research by Mr. E. S. Kim for the Master's thesis as a partial requirement for the M.S. degree from the University of Texas at Arlington, Arlington, Texas.