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A Subjective Evaluation of Noise-Shaping Quantization for Adaptive 332 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 36, NO. 3, MARCH 1988 A Subjective Evaluation of Noise-S haping Quantization for Adaptive Intra-/Interframe DPCM Coding of Color Television Signals BERND GIROD, H AKAN ALMER, LEIF BENGTSSON, BJORN CHRISTENSSON, AND PETER WEISS Abstract-Nonuniform quantizers for just not visible reconstruction quantizers for just not visible eri-ors have been presented in errors in an adaptive intra-/interframe DPCM scheme for component- [12]-[I41 for the luminance and in [IS], [I61 for the color coded color television signals are presented, both for conventional DPCM difference signals. An investigation concerning the visibility and for noise-shaping DPCM. Noise feedback filters that minimize the of quantization errors for an adaiptive intra-/interframe lumi- visibility of reconstruction errors by spectral shaping are designed for Y, nance DPCM coder has been reported by Westerkamp [17]. R-Y, and B-Y. A closed-form description of the “masking function” is In this paper, we present quantization characteristics for the derived which leads to the one-parameter “6 quantizer” characteristic. luminance and the color dilference signals that lead to just not Subjective tests that were carried out to determine visibility thresholds for visible reconstruction errors in sin adaptive intra-/interframe reconstruction errors for conventional DPCM and for noise shaping DPCM scheme. These quantizers have been determined by DPCM show significant gains by noise shaping. For a transmission rate means of subjective tests. In order to evaluate the improve- of around 30 Mbitsls, reconstruction errors are almost always below the ments that can be achieved by reconstruction noise shaping visibility threshold if variable length encoding of the prediction error is [l 11, we compare the quantization characteristics for just not combined with noise shaping within a 3:l:l system. visible errors for systems with and without noise shaping. Section I1 very briefly reviews adaptive intra-/interframe I. INTRODUCTION DPCM and describes thc details of the specific coding algorithm used in the subjective tests. In Section 111, the OR a transmission of color television signals at broadcast fundamentals of noise shaping are summarized and noise Fquality, a digital broad-band channel with a rate of around feedback filters are designed for Y, R- Y, and B- Y based on 30 Mbits/s is likely to be internationally standardized soon. In results from psychophysics literature. Section IV describes the order to reduce the rate of 216 Mbits/s of the digital studio subjective testing methodology that we used to determine 4:2:2 TV signal 111 to the desired transmission rate, data quantizers for just not visible distortions. Sections V and VI compression is required which, however, should result in a state the results of our subjective tests, and give quantizers for picture quality that is superior to the quality provided by just not visible quantization errors for the luminance and the today’s PAL, SECAM, or NTSC systems. A first step of data color difference signals u.ith conventional DPCM and with compression can be a sampling rate conversion to 10.125 MHz noise shaping DPCM. Finally, S’cction VI1 relates the subjec- for the luminance signal Y and to 3.375 MHz for the color tive test results to bit rate for both fixed and variable difference signals R- Y and B- Y, resulting in a 3: 1 : 1 system, wordlength encoding of the prediction error. as it has similarly been proposed, e.&., in [2]-[5] and investigated in [6], [7]. Furthermore, the horizontal and 11. THEADAPTIVE INTRA-/~NTEF.FRAME DPCM ALGORITHM vertical blanking intervals need not be transmitted, which In the following section, we will very briefly review leaves approximately 2 bits for each sample in the average. adaptive intra-/interframe DPCM and describe the details of Adaptive intra-/interframe DPCM with variable-length cod- the coding scheme that has been used in our subjective tests. ing of the prediction error signal is a source coding scheme Fig. 1 shows an adaptive intra-/interframe DPCM system. It that has been favored for 30 Mbit/s transmission of broadcast is based on the idea that :i television signal can be predicted quality TV signals in several publications, e.g., [2]-[51, [81- efficiently if the predictor is switched between two modes. [lo]. Additionally, a combination with noise shaping has been Inferframe Prediction: In regions of the picture where suggested to be an efficient means of reducing the visibility of the signal contains only small changes from frame to frame, reconstruction noise [ 1 I]. What picture quality results from a the amplitude of the current sample can be predicted accu- combination of the proposed algorithms at the given transmis- rately from the corresponding sample in the previous frame sion rate, however, has not been investigated yet. In order to [Fig. 2(a)], according to answer this question, quantization characteristics that lead to just not visible reconstruction errors in conjunction with the coding scheme envisaged have to be determined in subjective tests. Intraframe Prediction: In regions of the picture where For intraframe DPCM systems without noise shaping, the signal contains rapid motion, the amplitude of the interframe prediction error is large, and a better prediction is Paper approved by the Editor for Image Processing of the IEEE obtained by a linear combinatiori of the amplitudes of adjacent Communications Society. Manuscript received December I, 1986. This paper signal samples in the same field [Fig. 2(b)], according to was presented in part at the International Picture Coding Symposium, Stockholm, Sweden, June 1987. s,,,,= a,s; t a2s; + a3s; + a4sq B. Gird is with the lnstitut fur Theoretische Nachrichtentechnik und Inforinationsverarbeitung, Universitat Hannover, Germany. Adaptive intra-/interframe prediction has been discussed by H. Alrner, L. Bengtsson, B. Christensson, and P. Weiss are with the many authors, e.&., [21, [41, PI, [91, [171, [IS], [191. Research Department, Swedish Telecommunications Administration, Stock- Switching between intraframe prediction and interframe pre- holm, Sweden. diction can be controlled in two different ways. IEEE Log Number 8718639. 1) With feedback adaptation, the predictor is switched 0090-6778/88/0300-0332$01.OO 0 1988 IEEE GIROD et ai.: NOISE SHAPING FOR CODING OF COLOR TV SIGNALS 333 scheme that signals the adaptation state on the basis of a fixed block structure [SI, [9], [19]. Each block consists of nine horizontally adjacent luminance samples within one scan line 1S and the corresponding three R-Y samples and three B-Y samples (Fig. 3). Switching is encompassed by a comparison of the accumulated absolute values of the intraframe and the interframe luminance prediction errors e [9], [ 191, i.e., else (3) The adaptation mode is determined from the luminance signal TrQns_m?tter Receiver only and then adopted for the color difference signals. Fig. 1. Block diagram of an adaptive intra-iinterframe DPCM system with The quantization characteristic for just not visible quantiza- feedforward adaptation. tion errors is quite dependent on the choice of prediction coefficients a,, a2,a3, and a4 (2) [12]. Isotropic intraframe prediction yields the coarsest quantizer for just not visible reconstruction errors [20], [2 1 1. In order to keep reconstruc- tion errors below the visibility threshold not in the average, but everywhere in the picture, the isotropic predictor mini- mizes the prediction error power at edges of most unfavorable orientation. In terms of prediction error entropy, the isotropic predictor performs reasonably close to the optimum for typical images, such that it is equally useful for fixed wordlength u encoding and for variable wordlength encoding of the predic- FRAME N tion error. Table I lists the coefficients of isotropic intraframe predic- tors that are used in our codec for the luminance and for the color difference signals. It also states the horizontal and vertical bandwidths of the signals that have been assumed in the optimization of the isotropic predictor. The vertical bandwidth is characterized by the equivalent horizontal '22 '23 '2L bandwidth in MHz for an interlaced 625 line/50 Hz system. --1 Prediction coefficients were restricted to be multiples of 1/4. For coefficients that are multiples of 118, only a small improvement can be observed. III. DESIGNOF A NOISE-SHAPINGDPCM SYSTEM A. Reconstruction Noise Shaping by Additional Quantization Error Feedback FRAME N A standard DPCM system with memoryless quantizer usually produces white reconstruction noise [ 1 11 = a,s; + a,sj+ a3si+aLsL' (b) n=S'-S. (4) Fig. 2. Samples used for interframe prediction (a) or for intraframe Exceptions from the flat power spectrum are limit cycles of prediction (b) of the current sample So. the DPCM coder that can occur for a constant input signal S or slope overload that can occur at large signal discontinuities. based on previously transmitted information only. As this The reconstruction noise is perceived by the human visual information is equally available at transmitter and receiver, no system with its specific transfer function that depends on both additional adaptation information need be transmitted. The spatial and temporal frequency. Noise shaping fits the recon- adaptation algorithm has to be implemented at both transmitter struction noise spectrum to the frequency characteristic of the and receiver. human visual system such that larger quantization errors are 2) With feedforward
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