1、STD-ITU-R RECMN BT.20LB-ENGL 3998 4855232 053b488 573 8 Rep. ITU-R BT.2018 REPORT ITU-R BT.2018 STUDY OF THE SYSTEM C GHOST CANCELLING REFERENCE SIGNAL FOR THE EVALUATION AND CORRECTION OF LINEAR DISTORTION IN THE TELEVISION CHAIN (Question ITU-R 55/11) (i 998) Many countries are interested in impro
2、ving the operational quality of existing television broadcasting networks. The automatic correction in receivers of distortions that have accumulated in the TV chain is one of the most effective means of increasing the effective quality of the chain. For this purpose, Recommendation IT-R BT. 1 124 d
3、efines ghost cancelling reference (GCR) signais for correction of linear distortions in receivers, which can also be used for the correction of distortions in individual sections of complex TV chains, and which can also serve for the evaluation of distortions. At the present time, various enhancemen
4、t modules are being implemented in existing TV services. The correction of linear distortion is considered to be one of the most important enhancement modules. Some countries are currently using analogue systems with 6 MHz video bandwidth and intend in the future to use NICAM digital sound. So two b
5、andwidths for the luminance signal are expected to be in use: 6 MHz (without digital sound) and 5 MHz (when digital sound is used). So the question of 5 MHz and 6 MHz GCR signals is of interest. In 1996 (Doc. 1 lA142) was published presenting some results of studies of the GCR system C signal. This
6、report brings together the results of further studies (Doc. 11N80) on this subject. 1 Automatic correction of linear distortions as a part of the concept of enhanced analogue television The concept of enhanced analogue TV assumes the use of the wide-screen picture aspect ratio of 16:9, digital sound
7、, and improved image quality in comparison with conventional TV. The realization of this concept is based on the use of digital signal processing. Principles and some details of the construction of enhanced TV systems are described in ITU-R texts (Recommen- dation ITU-R BT. 1 11 8 - Enhanced compati
8、ble wide-screen television based on conventional television systems; Recommendation ITU-R BT. 1 197 - Enhanced wide-screen PAL TV transmission system (the PALplus system); Recommendation ITU-R BT. 1298 - Enhanced wide-screen NTSC TV transmission system, and Doc. 1 1 A/8). One of the most significant
9、 characteristics of enhanced TV is the improvement of image quality. This is achieved by means of: - - use of high quality sources of signal (component digital studios); use of intra-frame signal pre-processing and post-processing, allowing more effective separation of luminance and chrominance sign
10、als in the decoding process; correction in the receiver of linear distortions accumulated in the TV path (referred to in the ITU-R texts as “ghost cancellation”). - The automatic correction of linear distortions has common importance both for conventional and enhanced TV. By including a device for g
11、host cancellation in the receiver, linear distortions that have accumulated in multi-chain TV paths are removed rapidly, with consequent improvements in displayed luminance and chrominance resolution and in the decoding of teletext. 2 Recommendation ITU-R BT. 1 124 (Reference signals for ghost cance
12、lling in analogue television systems) defines three GCR signal systems - A, B and C. The question of GCR test signal standardization Rep. ITU-R BT.2018 9 For Europe, and for many other countries, signal C is of interest. The direct purpose of this signal is concerned with the reduction of echo signa
13、ls accumulated in the TV reception path. The GCR signal has wider application for the rapid estimation and correction of common linear distortions Gofaizen, 1995a and b and (Doc. 11M42). Correction of distortions is possible both in the television receiver, and at the input to each link in the distr
14、ibution and transmission chain. Taking into account that linear distortions can result in nonlinear effects and in worsening noise characteristics of the image, the use of automatic correction of nonlinear distortions does not exclude the necessity of controlling these distortions in each link. Thus
15、 the estimation of linear distortions is possible on various criteria. Use of computer technologies allows these to be achieved by computing methods. The studies in the Ukraine (Doc. 11A/80) took into account the following: - The possible optimization of the system C GCR signal by the use of alterna
16、tive window functions, suggested in work O.V. Gofaizen, 1995a and b and (Doc. llN42) with the objective of achieving an improvement in the accuracy of the estimation of distortions. Recognizing that the GCR signal is already standardized, any changes would need to be compatible with existing use of
17、the signal in a number of countries: aspects of the compatibility of any modifications would need to be investigated. An optimized GCR signal should have higher noise immunity relating to interference from the adjacent channels than the standard GCR signal (noise immunity gain should be appreciated
18、as the result of optimization). The introduction of an optimized GCR signal must not result in an increase in the cost of equipment. - - - Thus, during the course of choosing a GCR signal for some countries, various studies were felt to be appropriate in order to seek to provide the most effective s
19、olution bearing in mind these issues. 3 In Koo 19951 has described the properties of what has become known in the ITU-R as the system C GCR signal. This analysis was continued and is complemented in works Gofaizen, 1995a and b and (Doc. 11N42). The following is a recent alternative description and a
20、nalysis of the same system C signal, as derived during the course of latest studies: In work (Doc. 1 IN42) it is shown that the GCR signal may be represented by the formula: The mathematical description of the GCR signal and analysis of its basic properties I where: mT W(W) = w(t) e-Jodt -m T 2 q(t)
21、 = cosz 5 - 2 mT m-2 m-2 t s(t) = - sinc 7c - - mT mT -1 for o O sin x sincx = - X STD-ITU-R RECMN BT=2038-ENGL 3948 4855232 053b490 321 10 Rep. ITU-R BT.2018 The parameter values of this signal given in Recommendation ITU-R BT. 1 124 are: A = 0.30358 X V b = 0.2829 x 10-l2 s2/rad Q = 2.n x 5,5 x 10
22、6 rads Q= 2.n x 5 x 106 rads c = 0.9121 x lo6 rads. The parameters T and m in equation( 1) are: so that T = 94.5 ns and m = 36.4439. In this signal there are two window functions: - - W(o) in the frequency domain or its Fourier transform w(t), presented in the formulae, and P(w) in the frequency dom
23、ain or its Fourier transform p(t). This window function is not seen from the formulae. It is a rectangular function limiting the interval of product eJ W(o) when integrated by w. The structure of each of these window functions is shown in Figs. 1 and 2 respectively. The equation for the GCR signal i
24、n the time domain may be represented as Gofaizen, 1995a and b and (Doc. 11M42): g(t = f(t w(t P(t (2) where: 7c p(t) = - R sim S2t t w(t) = - 4b 63 : convolution sign. Here, function o(r) demonstrates the linear relationship between frequency change and time. The system C signal has the following in
25、herent characteristics Koo, 19951: - high energy, - - - - - - flat amplitude-frequency characteristic within the bandwidth of interest, smooth phase characteristic in the bandwidth of interest, GCR signal auto correlation characteristic is limited at the expense of convolution by function sinc Ch, a
26、t a given energy level the GCR signal duration is minimized, GCR signal spectral characteristic is not practically sensitive to change of sampling frequency and word length, GCR signai is real-valued, thereby simplifying the equipment needed for its use. STD-ITU-R RECMN BT*2038-ENGL 3778 4855232 053
27、6473 Ob8 11 Rep. IT-R BT.2018 FIGURE 1 The structure of window function W(w) in frequency domain and time domain n t f L - mT mT -x o x mT mT w c - 2x - 2R O T T - I I 0 mT * - (m - 4)2x O - 42n (m- .,_ mT Rap 2018-01 12 STDmITU-R RECMN ET-2018-ENGL 1998 m 4855212 053b492 TT4 Rep. IT-R BT.2018 FIGUR
28、E 2 The structure of window function P(o) in frequency domain and time domain -R R W i(): Fourier transform ofp(t) Rap 2018-02 4 Analysis of GCR signal parameters in the frequency and time domains GCR signal characteristics, evaluated using such model, are presented below. Figure 3 shows Fourier-pro
29、totype of window function w(t). Figures 4 and 5 describe window function W(o) in linear and logarithmic scales. W(w) decays to the following relative levels at the following frequencies: -6 dB at 5 MHz 35.4 ps) the residual level is less than -40 dB. Over a slightly wider interval (at r 36 ps) the r
30、esidual level is less than -60 dB. In the frequency domain, at f 5 MHz spectral density rapidly becomes insignificant. That is why in practice this signal is tolerant to changes of the frequency characteristic outside the nominal 5 MHz bandwidth. Besides, for these reasons, and also in connection wi
31、th use of a GCR signal with alternating polarity, the adjacent parts of the TV signal do not influence the measurement of distortions using the GCR signal. Other evaluations were made to assess the influence of certain factors: - Influence of signal restriction in the time domain According to the Re
32、commendation ITU-R BT.1124, the signal boundaries shall correspond to temporal samples i = 12.2 ps and i = 35.4 ps. Estimations show, that the difference of Fourier-transforms of the signal not limited in time, and limited to the specified limits, normalized to F(0) does not exceed 0.005 dB. A simil
33、ar result is obtained for a wider window area, limited to the temporal samples, between r 36 ps. Influence of bandwidth restriction of the Fourier-transform of the signal The inherent method of derivation of the GCR signal imposes two measures for restricting its bandwidth: use of window function W(
34、o), restricting the signal to 5 MHz at -6 dB, with rapidly increasing attenuation such that at 5.5 MHz it is less than -60 dB; use of integration boundaries (42, Q) for calculating the inverse Fourier-transform, appropriate to frequency range +5 MHz. This corresponds to multiplication by the appropr
35、iate rectangular window, accepting the peak level between these frequencies, and zero level outside their limits. - - - STD-ITU-R RECMN T.ZOL5-ENGL L998 m 4855212 O536497 586 m 17 Rep. ITU-R BT.2018 Use of these two windows results in the following. Spectrum restriction caused by the second window o
36、ccurs at a very small spectral density level of the signal, owing to its attenuation by the first window. Therefore, the influence of first window function on the GCR signal is insignificant. Use of the first window with a very narrow transition area leads to the appearance of oscillations with the
37、same amplitude as that of the final part of the signal. This is a shortcoming of the GCR signal, but the compromise accepted ensures the complete control of distortions in the frequency range 0-5 MHz, having sufficient energy reduction above 5.1 MHz. This is very important for an enhanced TV system,
38、 since the frequency above 5.1 MHz is available for the transmission of a digital sound signal. - On choice of window function “(O) It is clear, considering the aforesaid, that the form of the window function does not have a major effect on the GCR signai properties. Nevertheless, it is desirable to
39、 estimate the value of such an effect, taking into account that a number of studies in the world were devoted to a choice of window functions in connection with synthesizing finite impulse response (FR) filters. For example, it is generally considered that a Hamming window function is better than a
40、cos2 window. A wider class of windows - the Kaizer window, makes it possible to simulate an almost exact approximation of windows such as Hamming, Blackman, and others, by varying its parameters. Figure i0 represents two realizations of window W(w) shown on a logarithmic scale. The first was obtaine
41、d on the basis of a cos2 window, and the second on the basis of a Hamming window. Figure 11 shows the differences in amplitude- frequency response achieved between these realizations, normalized to zero frequency amplitude. It is clear that, in the range 5-6 MHz, the second realization achieves some
42、 10 dB greater attenuation than the first. At higher frequencies, the second realization is inferior. However, this is not significant, as the relative levei of Fourier-transforms appear to be I below -60 dB. These estimations may be helpful for the construction of GCR signals for new applications.
43、50 O FIGURE 10 Hamming and cos2 windows on logarithmic scale , Hammine 1 - 200 I l I l I , I -12 -10 -8 -6 -4 -2 O 2 4 6 8 10 12 0/2x (MHz) Rap 2018-10 STD*ITU-R RECMN BT-2018-ENGL 18 20 O - -20 u - -40 - 60 - 80 1998 W 4855212 053b498 412 Rep. ITU-R BT.2018 FIGURE 1 i Difference of Hamming and cos2
44、 windows in logarithmic scale I I I l l I I I l I I I I l , I -12 -10 -8 -6 -4 -2 O 2 4 6 8 10 12 d2x (MHz) Rap 201 8- 11 It seems from this analysis that the window function used by the System C GCR signal as described in Recommen- dation ITU-R BT.1124 is optimal. 6 Parameters of GCR signal for the
45、 6 MHz bandwidth channel According to the draft report on the enhanced SECAM TV system (Doc. 11N8) two modes of operation of the enhanced SECAM system are envisaged: transmission of only analogue sound, and transmission of both analogue and digital sound. In the case of the use of both analogue and
46、digital sound, the video signal bandwidth is nominally 5.1 MHz, and for this purpose the GCR signal as described in Recommendation ITU-R BT. 1124 ideally fits. In the case of the conventional SECAM system or the enhanced SECAM system with only analogue sound, the control and correction of linear (in
47、cluding ghost) distortions should ideally be made in the frequency range up to 6 MHz. In this connection, it is proposed to define the GCR signal for this bandwidth. The following parameter values are suggested for a system C GCR where it is required to have a bandwidth of 6 MHz: A = 2.7 x 10-7 v b
48、= 0.23 x s2/rad c = 0.9121 x lo6 rads 0 = 2n x 6,25 x lo6 rads RI= 2.n X 6 x lo6 rads. Using the above values, m and Tare derived to be: m = 43.332 T = 79.487 ns. STD-ITU-R RECMN BT.20LB-ENGL 1998 D 4855232 053b471 359 Rep. ITU-R BT.2018 19 Figures 12, 13 and 14 represent the amplitude-frequency cha
49、racteristic of window function W(w) for 6 MHz bandwidth on linear and logarithmic scales. FIGURE 12 Amplitude-frequency characteristics of window function W (O) for 6 MHz bandwidth shown on a linear scale - 0.2 I I I I l l l l I -10 -8 -6 -4 -2 O 2 4 6 8 10 WX. (MHz) Rap 3318-12 FIGURE 13 GCR signal spectrum for 6 MHz channel bandwidth at QI = 2x x 6 x 106 radis, R = 2x x 6.25 x 106 rads I 46 8 10 -10 -8 -6 -4 -2 O 2 d2x (MHz) Rap 201 84 3 STD-ITU-R RECMN BT-2038-ENGL 3998 I 4855232 0536500 9TO I 20 50 O - 100 - 150 Rep. ITU-R BT.2018 FIGU
