1、Rep. ITU-R BT.2018 1 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) (1998) Many countries are interested in improving the operational quality of existing television bro
2、adcasting 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 ITU-R BT.1124 defines ghost cancelling reference (GCR) signals for corr
3、ection 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 enhancement modules are being implemented in existing TV services.
4、 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 bandwidths for the luminance signal are expected to be in
5、 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. llM42) was published presenting some results of studies of the GCR system C signal. This report brings together the results of further studies (Doc
6、. llM80) 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, and improved image quality in comparison with convention
7、al 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 1 18 - Enhanced compatible wide-screen television based on conventional televisio
8、n systems; Recommendation ITU-R BT. 1197 - Enhanced wide-screen PAL TV transmission system (the PALplus system); Recommendation ITU-R BT.1298 - Enhanced wide-screen NTSC TV transmission system, and Doc. 11M8). One of the most significant characteristics of enhanced TV is the improvement of image qua
9、lity. 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 signals in the decoding process; correction in the receiver of linea
10、r 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 ghost cancellation in the receiver, linear distortions that have
11、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 cancelling in analogue television systems) defines three GCR signal s
12、ystems - A, B and C. The question of GCR test signal standardization 2 Rep. ITU-R BT.2018 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 signals accumulated in the TV reception path. The GCR signal has wide
13、r application for the rapid estimation and correction of common linear distortions Gofaizen, 1995a and b and (Doc. llM42). Correction of distortions is possible both in the television receiver, and at the input to each link in the distribution and transmission chain. Taking into account that linear
14、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 the estimation of linear distortions is possible on various cri
15、teria. Use of computer technologies allows these to be achieved by computing methods. The studies in the Ukraine (Doc. llM80) took into account the following: The possible optimization of the system C GCR signal by the use of alternative window functions, suggested in work O.V. Gofaizen, 1995a and b
16、 and (Doc. llM42) 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 the signal in a number of countries; aspects of the compatibility o
17、f 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 as the result of optimization). The introduction of an optimized GC
18、R 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 solution bearing in mind these issues. 3 In Koo 1995 has described the pro
19、perties 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. llM42). The following is a recent alternative description and analysis of the same system C signal, as derived during the course of lates
20、t studies: The mathematical description of the GCR signal and analysis of its basic properties In work (Doc. llM42) it is shown that the GCR signal may be represented by the formula: where: mT W(ci) = w(t) e-Jutdt -mT 2.n t q(t)= cos - - 2 rnT rn-2 rn-2 s(t) = - sinc .n - rnT rn -1 for ci) t T O O O
21、 - sin x sincx = - X Rep. ITU-R BT.2018 The parameter values of this signal given in Recommendation ITU-R BT. 1124 are: A = 0.30358 x lop6 V b = 0.2829 x 10-l2 s2/rad Q = 2.n x 53 x lo6 rad/s QI= 2.n x 5 x 106 rad/s c = 0.9121 x lo6 rad/s. The parameters T and m in equation( 1) are: 3 Ql rn=-+2 C so
22、 that T = 94.5 ns and m = 36.4439. In this signal there are two window functions: - - W(U) in the frequency domain or its Fourier transform w(t), presented in the formulae, and P(u) in the frequency domain or its Fourier transform p(t). This window function is not seen from the formulae. It is a rec
23、tangular function limiting the interval of product ej sim (o)b O2 W(U) when integrated by (U. The structure of each of these window functions is shown in Figs. 1 and 2 respectively. The equation for the GCR signal in the time domain may be represented as Gofaizen, 1995a and b and (Doc. llM42): where
24、: .n p(t = - Q sinc at t O(t) = - 4b O : convolution sign. Here, function (t) demonstrates the linear relationship between frequency change and time. The system C signal has the following inherent characteristics Koo, 19951: - high energy, - - - - - - flat amplitude-frequency characteristic within t
25、he 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 Qt, at a given energy level the GCR signal duration is minimized, GCR signal spectral characteristic is not practic
26、ally sensitive to change of sampling frequency and word length, GCR signal is real-valued, thereby simplifying the equipment needed for its use. 4 Rep. ITU-R BT.2018 FIGURE 1 The structure of window function W(w) in frequency domain and time domain - mT O mT - mT T -n: o n: mT mT _ - 2n: O 2n: - (m
27、- 4)2n: O (m - 4)2n: mT mT Q(o), S(o), W(o): Fourier transform of q(t), s(t), w(t) Rap LO 18-01 Rep. ITU-R BT.2018 5 FIGURE 2 The structure of window function P(w) in frequency domain and time domain P(o): Fourier transform ofp(t) Rap 2018-02 4 Analysis of GCR signal parameters in the frequency and
28、time domains GCR signal characteristics, evaluated using such model, are presented below. Figure 3 shows Fourier-prototype of window function w(t). Figures 4 and 5 describe window function W(m) in linear and logarithmic scales. W(m) decays to the following relative levels at the following frequencie
29、s: -6 d at 5 MHz 35.4 ps) the residual level is less than 40 dB. Over a slightly wider interval (at t 36 ps) the residual 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 f
30、requency characteristic outside the nominal 5 MHz bandwidth. Besides, for these reasons, and also in connection with 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
31、 assess the influence of certain factors: - Influence of signal restriction in the time domain According to the Recommendation ITU-R BT. 1124, the signal boundaries shall correspond to temporal samples t = 12.2 ps and t = 35.4 ps. Estimations show, that the difference of Fourier-transforms of the si
32、gnal not limited in time, and limited to the specified limits, normalized to F(0) does not exceed 0.005 dB. A similar result is obtained for a wider window area, limited to the temporal samples, between t 36 ps. Influence of bandwidth restriction of the Fourier-transform of the signal The inherent m
33、ethod of derivation of the GCR signal imposes two measures for restricting its bandwidth: use of window function W(m), restricting the signal to 5 MHz at -6 dB, with rapidly increasing attenuation such that at 5.5 MHz it is less than 40 dB; use of integration boundaries (-Q, Q) for calculating the i
34、nverse Fourier-transform, appropriate to frequency range I5 MHz. This corresponds to multiplication by the appropriate rectangular window, accepting the peak level between these frequencies, and zero level outside their limits. - - - 10 Rep. ITU-R BT.2018 Use of these two windows results in the foll
35、owing. Spectrum restriction caused by the second window occurs 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 transiti
36、on area leads to the appearance of oscillations with the 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
37、.1 MHz. This is very important for an enhanced TV system, since the frequency above 5.1 MHz is available for the transmission of a digital sound signal. - On choice of window function W(m) It is clear, considering the aforesaid, that the form of the window function does not have a major effect on th
38、e GCR signal properties. Nevertheless, it is desirable to 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 (FIR) filters. For example, it is generally c
39、onsidered that a Hamming window function is better than a 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 10 represents two realizations of windo
40、w W(m) shown on a logarithmic scale. The first was obtained 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,
41、in the range 5-6 MHz, the second realization achieves some 10 dB greater attenuation than the first. At higher frequencies, the second realization is inferior. However, this is not significant, as the relative level of Fourier-transforms appear to be below 40 dB. These estimations may be helpful for
42、 the construction of GCR signals for new applications. FIGURE 10 Hamming and cos2 windows on logarithmic scale 50 O - 50 h F4 v - 100 - 150 - 200 I l l l l l l l l l l Hamming 1 -12 -10 -8 -6 -4 -2 O 24 6 8 10 12 w/2n (MHz) Rap 2018-10 Rep. ITU-R BT.2018 11 20 O h F4 -20 v - 40 - 60 - 80 FIGURE 11 D
43、ifference of Hamming and cos2 windows in logarithmic scale -12 -10 -8 -6 -4 -2 O 2 4 6 8 10 12 d2n (MHz) Rap2018-11 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 According to the draft report on the e
44、nhanced SECAM TV system (Doc. llA/8) two modes of operation of the enhanced SECAM system are envisaged: transmission of only analogue sound, and transmission of both analogue and digital sound. Parameters of GCR signal for the 6 MHz bandwidth channel In the case of the use of both analogue and digit
45、al 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 (includi
46、ng 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 = 0.2
47、3 x 10-l2 s2/rad c = 0.9121 x lo6 rad/s Q = 2.n x 6,25 x lo6 rad/s QI= 2.n x 6 x lo6 rad/s. Using the above values, m and Tare derived to be: m = 43.332 T = 79.48711s. 12 Rep. ITU-R BT.2018 Figures 12, 13 and 14 represent the amplitude-frequency characteristic of window function IV( for 6 MHz bandwi
48、dth on linear and logarithmic scales. FIGURE 12 Amplitude-frequency characteristics of window function W6 (a) for MHz bandwidth 1.2 1 o. 8 O. 6 0.2 O - 0.2 shown on a linear scale I 1 1 1 1 1 1 1 1 -10 -8 -6 -4 -2 O 2 46 8 10 a/2n (MHz) Rap 2018-12 FIGURE 13 GCR signal spectrum for 6 MHz channel ban
49、dwidth at fil = 2n x 6 x lo6 rad/s, fi = 2n x 6.25 x lo6 rad/s 50 O h F4 g -50 - 100 - 150 - 200 -10 -8 -6 -4 -2 O 2 46 8 10 d2n (MHz) Rap 2018-13 Rep. ITU-R BT.2018 600 400 E a 4 0- v 4 200 B .3 3 13 - - - h F4 v FIGURE 14 GCR signal spectrum for 6 MHz channel bandwidth at fil = 2n x 6 x lo6 rad/s and at unlimited integration O - 50 100 -150 I I l l l l l l l l -10 -8 -6 -4 -2 O 2 46 8 10 d2n (MHz) Rap 2018-14 Figures 15 and 16 illustrate 6 MHz GCR signal lines A and B for positive and negative polarity. FIGURE 15 GCR line A for 6 MHz bandwidth -200 ci -400 I I l l l
