1、CCIR RECMN*4b4-2 92 m 4855212 05L7b3 924 m Rec. 464-2 141 RECOMMENDATION 464-2 PREEMPHASIS CHARACTERISTICS FOR FREQUENCY-MODULATION SYSTEMS FOR FREQUENCY-DIVISION MULTIPLEX TELEPHONY IN THE FIXED-SATELLITE SERVICE (Question 27/4) (1970-1982-1992) The CCIR, considering a) division multiplex telephony
2、 signal is the same with and without pre-emphasis; that the pre-emphasis characteristic should preferably be such that the r.m.s. deviation due to the frequency- b) threshold the thermal noise is highest in the top channel and decreases with decreasing baseband frequency; that in a frequency-modulat
3、ion system for frequency-division multiplex telephony operating well above c) decade, operating well above threshold the thermal noise is constant over the whole baseband; that in a phase-modulation system, or in a frequency-modulation system with pre-emphasis at 20 dB per d that the thermal noise i
4、n the highest channel of a phase-modulation system is approxhately 4.8 dB lower than the corresponding channel of a frequency-modulation system, assuming that both systems operate well above threshold and are adjusted to have the same multi-channel r.m.s. frequency deviation; e that the reduction in
5、 frequency deviation with decreasing baseband frequency in a phase-modulation system makes such a system more sensitive to low frequency noise, especially that arising in threshold extension demodulators operating near to threshold; 0 that for earth stations normally operated above threshold the eff
6、iciency of use of satellite transmitter power is practically unaffected by changes in the range of pre-emphasis characteristic from below 6 dB to about 8 dB, but the efficiency of use of radio-frequency bandwidth increases slightly with an increase in pre-emphasis range; s the material presented in
7、Annex 1, recommends 1. that in systems in the fixed-satellite service employing frequency modulation, radio-frequency carriers with capacities of 12 or more telephone channels should be used with pre-emphasis and should use the same normalized pre-emphasis characteristic; 2. that the preferred pre-e
8、mphasis characteristic is given by the expression: i Relative frequency deviation of the test tone = 5 - 10 log at the frequency dB 142 where: CCIR RECMN*Y64-2 92 W Y855212 0518764 860 W Rec. 464-2 f,. = 1.25 f, : resonant frequency of the network ftn, : highest telephone channel baseband frequency
9、of the system f: baseband frequeucy. The variation of deviation with frequency is shown in Fig. 1; FIGURE 1 he-emphasis characteristic for telephony 6 5 a 3 - m2 0 -1 C O - - al 3 wo al -1 cr: .- u J - -2 -3 -4 -5 O. Normalized frequency v/, 1.0 3. that the tolerance on the frequency response of the
10、 pre-emphasis characteristic, and also on the de-emphasis characteristic should be such that, within the nominai upper and lower limits of the baseband, the departure of the characteristic of a practical network from the theoretical characteristic should be confined within a variation of IcO.1 + 0.0
11、5ft,) dB, f being the baseband frequency and f, the nominal maximum frequency of baseband. This corresponds to cotnponent tolerances of about 3I 1 % for resistors and about 310.5% for capacitors and inductors. Further, the maguitude of the departure should exhibit no rapid variations within this fre
12、quency range. Note 1 - It is recognized that it may be desirable to achieve the pre-emphasis characteristic by inserting a network at different places in various types of equipment. An example of pre-emphasis and de-emphasis network, to work between a constant-voltage source and M open-circuit load,
13、 is shown in Fig. 2.3) and 2b), respectively. An example to work between matched resistive input and output impedances is shown in Fig. 3a) and 3b), respectively. CCIR RECNN*464-2 42 W 4855232 0538765 7T7 W Rec. 464-2 143 Note 2 - In the expression for the relative deviation as indicated in 0 2, it
14、should be noted that the frequency at which the deviation with pre-emphasis corresponds to that without pre-emphasis is 0.61320f,. It may be convenient to adopt this frequency for testing the loss between baseband terminai points of systems when these are not in service. Note 3 - It is recognized th
15、at it may sometimes be desirable to use a different pre-emphasis characteristic by agreement between the administrations concerned. FIGURE 2 Pre-emphasis and de-emphasis networks to work between a constant-voltage source and an open-circuited load A R = 1.81 Ro Where f, is the hlghest baseband frequ
16、ency a) Re-emphasis network R I R = 1.81 Rn b) De-emphasis network 144 CCIR RECMN*464-2 92 4855232 O538766 633 Rec. 464-2 FIGURE 3 Pre-emphasis and de-emphasis networks to work between matched resistive input and output impedances b Ro - kl _c n I n u) he-emphasis network Ro- l: fr = 1.25 fmat = - -
17、 2x 1/ L1C1 I- 2x L2C2 E = 0.79 Ro O. 79 Ri = 1.81 Ro Rz 0.01 Ro R3 = R4 = Ro RO Rs = - 1.81 1 = 2x L4C4 b) De-emphasis network 1. ntr CCIR RECMN*464-2 92 m 4855232 0538767 57T m Rec. 464-2 145 ANNEX 1 Use of pre-emphasis in frequency-modulation systems for frequency division multiplex telephony and
18、 television in the fixed-satellite service duction The use of pre-emphasis in systems in the futed-satellite service for frequency-division multiplex telephony using frequency modulation results in a useful improvement in the signal-to-noise ratio in the higher frequency channels of the system and t
19、hus enables the space-station transmitter power and bandwidth requirements to be reduced. The use of pre-emphasis for television modifies the energy distribution in the radio-frequency emission of systems in the fixed-satellite service, in such a way as to reduce, substantially in some circumstances
20、, the possibility of interference within and between systems in the fixed-satellite service and between systems in the fixed-satellite service and radio-relay systems using the same frequency bands. The use of pre-emphasis for television may also enable the effective frequency deviation of the syste
21、m in the fixed-satellite service to be increased, thereby improving the signal-to-noise ratio; however, too large an increase in deviation could offset the reduction of interference potential. The deviation and pre-emphasis used to obtain the best possible transmission of some television signal stan
22、dards may differ appreciably from those recommended for telephony. The use by different administrations of the facilities offered by active systems in the fixed-satellite service, including the shared use of space-station repeaters, would be facilitated by the use of agreed pre-emphasis characterist
23、ics for such systems employing frequency modulation. 2. Telephony The effect of pre-emphasis will be to improve the signal-to-noise ratio in the high frequency channels and to reduce it in the low frequency channels. This may in turn affect the carrier-to-noise ratio at which the noise in the worst
24、channel reaches 50 O00 pWp at a point of zero relative level. Each of these effects will have repercussions on the satellite power and bandwidth required to meet the noise objectives of Recommendation 353. Furthermore, the optimum characteristic for a system operated at or below threshold for a cons
25、iderable proportion of the time may not be the same as that for systems which normally operate above threshold. The threshold margin of present satellite systems is generally sufficient to prevent them from operating below threshold for all but very small proportions of the time and the same is expe
26、cted to be true for future systems. For general use, therefore, a pre-emphasis characteristic with a relatively wide range of attenuation will be optimum. Measurements of signal-to-noise ratios in an operational system have confirmed that the network described in this Recommendation which has an 8 d
27、B range of attenuation, gives satisfactory results in practice. However, for systems operating nearer to threshold, a narrower range of attenuation may be found to be optimum. 3. Television Information on the application of pre-emphasis to frequency-modulation systems transmitting television cill be found in Recommendation 405 and CCIR Volume XII.
