1、SECTIOP I CCIR VOLUMEMIII 90 4855212 0509517 T = Rec. 246-3 3C: DIGITAL TRANSMISSION 3C a: Radiotelegraph circuits RECOMMENDATION 246-3 49 FREQUENCY-SHIFT KEYING * (Question 8/3) (1951-1953-1956-1959-1966-1970-1974) The CCIR, CONSIDERING (a) that frequency-shift keying is employed in radiotelegraphy
2、 in the fixed service; (6) that it is desirable to adapt the frequency-shift used to the modulation rate; (c) that it is desirable to standardize the main operating characteristics of systems employing frequency-shift keying; (d) that various technical factors influence the choice of operating chara
3、cteristics in such systems, in particular: - - - economy of bandwidth and the consequent need to control the shape of the transmitted signals, signal distortion due to propagation conditions, instability of the characteristics of certain transmitter and receiver elements (such as oscillators, filter
4、s or discriminators), this instability being one of the reasons for the. relatively large shift still employed in some existing types of equipment; (e) that for synchronous transmissions .using limiter-discriminator detection, a modulation index of about 0.8 is desirable for obtaining low bit error
5、rates (see Recommendation 436 and Report 198, Geneva, 1963) and that for asynchronous (start-stop) transmissions, a modulation index between 1 and 2 is more appropriate; (f) that for synchronous transmissions using filter-assessor detection, a sufficiently high value of frequency- shift is desirable
6、 to take advantage of frequency diversity effects; (g) that difficulties can arise from the use of terms “mark” and “space” on teletype.circuits and also that the CCIT, at its VIIth Plenary Assembly (1953), issued Recommendation 1.4 introducing new terms; these terms have been published by the ITU i
7、n the “List of Definitions of Essential Telecommunication Terms”, Part I, General Terms, Telephony, Telegraphy, June 1957, UNANIMOUSLY RECOMMENDS 1. that for freque-ncy-shift systems working on two conditions only (Le. single channel or time division multiplex systems) and operating between about 3
8、MHz and 30 MHz, the value of the frequency-shift employed should be the lowest compatible with the maximum modulation rate regularly used, the propagation conditions and the equipment stability; * For the use of frequency-shift keying in the maritime mobile service, see Appendix 38, 0 (c) of the Rad
9、io Regulations. 7 - - COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services- _CCIR VOLUME*III 90 4855212 0509518 1 - 50 Rec. 246-3 2. stability * and selectivity, the following values of frequency-shift in Table I are preferred for new syst
10、ems: that for services where the transmitting equipment and the receiving equipment are of sufficient high TABLE I Frequency-shift Maximum modulation rate (baud) Synchronous Asynchronous (Hz) - 50 70 100 50 and 15 85 200 100 170 - 200 340 3. that for systems using filter-assessor detection (see Repo
11、rt 345) or where the achievement of the necessary stability or receiver selectivity is impractical, the preferred values of frequency-shift are 200 Hz, 340 Hz, 400 Hz* and, for modulation rates above 250 baud, 500 Hz. The values of 140 Hz, 280 Hz and 560 Hz may be used provisionally, but 560 Hz shou
12、ld not be adopted for new systems. The value of the frequency-shift should, if possible, be maintained within f 3% of its nominal value and, in any case, within f 10%; 4. * that the following equivalence among the various terms indicating circuit condition be adopted: (Table II is in accordance with
13、 CCIT Recommendations. U.l, see Fascicle VIL1 and V.l, see Fascicle VIII.l.) TABLE II Circuits using teleprinter or punched tape equipment Frequency Circuits Emitted using signal (2) 7-unit Data Telex Morse code of International Telegraph em iss i on Alphabet No. 2 Mark Free line condition Idle circ
14、uit condition B O Higher No Start perforation A(i) Mark Stop Perforation Z(1) Y 1 Space frequency Lower frequency Space (1) on a wire circuit. (2) on a radio channel. * In the absence of a Recommendation on the stability required for narrow-band frequency-shift keying a provisional value of 12 Hz ma
15、y be used for the maximum permissible overall frequency error, including modulator, demodulator and translating stages at both ends of the circuit. The value 170 Hz is used in the maritime mobile service (see Appendix 38, 4 (c) of the Radio Regulations). When modification Df equipinent is necessary, it is recognized that it may take some time before the recommendations of these paragraphs can be implemented on circuits between different Administrations. I * * COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services
