220.127.116.11.1 Introduction. A modulation plan sets forth the development of a band of frequencies called the line frequency (i.e., ready for transmission on the line or transmission medium). The modulation plan usually is a diagram showing the necessary mixing, local oscillator mixing frequencies, and the sidebands selected by means of the triangles described previously in a step-by-step process from voice channel input to line frequency output. The CCITT has recommended a standardized modulation plan with a common terminology. This allows large telephone networks, on both national and multinational systems, to interconnect. In the following paragraphs the reader is advised to be careful with terminology.
18.104.22.168.2 Formation of the Standard CCITT Group. The standard group, as defined by the CCITT, occupies the frequency band of 60 kHz to 108 kHz and contains 12 voice channels. Each voice channel is the nominal 4-kHz channel occupying the 300-Hz to
3400-Hz spectrum. The group is formed by mixing each of the 12 voice channels with a particular carrier frequency associated with each channel. Lower sidebands are then selected, and the carrier frequencies and the upper sidebands are suppressed. Figure 4.19 shows the preferred approach to the formation of the standard CCITT group. It should be noted that in the 60-kHz to 108-kHz band, voice channel 1 occupies the highest frequency segment by convention, between 104 kHz and 108 kHz. The layout of the standard group is illustrated in Figure 4.19. Single sideband suppressed carrier (SSBSC) modulation techniques are utilized universally.
22.214.171.124.3 Formation of the Standard CCITT Supergroup. A supergroup contains five standard CCITT groups, equivalent to 60 voice channels. The standard supergroup, before further translation, occupies the frequency band of 312 kHz to 552 kHz. Each of the five groups making up the supergroup is translated in frequency to the supergroup frequency band by mixing with the appropriate carrier frequencies. The carrier frequencies are 420 kHz for group 1, 468 kHz for group 2, 516 kHz for group 3, 564 kHz for group 4, and 612 kHz for group 5. In the mixing process, in each case, the difference is taken (i.e., the lower is selected). This frequency translation process is illustrated in Figure 4.20.
126.96.36.199 Line Frequency. The band of frequencies that the multiplexer applies to the line, whether the line is a radiolink, wire pair, or fiber-optic cable, is called the line frequency. Some texts use the term high frequency (HF) for the line frequency. This is not to be confused with HF radio, which is a radio system that operates in the band of 3 MHz to 30 MHz.
The line frequency in this case may be the direct application of a group or supergroup to the line. However, more commonly a final frequency translation stage occurs, particularly on high-density systems.15 An example of line frequency formation is illustrated in
15"High-density" meaning, in this context, a system carrying a very large number of voice channels.
Figure 4.21. This figure shows the makeup of the basic 15-supergroup assembly. Its capacity is (15 x 60) 900 voice channels (Ref. 7).
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