 follows: 20,300 23,400 Hz When we take the sum, as we did previously, and filter out all other frequencies, we say we have selected the upper sideband. Therefore we have a triangle facing to the right, and we call this an upright or erect sideband. We can also take the difference, such that 20,000 Hz 20,000 Hz 300 Hz 3,400 Hz 19,700 Hz 16,600 Hz and we see that in the translation (mixing process) we have had an inversion of fre- quencies. The higher frequencies of the voice channel become the lower frequencies of the translated spectrum, and the lower frequencies of the voice channel become the higher when the difference is taken. We represent this by a right triangle facing the other direction: 16,600 19,700 Hz This is called an inverted sideband. To review, when we take the sum, we get an erect side- band. When we take the difference, frequencies invert and we have an inverted sideband represented by a triangle facing left. Again, this modulation technique is called single-sideband suppressed carrier (SSBSC). It is a type of amplitude modulation (AM). With conventional AM, the modulation pro- duces two sidebands, an upper sideband and a lower sideband, symmetrical on either side of the carrier. Each sideband carries the information signal. If we tune to 870 kHz on the AM dial, 870 kHz is the frequency of the RF carrier, and its sidebands fall on either side, where each sideband is about 7.5 kHz wide. Thus a radio station on the AM dial requires about 15 kHz of spectrum bandwidth. In our case, there are also two sidebands extending about 3.4 kHz either side of the carrier frequency. Of course, the carrier frequency is the local oscillator frequency, which is suppressed at the output, as is the upper sideband. All that remains is the lower sideband, which contains the voice channel information. 4.5.2.3 CCITT Modulation Plan 4.5.2.3.1 Introduction. A modulation plan sets forth the development of a band of fre- quencies called the line frequency (i.e., ready for transmission on the line or transmission TRANSMISSIO