GB1284318A - A device for forming a multi-channel primary group for a carrier frequency transmission system - Google Patents

Abstract

1284318 Frequency division multiplexing CIT-COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS 6 Aug 1970 [8 Aug 1969] 37966/70 Heading H4R In each channel of a multi-channel transmission system the input frequency is modulated with a common frequency F1, a filter passes the resulting common intermediate frequency band and a second modulator modulates the intermediate band with one of a set of carrier frequencies; the carrier frequencies of said set being consecutive multiples of a common basic frequency F0, F1/F0 being expressible as P/Q with P and Q both integers less than 100, substantially half of the channels using inverse modulation and the remainder using direct modulation. The invention can be used to send 16 channels each of 3 KHZ in a 48 KHZ band. In the preferred form, each channel transmits a voice band of 200-3050 HZ which is passed through a band pass filter 1 tuned to this band, modulated at 2 by a fixed frequency 14,850 HZ to produce signals in a band 15,050 to 17,900, passed through a second band pass filter 3 tuned to this band, modulated using direct or inverse modulation and a frequency Fn of a set of thirteen carrier frequencies to produce a transmission band of width slightly less than 3KHZ (e.g. 60À100 to 62À950 KHZ), combined with three adjacent and similarly produced transmission bands in a four resistive input amplifier 6 to produce a group of width less than 12 KHZ and after passing through a group band pass filter 74 combined with three further group bands in a four resistive input amplifier 9 to produce the required 16 channel band. Even numbered channels use normal modulation and the lowest eight carrier frequencies while odd numbered channels use reverse modulation and the highest eight carrier frequencies, the three middle carrier frequencies being used for two channels with different types of modulation. Within each group the carriers frequencies Fn used are outside the range of that group band pass filter 7 1 to 7 4 . The carrier frequencies are 48 to 120 KHZ in increments of 6 KHZ and can be produced by frequency multiplying 6 KHZ. A common oscillator operating at 2376 KHZ is divided by 396 and 160 by frequency divider chains (Fig. 5, not shown) to give the basic frequency 6 KHZ and the common frequency F1 respectively. Integrated circuits may be used for the frequency dividers.