GB1603436A - Frequency multiplex telecommunication systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO FREQUENCY MULTIPLEX TELECOMMUNICATION SYSTEMS (71) We, SIEMENS AKTIEN GESELLSCHAFT, a German Company of Berlin and Munich, German Federal Republic, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement :- The invention relates to frequency multiplex telecommunication systems of the type having telephone channels combined to form a basic primary group in a carrier frequency communications transmission system, and provides for the transmission of a sound broadcast programme having a bandwidth approximately equal to that of a pregroup three telephone carrier frequency channels to be conveyed in said basic primary group.

Experience has shown that the technical outlay and space requirement needed for devices to be used for carrier frequency transmission of sound broadcast programmes is to a considerable extent determined by the requisite characteristics demanded for the associated filters and carrier supply devices. The high quality which is required can only be reasonably guaranteed if many of the known means for protection against impermissible noise values and cross-talk values, fluctuations in level and frequency shifts are employed in the system. In the previously used transmission devices a notable reduction in technical outlay and space requirement has been achieved by dispensing with compandors and pilot level regulation, for example, with the consequent disadvantages, especially in transmissions employing transportable equipment. Since endeavours are now being made to distinguish only between lines having a 15 kHz bandwidth and those having a nominal 5 kHz bandwidth for sound broadcast programme transmission, a new problem is presented to provide for better methods of utilizing carrier frequency transmission for sound broadcast programme channels having the wider-bandwidth.

One object of the present invention is to provide a carrier frequency sound broadcast programme transmission system which can be operated at a high speed and in a simple fashion for transmission in the primary group position of a carrier frequency system, maintaining adequate sound quality, whilst being substantially insensitive to interference and effected in a manner which does not disturb the carrier frequency communications transmission system itself.

The invention consists in a frequency multiplex telecommunication system in which means are provided to effect carrier frequency sound broadcast programme transmissions or carrier frequency channels each having a bandwidth substantially equal to that of a pregroup of three telephone channels, and located in a band forming a fundamental primary group of the carrier frequency communications transmission system, the bandwidth of each sound broadcast programme channel being substantially 7 kHz, each sound broadcast programme channel having a pilot signal added in the low frequency state above its carrier-modulated sound broadcast programme frequency band, each sound broadcast programme channel being converted into the carrier frequency state by means of mutually identical quadrature modulators, the carrier frequencies for conversion being selected such that in each transmission state all the signal frequencies of the sound broadcast programme channel to be transmitted are spaced by more than I kHz from the frequency band limits specified for the respective carrier frequency communications transmission channel.

As a result of these measures a carrier frequency sound broadcast programme transmission system is obtained which can be operated at high speed and in simple fashion but which only requires a low technical outlay, for one to four monophonic sound broadcast programme channels in a carrier frequency fundamental group with a greater sound frequencv bandwidth than 5 kHz, employing three carrier frequency telephone channels in respect of each sound broadcast programme channel, and predominantly suitable for high-speed, transportable sets.

This system can also be constructed using simple lumped circuit band-pass filters in order to effect the necessary selection of the individual carrier frequency sound broadcast programme channels.

All the requisite pilots and carrier frequencies can advantageously be derived from a transmitting-end quartz crystal generator, one of the derived pilots serving for synchronisation for a receiving end quartz crystal generator.

If the sound broadcast programme channels are transmitted as frequency inverted signals, the carrier frequencies are advantageously selected to be 70.5 kHz, 82.25 kHz 94 kHz and 105. 75 kHz and a pilot frequency of 7833 1/3 Hz is selected.

The invention will now be described with reference to the drawings, in which :- Figure I is a frequency allocation chart; Figure 2 schematically illustrates a transmitting arrangement for working to the plan shown in Figure 1 ; and Figure 3 schematically illustrates the associated receiving arrangement.

In the frequency plan shown in Figure 1, the individual sound broadcast programme channels KI to K4 are transmitted in the inverted state, as lower side-bands of respective carrier frequencies. The most stringent demands are placed upon the upper frequency channel, and the selection filters are designed for more than a I kHz frequency gap between the pass band and the specified channel limits, and preferably a 2% frequency gap, which makes the filter design quite viable. The other filters have to meet less demanding requirements. The carriers 70.5 kHz, 82.25 kHz, 94.0 kHz and 105. 75 kHz are obtained by multiplying the fundamental frequency (11. 75 kHz) by the factors 6 to 9. Therefore it is easily possible to derive these from a common quartz crystal frequency of 5.922 mHz using frequency division by corresponding factors, with 7 as the highest prime number, which requires only a low circuitry outlay if TTL design techniques are employed. The pilot for each sound broadcast programme channel (7833 1/3 Hz) is obtained from the carrier frequency 94.0 kHz using division by 12. On account of the proven advantages of use of quadruple carrier frequencies for the operation of quadrature modulators, 282 kHz, 329 kHz, 376 kHz and 423 kHz are the preferred measurable carrier frequencies and 23.688 MHz is the preferred quartz crystal frequency. Quartz crystal generators in these frequency positions can be adequately stabilise without the use of a thermostat (error < 2x 10~5) and are easily synchronised at the receiving end by means of known AFC-circuits. The rigid linking of all carrier frequencies and pilot frequencies at the transmitting and receiving ends substantially eliminates frequency deviations at the transmitting end when only one single of the four incoming pilots is used to synchronise the associated quartz crystal generator at the receiving end. Furthermore an adequate reduction is achieved in permissible frequency errors in the carrier frequency transmission systems, which errors are additively similar for a carrier frequency fundamental group connection, as will be explained for the quoted example.

With the common fundamental frequency fl= (11. 75 kHzAf,) the following carriers and pilots are used at the transmitting end: Carrier 4=6 carrier 3=7 carrier 2=8 f,, and carrier 1=9 fol, whist the pilot frequency 2 =-fol.

3 At the receiving end, the following pilots modified by the frequency shift of the carrier frequency transmission (, Af, are then received in the carrier frequency state:

16 Channel 4=-f, AfT 3 19 Channel 3= fl~AfT 3 22 Channel 2=-f, AfT 3 25 Channel l= fl-Afr 3 The reconversion into the audio frequency state is carrier out in similar fashion at the transmitting end by carriers which are rigidly linked via the fundamental frequency f,. The carrier frequency f, can be determined via a AFC circuit by the pilot in channel 1 or 2 or 3 or 4:

In the demodulation of the carrier frequencies derived therefrom (6-, 7-, 8-or 9-times the respective numerical values) with the four possibilities different frequency errors occur in the four sound broadcast programme channels, as listed in the following table :

AFC Channel 4 Channel 3 Channel 2 Channel I 2 5 8 fl Channel 4-4Af,)-- (Af,)-- (Af,)- (Af,) 16 16 16 16 1 2 5 8 Channel-- (fT)- (efT)- (clfT)- (efT) 19 19 19 19 4 1 2 S Channe ! 2- (ofT)-- (AfT)--AfT)--- AfT) 22 22 22 22 7 4 I 2 Channell-- (ofT)- (Afr)- (fT)- (lfT) 25 25 25 25 The table indicates that the frequency shift of 2 Hz permissible for carrier frequency systems can be constantly kept under 0.5 Hz when a plurality of channels are operated, even when only one individual, but suitable channel is operated with AFC.

By way of an exemplary embodiment of the operation of two sound broadcast programme channels, Figure 2 illustrates part of a transmitting station and Figure 3 a corresponding part of a receiving station.

An amplifier I provides level matching and pre-amphasis, and feeds a low frequency lowpass filter 2 (e. g. 6.4 kHz), after which there is in each channel a transmitting amplifier 3 with pilot input coupling, a transmitting modulator 4 and a compandor 6 corresponding to CCITT Rec. J. 31, is inserted between two simple transmitting band-pass filters 5 and 7, each with low blocking attenuation. A common amplifier 9 having four inputs for coupling a maximum of four channels (sound or telephone) provides facilities for matching the devices in the following carrier frequency transmission system. A common generator 8 serves to produce the pilot and the four carriers. The receiving station shown in Figure 3 is provided with a similar construction, a common carrier frequency receiving amplifier 10 having matching facilities for the feeding carrier frequency system, and four decopuled outputs for the operation of the maximum of four sound broadcast programme channels, each comprising receiving band-pass filters 12, 13, which govern the channel selection, followed by a compandor 6 that is operated as an expandor, with associated level regulation (AGC) provided from a pilot receiver 15 that is fed from a receiving modulator 14 and feeds a low frequency low-pass filter 2, that is followed by an output amplifier 16 provided with deemphasis. In a manner similar to the common generator 8 at the transmitting station, a common generator 11 which serves to produce the maximum of four demodulation carriers, which in this case is synchronised by one of the maximum of four received pilots.

For a carrier frequency transmission of the sound broadcast programme channels in the normal state, i. e. non-inverted, a similar construction can be used, but with a different fundamental frequency (12. 25 kHz) and with the factors 5 to 8. The devicc outlay and the results are similar to those in the example of the carrier frequency sound broadcast programme transmission in the inverted state, which has been described in detail above.

Claims (4)

1. WHAT WE CLAIM IS : I. A frequency multiplex telecommunication system in which means are provided to effect carrier frequency sound broadcast programme transmissions on carrier frequency channels each having a bandwidth substantially equal to that of a pre-group of three telephone channels, and located in a band forming a fundamental primary group of the carrier frequency communications transmission system, the bandwidth of each sound broadcast programme channel being substantially 7 kHz, each sound broadcast programme channel having a pilot signal added in the low frequency state above its carriermodulated sound broadcast programme frequency band, each sound broadcast programme channel being converted into the carrier frequency state by means of mutually identical quadrature modulators, the carrier frequencies for conversion being selected such that in each transmission state all the signal frequencies of the sound broadcast programme channel to be transmitted are spaced by more than I kHz from the frequency band limits specified for the respective carrier frequency communications transmission channel.
2. 2. A system as claimed in Claim 1, in which all the requisite pilot signals and carrier frequencies are derived from a common transmitting end quartz crystal generator, and that one of the derived pilots serves to provide synchronisation of a further common quartz crystal generator at the receiving end.
3. 3. A system as claimed in Claim I or Claim 2, in which a maximum of four sound broadcast programme channels are transmitted, each in the inverted state, the carrier frequencies 70.5 kHz, 82.25 kHz, 94.0 kHz and 105. 75 kHz being used, with a pilot frequency of 7833 1/3 Hz.
4. 4. A frequency multiplex telecommunication system with a sound broadcast programme transmission provision in its primary group, substantially as described with reference to Figures 2 and 3.