US1907109A

US1907109A - Radio signaling system

Info

Publication number: US1907109A
Application number: US501825A
Authority: US
Inventors: Hinton Leslie Turner
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1930-01-02
Filing date: 1930-12-12
Publication date: 1933-05-02
Anticipated expiration: 1950-05-02
Also published as: DE609540C; FR733626A; NL34797C

Description

T E Q T w T 1% 4 0 I INVENTOR L. 7i H/NTON BI ATTORNEY May 2, 1933.

Patented May 2, 1933 UNE'I'ED STATES PATENT OFFICE LESLIE TURNER HINTON, OF PAR-IS, FRANCE, ASSIGNOR TO WESTERN ELECTRIC COM- PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK RADIO SIGNALING SYSTEM This invention relates to signaling systems and particularly to such systems wherein intelligencewhich may be in the form of s1 eech, music, pictures, etc, is transmitted by means of modulated carrier waves, the latter being subsequently demodulated to reproduce the original signals.

In such a high frequency signaling system a certain quality degradation of the transmitted signal occurs and one of the objects of the present invention is to reduce this quality degradation and to improve the standard of quality of the transmitted signals.

A substantial portion of this quality degradation results from the production in such signaling systems of spurious harmonic fre quencies, some of them falling within the signal frequency band and thus causing distortion.

The analysis of the production of such spurious harmonics is somewhat complicated but in order to render the following more clear a few explanations will be given.

In a high frequency signaling system the received signal usually consists of a carrier wave and of two side bands. It is a known fact that particularly in short wave operation a selective fading effect develops. This effect is usually explained by the phase dif ferences in the received signals when they are received over different space channels. The phase differences vary with the weather, the

, time of the day, and with other conditions,

and produce a marked change in the attenuation of relatively narrow hands over the whole width of the band received. 'For example, assuming that there is in transmission a carrier frequency and side bands each having 3 kilocycles band width, from time to time narrow bands within the total of 6 kiloeycles will have their attenuation considerably increased. It has been found that a narrow band containing the carrier frequency might fade while the frequency in the side bands say, 1100 cycles from the carrier hand on each side, will not experience the same attenuation.

Considering now what happens in the receiving circuit, it will be seen that if a single frequency of 1100 cycles is transmitted, the

501,825, and in Great Britain January 2, 1930.

receiver picks up three frequencies, viZ., 0+ 1100, C and C 1100, where C is the carrier frequency. Usually the amplitude of the carrier wave is considerably greater than that of the side bands. The amplitude of the 1100 cycles received is afunction of the amplitude of the carrier wave and of the side bands whereas that of the unwanted modulation products of 2200 cycles is a function of the amplitude of the side band alone.

Under the selective fading conditions mentioned above, the amplitude of the carrier wave may decrease considerably while the amplitude of the side bands may remain unchanged. Then the ratio of the received signals of 1100 cycles to the one of 2200 cycles will be considerably changed, that of 2200 cycles appearing in such an amount that it harms the quality of the transmit-ted signals.

It has been noted that this effect is more noticeable in telephony than in telegraphy, probably due to the production of new and unintelligible frequencies. In the case of a high frequency signaling system using signal frequency band inversion it should be noted that the spurious harmonics produced may have a distorting eifect different from the one they have in a system which does not use signal frequency band inversion. In some cases the spurious frequencies in question are more objectionable to the ear, in a system using frequency band inversion than in a system dispensing with frequency band inversion. This efiect may be explained as follows in the case of telephony.

In a high frequency signaling system, harmonics of the original frequencies are produced whenever a modulated wave is demodulated in a device operating by means of a nonlinear characteristic. For example, these harmonics are always produced in the usual types of receivers employing an ordinary detector. Usually the predominant harmonic is the second (i. e. 2f where f is the fundamental of the si al frequency) and the ratio of the ampiitn" e of the second hannonic to the amplitude of the fundamental is a function of the percentage modulation.

With ordinary systems of transmission, however, the degradation of quality due to hand and if the filters used have very sharp the interaction of these harn'ionics is not parti cularly serious, the frequencies produced are definitely in harmonic relation to the signal frequency and the ear is able to accept them, the net result being a slight (lOCIOLlEK. in the apparent shrillness or hardness of tone in the case of telephony. lVhen inverted speech is transmitted on the other hand, the harl'nonics produced. in the receiving detector are lnn-iuwm' of the inverted frequencies and consequently will have no simple relation to the signal frequencies after the Whole band has been re-invertcd. The ear is therefore unable to accept such spurious fre quencies and per-(hives them as objectionable and interfering noises.

It should be noted that in any signaling system utilizing a modulated carrier wave (i. e. using inverted or non-inverted signal frequency bands) other spurious frequencies which are not harmonics of the original frequencies are introduced. This occurs whenever, as in the case of music broadcasting, more than one frequency is transmitted at a time. In such case the various signal frequencies beat together, giving terms whose frequencies are equal to the sums and differences of all the original signal. frequencies taken together in pairs. This a degradation of quality which. is often noticeable in the reception of ordinary broadcasting.

According to a feature of the present invention, means are provided for minimizing the influence of spurious harmonics in a high frequency signaling system.

According to another feature of the present invention the spurious harmonics are reduced by suitably choosing the relative positions in the frequency spectrum of the carrier frequency'and of the signal frequency hand.

If the spacing frequency, that is to say, the distance in the frequency spectrum between the carrier frequency and the lower frequency of the signal frequency band, is different from zero, some parasitic intermodulation products falling within the signal frequency band are cut out.

If the spacing frequency is equal to the frequency band Width of the signal frequency cut-off frequencies, substantially all these spurious or parasitic inter-modulation prodnets are cut out.

One advantage of the present invention is that the present method for reducing the influence of spurious harmonies may be applied eitherto a secret signaling system wherein the whole frequency band is inverted, or to a system wherein only some sub-bands of the signal freqnency bands are inverted. In either of the cases an improvement in the quality of the transmitted signals is obtained.

It should be emphasized that although the present system is particularly suitable for a fsecret signaling system using signal frequency band inversion, the method of choosing the relative position in the frequency spectrum of the carrier fre 1;lency and of the sigz'ial frequency band, may he used in any signaling svstmn :2 nd will improve the quality of transmission.

According to the invention a high frequency signaling system is provided with the side bands located at such distance from the carrier band that undesirable frequencies are substantially eliminated.

The invention will be described. in connection with the accon'rnanying drawing which shows, in y 'wa y of example, some arrangements for carrying the invention into effect.

Referring to the drawing, Figs. 1 and 2 il lustrate the position in the frequency spectrum of the side band and of the carrier frequency.

Fig. 3 illustrates the position of the side hands when they have been spaced from the carrier frequency.

Figs. 4c and 5 show improved high frequency signaling systems.

l-V hen transmitting a signal frequency band of width a, spurious frequencies are produced in the received signals occupying a hand of frequency of width 2a. As shown in Fig. 1, the original signal frequency band occupies the position 1 in the frequency spectrum whereas 2 represents the range covered by the spurious frequencies which are obtained during selective fading conditions.

The relation of the carrier frequency 3 with regard to the side bands 4t and l is also shown in Fig. 1 and it should be noted that the side bands 1- and 9 occupy a position in the frequency spectrum immediately adjacent to that of the carrier frequency.

It may be shown, as illustrated in Fig. 2, that if the side bands are separated from the carrier wave in the received signals so that if the width of the side band is (1 cycles, then the lowest speech frequency in the side hand must be at a cycles from the carrier Wave G and under this condition the useful received signal will be between a and 2a cycles, While the undesirable frequencies will be between 2a and 4a cycles.

Therefore the useful signals can be separated from the useless ones by means of filters or other known devices producing a similar efl'ect.

In order to determine the separation of a number of side bands from the frequency of a carrier wave, it may be assumed that the width of the side bands nearest to the carrier is a cycles as shown in Fig. 3. There is a separation equal to a cycles of those next adjacent 5 cycles next to the carrier frequency and consequently alternate bands, one useful and the other useless, are produced and will follow the law 6;, 2a, 6, 2b. In Fig. 3, C is a line of symmetry; the complete drawing to the left of G is not shown,

being identical with that shown to the right flay,

The width of the useful baml 0 should not be greater than that of the band a.

Fig. 4; shows the system schrnnatical iy in the form applied to a simple tckqihouc systmn. A band having a frequency range from 0 to 3 kilocycles is increased in frequency by known means such as by a modulator MOD and selection of a side band by means of the filter F, and demodulators if necessary and is fed to the transmitter T as a frequency band from 3 to (i hilocycles. Here it is combined with the carrier frequency by any of the known methods and it is transmitted to the receiving point. which is provided with a detector 1 the useful received signal will be comprised between 3 to 0 kilocycles and the useless one between 6 to 12 lrilocycles. The two bands are then separated by a filter F which allows the passage of the useful signal alone. By means of a frequency step-down device which may comprise a filter F and a frequency changing device FG (which may be similar to the stepup arrangement used at the transmitting end) the frequency band is brought to the position of 0 to 3 kilocycles, thus giving a frequency band at the receiving point having the same position in the frequency spectrum the ori inal frequency band at the sending point.

*ig. 5 shows an alternative way for modifying the position of a signal frequency band in the frequency spectrum so as to reduce spurious frequencies.

Referring to Fig. 5 the speech signals are applied to an apparatus T for transforming sound vibrations into electrical vibrations. The said electrical vibrations are applied to a low pass filter LP having a cut-off frequency of 3000 cycles per second. The filtered signals thereafter are applied to a modulator MODl, supplied with a modulating frequency of say 20 kilocycles, and the upper side band thus produced is selected by means of a filter F having a suitable pass range, for instance a pass range between 20 and 23 kilocycles. This side hand then enters a demodulator MOD2 supplied with a carrier frequency of say 26 kilocycles the lower side band being then selected by means of a low pass filter LP having a cut-off frequency of 6000 cycles. In the output of this filter there is obtained an inverted speech band with a frequency range of 3000 to 6000 cycles. This is applied to the transmitter T and sent to the receiving station R. In the receiving inverter, the inverted speech is applied to a high pass filter HP having a cut-ofi frequency of 3000 cycles and is thereafter applied to a modulator MOD.1 supplied with a carrier frequency of 20 kilocycles, the upper side band being selected by means of a band pass filter BF of suitable range, for example a. pass filter of 23 to 26 kilocycles. The out At the receivin .ioint' put of filter BF is applied to a demodulator MOD2 which is supplied with a carrier frequency of 26 lrilocycles, the lower side band being selected by means of a low pass filter Ll" with a cut-oil frequency of 3000 cycles, normal speech being ()btt-tll'md after this operation. The normal. speech vibrations are applied to a device T adapted to transform electrical vibrations into sound waves.

The beat notes between the side hands, when the carrier frequency fades out, cause, as it has been pointed out above, spurious harmonic frequencies and particularly a strong second harmonic, but this spurious frequency is now outside the gnul frequency range in the output of the receiving inverter.

The present systems permit an increase in the degree of modulation at the transmitter since the spurious harmonic frequencies are no longer troublesome. In certain cases the present system. allows a reduction of the signal to noise ratio, that is to say a reduction of the ratio of the received field strength to the field strength due to static inasmuch as this reduction of signd strength is offset by the increased perccn modulation that may be used.

The principle of suitably selecting the positionin the frequency spectrum of the carrier frequency and of the signal frequency band for reducing disturbing intermodulation products and for providing the facilities above mentioned is of considerable importance and the broad idea underlying this invention may easily be applied to other schemes of modulation such as frequency or phase modulation systems.

What is claimed is:

1. The method of signaling which comprises generating a pair of sidebands displaced by a frequency interval from the neighborhood of an associated carrier Wave, transmitting said carrier and displaced sidebands, detecting displaced signals therefrom with incidental production of combination frequencies due to inter-modulation between the two sidebands, said combination frequencies lying mainly outside the frequency range of said signals due to said displacement of the sidebands, selecting said signals and translating them to their original frequency range substantially free from said combinat'ion frequencies.

2. The method of transmission of signals employing difierent bands of modulated waves, characterized in this, that the frequencies of the unmodulated waves relative to frequencies of said signals, are so selected that the side-bands generated by modulation of said waves by said signals are separated in the frequency spectrum by at least twice the Width of a single side-band.

3. A method in accordance with claim 2, characterized in this, that unmodulated waves of two different frequencies are supplied, the Waves resulting from modulation of the Wave of one frequency by signals being freed of any component of the unmodulated wave frequency, and then used to modu- 5 late the Wave of said other frequency.

4:. A method in accordance With claim 2, characterized in this, that unmodulated Waves of two different frequencies are supplied, the Waves generated by modulation of one of said frequencies by signals being filtered to suppress all components except the lower side-bands, and then used to modulate the wave of said other frequency.

5. A high frequency signaling system using signal frequency inversion, wherein the transmitting wave comprises a carrier Wave and side-hands, said side-bands inversely related in frequencies to the signal frequencies, and spaced from said carrier and each other in the frequency spectrum by at least twice the Width of one of said side-bands.

6. In a secrecy signaling system, employing signal. frequency band inversion, the method of substantially eliminating effects in the received and translated Waves of the quality degrading combination and intermodulation frequencies produced in transmission, which comprises spacing the inverted signal frequency bands, in the transmitted Waves, at least twice the signal frequency range apart, and separating said quality degrading frequencies from the useful components of the received Waves by frequency e discriminating means.

In Witness whereof, I hereunto subscribe my name this 18th day of November, 1980.

LESLIE TURNER- HINTON.