US2219936A

US2219936A - Signaling system

Info

Publication number: US2219936A
Application number: US228911A
Authority: US
Inventors: Percival William Spencer
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1937-09-13
Filing date: 1938-09-08
Publication date: 1940-10-29
Anticipated expiration: 1957-10-29

Description

Patented Oct. 29, 1940 I UNITED STATES PATENT OFFICE SIGNALING SYSTEM Britain I Application September 8, 1938, Serial No. 228,911

In Great Britain September 13, 1937 .3 6 Claims.

This invention relates to electric wave filtering systems and is concerned with the use of such systems in connection with signals covering s relatively wide ranges of frequency.

5" It is sometimes required to divide a band of signal frequencies into two smaller bands, such that when these bands are re-combined the original signal band is obtained. Thus, in the a, case of the transmission of television signals by 10 cable, it is desirable to employ a simple concentric cable over which a wide band of frequencies covering both sound and vision frequencies is to be transmitted. Such a cable is, however, liable to interference at low frequencies from power supplies and the like and one method of preventing such interference is to employ a balanced line enclosed in a suitable conducting sheath, but this method is expensive.

The object of the present invention, therefore, is to facilitate the separation from a wide band of frequencies of a relatively small band of frequencies, and to make provision for re-combining the bands in order to obtain the original sig- ;,nal without loss of any frequencies.

According to the present invention, in an electric wave filter system, provision is made for selecting from a wide band of signal frequencies, a partial band of given width and for subselquently including said partial band within said wide band, the output from a'filter employed to effect selection'of said partial band being subtracted from said wide band to provide a second partial band, the Whole of the frequencies in said wide band being delayed before the subtraction is effected, by a period equal to the time delay of said filter. The said partial frequency band may be at the lower frequency end of the wide band and the filter employed in this case is of the low pass type.

In applying the invention to a high frequency signaling system, provision is made for selecting a partial frequency band from a wide band of ,frequencies, said partial band being used to modulate a carrier wave having a fundamental frequency at or above the highest frequency in said band, the carrier wave after modulation, together with the signals in the whole of the wider frequency band being applied to a conductor connected to two parallel channels the first of which includes means for subtracting from the wider frequency band, a-band of frequencies equal in width to said partial band, the second 55 ;-of said channels including means for selecting the modulated carrier wave and obtaining therefrom by rectification the modulation frequencies which are added to the portion of the wider frequency' band pass through the first channel.

The wider frequency band is passed through the first of said channels which includes a band stop filter for rejecting the modulated carrier wave, a low pass filter and a delay network having a time delay equal to that of said low pass filter in order that the frequencies in said band not passed by said low pass filter shall be subjected to equal delay to the frequencies passed by said low pass filter. The second of said parallel channels may include a tone correcting cirout for compensating the loss of high frequencies and a low pass filter, the delay due to which is compensated in the first of said channels by a further delay network, the outputs from the two channels being combined.

In the case of the provision of a low pass filter, the output of which is subtracted from the original signal represented by the wider frequency band delayed by an amount equal to the delay of the low pass filter in order to obtain the equivalent of a high pass filter, there is a difference from the output of an ordinary high pass filter in that when the outputs are combined the original signal is reproduced exactly. The output of the low pass filter need not cutoff very sharply above its cut-off frequency, and the difference between this and the original signal need not cut-off sharply below this frequency. Thus, considering two constant K sections of a low pass filter having a cut-off frequency of 100 kilocycles per second, the output from such a filter will fall to about 1% at 175 kilocycles per second, which is sufficient for most practical purposes. Now at frequencies below about 50 kilocycles per second the time delay approaches a constant delay of four radians of 100 kilocycles per second, that is to say about 7 micro-seconds. The original sign-a1 must, therefore, be delayed by about 7 micro-seconds, and the output of the low pass filter subtracted from it. The difference output will be flat down to 1'75 kilocycles and will be very low below kilocycles per second. Between 50 and 175 kilocycles per second, the response will be of a complicated nature. This 50 fact is, however, not of importance provided that when the difference output is added later to the output of the low pass filter, the original signal is reproduced exactly.

In order that the invention may be more clearly understood and readily carried into effect, a portion of a television transmission system embodying the invention will now be described by way of example with reference to the accompanying drawing.

Referring to the drawing, the total transmission range of signals is represented. by the line I, extending between and 3'n1egacycles. Signals within this wide range are represented as dividing at the point 2 and being transmitted through conductors 3 and 4, the conductor 3 including a low pass filter represented by a rectangle 5, and which passes signals from 0 to 100 kilocycles per second. The rectangle 6 represents a mixing circuit in which the frequencies passed by the filter 5 are caused to modulate a carrier wave having a frequency of 3.5 megacycles per second. The whole of the original signal frequency band width of 0-3 megacycles conveyed by conductor 3 is added to the modulated carrier wave from the mixer 6 and they are together fed to a concentric cable I, the line 8 representing the increased frequency range of the signals in the conductors between the mixer 6 and cable I including the range from 0 to 3 megacycles and the added carrier frequency of 3.5 megacycles with the modulation frequencies which will be of the order plus and minus 175 kilocycles. A frequency range equal to that represented by the line 8 is represented by the line 9 at the receiving end of the cable. The cable I feeds two filter channels through conductors I0 and II respectively. The rectangle I2 represents a circuit sharply tuned to 3.5 megacycles per second, this circuit being succeed ed by a rectifier represented by the rectangle I3. The tuned circuit I2 serves to remove frequencies within the range extending from Q to 3 megacycles per second, so that beats between signal components within this range are of negligible amplitude at the output of rectifier I3. The output from rectifier I3 passes to a correcting circuit represented by the rectangle I4 which serves to restore the modulation frequency characteristic by emphasizing the higher frequencies up to say 175 kilocycles, the circuit I4 having a frequency characteristic which is substantially the inverse of the sharply tuned circuit I2, but transferred from the carrier frequency to zero frequency. The correcting circuit I4 is followed by a low pass filter represented by the rectangle I5, this filter serving to remove all frequencies not required such as those due to beats between the carrier wave and the original signal. This low pass filter may comprise two sections, and may conveniently have a cut-off frequency of 250 kilocycles per second. A band pass circuit could be employed in place of the sharply tuned circuit I2 and in this case, the only circuit required to follow the rectifier would be a band stop filter, which would serve to remove the carrier and its side-band frequencies.

The conductor I0 includes a band stop filter represented by the rectangle I6, and which is effective to prevent the transmission of the carrier wave of 3.5 megacycles per second, and its modulation frequencies. The output from the filter I6 is thus represented by the line I! extending over a frequency of from 0 to 3 megacycles per second. Following the filter I6 is a low pass filter represented by the rectangle I8 connected in parallel with a delay network I9, the time delay of which is substantially equal to that of the filter I8. This filter passes frequencies from 0 to 100 kilocycles per second, and the output from it is subtracted from the range of frequencies passed through the delay network I9, the resulting output being represented by the line 20 extending over a frequency range of from 50 kilocycles per second to 3 megacycles per second. This frequency range is subjected to delay in a network represented by the rectangle H, the output from which is combined with the output from the low pass filter I5, the combined output being represented by the line 22 identical with the original signal extending over the frequency range of from 0 to 3 megacycles per second.

The delay network I9 may conveniently consist of 150 sections and have a cut-off frequency of 7.5 megacycles per second. The delay network 2| is provided mainly to compensate for the delay caused by the low pass filter I5 or for the band pass circuit preceding it. The delay network 2I may conveniently consist of 60 sections and have a cut-off frequency at 7.50 megacycles per second.

It will be seen that the frequency range of 0 to 3 megacycles is present in the channel including the conductor 4, cable I, conductor I0 and the filter circuits I8 and I9, subtraction of the 0-50 kilocycle range only being effected when the output from the low pass filter I8 is subtracted from the output from the delay network I9. The lower frequency band on the other hand is present as a band of modulating frequencies in the channel including the conductor 3, cable I, conductor Ii and circuit I2 and after de-modulation by the rectifier I3, the lower frequency range is passed on through circuits I4 and I5 to be added to the output from the delay network 2|. The various delay networks included in these channels are so arranged that the total delay in each channel is equal.

It will be appreciated that whereas constant K sections have been referred to as examples in the filters described, filters may be employed having m derived sections with m less than one, in order to obtain a sharper cut-off or with m greater than one to obtain more uniform time delay throughout the frequency pass range. Again, the delay network need not consist of physical coils and condensers but other means may be employed. Again, the delay may be produced by supersonic waves in a suitable liquid, the supersonic waves being produced and detected by a quartz crystal. For this purpose, it is desirable to change the pass band to a carrier frequency of say 10 megacycles per second, in order toobtain a fiat characteristic.

Instead of employing low pass filters having a cut-off frequency of 100 kilocycles per second, it is possible to employ the difference between the output of a high pass filter and the original signal. Such a method has the advantage that the compensator delay filter I9 is not required. On the other hand, a much wider frequency range would be required for the side-bands of the modulation frequencies, owing to the nature of the phase change over the pass band of a high pass filter.

In the particular example described, while the use of a low pass filter has been described, it will be understood that band pass or high pass filters may be employed.

I claim:

' 1. In the method of transmitting electrical signals having a wide band of frequency components with a predetermined minimum frequency and a predetermined maximum frequency, the steps of segregating frequency components lying between the predetermined minimum frequency and a predetermined value of frequency intermediate the predetermined minimum and maximum frequency, producing carrier wave energy whosefrequency is at least as great as the predetermined maximum frequency, modulating the produced carrier wave energy by the segregated frequency components, transmitting together over a single transmission path the modulated carrier wave energy and the entire range of electrical signals, receiving the transmitted energy and signals, separating the received modulated carrier wave energy from the received electricalsignals, removing frequency components lying between the predetermined minimum and-asecond predetermined intermediate value of frequency from the received electrical signals, demodulating the received carrier Wave energy, and combining the demodulated carrier wave energy with that portion of the electrical signals not removed, whereby all of the original frequency components of the band width are reproduced at the receiving point.

2. In the method of transmitting electrical signals having a Wide band of frequency components with a predetermined minimum frequency and a predetermined maximum frequency, the steps of segregating frequency components lying between the predetermined minimum frequency and a predetermined value of frequency intermediate the predetermined minimum and maximum frequency, producing carrier wave energy whose frequency is at least as great as the predetermined maximum frequency, modulating the produced carrier wave energy by the segregated frequency components, transmit-ting together over a single transmission path the modulated carrier wave energy and the entire range of electrical signals, receiving the transmitted energy and signals, separating the received modulated carrier wave energy from the received electrical signals, removing frequency components lying between the predetermined minimum and a second predetermined intermediate value of frequency from the received electrical signals, demodulating the received carrier wave energy, modifying the amplitude of the demodulated carrier wave energy as a function of frequency, and combining the modified demodulated carrier wave energy with the portion of electrical signals not removed, whereby all of the original frequency components of the band width are reproduced at the receiving point.

3. In the method of transmitting electrical signals having a wide band of frequency components with a predetermined minimum frequency and a predetermined maximum frequency, the steps of sepregating frequency components lying between the predetermined minimum frequency and a predetermined value of frequency intermediate the predetermined minimum and maximum frequency, producing carrier wave energy whose frequency is at least as great as the predetermined maximum frequency, modulating the produced carrier wave energy by the segregated frequency components, transmitting together the modulated carrier Wave energy and the entire range of electrical signals, receiving the transmitted energy and signals, separating the received modulated carrier wave energy from the received electrical signals, transmitting the separated electrical signals over two different paths'introducing electrical delay in one of said paths, attenuating frequency components above a predetermined value along the other of said two paths, combining the delayed signals and the signals attenuated, delaying the combined signals, demodulating the received carrier wave energy, and combining the demodulated carrier wave energy with that portion of the electrical signals not removed, whereby all of the original frequency components of the band width are reproduced at the receiving point.

4. A system for transmitting electrical signals having a wide band of frequency components with a predetermined minimum frequency and a predetermined maximum frequency, comprising means for segregating frequency components lying between the predetermined minlmum frequency and a predetermined value of frequency intermediate the predetermined minimum and maximum frequency, means for producing carrier wave energy whose frequency is at least as great as the predetermined maximum frequency, means for modulating the produced carrier wave energy by the segregated frequency components, means for transmitting together over a single transmission path the modulated carrier wave energy and the entire range of electrical signals, means for receiving the transmitted energy and signals, means for separating the received modulated carrier wave energy from the received electrical signals, means for removing frequency components lying between the predetermined minimum and a second predetermined intermediate value of frequency from the received electrical signals, means for demodulating the received carrier wave energy, and means for combining the demodulated carrier wave energy with that portion of the electrical signals not removed, whereby all of the original frequency components of the band width are reproduced at the receiving point.

5. A system for transmitting electrical signals having a wide band of frequency components with a predetermined minimum frequency and a predetermined maximum frequency, comprising means for segregating frequency components lying between the predetermined minimum frequency and a predetermined value of frequency intermediate the predetermined minimum and maximum frequency, means for producing carrier wave energy whose frequency is at least as great as the predetermined maximum frequency, means for modulating the produced carrier wave energy by the segregated frequency components, means for transmitting together over a single transmission path the modulated carrier wave energy and the entire range of electrical signals, means for receiving the transmitted energy and signals, means for separating the received modulated carrier wave energy from the received electrical signals, means for removing frequency components lying between the predetermined minimum and a second predetermined intermediate value of frequency from the received electrical signals, means for demodulating the received carrier wave energy, means for modifying the amplitude of the demodulated carrier wave energy as a function of frequency, and means for combining the modified demodulated carrier wave energy with the portion of electrical signals not removed, whereby all of the original frequency components of the band width are reproduced at the receiving point.

6. A system for transmitting electrical signals having a wide band of frequency components with a predetermined minimum frequency and a predetermined maximum frequency, comprising means for segregating frequency components lying between the predetermined minimum frequency and a predetermined value of frequency intermediate the predetermined minimum and 15 maximum frequency, means for producing carrier wave energy whose frequency is at least as great as the predetermined maximum frequency, means for modulating the produced carrier wave energy by the segregated frequency components, means for transmitting together the modulated carrier wave energy and the entire range of electrical signals, means for receiving the transmitted energy and signals, means for separating the received modulated carrier wave energy from the received electrical signals, means for transmitting the separated electrical signals over two difierent paths, means for introducing electrical delay in one of said paths, means for attenuatlng frequency components above a predetermined value along the other of said two paths, means for combining the delayed signals and the signals attenuated, means for delaying the combined signals, means for demodulating the received carrier wave energy, and means for combining the demodulated carrier wave energy with that portion of the electrical signals not removed, whereby all of the original frequency components of the band width are reproduced at the receiving point.

WILLIAM SPENCER PERCIVAL.