US 2203758 A
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June 11, 1940. WALKER I 2,203,758
TELEVISION SYSTEM Filed March 11, 1939 2 Sheets-Sheet FREQUENCY FREQUENCY FREQUENCY I l I I FREQuE/vc? INVENTOR L.E. Q. WALKER ATTORNEY Patented June 11, 1940 PATENT OFFICE TELEVISION SYSTEM Louis Edward Quintrell Walker, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application March 11,
1939, Serial No. 261,260
In Great Britain March 14, 1938 3 Claims.
This invention relates to television systems and more particularly to television and accompanying sound systems of the kind wherein video signals representative of a vision programme and sound signals, representing accompanying sound, are transmitted as: modulations of two different carrier frequencies. For example, in present television systems, two adjacent short wave lengths are employed, one as the carrier of each transmitting video signals and the other as the carrier of accompanying sound signals, sothat, in effect, installation comprises two transmitterscne operating on each Wave length, and each cooperating receiving installation comprises two receivers, one operating on each length. i q l One of the most difiicult problems met with, both in television transmitting and television reception, is that of satisfactorily and substantially uniformly handling and amplifying the video signals, the difficulty arising by reason of the wide frequency band occupied-by the video signals in modern high fidelity systems. For example, in known transmitters and receivers the video signal amplifiers, both at the transmitter and receiver, are required to give reasonably uniform amplification over a frequency range extending up to (very approximately) 3 megacyclesan obviously difficult requirement. hand the sound signal amplifiers are required to give substantially uniform amplification only up to about 10,000 cycles persecond--an easy requirement.
The present invention seeks to reduce the above difliculties and this object is achieved, in
, a television and accompanying sound system making use of two carrier frequencies, by dividing the video signal band into two sub-bands and transmitting the lower sub-band only on one carrier frequency, the other sub-band being shifted downwards in the frequency spectrum by, an amount substantially equal to its own lowest frequency and transmitted on the other carrier. The sound signal band is also transmitted on one of these two carriers.
A preferred form of television and sound transmitter in accordance with the present invention comprises means for sub-dividing the total video signal band into two equal or approximately equal sub-bands, a lower sub-band and an upper sub-band, heterodyne ,means for shifting the upper half sub-band so that it extends from zero up to a maximum frequency equal to approximately half the largest frequency in thetotal video band, means for eliminating On the other from the shifted sub-band a range extending from zero up to the maximum sound frequency required to be transmitted so as to produce a resultant limited shifted sub-band, means for modulating a second carrier frequency with the limited shifted sub-band and also with the sound signal band, and means for transmitting the two modulated carrier frequencies.
A preferred form of television receiver in accordance with the invention is the counterpart of the preferred form of. transmitter just described; i. e., the lower sub-band obtained by demodulating one carrier is applied to the vision reproducer (usually a cathode ray tube) while the shifted upper side band obtained by demodulating the second carrier is heterodyned back to its original position in the frequency spectrum, and, after such shifting back, is also applied to the vision reproducer, the sound signals, which are also obtained by demodulating the second carrier being separated out by suitable filter means and utilized as in the ordinary way for sound reproduction.
Thus the invention [considerably simplifies the problem of design of and substantially cheapens the cost of the video signal amplifiers both at transmitter and receiver by, in effect, sharing the video signal band between the two operating wave lengths the second of which is already provided for carrying the sound signals.
Consider the case of a television and accompanying sound installation required to transmit video signals extending from 0 to n cycles per second and sound signals extending from 0 to m cycles per second. In practice m isvery small compared to n. The video band is divided by any convenient known filter means into two subbands 0 to 11/2 and n/Z to n. One carrier frequency (f) is modulated with the lower sub-band 0 to n/2 and transmitted, the resultant modulated carrier extending from f'n/2 to f+n/2. The upper video sub-band 71/2 to n is: heterodyned with a frequency n/2 to shift it to 0 to IL/2 and the small band of frequencies 0 to m is abstracted therefrom. A second carrier frequency f is modulated with the shifted limited video sub-band m to 11/2 and also with the sound signals in the band 0 to 112 so that the resultant modulated carrier extends from jn/2 to f+n/2, the small central portion f'm to f+m of this band corresponding to sound signals.
The abstraction of a small band of frequencies from the shifted upper video sub-band to ac-" commodate the sound signals, does, of course, involve someloss of video signals but, in practice,
n/2 is so large compared to n that this has negligible effect upon the final result. Moreover, it is possible so to arrange matters that the abstracted small band is one of the idle regions (i. e. the regions of very small energy content) in the video Signal spectrum. This expedient may, however, involve that the two sub-bands into which the total video band is divided, are not exactly equal. The invention is, however, not limited tomaking the sub-bands equal, though, other considerations apart, equality is-to be preferred. 7
A cooperating receiver for a transmitter as above described is the counterpart of the said transmitter; the appropriate video sub-band is shifted back to its original position in the frequency spectrum and applied together with the other video sub-band to the vision reproducer apparatus. The sound signals, separated out from the demodulation signals derived from the appropriate demodulator, are applied to the sound reproducing apparatus.
The invention is illustrated in and further explained in connection with the accompanying graphical and diagrammatic drawings wherein:
Figures 1-4 show curves of band widths to facilitate in explaining the invention, and
Figures 5 and 6 show schematic diagrams of a television. transmitting and receiving system, respectively.
In the drawings Figure 1 represents in conventional manner the total frequency spectrum of a television signal extending fro-m 0 to n cycles per second and Figure 2 shows how the said spectrum is to be split into two bands, one extending from 0 to n/Z cycles per second and the other extending from 12/2 to n cycles per second. Figure 3 represents in conventional manner the carrier and the two side bands obtained by modulating one carrier j (the vision carrier) with the band 0 to 71/2 of Figure 2, the whole modulated range thus extending from j-n/Z to f-l-n/Z. Figure 4 represents in conventional manner the second carrier f (the sound carrier) modulated with a band 0 to n/Z obtained by heterodyning the upper band n/2 to n of Figure 2 with a frequency n/2 the modulated vision range obtained extending from j-n/2 to f+n/2 with the exception of the central range f'7TL to j'+m, this central range being removed to accommodate the sound signal modulated range.
Figure 5 shows in block diagram form a transmitted in accordance with the invention. In Figure 5 rectangle 1 represents a source of vision signals extending from 0 to n cycles per. second. Output from I is fed in parallel to two filters 2, 3, the former passing the range n/2 +m to n and the latter passing the range 0 to 11/2. 4 represents a heterodyne of frequency n/2 which heterodynes the output from 2 to produce the range m to n/Z, which range is employed at 5 to modulate a carrier for transmission from the aerial 6. The sound signals in the range 0 to m are derived from a source '1 and are also employed at 5 to modulate the carrier f for transmission from the aerial ii. The output from filter 3 is used to modulate the carrier 1 for transmission from the aerial 9.
Figure 6 represents in block diagram form a receiver in accordance with this invention. Signals picked up upon a receiving aerial 9 arefed to a receiver proper 3' for the carrier 1 the demodulated output of this receiver operating, in any well known convenient Way, a cathode ray tube; vision reproducer I 'over the range from 0 to n/2. Signals picked up on a second aerial B" are fed to a second receiver proper 5' for the carrier f and the demodulated output from 5 is divided into two ranges 0 to m and m to n/2 by two filters l (l and 3 respectively, the former passing the range 0 to m and the latter the range m to n/Z. The output from 3 is heterodyned at 4 by a frequency 11/2 to produce therange m.+n/2 to n, this range being passed to the tube l. The sound signals passed by the filter ID are fed direct to the loudspeaker 1.
It is to be understood that the filters to be used in this invention (although shown schematically) may be constructed in any manner such as shown, for example, in the article by Otto J. Zobel, entitled Distortion Correction in Elec trical Circuits with Constant Resistance Recurrent Networks to be found in the Bell System Technical Journal for July, 1928, page 438 et seq.
Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed I declare that what I claim is:,
1. A television picture and sound transmitter comprising means for sub-dividing the total picture signal band into two approximately equal sub-bands, a lower sub-band and an upper subband, a source of oscillations of a constant frequency substantially equal to the lowest frequency in the upper sub-band, heterodyne means including said source of oscillations for shifting the upper sub-band so that it extends from substantially zero up to a maximum frequency equal to approximately one-half the largest frequency in the total picture signal band, means for eliminating from the shifted sub-band a range of frequency extending from zero up to the maximum sound frequency required to be transmitted was to produce a resultant limited shifted sub-band, two separate carrier frequencies, means for modulating one of said carrier frequencies with the lower sub-band, means for modulating the other of said carrier frequencies with the limited and shifted upper sub-band, and also with the sound signal band, and means for transmitting the two modulated carrier frequencies.
2. A system as defined in claim 1 which. includes also a television picture and sound receiver comprising means for demodulating said one carrier to produce the lower sub-band of picture signals, means for demodulating theiother'carrier to produce the sound signal band in the shifted upper sub-band of picture signals, high and low pass filter means for separating the sound signals' from the shifted picture signals, a source'of oscillations of a constant frequency substantially equal to the highest frequency occurring in the lower sub-band, heterodyne means including said source of oscillations for shifting the upper subband to its proper frequency spectrum position, means for applying both the picture sub-bands to a picture reproducing means and .means for applying the sound signals to a translating device.
3. A television picture and'sound transmitter comprising means for dividing .the picture signal band into two approximately equal sub-bands, a lower sub-band and an upper sub-band, heter odyne means including a source of oscillations substantially equal to the lowest frequency in the upper sub-band for shifting the upper sub-band whereby its frequency range varies from substantially'zero to a maximum frequency substantially equal to approximately one-half'the highest fre-f quency present in the original picture signal band,
means'for'suppressing fromthe shifted sub-band 75 all frequencies from substantially zero to approx imately the maximum sound frequency existing in the sound spectrum whereby a limited and shifted sub-band will result, two separate anddisplaced carrier frequenciesmeans for modulating one of said carrier frequencies with the lower subband, means for modulating the other of said carrier frequencies with the limited and shifted upl per sub-band, and also with the sound signal band, and means for transmitting the two modulated carrier frequencies. i
LOUIS EDWARD QUINTRELL WALKER.