US2146804A - Television system - Google Patents

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Feb. M, i939. H. M. DOWSETT ET AL Filed Dec. 6, 1934 Patented Feb. 14, 1939 UNITED STATES TELEVISION SYSTEM Harry Melville Dowsett and Louis Edward Quintrell Walker, London, England, assignors to Radio Corporation of America, a corporation of Delaware Application December 6, 1934, Serial No. 756,293 In Great Britain December 6, 1933 12 Claims.

Thisl invention relates to television systems and has for its object to provide an improved television system wherein the width of the frequency band transmitted may be substantially reduced ,-5 with the aid of cheaper and simpler apparatus tributed over this wide band but is, to a large extent, concentrated at frequency bands occurring at and near the scanning line frequency and harmonics thereof. For example, to take the case of a television transmitter wherein fifteen pictures are transmitted per second and each picture scanned with fifty lines, the majority of the energy in the television signals will occur in comparatively narrow bands at and near the scanning line frequency of 750 cycles per second and harmonics thereof, the energy content becoming practically negligible at 15,000 cycles per second. Satisfactory television transmission can be obtained, accordingly, if all that is transmitted is signal energy from these sub-bands i. e. if merely the scanning line frequency sub-band energy and energy from a number of the harmonic sub-bands is transmitted. Practically speaking the whole television signal will have been sent if energies from the sub-bands up to and including the 20th harmonic sub-band (that about 15,000 cycles per second if 750 cycles per second is the scanning line frequency) are sent; that is to say, it is unnecessary to consider energies of about the 20th harmonic in frequency and in some cases, where a high degree of delity is not required, one can stop short at lower harmonic frequency sub- 40 bands, for example the 10th or 15th harmonic sub-band. 1

Numerous proposals have been made to utilize the phenomenon of concentration of television signal energy in bands about the scan line frequency and harmonics thereof to reduce the overall band width necessary for transmission; for example, it has been proposed to divide the signals resulting from scanning the subject of transmis- Vsion into a plurality of sub-bands or components occurring round the scanning line frequency and harmonics thereof and to heterodyne each such band or component with a distinct injected locally generated signal of predetermined frequency so that, as a result of heterodyning, the bands of 55 concentrated energy, which occur at relatively widely spaced intervals in the frequency spectrum of the original television signal, are brought more closely together and the frequency band for transmission thereby reduced. It will be appreciated, however, that systems such as those just described have the serious practical objection that somewhat complicated and expensive filter and heterodyne apparatus is necessary and Vthe principal object of the present invention is to provide an improved system which enables narrowing of the transmission band to be obtained on the known general principles above outlined without the use of complex or expensive lter and heterodyne apparatus.

Regarded in its broadest aspect the invention consists in transforming a band of television signals derived from scanning a subject for transmission into a lower frequency band of narrower extent by a method which includes heterodyning said band of signals with heterodyne energy comprising fundamental and harmonic frequency energy, said fundamental frequency being relatively close to the fundamental scanning line frequency.

In a preferred method in accordance with the invention television signals derived from scanning a subject of transmission are heterodyned with heterodyne energy of a fundamental frequency=(1-lc)w (where w is the scanning'line frquency and lc is less than the reciprocal of the numerical designation of the harmonic about which the highest frequency sub-band required- Vto be considered occurs), the energy resulting from heterodyningbeing applied to a 'squarelaw or approximately square law rectifier, and the narrowy lower band of frequency resulting from rectification being transmitted. Thus, if the subbands to be considered are that occurring round the scanning line frequency, and that occurring round the harmonics thereof up to the 20th harmonic Ic would be chosen less than 1/20 and the fundamental frequency' of the heterodyne oscillator would be some frequency between .95o and w.

Referring now to the drawing in which Fig. 1 shows schematically in block diagram form a television transmitter embodying the in- Vention; i

Fig. 2 shows ceiver also embodying the invention; and

Fig. 3 shows a graph of the relation between amplitude and frequency of the signals for use in explaining the invention; theinvention will be described in detail. y

Y In one way of carrying out the invention a subject for transmission is scanned 11o-produce telein similar form a television revision signals in any well known Way e. g. by means of a synchronous motor driven scanning apparatus-for example a Nipkow disk I--and associated photoelectric cell 3. The energy in these television signals will be concentrated in bands occurring round the scanning line frequency and harmonics thereof. Let the television signal at any instant be represented by the series:

(w7f) =A1 sin wt-I-Az sin ZwI-l-As Sin Swt-l- (1) Where w is the scan line frequency and t represents time. This signal is heterodyned by any of means 1 Well known in the art with a locally generated oscillation from the generator 5, of the form:

where w' is the fundamental frequency.

the resultant or combined wave fuit) |-,(wt) may be written:

This resultant wave is rectified or demodulated by any convenient known device 9 of square law demodulation characteristics.

Let the relation'between output current and input voltage of the rectifier be of the form:

i(output current) :au-i-aie-i-azez ThisV signal is that which is, in carrying out the invention, transmitted for example by being employed to modulate a radio transmitter l I.

Comparing (l) with (4) it will at once be seen that the original signal band (1) of width has been transformed into a lower frequency narrower band of width the individual amplitudes in the original band 'having been squared. If

In@ 21r is mad'e less than the bands will not overlap and for this, of course,

nk must be less than unity, that is to say k must be less than The local oscillations from the generator 5, that is to say, the injected signal (2) can be obtained in any convenient manner, for example by means of a multi-vibrator, but it is preferred, in the usual case where a synchronous motor is employed for driving a scanning disk I or equivalent scanning mechanism, to provide on said motor a subsidiary Winding giving an impure Wave of a fundamental frequency w this subsidiary winding being in addition to the usual winding provided for synchronization purpose and being of frequency w.

It will be now appreciated that the result achieved is that substantially the whole of the useful part of the energy in the original television signal band is reduced to a band extending only from a low frequency (for example about 50 cycles `per second) to a frequency below the scan line frequency (for example 750 cycles per second) and thus great compression of the frequency spectrum is obtained without the use of any filters for dividing thetelevision signals into sub-bands and without the use of any expensive or complicated heterodyne apparatus.

At the receiving end of the system, frequency transposition is effected in a manner analogous to that effected at the transmitter. More specifically, following the lines of the mathematical statement already given, a local oscillation from the generator 23 of form:

July 1928 at page 438 et seq. The resultant Wave may be shown to be of the form:

This wave is passed through a square root law detector 2|. Let the output current (i') input voltage (e) relation of this detector be of the form:

Then the output signal obtained at output loads 25 is of the form:

Signal (7) is obviously signal (1) (the'original signal) multiplied throughout by V55 with a constant term bn added, and with a series vzfnsin(bmah/52.425111(l-mzwH- added.Y The terms azAa sin (l-kmwt can be abstracted and the original signal thus obtained utilized for picture reproduction in any l)'Well known' Way. K Y A In Ythe claims which follow the mathematical symbols have, of course, the meanings they have as employed in the body of the specification.

Having thus described the invention, what is claimed is:

1. In a television transmitter wherein electrical energy of line scanning frequency and harmonics of a plurality of orders is produced, the steps in the method of transforming a band of television signals derived from scanning a subject for transmission into a lower and narrower frequency band comprising producing independently electrical wave energy comprising fundamental and harmonic frequency components, said fundamental frequency being equal to l-lc times said produced line scanning frequency where 7c is less than the reciprocal of the highest order of the produced line scanning harmonics, and heterodyning said band of signals with said independently produced energy.

2. The method claimed in claim 1 which includes the additional steps of demodulating the resultant heterodyned energy, producing thereby a plurality of bands of frequencies, and transmitting only one of said produced bands of frequencies.

3. The method claimed in claim l which includes the additional steps of demodulating the resultant heterodyned energy, producing thereby a plurality of bands of frequencies, and transmitting only the lowest of said produced bands of frequencies.

4. The method claimed in claim l wherein the difference between the fundamental heterodyning frequency and the line scanning frequency is less than 5% of the line scanning frequency.

5. A television receiver for operating from received signals from a television transmitter wherein electrical wave energy of line scanning frequency is produced comprising a local oscillator for producing wave energy having fundamental and harmonic frequency components, said fundamental frequency being only less than 5% different from twice the said line scanning frequency means to combine energy from the local oscillator with said received signals in predetermined phase relation, means to detect the combined energy and signals, and means to derive detected energy from the detecting means.

6. A television receiver, for operating from received signals from a transmitter wherein the signalling energy is transformed to a lower and narrower band of frequencies than the derived band of television signals from the scanning of a subject comprising a local oscillator for producing wave energy, said produced energy having fundamental and harmonic components, the frequency of said fundamental component being only less than 5% different from twice the frequency of the highest frequency in the said transformed frequency band, means for adding the locally produced energy out-of-phase with the received signal energy, means for demodulating the said added energies, and means for utilizing the resultant demodulated energies.

'7. A television transmission system comprising means. to convert a light image to electrical signals, means to generate wave energy of fundamental frequency and a plurality of harmonics thereof, means to combine the wave energy with the electrical signals, means to detect the combined energy, means to modulate carrier wave energy by a portion of the detected combined signals and energy, and means to transmit the modulated carrier wave energy.

8. A television system comprisingmeans to convert a light image to electrical signals, a rst means to generate wave energy of fundamental frequency and a plurality of harmonics thereof, means to combine the wave energy with the electrical signals, means to detect the combined energy, means to modulate carrier wave energy by a portion of the detected combined signals and energy, means to transmit the modulated carrier wave energy,'means to receive the transmitted modulated carrier wave energy, means to generate a second Wave energy of fundamental frequency and a plurality of harmonics thereof, means to combine the received modulated carrier wave energy with the energy from the second generating means in predetermined phase relation, means to detect the combined energies, and means to derive a portion of the detected energy.

9. A television system comprising means to convert a light image to electrical signals, a first means to generate wave energy of fundamental frequency and a plurality of harmonics thereof, means to combine the wave energy with the electrical signals, means to detect the combined energy, means to modulate carrier wave energy by a portion of the detected combined signals and energy, means to transmit the modulated carrier wave energy, means to receive the transmitted modulated carrier wave energy, means to generate a second wave energy of fundamental frequency and a plurality of harmonics thereof, means to combine the received modulated carrier wave energy with the energy from the second generating means in predetermined phase relation, means to detect the combined energies, and means to derive from the detected energy energy representative of the electrical signals.

10. A television receiver comprising means to receive transmitted signals, means to generate signals having a fundamental frequency and a plurality of harmonics thereof, means to combine the received signals with the generated signals in predetermined phase relation, a square root law detecting means for detecting the combined signals, and means to derive from the detecting means energy proportional to only one of the components of the received signals.

11. In a television transmitter wherein electrical energy of line scanning frequency and harmonics of a plurality of orders is produced, the steps in the method of transforming a band of television signals derived from scanning a subject for transmission into a lower and narrower frequency band comprising producing independently electrical wave energy comprising fundamental and harmonic frequency components, said fundamental frequency being equal to l-lc times said produced line scanning frequency Where 7c is less than 'the reciprocal of the highest order of the produced line scanning harmonics, heterodyning said band of signals with said independently produced energy, and transmitting the entire reduced band of signals over a single channel.

l2. A television receiver comprising means to receive transmitted signals, means to generate signals having a fundamental frequency and a plurality of harmonics thereof, means to combine the received signals with the generated signals in degrees out of phase relation, a squarel root law detecting means for detecting the combined signals, and means to derive from the detecting means energy proportional to only one of the components of the received signals.

HARRY MELVILLE DOWSETT. LOUIS EDWARD QUINTRELL WALKER.

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