US1706538A

US1706538A - Testing television and like system

Info

Publication number: US1706538A
Application number: US200702A
Authority: US
Inventors: Mertz Pierre
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1927-06-22
Filing date: 1927-06-22
Publication date: 1929-03-26
Anticipated expiration: 1946-03-26

Description

March 26, 1929. P, MERTZ 1,706,538

TESTING TELEVISION AND LIKE SYSTEM Filed e 22 1927 T557' FREQUENCY FREQUENCY FREQUENCY FREQUENCY v /N VEN ron HEME MEA-'rz 5 Y M A Tran/VE Y Patented' vMar. 26, 1929,.

UNITED STATES PATENT OFFICE'.

PIERRE MEBTZ, OF BELLEROSE MANOR, NEW YORK, ASSIGNOB TO AMERICAN TELE- PHONE, AND TELEGRAPH COMPANY, A CORPORATION OF NEW ,YORIL TESTING TELEVISION AND LIKE SY STEM.

Application led .Tune 22, 1927. Serial No. 200,702.

This invention relates to electrical testing, I

and aims particularly to facilitate investigation ofy transmission properties of television and telephotographic systems.

In accordance with the invention a special design is transmitted over the system and the`distortion in the reproduction at the receiver noted. This gives a rapid test of the overall characteristics, especially when used with television systems. 0 For example, a design may be used which causes the signal sent to the transmission channel connecting the television transmitter and television receiver to be a sine wave of constant amplitude and of frequency constant for any line element of the design or picture but different for `different line elements. In the case in which', the design is scanned in successive straight parallel lines, the design may be a set of alternate black and White converging bands which will extend across the field. l

If there is phase distortion due to different phase delay at the various frequencies, it will be apparent in the received picture by relative displacements of given tone values or intensities along the line elements corresponding tothe different frequencies, respectively.

Amplitude"\y distortion would be apparent from varying contrast from line element to line element of the received picture, throughout the range of line elements correspondingfto different frequencies, respectively.

Other objects and aspects of the invention will be apparent from the following description and claims.

Figure 1 is a block diagram of a television system; Fig. 2 is a conventional type of representation of a design for creating desired testing currents in the system; Fig. 3 shows conventionally a character of image produced from the original design shown in Fig. 2 under conditions in the system which are' indicated by the diagram constituting Fig. 4; Fig. 5 shows conventionally a character of image produced from the original design shown in Fig. 2 underconditions in the system which are indicated by the diagram constituting Fig. 6; Fig. 7 shows conventionally another original design for creating desired testing currents inthe system; Fig. 8 is a set of curves'indicating operating conditions which maybe desired in the test; Fig. 9 shows conventionally a third original the deslgn for creating desired testing currents 1n the system; and Fig. 10 shows conventionally a character of image produced from the pattern of Fig. 9 under conditions in the system Whlch are indicated by the diagram constituting Fig. 11.

'Any desired type of picture transmission or television system may be used in the practice of, the invention. Fig. 1 indicates a television system which ymay be so used. Preferred forms of television systems are disclosed 1n lthe copending applications of Frank Gray, SerialNo. 181,538, filed April 6, 1927, and J. WV. Horton, Serial No. 181,- 5170., filed April 6, 1927. In Fig. 1 the televlslon transmitting apparatus 1 at a sending statlon and the television receiving apparatus 2 at the receiving station are connected bya wire or radio transmission channel 3. All of these parts of the system may be, for example, such as those disclosed in the Gray appllcation just mentioned.

To quickly test overall transmission characteristics of the system a picture may be transmitted over the system which will give, for the signal transmitted to the line, a sinusoidal current of constant amplitude and of a frequency which is const-ant for any line element of the picture scanned but changes slightly as each new line element across the picture is scanned. Th'e picture may be, for example, of the general type conventionally indicated in Fig. 2, wherein the upwardly l pointing arrow indicates the direction of scanning. In the figure, the changes from black to white are sudden; but it is to be understood that in the test design' actually transmitted, the changes are gradual, and such as to give a pure sine wave and not the square wave which would result if the changes from black to white were sudden. This statement holds for all of the test designs described herein. f

If there is overall phase distortion in the system due to different phase delay at the various frequencies, at the receiving end of the system the successive line elements of the image will not line up in phase, but "will be out of phase, for example, as indicated in Fig. 3 wherein the frequencies near the middle of the frequency range are retarded along the time axis, relative to the higher and lower frequencies. Ther deviation at each frequency will be proportional to the phase delay B/w at that frequency. lThe i quantity w is 21rtimes the frequency in cyrange of frequencies is to be kept, the inter-l the image.

cles per second, and the quantity B is the phase change, in radians of angle, experienced byl the sinusoidal signal wave due to transmission through the system.

yThe phase delay can be plotted roughly against frequency from the appearance of This has been done 4in Fig. 4, wherein the graph 15 corresponds with Fig. 3. If thevphase delay were uniform for all of the frequencies represented by the various line elements of the` picture, the graph 15 would be a horizontal strai ht line.

If there is amplitude distortion, the successive line elements of the picture will show a varying contrast throughout the range of frequencies, for example, as indicated in Fig. 5, the contrast ybeing greatest for those frequencies attenuated the least. The variation in contrast has been conventionally shown by widening the black lines to indicate increased contrast, and narrowing them toindicate decreased contrast. The attenuation-frequency curve 16 corresponding to Fig. 5 is roughly shown in.Fig. 6. The attenuation test is useful for showing the general trend of the attenuation throughout the frequency range, and is sensitive to Shar irregularities in'the attenuation.

With regard to phase delay measurements as described above in connection with Figs. 3 and 4:, sharp deviations (that is, within a short frequency range) in the phase delay are easily observed, but gradual deviations over a large frequency range are more difficult to see.

The method of the delay test may be modilied, with resulting increase of accuracy, by using a design such as that conventionally indicated in Fi .I 7 insteadl'of the design of Fig. 2. The direction of scanning is the same as in Fig. 2. The design of Fig. 7 is such that in transmitting the sine waves a standard comparison frequency is alternated eachtime with the testing frequency. With -this design a gradual deviation in thedelay may be more easily detected. If the same val between successive frequencies can be maintained small by increasingthe size ,of the design.

The succession in frequencies` can be so taken that the-length of a linegelement of the picture corresponds to an integral- 'number of cycles, thus avoiding any discontinuity in going from one line elementto the next,

' except when the frequency .changeoccurs 'Inhis eliminates the obscuring. effect` of the transients which would'be caused by these discontinuities.

The accuracyof the .delay tests with the designs ofFigs. 2 and 7, can be increased by using, in'the receiving .apparatus 2, nonlinear transmission characteristics, for ex-fT ample, in the vacuum tubes, magnetic cir- '.'sanie relative points cuits, or receiving lamp. Referring to Fig. 8, such non-linear characteristics can be made to change a sinusoidal variation such of variation indicated by curve 22 can bel obtained, for example, by employing an amplifier which amplifies high instantaneous magnitudes of signals more than the lower instantaneous magnitudes. of variation indicated by curve 21 can be` obtained, for example, by an amplifier or network which has higher transmission efliciency for low amplitudes of signals than for the higher amplitudes. If the elements with the non-linear transmission characteristics are employed, introduction of additional phase distortion through these elements should be guarded against.

To test envelope delay (dB/das) instead of phase delay (B/w), still another pattern, such, for example, as that conventionally indicated in Fig. 9, can be employed. The

in Fig. 5, variation in contrast is indicated by widening the black lines to indicate increased contrast, and narrowing them to indicate decreased contrast. With this design the current sent out on the line will not be of constant amplitude,l but will be modulated. That is, each line element of the picture generates a wave of constant frequency and varying amplitude, the frequency varying slightly from line element to line element., and the envelopes of all-of the Waves having the same frequency. The waves, as generated,-have their greatest amplitudes correspond to the same relative points (namely the center points) in the line elements which produce the waves, respectively; and the waves, as generated, have their least amplitudes correspond to the (namely, the end points) in thei. line elements which produce the waves respectively. At the receiving end of the system, in the imago, which may have some such appearance as conventionally indicated by Fig. 10, for example, the maxima or minima of the modulations will trace out the envelope' delay versus frequency curve, as indicated by the

dotted line graphs

31 and 32, respectively, in Fig. 10. The envelope delay versus frequency curve is drawn as a full line curve 30 in Fig. 11. This curve indicates the relative times at which the envelopes of the variations, at the receiving end of the system, due to the various line elements of the 'picture in Fig.

The form 9 reach their maximum and minimum values.

In general, the accuracy of the envelope delay test is less than theaccuracy of the phase delay tests. However, in those cases where the phase delay varies a small amount in a very narrow frequency interval, the irregularity in the envelope delaywill be much more prominent than in the phase delay. In such cases, though the accuracy of the envelope delay test would be less, the sensitivity in detecting the irregularity would he greater.

lVhat is claimed is:

y 1. The method of operating a wave transmission system which comprises transmitting through the system Waves which differ from each other in a known regular manner as regards a. characteristic common to all of them, and producing a figure from the transmitted waves Which-is representative of them.

2. The method of operating an electrooptical system which comprises transn'iitting through the system waves of different frequencies, respectively, and producing from vthe waves transmitted a design from which frequency characteristics ofthe system are apparent by inspection. a

3. The method of determining the relative effects which a Wave transmission system produces upon a given characteristic of waves which are respectively different as regards another given characteristic, which method comprises transmitting through the system, in rapid succession, Waves which differ in a known regular manner as regards said second characteristic,A and producing a design from the transmitted Waves.v

4'. The method of determining overall characteristics of an electro-optical system at various frequencies Which comprises transmitting through the system Waves of different frequenciesl respectively, in rapid succession, and producing from the Waves transmitted a design indicative `of the relative effects of the transmission'upon t-he waves of different frequencies, respectively.

5. The method lof determining frequency characteristics of a television system, which comprises transmitting through thesystem, in rapid succession, waves the respective frequencies of which differ in a known regular manner, and producing from the transmitted vwaves a design from Which frequency characteristics of the system are apparent by inspection.

6. The method vof operating an electrooptical system Which comprises viewing through the system a pattern which generates sine waves of constant amplitude and systematically changing frequency. 4

7. The .method of operating an electrooptical system which comprises transmitting through the system a picture containing line elements that generate sine waves which are all of the same amplitude but the frequency 4of which changes from element to element so .that the frequency change is gradual.

8.v The methodv of operating a picture transmission system which comprises successively transmitting through the system parallel adjacent line ele-ments of a picture, each of which generates an integral number of cycles of a wave thatl has a different fre quency in the ease of each line element.

9. The method of operating a picture transmission system including a sending station, a receiving station and a wave transmission channel for transmission between said stations, Which method comprises transmitting from said sending station through said transmission channel sinusoidal variations of constant amplitude but of changing frequency, changing the sinusoidal variations into variations of a relatively peaked Wave form, and producing an image from the latter variations.

. 10. The method of testing a picture transmission system which comprises successively transmitting through the system line ele- PIERRE MERTZ.