RESEARCH DEPARTMENT CO=CHANNEL 1NTERFERENCE BETWEEN TELEV 1S I ON SIGNALS OF THE SAME OR 01 FFERENT STAN'DAROS Report Moo T=08~ (i... /U) THE BRIT.ISH BROADCASTING CORPORATION ENGINEERING DIVISION RESEARCH DEPARTMENT CO - CHANNEL INTERFERENCE BETWEEN TEL~VISION SIGNALS OF THE SAME OR DIFFERENT STANDARDS ( 1982/ U ) G, Fe Newel1, A, t·L L L Eo LW , Taylof1". 1.10 A" ( D, Mau rice) This Report is the propert1 ot the British BroadcastlD& Corporation and aay Dot be reproduced in any form without the written perm18s1on ot the Corporation. Report No" T-084 CO~CHANNEL INTERFERENCE BETWEEN TELEVI SION SIGNALS OF THE SAME OR DIFFERENT STANDARDS Section Title Page SUMMARY 0 1 INTRODUCTION 1 THE EXPERIMENTAL PROCEDURE c 2 201 The Test Conditions 0 2 202 Description of Apparatus 3 203 The Subjective Criterion 5 MEASUREMENTS AND RESULTS, 6 301 Co- Channel Interference with Large Offset 6 302 Co- Channel Interference with Small Offsets 10 303 Precision Offset" " 0 G " "" 0' 0 0 0 0 14 304 The Effect of Movement in the Wanted Picture 18 305 Comparison between the 625- line and the 525-line Standards 0 19 SUMMARY OF RESULTS AND CONCLUSIONS 19 REFERENCES 0 21 May 1962 Report No , T-084 ( 1982/14) CO ~ CHANNEL INTERFERENCE BETWEEN TELEVISION SIGNALS OF THE SAME OR DIFFERENT STANDARDS SUMMARY This report describes measurements of the ratios of the amplitudes of wanted and interferin~ si~als which produce a particular subjective ~rade of interference, The measurements were made in order to ascertain the required protection ratios for mutual interference between two stations each operatin~ on any of the three television standards at present in use in Western Europe; 405 lines, 625 lines and. 819 lines all with 50 fields per second, The results show that? if the frequency difference between the wanted and interferin~ carriers can be "maintained with a stability of less than 2 cls (precision offset), each of the three wanted signal standards require the same protection, In these conditions there is an advanta~e of up to 6 dB in havin~ negative modulation for the wanted signal, If the offset freque~cy cannot be maintained to within a few cycles per second, the required protection is smaller for the systems with the greater number of lines per field, A comparison between 525 lines, 60 fields per second and 625 lines, 50 fields per second is included, L INTRODUCTION Many workers have published the results of measurements of the relative amplitudes of a wanted television signal and an interfering signal for a given subjective grade of interference, Most of these measurements have been concerned with interfering signals which were either unmodulated, modulated with tone or modulated with a television si~al having the same line- and field-frequency as the wanted signal, The measurements to be described here were carried out in order to examine more carefully the effect of the modulation carried by either the wanted or the interfering si~al ; to answer, for example the questions : (a) What is the effect of the polarity of the modulation of either signal? (b) What is the effect of the number of lines per field? (c) What difference does it make if the two field frequencies are not exactly identical? 2 2. THE EXPERIMENTAL PROCEDURE 2.1 The Test Conditions The subjeotive assessments were made by sixty observers, men and women, who were not direotly oonoerned with work in the television laboratories. These observers each made assessments for a period of about one half-hour on about twelve oooasions - not more frequently than onoe in anyone week. Most of the measurements were made with the wanted "programme" oonsisting of a still pioture obtained from a 3! by 3! in (82·6 x 82·6 mm) flying-spot slide­ soanner. Fig. 1 shows the pioture used. A few measurements were m&de with the wanted "programme" oonsisting of motion piotures obtained from a. 35 mm flying-spot teleoine maohine, so that the effeot of movement in the wanted pioture oould be assessed. 'l'he observers were seated in groups of five a.t ~ time in front of a reoeiver and distant from it between four and six times the pioture height. All the receivers had pioture tubes with a. 21 in (53 om) dia.gonal and were adjusted to have a peak brightness of about fifteen foot-lamberts with an ambient illumination of about one Fig. I ~ Still picture used for tests 8 half foot-lambert, The observers were asked to assess the subjective grade of interference presented to them, using a six-point scale, which will be described in Section 2,8, The assessment.s were individual, no co-opera.tion or discussion of the effects being permitted during the test period, Each observer wrote his or her assessment a.gainst the appropriate test number on a form provided for this purpose, No instruction was given to the observers to assume that the interference was present for only part of the time; they were presented with the interference condition for about a half minute and were asked to assess its subjective effect, In order to restrict the number of measurements made in this manner, some preliminary tests of an exploratory nature were carried out. These were made by six experienced observers each operating a two-position sm tch which was held in the hand and used to indicate whether the assessment was favou.rable or unfavourable. By means of a meter the test supervisor was able to observe the proportion of favourable assessments. By adjustment of the interference level he could quiokly find the ratio of signal ampli tude.s corresponding to the median assessment. This procedure helped to restrict the range of interference conditions over which the larger-scale tests had to be made. ' A small number of tests were made in which two picture monitors wer~ used and the observers were asked to indicate when the different interferenoe conditions shown on the two monitors were equally visible. These tests were carried out in order to provide some cross-checks on the results of the main series of tests. The assessments were made with the wanted signal unaccompanied by a sound carrier although such a carrier was sometimes used in order to assist the correct tuning of the receivers, The amplitude of the wanted signal was adjusted to produce a fringe-area signal-to-noise ratio in the displayed picture, The ampli iiude of the d o a , P4 composite signal was greater than that of the r om. s . noise in the video stages by 27 dB for a 405-line receiver, 29 dB for a 625-line receiver and 81 dB for an 819-line receiver. A number of tests were also made with high sign.al-to-noise ratios. It was found that noise tended to mask the interference pattern. The extent of this masking was such that the amplitude of the interfering signal could be SOme 2 dB greater when the noise was presen t o 2. 2 Description of Apparatus The picture sources available included: (a) A 405-line 35 mm flying-spot film-scanner, (b) A 405-line 3~ x 3~ in [82' 6 x 82' 6 mm i flying- spot slide-scanner, (c) A 625-1ine caption-scanner, (d) A multi-standards flying- spot scanner for either 35 mm film or 3~ x 3~ in {82 ' 6 x 82 "' 6 mm ) slides" This scanner could operate on any of the required standardsc (e) A 405-line monoscope channel producing a Test Card "C" waveform. 4 Measurements oould be made with anyone of the three standards as the wanted sigaal a.nd anyone of them as the interfering signal with the exoeption of an 8i9-line signal interfering with an 8i9-line signal. In a.ddi tion, two camera channels were available for a short time. These were 4~ in (114 mm) image orthicon channels? one operating on one of the European standards (625 lines? 50 fields per second) and the other on the U,S. standard (525 lines? 60 fields per second). A comparison between these standards was made . The receivers were: (a) Two similar? good domestic receivers produced in the U. K. for the 405-line system, (b) Two similar? good domestic receivers produced in Holland for the 625-line system. (c) Two similar? good domestic receivers produced in France for the 819-line system. These receivers were ~ of course? all of the vestigial-sideband type? but the modulated signals used in the tests were double-si deb and., Some of the tests were repeated with vestigial-sideband filters included in the modulator output circuits, in order to check the effect of the unwanted sidebandso No difference was observed. The 405-line receivers were capable of operating with either flywheel or hard-lock line synchronization. Comparison of the two methods of synchronization showed that? when a low-frequency component was present in the video interference beat-note, the flywheel synchronizing circuit increased the susceptibility to inter­ ference. On the other hand? when the frequencies involved were too high for the time-constant circuit to follow, the flywheel operation proved an advantage. The differences in the protection ratios required with the two circuits is never greater than 2 dB, and is felt to be negligible in general effect. Fig. 2 shows the general circuit arrangement. The carrier of the wanted signal was generated by a crystal oscillator and could be controlled in frequency over ± 100 c/s by means of an a. f . c , circuito This enabled the offset (ioe o the frequency difference between the two carriers) to be locked accurately to a sub-harmonic of the line-scan frequency or to a low-frequency oscillator which could be easily and accurately adjusted over a small range of frequencies.