1、= 3404583 00bOb49 383 - * EN 60756 Januay 1W UDC 621.397.037.732 Descriptors: Stability, time base, magnetic videorecording . b English version Non-broadcast video tape recorders Ifme base stability (IEC 756 : 1991) Magntmcopes utiliss hors de la radiodiffmion - Videobandgere ftir den Gebrauch auBer
2、hab des Stabilit de base de temps Rundfunks - zeitbasisstabilitat (CE1 756 : 1991) (IEC 756 : 1991) This European Standard was approved by CENELEC on 1992-12-09. CENELEC membeB are bound to comply with the CENCENELEC intemai Regulations which stipuiate the conditions for giving this European Standar
3、d the status of a nationai standard without any alkraion. Up-to-date liste and bibliogmphicai references concerning such nationai standards may be obtained on application to the Centrai Secretariat or to any CENELEC member. This European Standard exists in three officiai versions (English, French, G
4、erman). A version in any other language made by transkition under the responsibility of a CENELEC member into its own language and notjfed to the Central secretariat has the same status as the officiai versions. CENELEC members are the national electrotechnical committees of Austria, Belgium, Denmar
5、k, FTnland, France, Germany, Greece, Iceland, Ireland, Itaiy, Luxembourg, Netheriands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom. CENELEC European Committee for Electrotechnica Standardization Comit Europen de Nonnalisation Electrotechnique Europisches Komitee fur Elektrotechni
6、sche Normung Centrai Secretariat: rue de Stassart 35, EL1060 Brussels O 1993 copyright reserved to CENELEC members Ref. NO. EN 60756 : 1993 E - i.*- .* m 3qOY583 0060650 OT5 m Page 2 EN 60766 : 1993 Foreword The CENELEC questionnah procedure, performed for nding out whether or not the intemationai s
7、tandard IEC 756 : 1991 couid be accepted without textual changes, has shown that no common modifications were necessary for the acceptance as European standard. The reference document was submitted to the CENELEC membeis for formal vote and was approved by CENELEC as EN 60756 on 9 December 1992. Thi
8、s European stanw supersedes HD458s1: 1985. The foliowing dates were xed: - latest date of publication of an identical national standard (dop) 1993-1201 latest date of withdmwal of - conflicting national standards (dow) 1993.1241 3404583 00bOb5L T3L _- 756 O IEC NON-BROADCAST VIDEO TAPE RECORDERS - T
9、IME BASE STABILITY 1 Scope and object This International Standard specifies the time base errors of the monochrome as well as of the colour composite video signal reproduced from two head helical-scan domestic video recorders, recording one field on each track. This standard gives characteristics an
10、d maximum figures of the time base errors to make it possible to design the horizontal flywheel of television receivers so as to ensure acceptable stability on the screen. 2 Definitlons concerning tlme base stability 2.1 Gap Loss of video signal during a certain period of each field. 2.2 Phase step
11、2.2.1 Due to a difference between the length of the head path and the length of the recorded track, a phase step occurs at the end of the gap respectively at the time of switching from one track to the next. The sign of the phase step depends on the sign of the difference of lengths. 2.2.2 Due to a
12、deviation of the angle between the two video heads from the theoretical value of 180, a phase step occurs at the end of the gap respectively at the time of switching from one track to the next. The sign of the phase step alternates from one field to the next. 2.3.1 The mean value of the line frequen
13、cy is determined by the source to whlch the recorder is locked. If the frequency of this source deviates from the frequency of the source used during recording, the mean value of the line frequency of the playback signal will differ from its original value. If the machine is not locked to an externa
14、l reference, the deviation will depend on the difference between head speed during recording and that during playback. 2.3.2 Because neither head nor tape velocities are constant, a jitter of the line frequency occurs containing various frequency components which depend on mechanical parts of the ta
15、pe deck and varying tape properties. 2.4 2.4.1 Depending on the modulation system used in the colour signal processing, a deviation of the mean value of the colour carrier frequency from its original value may occur. Deviation of colour carrier frequency 1 756 0 IEC 2.4.2 Depending on the modulation
16、 system used in the colour signal processing the jitter of the colour carrier frequency will be a certain percentage of the jitter of the line frequency. 2.5 Relative displacement Relative displacement is defined as the ratio of the horizontal displacement of a picture element to the line interval.
17、3 Measurement of tlme base errors 3.1 Gap The timing of the gap relative to the vertical sync pulse, the gap duration and signals during the gap shall be measured with an oscilloscope. 3.2 Phase Step The phase step due to a difference of lengths and the phase step due to a deviation of angles are su
18、perimposed and shall be measured on the screen of a monitor or of a televfsion set. This shall be done by measuring the horizontal deviation of a vertical bar at the end of the gap with respect to the horizontal position of the vertical bar at the beginning of the gap respectively before and after t
19、he time of switching from one track to the next. 3.3 Deviation of line frequency 3.3.1 The mean value of the line frequency shall be measured by a counter or a fre- quency discriminator having a long time constant with respect to the line frequency. 3.3.2 The jitter of the line frequency shall be me
20、asured by a frequency discriminator or by measuring the line period. The jitter shall be determined by the percentage of peak-to-peak deviation from the average value AfH,+fH and weighted by a weighting curve due to the fact that the reaction of the horlrontal flywheel of television receivers depend
21、s on thefrequency of the jitter. 3.4 Deviation of colour carrier frequency 3.4.1 The mean value of the colour carrier frequency shall be measured by a frequency counter. 3.4.2 The method of measuring the jitter of the colour carrier frequency is under con- sideration. 4 Maximum values of time base e
22、rrors 4.1 Gap The centre of the gap should lie three to 15 lines before the leading edge of the vertical sync pulse. The duration of the gap should be less than five lines. The signal during the gap shall be a constant level between black level and white level upon which an unwanted 2 = 3404583 0060
23、653 804 756 O IEC signal (e.g. noise) may be superimposed. The peak-to-peak amplitude of the unwanted signal shall be less than 60 % of the sync amplitude. For machines with switched video heads, the following specifications shall apply: a) no signal gap shall occur; b) the switching position shall
24、be five to ten lines before the start of vedical sync. If no disturbance in the sync occurs during switching, this figure may be O to ten lines; c) the overlap duration shall be two lines minimum, including the switching point. 4.2 Phase step The total phase step shall not exceed: f6 ps f 15 ps for
25、525 line/60 Hz systems; for 625 IineAO Hz systems. The phase error shall not exceed 1 vs in both cases. NOTES I 2 Phase step is defined in 22.1. Phase error is defined in 2.2.2. 4.3 Deviation of line frequency 4.3.1 Deviation of the mean value of line frequency from its nominal value shall be less t
26、han: f 4 % f0,5 % i0,5 V0 for monochrome recorders; for colour recorders for 625 linel50 Hz systems; for 525 line/6O Hz systems. 4.3.2 Peak-to-peak deviation of line frequency shall be less than: i0,3 Of$ f0,2 Yo for 525 llne/60 Hz systems; for 625 line/50 Hz systems. The percentage of peak-to-peak
27、deviation of the line frequency from its average value (jitter) is weighted by the weighting curves shown in clause 5. 4.4 Deviation of colour carrier frequency 4.4.1 The deviation of the colour carrier frequency shall not exceed: f50 Hz f 150 Hz fl kHz for the NTSC system; for the PAL system; for t
28、he SECAM system. 4.4.2 The limits for the jitter of the colour carrier frequency are under consideration. 3 3404583 00bOb54 740 756 0 IEC 5 Weighting curves The weighting curve represents the response of the flywheel circuit of the TV receiver to jitter and shows at each frequency the relative displ
29、acement with respect to the jitter of the playback signal. Therefore, the movement on the screen corresponds to the jitter of the line frequency of the playback signal weighted by the weighting curve. 5.1 Weighting curve for CCIR System 525160 1- 10 50 0.5 1 lhe weighting curve Is based on a horizon
30、tal flywheel of the television receiver having a minimum gain of 16 000, a resonant frequency of 160 Hz and a damping factor 1,6. 5.2 Weighting curve for CCIR System 625150 The weighting curve is based on a horizontal flywheel of the television receiver having a minimum gain of 30 000, a resonant fr
31、equency of 160 Hz and a damping factor of 2. This flywheel produces a maximum overshoot of 20 lines after a phase step. There is only one overshoot having an amplitude of 5 % of the value of the phase step. This design ensures that a phase step occurring before the vertical sync pulse will cause neg
32、ligible flagging“ at the top of the picture. 4 3404583 00b0655 b87 m _- . 756 0 IEC 6 Time base stablllty at playback speeds different from the nominal value Playback modes using tape speeds other than nominal are known as “trick“-modes. Examples are stop motion, fast motion (picture search) or slow
33、 motion in both forward and reverse directions. These modes affect the time base of the playback signai and cause deviations from the values specified in clause 4. 6.1 Signal transitions 6.1 .i Signai transitions due to head switching at the end of a scan (indicated by S in figures 1 and 2). This si
34、gnal transition is described in 2.1, 3.1 and 4.1. 6.1.2 Signai transitions due to “trick“-modes (indicated by Tin figures 1 and 2). The following cases are distinguished: a) if one head is used per scan (see figure 1); b) if more than one head is used per scan (see figure 2); c) if special arrangeme
35、nts are used (ag. actuators). 6.1 -2.1 Number of signal transitions The maximum number of signal transitions per field depends on the ratio of the tape speed used during trick-mode to the nominal tape speed, but the mean line frequency shall not exceed the limit indicated in 6.2.1. 6.1.2.2 Position
36、of signai transitions The position of the signal transitions with respect to the vertical synchronizing pulse is not defined and may vary as a function of Ume. In successive fields the position is not necessarily the same. 6.1 -2.3 Depending on the characteristics of the disturbances of the sync sig
37、nal the television receiver or monitor could detect an unwanted vertical synchronizing pulse producing a vertical instability of the picture. Distortion of the synchronizing signai It is, therefore, necessary to check the playback signai by means of a test circuit. The test circuit shall consist of
38、a sync separator followed by an integrating network having a time constant of 15 ps. The output signal of the test circuit shall be measured by an oscilloscope (see figure 3). During a time period of 50 lines in front of the natural or artificial vertical sync pulse (see 6.3.2). no unwanted signal s
39、hall occur having an amplitude of more than 50 “/o of the amplitude of the natural or artificial vertical sync pulse at the output of the test circuit. 5 M 3404583 00bOb5b 513 M 7s o IEC 6.2 Horizontal synchronizing signal 6.2.1 Deviation of line frequency The deviation of the mean value of line fre
40、quency from the nominal value for the television system considered during playback in “trick“-modes shall be less than f2 %. 6.2.2 Number of lines per field During playback in “trick“-modes the number of lines per field differs from the nominal value of the television system in use. lhe amount of de
41、viation depends on the ratio N of the tape speed during “trick“-mode to the nominal value, .e. the tape speed during normal playback. a) In the case of N + 1 and N S - 1 (fast forward mode or fast reverse mode) the number of lines is the same in each field. The deviation from the nominal value for t
42、he television system considered shall be less than I8 %. b) In the case of N between +1 and -1 (slow forward mode or slow reverse mode and stop motion) the number of lines per field is the same in each field or may change in a certain sequence, depending on the design of the video recorder. The maxi
43、mum deviation from the nominal value for the television system considered is twice the H deviation used by the recording format and therefore three lines for a system having 1,5 H deviation between adjacent tracks. The sequence of change from field to field, if any, depends on the tape speed and is
44、not predictable. In any case, a number of fields with the nominal number of lines will occur, followed by one or more fields with a different number of lines. NOTE - Frequently the position of the artifidal vertical sync pulse (898 6.3.2) is changed by up to two lines in some fields In order to achi
45、eve a better vertical stabiliy of the picture on the television receiver or monitor. 6.3 Vertical synchronizing signal 6.3.1 Deviation of field frequency The deviation of the mean value of field frequency from the nominal value for the television system consdered during playback in trick“-modes shal
46、l be less than i6 %. 6.3.2 Artificial vertical sync pulse In order to avoid disturbance of the vertical synchronization of the television receiver or monitor during playback in “trick“-modes, an artificial vertical sync pulse may be used. 6.3.2.1 Position of artificial vertical sync pulse The leadin
47、g edge of the artificial vertical sync pulse shall be positioned zero to ten lines in front of the leading edge of the original vertical sync pulse. 6 = 3404583 ClObOb57 45T D 7560 IEC The original vertical sync puise may be suppressed or replaced by horizontal sync pulses. NOTE - The position of th
48、e artifkial vertical sync pulse may be changed in some fields to achieve better stability of the picture on the screen (see 6.2.2 b). 6.3.2.2 The artificial vertical sync pulse may be a single pulse without line information. Shape of artificial vertical sync pulse The rise time shall be less than 2
49、ps. The amplitude shall be equal to that of the original vertical sync pulse. The recommended duration is: - - 3 to 11 lines for 525/60 systems; 2,5 to 10 lines for 625/50 systems. Figure 1 - Signal transition (one field) - One head is used per scan (see 6.1.2 a) Example of fast forward at five times normal speed. T = signal transitions due to *trick“-mode. Total duration. A = period of total breakdown of signal. S = V = verticalsyncpulse. H = horizontal sync pulses. NOTE - The position of the signal transitions Tis not necessarily as shown in this figure (see 6.1
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