1、BRITISH STANDARD BS 7537-2: 1991 IEC 1055-2: 1991 Measurement techniques and operational adjustments of broadcast VTRs Part 2: Guide to special mechanical measurements and alignmentsBS7537-2:1991 This British Standard, having been prepared under the directionof the Electronic Equipment Standards Pol
2、icy Committee, was published underthe authority of the Standards Board and comes intoeffect on 29November1991 BSI 10-1999 The following BSI references relate to the work on this standard: Committee reference EEL/22 Draft for comment 88/32442 DC ISBN 0 580 20289 5 Committees responsible for this Brit
3、ish Standard The preparation of this British Standard was entrusted by the Electronic Equipment Standards Policy Committee (EEL/-) to Technical Committee EEL/22, upon which the following bodies were represented: Association of Professional Recording Services Ltd. Audio Engineering Society British Br
4、oadcasting Corporation British Kinematograph, Sound and Television Society British Tape Industry Association Independent Broadcasting Authority Independent Television Association National Council for Educational Technology (NCET) Royal Photographic Society Royal Television Society Sound and Communic
5、ation Industries Federation Amendments issued since publication Amd. No. Date CommentsBS7537-2:1991 BSI 10-1999 i Contents Page Committees responsible Inside front cover National foreword ii Section 0. General 0.1 Scope 1 Section 1. Transverse-track VTRs 1.1 Mechanical adjustments of headwheels 1 1.
6、2 Special mechanical measurements 2 Section 2. B format VTRs 2.1 Special mechanical measurements 5 Section 3. C format VTRs 3.1 Mechanical measurements and adjustments 5 3.2 Special mechanical measurements 7 Section 4. U-matic H format VTRs 9 Annex A (informative) Bibliography 10 Figure 1 Positions
7、of the tracks on television magnetic tapes. The magnetic coating of the tape faces the observer 4 Figure 2 Measurement of track straightness 8 Figure 3 Graph of peak deviation against track number 8 Publication(s) referred to Inside back coverBS7537-2:1991 ii BSI 10-1999 National foreword This Part
8、of BS7537has been prepared under the direction of the Electronic Equipment Standards Policy Committee and is identical with IEC1055-2:1991 “Measurement techniques and operational adjustments of broadcast VTRs Part2: Special mechanical measurements and alignments”, published by the International Elec
9、trotechnical Commission (IEC). This British Standard describes the mechanical adjustments and special mechanical measurements which are peculiar to the types of VTRs used in television production and ENG, and are recommended for adoption within the European Broadcasting Union (EBU). It is important
10、for these machines to be adjusted and measurements checked to be within the recommended standards to avoid compatibility problems during international programme exchanges. These recommendations were based on the results of investigations conducted by EBU Sub-groupG2 (Television tape recording) and w
11、ere published by the EBU in document Tech.3219-4 “Special mechanical measurements for television tape recorders”. Section1, concerning transverse track VTRs, comprises the corresponding passages of EBU document Tech. 3219 (1976) suitably updated. Sections2and3are concerned with B format and C format
12、 VTRs using25.4mm tape. Section4is concerned with U-matic H-format using19mm tape. This type of format is not recommended for international exchanges, but it is in widespread use for ENG. For the purposes of this British Standard, any reference to IEC page numbers in the text should be ignored. Copi
13、es of EBU documents may be purchased from the Technical Centre of the European Broadcasting Union, Cas Postale67, CH-1218 Grand-Saconnex, Geneva, Switzerland. Telephone +41227172111Telefax +41227985897. A British Standard does not purport to include all the necessary provisions of a contract. Users
14、of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Cross references International Standard Corresponding British Standard IEC 1055-1:1991 BS 7537 Measurement techniques and operational adju
15、stments of broadcast VTRs Part 1:1991 Guide to operational adjustments on analogue composite broadcast VTRs (Identical) Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1to10, an inside back cover and a back cover. This standard has been updated (se
16、e copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS7537-2:1991 BSI 10-1999 1 Section 0. General 0.1 Scope This technical report defines the special mechanical measurements for television tape recorders. Section 1. Tra
17、nsverse-track VTRs 1.1 Mechanical adjustments of headwheels When a new headwheel is accepted and installed on the machine, the mechanical adjustments should be checked. This concerns three main points: a) mutual position of headwheel and vacuum guide; b) headwheel tachometer phase; c) position of co
18、ntrol-track head. The first point should be frequently checked during the life of the headwheel; this should, in fact, be a routine operational check. On the other hand, the second and third points should be checked only when a new headwheel is accepted. 1.1 a) Relative positions of the headwheel an
19、d vacuum guide If, on replay, any of the geometrical factors is incorrect, the precise relationship between the video heads and the tape that existed during the recording of the alignment tape cannot be reproduced. The most common errors are quadrature, skew and scallop 1, 2 1) . These take the form
20、 of time base errors which are caused by the video heads being incorrectly positioned. If the penetration of the head is incorrect, “skew error” is produced; the vertical elements of the image are distorted so as to form a series of saw-teeth, each of which corresponds to a head sweep. “Scallop erro
21、r” is caused by incorrect setting of the guide height and the vertical elements are distorted so as to form a series of parabolic arcs. If the heads are not exactly at an angle of90 to one another, equal horizontal displacements occur for each line associated with a head sweep and the defect is call
22、ed “quadrature error”. It should be mentioned here that it is quite common for the guide height to be adjusted to give maximum flatness of the oscilloscope display of the FM amplitude. If the machine is being aligned to play a particular tape and this method produces the most acceptable pictures whe
23、n playing that tape, then obviously discretion is used in adopting a compromise. All modern recorders have time base correctors which correct for these errors and generally the oscilloscope error display provided by the machine provides the most sensitive means of optimization. Before attempting the
24、 following adjustments, the machine and heads should be at their normal operating temperature. The guide position should be in the fixed condition during the adjustment and the fixed guide-position and guide-height screw controls adjusted until skew and scallop errors have been eliminated as seen on
25、 a picture monitor displaying the demodulator output (i.e.before monochrome timing correction) and on the oscilloscope monochrome timing-error display. There should be no visible timing error once these adjustments have been made, but occasionally quadrature errors may be discerned. On some equipmen
26、t the relative positions of the heads may be changed to minimize this. The guide height and position should preferably not be changed until the next alignment (this is feasible on machines equipped with velocity compensators). The presence of co-planarity errors can also be checked at this stage. Th
27、is is done by replaying the alignment tape and examining the amplitude of the FM output on the oscilloscope display, while rocking the tracking control. If the amplitudes of the FM signals produced by the four video heads do not change simultaneously in the same way, co-planarity errors are present.
28、 They can be measured by making the video tracks visible on the tape by means of a magnetic developing powder, lifting the pattern off the tape by means of a transparent adhesive tape and transferring it to a glass slide which is optically projected on a large screen. 1) The figures in square bracke
29、ts refer to the bibliography, Annex A, page10.BS7537-2:1991 2 BSI 10-1999 1.1 b) Tachometer phase The alignment tape is reproduced and the tachometer dot display is examined on a video monitor showing the demodulator output and operated in the pulse cross mode. If the tachometer dots are not in the
30、correct position as described in document Tech.3219-2, Chapter1, item2.1, the mechanical position of the headwheel tachometer pick-up head can be adjusted by means of the appropriate adjustment screws (thisis normally considered to be a factory alignment). 1.1 c) Position of the control-track head I
31、n order to check the position of the control-track head relative to the headwheel, a tape can be recorded, magnetically developed, and the recording checked to ensure that it conforms to the standard by means of a travelling microscope. Alternatively, the following method can be used. A portion of e
32、rased tape is threaded through the tape path and appropriate tension is applied to the tape. The tape must not, however, pass between the capstan and the pinch roller. A video signal is fed to the machine and the machine is switched to record, but is stopped immediately after the guide closes. When
33、the tape is magnetically developed, a well-defined pulse will be visible which defines the position of the control-track head gap. The rotating heads will have produced a visible scratch on the tape defining the plane of the pole tips. The distance between the centre of the scratch produced by the h
34、ead-wheel pole tips and the centre of the control-track head mark can be measured by means of a travelling microscope. It should be17.80mm. The method of verification in which the alignment tape is played back and the timing relationship between the demodulated vertical synchronizing signal and the
35、control track is checked on an oscilloscope, is not recommended because it is critically dependent on an extremely accurate centring of the tracking control. 1.2 Special mechanical measurements Methods for the checking and adjustment on the tape-machine of several mechanical characteristics which wi
36、ll normally be the subject of periodic operational attention have been described in1.1 of section1. In the present clause, methods are described for the checking of other mechanical parameters of television tape-machines, which will normally be measured on the occasion of acceptance tests or periodi
37、c maintenance. 1.2 a) Spacing of the video tracks The EBU Standards 1 specify the spacing of the video tracks to an accuracy of 0.1%. Such a high degree of accuracy is necessary to achieve satisfactory splicing and to facilitate electronic editing of recorded tapes; to obtain the necessary accuracy,
38、 however, special methods of measurement are required. Dimension J (seeFigure 1), which is the tape length covered by four consecutive video tracks, is generally calculated from a measurement carried out on a much longer length of tape, on which the video tracks have been made visible (or developed)
39、 by means of a suspension of ferro-magnetic powder. In this measurement, reference is often made to the spacing of the edit pulses, which are more readily identified and are related to the video tracks. Four measuring methods have been successfully employed within the EBU. These give comparable resu
40、lts. Method 1: The spacing of the edit pulses is measured on a calibrated base 4 m long. The tape is measured under a tension of 1.5 N, but the measure is corrected to take into account the corresponding elastic elongation. Method 2: The spacing of the edit pulses is measured on a calibrated base ab
41、out 1 m long, with tape tensions of 2 and 4 N; the result is then linearly extrapolated for the case of zero tape tension. At the nominal track spacing, and for zero tape tension, 40 edit pulse intervals or 800 J intervals, are contained in a tape length of 1 270.00 mm. Method 3: The spacing of the
42、edit pulses is measured against a calibrated base about 1 m long, with the tape lying in a horizontal plane under (supposedly) zero tension. Method 4: The tape is allowed to settle on a cleaned plastic plate. Intervals of 10 J are repeatedly measured by means of a travelling microscope having a rang
43、e of 10 cm, and a running mean is calculated.BS7537-2:1991 BSI 10-1999 3 1.2 b) Spacing between the audio and cue heads and the video headwheel The distance between the audio and cue heads and the video headwheel, which is specified in the EBU Standards as being235 1.3mm, is a characteristic of some
44、 importance, particularly where the EBU time and address code is used for accurate editing. Cumulative tolerances on cure-track head positions in successive copies of video signals and address signals could, in fact, destroy the relationship between a specific television picture and a specific addre
45、ss. The method of measuring the audio-to-video spacing is similar to that described in1.1 c) of this section for the measurement of the distance between the video headwheel and the control-track head. In order to improve the visibility of the magnetic pattern developed on the tape, it is advisable t
46、o record the audio and the cue tracks to saturation. 1.2 c) Position of the audio, cue and control tracks The position of the audio, cue and control tracks, in a direction perpendicular to the tape axis, is specified in the EBU standards. Figure 1, taken from these standards, is reproduced here for
47、the readers convenience. The method of measurement that is generally adopted employs a travelling microscope; the tape is placed on its bed after the recorded magnetic pattern has been “developed” and made visible by means of a suspension of ferro-magnetic powder. It is advisable that the audio and
48、cue tracks should be recorded to saturation in order to make their pattern more easily visible.BS7537-2:1991 4 BSI 10-1999 Figure 1 Positions of the tracks on television magnetic tapes. The magnetic coating of the tape faces the observer Dimensions of video, audio, cue and control track record Dimen
49、sion A B C D E F G H I J K L M W min 0.00 1.02 1.47 1.98 2.21 29.1 48.31 48.79 50.50 0.240 0.005 50.70 mm 1.5875 max 0.10 1.24 1.57 2.16 2.39 29.3 48.62 49.02 50.70 0.265 + 0.005 50.80 mm = 90 33 3 J 4 -BS7537-2:1991 BSI 10-1999 5 Section 2. B format VTRs An optimal record and reproduction signal quality can be guaranteed only if all parts involved in the tape path are subject to periodic operational attention. Routine operational work is limited to a dally cleaning and demagnetizing of the tape guides, The edges of the running tape in the tape p
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