SMPTE Type C Helical-Scan Recording Format By DAVID K. FIBUSH About two years ago there was little prospect that a 1-in helical-scan videotape recorded marily to eliminate the need to review all on one manufacturer’s machine could ever be played back on another manufacturer’s aspects of the format each time a change VTR. In a single year, however, this situation has changed and the S M P T E Standards is made. Typically, the format documents Committee has approved the basic format documents for high-quality I-in helical-scan include a document showing record loca- VTRs capable of interchanging tapes. Topics covered in detail in the paper include: the tions on the tape, a mechanical configura- process of developing a standard, with a list of the relevant ANSI and S M P T E tion document, and a number of electrical documents; helical video format parameters, with definitions and criteria for their parameter documcnts for video, audio, selection; location of tape tracks; video and sync records, with sources and magnitudes control track, ctc. Many pcoplc are famil- of various time-base errors; limitation of vertical-interval dropout; the requirement for iar with the record location drawing as this six heads; the contraints of video track straightness and longitudinal track dimensions; information is widely publicized to show audio and control-track recording; and video signal processing. I t is expected that the the users the available channels and some new SMPTE Type C format will provide a practical solution to the users’ requirements of the features of the VTR. This drawing for tape interchange. for the Type C format is shown in Fig. 1 and contains information that is part of C98.19. Each Type C document is listed by Introduction ferent manufacturers. However, early in ANSI number or SMPTE Rccommcndcd In an amazingly short period of time, 1978 preliminary interchange tapes were Practice number and title later in this scc- SMPTE committees have reached agrcc- exchanged with excellent rcsults. tion. Reference to format documents will ment on a proposed tapc recording format Development of a Standard be by these numbers. for high-quality 1- in helical-scan videotapc Duc to the electromechanical nature of recorders. Starting in January 1977 with Writing a standard proceeds in a num- ber of well defined steps. First, a Working VTRs, it is necessary to specify certain tape a “white paper” submitted to the SMPTE transport and video head scanning system by ABC and CBS, and ending with ap- Group is formed to work out the technical details and draft the various documents paramcters in ordcr to guarantcc good tapc proval by the SMPTE Standards Com- interchange. Ideally, accurate record lo- mittee, the basic format documents for that define, in this caw, a tapc format. The draft is next modificd and approved by the cations on tapc could be specified to clim- 525-linc NTSC systems have becn com- inate the need for specification of VTR pleted in less than one calendar year. parent engineeringcomrnittee (donc for thc Type C format by thc SMPTE Committee mechanical paramcters. However, due to With major improvements in quality and limitations on measurement riccuracy and on Video Recording and Reproduction the resulting expanded use of I-in helical the mechanical properties of tape, this VTRs, it was clear that a common format Technology on 7 December 1977). After the SMPTE Standards Committee ap- approach is not fcasiblc. The mechanical was needed if the users’ demands for tape pararncters and some basic system con- proves the documents (as they did, in this interchange were to bc satisfied. Through cepts are contained in C98.19. organizations such as the SMPTE. tech- case, also in December 1977), it becomes listed below arc thc Typc C format nical experts can work together to produce possible for the first time to distribute the documents that were complete by thc cnd a format standard of sufficient merit to proposed format documents to all intcr- of 1977. These were published in the serve major portions of the nccds of ested partics. This is done formally in the S M P T E Journal of March 1978 and arc cquipment users and manufacturers. This SMPTE Journal. whcrc comments are presently available from SMPTE Head- is exactly what is happening with the Type solicited.* After a reasonable timc for quarters. C format, whcrc major users were intcr- comment, the SMPTE Board of Governors ANSI C‘9R.IX: Draft American Na- ested in the features of formats charac- votes to approve or disapprove the format tional Standard Basic System and Trans- teristic of VTR equipment of various documents as SMPTE Recommended port Geometry Paramctcrs for Onc-Inch manufacturers. An SMPTE Working Practices or - for those considered lcast Group of technical experts sponsored by a likely to changc - to send them on to the Type C Helical-Scan Video Tape Re- corders. wide variety of VTR users and manufac- American National Standards Institute for turers worked out the technical details of possible approval as ANSI Standards. ANSI C‘98.19: Draft American Na- tional Standard Dimensions and Locations a format utilizing thc best features of scv- Where appropriate, approved documcnts of Records on One-Inch Typc C Hclical- era1 VTR manufacturers’ formats. The are sent to the IEC or IS0 as U S . docu- Scan Video Tape Recordings. new Type C format promises to be impor- ments to be considered for international ANSI C‘Y8.20: Draft American Na- tant to the interchange of vidcotapc pro- standardization. tional Standard Frequency Responsc and grams in the next ten ycars. Already, sev- At each step in the path towards stan- eral manufacturers are planning to build dardization, all negative tcchnical or edi- Refercncc Level of Audio Records for VTRs to the new format, and acceptance torial comments are Considered and, whcrc One-Inch Type C tlclical-Scan Video by users is tremendous. appropriate, alterations are made to the Tapc Rccorders. Approval of the Type C format as for- documents. Standards and Recommcndcd S M P T E RP 85: Proposed SMPTE mal Standards and Rccomrnended Prac- Practices arc automatically reviewed every Recommended Practice Specifications tices by the Amcrican National Standards five years, but important changes may be Tracking Control Rccord for One-Inch Typc C Helical-Scan Video Tapc Rc- Institute and SMPTE respectively is pro- incorporated as soon as they are identified. ceeding in a timely manner. As of the end As cxperience is gained with any new for- cordi ngs. of 1977, no programs had yet been inter- mat, it is quite possible that amendmcnts S M P T E RP 86: Proposed SMPTE changed between machines built by dif- will be needed in thc carly years of its use. Recomnicndcd Practice Video Reference This was true with the quadruplcx format, Carrier Prequcncics and Pre-emphasis Prc\cnlcd on 3 February 1 Y 7 X a t thc 12th Annual in which minor changes are bcing made Characteristics for One-Inch Type C He- SMPTE Television Conference in Allanla by David K. cvcn today. lical-Scan Video Tape Recordings. Fibusti. Ampcx Corp.. 401 Hroadwny. Redwood City. A number of separate documcnts are SMPTE Working Groups are presently CA 94063. Copyright GI by the Society of Motion Picture and Television Engineers. Inc. Previously used to define a videotapc format, pri- developing draft standards and rccom- publishcd in uncdited form in the SMPTE book Onc- mended practiccs for tape, reels, test ma- Iiwh Helical Video Recording. * See pp. I62 168 in the March I Y 7 X Jourtral terials and time-code recording. -TAPE MOTION -TAPE MOTION AUDIO 2-0.8 0.8 AUDIO I 0.8 DROPOUT 0.4 17 t V I D E O RISE 24.4 'i' 7 3 .? TAPE TRAVEL DURINQ O N E FIELD ~ ~~ ____ - TAPE REFERENCE EDGE J" 0.1 NOTE ALL LDIMENSIONS I N MILLIMETERS Fig. 3. Video track parameters. (NOT TO SCALE) Fig. 1. Record locations and dimensions for Type C format videotape. Drawing is not to scale; linear dimensions are in millimeters; and values are H O R I Z O N T A L SYNC nominal, to nearest 0.1 mm. P U L S E RECORD UPPER DRUM VIDEO TIP LOWER DRUM TAPE REFERENCE EDGE TAPE ENTRANCE/ (QUIDED EDGE) GUIDE Fig. 2. Configuration of Type C scanner. Fig. 4. Sync pulse line-up. Helical Video Format Parameters the tape with the tape under normal ten- the sync pulse alignment error be very close A complete description of the video sion. to zero to insure minimum phase jump in portion of a helical-scan tape format in- Effective drum diameter (D):A value H-sync pulse timing between tracks during volves many interrelated geometric pa- of drum diameter used in format calcula- still-frame operation. This restriction rameters. These parameters and other tions to take into account tip projection and provides the best possible slow-motion important itcms are dcfined below. air film (if any). The effective value is picture without special accessories and Scanner: A mechanical assembly con- typically equal to or greater than the actual eases design of special slow-motion ac- taining a drum, rotating pole tips, and value by 0.004%. cessories which are required for further tape-guiding elements. Writing speed (oh): The relative video improvement in performance. These spe- Drum: A cylindrical column around pole tip-to-tape speed. cial accessories, working in conjunction which tape is at lcast partially wrapped in Sync pulse alignment error (L,$):T he with an appropriately designed time-base order to form the head-to-tape intcrface of distance along a video track between a corrector, can provide an output signal a videotape recording system.