STUDIO

Details of the Fostex DAT and timecode systemHeadsStandard DAT machines have a rotary headassembly with two heads on it at opposite sides.Tape wrap around the rotary head is 90 °. TheFostex assembly places a total of four heads onthe drum set up in two pairs with a spacing of92.1° between heads. Each pair of heads islocated vertically on the drum to allow them tocover the same track stripe on the tape.According to the application, either of the paircan read the tape (play head) and the other canwrite to the tape (record head). Thus, read afterwrite or off -tape monitoring can be done if headlA records and 1B plays back, this is equivalentto a conventional recorder.If lA plays off the tape what is there, then 1Brecords to it, it is possible to digitally crossfadebetween previously recorded material and newmaterial. The same is true of timecode, exceptthat there is no crossfade, but it is possible toread timecode off the tape, add an offset orother information to it, then record it to exactlythe same tape position.TimecodeIn deciding how to put timecode on DAT, thereare two initial options. It might seem logical atfirst to put it on one of the analogue auxiliarytracks at the top or bottom edge of the tape butthere are several problems here. First the tapeedge is most susceptible to damage. Secondly,the DAT tape moves very slowly, only '/b thespeed of a compact cassette, so getting the highfrequencies necessary for SMPTE code on andoff the tape is difficult. Third, this highfrequency problem gets even worse when oneconsiders the 100x normal speed winding rateof DAT and trying to read timecode at thatspeed.Therefore, it was decided to put the timecodeinto the digital subcode area of the DAT track.Here another problem occurs. DAT has a framerate of 33.3 frames/s compared to SMPTE,which ranges from 30 frames /s (NTSC) down to24 frames /s for motion picture film. Therefore itis not possible to easily match the rate of one tothe other. Fostex took the simple decision tojust write to the tape the last SMPTE frameavailable. This means that sometimes there areSMPTE frames missing but that is relativelyeasy to sort out afterwards if the SMPTE clockinformation can be maintained. This isaccomplished by recording the number of clockticks between DAT frames at whatever SMPTEframe rate is used, along with the time errorbetween SMPTE clock edges at the DATframing times. This allows for perfectregeneration of the SMPTE clock and timecode.The alternative method of recording timecodeon DAT seems to be to use a frame converter,changing the frame rate of the incomingSMPTE timecode to fit the DAT rate, thenchanging it again on playback. Unfortunatelythis is more difficult. However, the proposedstandard implies some method like this one, sinceit requires the DAT machine to be able to replayat a different SMPTE rate than was used onrecord. Time will tell what is actually adopted asa standard. The earliest date for this is a yearfrom now.References1 H Yamazaki, T Ketori, T Morita, S Okazawa,H Nogima and Y Abe, Timecode in sub dataarea of R -DAT, AES Pre -print 25892 Luc Baert, Luc Theunissen, Guido Vergult,Digital audio and compact disc technology,Heinemann Newnes 19883 Preliminary manual, Fostex D -20GUIDEGUIDER -DAT FRAME30mSECAUDIO DATAFostex DAT head assemblyHEADROTATION.-- BO-- 33.3-SMPTE DATABITSTO 417m$ECFISMPTETIMECODE DATABIT CLOCK OFSMPTE CODEGUIDEGUIDEn BITS41-TAPETAPE TRAVELTime relationship of SMPTE and DAT digital dataStandard DAT head assembly.- AUXILIARY TRACK 2INPUTFRAMESMPTE /EBU-. CONVERTERTIMECODER- DAT- .FRAMECONVERTEROUTPUTSMPTE /EBUTIM ECODE-- SUBDATA AREA 2-ATF 2MAIN DATA AREAATF 1-SUBDATA AREA 133.3 FRAME /SEC 33 3FRAME/SEC.- AUXILIARY TRACK 1Frame conversion between SMPTE /EBU code and RDATimplied by a proposed standardTrack format of RDAT106 Studio Sound, June 1989