|DVCPRO's DV Playback||
This article was originally posted 20 February 1998 on the DV-L mailing list and is republished here by permission. The following content is copyright (c) 1998 by Jan Crittenden and Tony Sangiovanni, Panasonic Broadcast & Digital Systems.
Note: D-7 is the SMPTE's designation of the DVCPRO format.
There has been discussion about DV, DVCAM, and DVCPRO recording, playback, track pitch, track width, etc. And many theories have been proposed about how all of this is possible on a DVCPRO machine. Some have speculated that the DVCAM is only a 10 micron track yet has a guardband to increase the track pitch to 15 microns. Well I know there is no guardband in any of the DV formats but was uncertain about how this works. So I went to engineering. One of our guys, Tony Sangiovanni (if you know him, tell I gave him credit) has written for us an elegant explanation of how all this is accomplished. If you know Tony, you will hear him talking, he has this wonderful New Yawk accent with a little slang thrown in. You gotta love the guy, he is great. So here is the explanation:
a) Record heads --- 18 Microns
b) Playback heads -- 24 Microns
The tape speeds for DV is 17mm per second, D-7 is 34 mm/sec, DVCAM is 28 mm/sec.
The drum speed is a constant 9000 RPM for all formats!
In our deck, during D-7 playback, the tape is moved along through the mechanism at 34 mm/sec, and the playback heads are employed. The reason the [playback] heads are larger than the tape track WIDTH (not pitch), is for interchange tolerance (otherwise known as "slop"). Because the adjacent tracks are azimuth recorded to each other, even if the playback head "reads" the adjacent track, the heads will not "see" it, because of the phase cancellation (azimuth). There is no "guardband" between tracks.
When the D-7 deck is used to playback DV, the record heads, not the playback heads are used...........Why???
During DV playback, the tape is moved along at only 17mm/sec (speed is slowed) and the drum rotation is a constant 9000 RPM, so now things are relative, but the record heads (exactly 18 microns) "read" across just under two tracks of video (actually 1.8 tracks, 18 divided by 10). The azimuth phase difference between the two adjacent tracks, lets the bigger head only "see" one track.
The record heads must be made exactly to the size of the format intended to be recorded this keeps things within a "tolerance". That's why our decks only record DVCPRO, if we were to RECORD another format (DV or DVCAM), we would need other set of heads, of that size (either 10 or 15 microns) mounted onto the head assembly, for that purpose only.
If the bigger playback head was used, the 24 micron head would "read" across three adjacent tracks (actually 2.4 tracks, 24 divided by 10), but the first and third tracks would be in the same phase, and interfere with each other. That's the reason for using the record head for playback.
During DVCAM playback, tape runs at 28 mm/ps, 9000 RPM, but now the 24 micron playback head is used. it "reads" across two tracks (24 microns, D-7 head width) divided by 15 (DVCAM track width) for 1.73 tracks.)
Works fine, lasts a long time.........Till the heads clog!
Panasonic Broadcast & Digital Systems
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Copyright (c) 1998, 1999 by Adam J. Wilt.
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Last updated 19 June 1999.