[QuadList] Head switch

[rabruner at aol.com]

In reference to the Sony U-matic machines, the scanners in the 1000 and 2000 series machines ( 1600, 2850, etc.) did not have motors in the scanners.  Besides the capstan motor, there one one big motor (Called "the AC Motor" run directly off the power line, which drove the reel tables and the scanner through a series of belts. The belt to the scanner was a polished flat belt with low friction.  There was an eddy current brake on the scanner that would slow the scanner down and rgulate its speed and phase, causing the belt to slip.  This caused a great deal of chatter in terms of time base and the switch from one head to another would bounce around over a line. It was nominally set 6,5 lines before vertical sync. If it were set too close to vertical sync, there was a real possibility that it would would into the sync pulse and cause break up.  Note that on record the tach pulse phase was set to this same point.  The 3000 series portable machines had direct drive motors in the scanner, and were probably set up this way for the sake of interchange. The head switch, though nasty, was in overscan on most TV sets and was only an issue to people who watched video on underscanned monitors. 
    When the BVU series machines came along there were a number of improvements.  The scanners now had motors and were servo'd directly for speed and phase, the AC motor now could be serviced for reel tension and reverse drive, and there was a two phase tach in the head now, which gave them information on which head was on the tape and let them always record and play the same field with the same head.  The switching stability was much improved and could not be put 2-1/2 lines ahead of V synch without much worry that it would cause trouble in that area. Moving the switch pulse out of the picture also allowed Sony to ad a head to write time code to the tape over the video in that same spot.  Because the video and the TC were cross polarized, there was not much cross talk on BVU machines, but BVU tapes played in VO-2nnn would be noisy at the bottom of the picture. VO Machines would playback BVU recordings with the switch position in the vertical interval, so at times, there were cases of an old worn belt or pulley causing the system to be unstable during  PB.
     My guess is that Ampex put DO outside of vertical interval in the picture area because it was to big to co exist with the sync pulse in VI



Attached Message


From:
C. Park Seward <park at videopark.com>

To:
Quad List <quadlist at quadvideotapegroup.com>

Subject:
Re: [QuadList] Blanking Issues

Date:
Sat, 6 Feb 2010 14:53:54 -0800


Hi Don, 


So why was head switching ever put in the active picture? 




Best,
Park



C. Park Seward
Visit us: http://www.videopark.com




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