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   From: stephen@junkmailblocker.sptv.demon.co.uk   
      
   "Martin Underwood"  wrote in message   
   news:EZHOa.1217$pQ2.97660@newsfep1-win.server.ntli.net...   
   > "R. Mark Clayton"  wrote in message   
   > news:beffl5$31l$1@hercules.btinternet.com...   
   > > "Martin Underwood"  wrote in message   
   > > news:j9AOa.10723$4O4.1016259@newsfep2-win.server.ntli.net...   
   > > > "Steve Roberts"  wrote in message   
   > > > news:3f0ac941.25351983@news.reith.bbc.co.uk...   
   > > > > On Tue, 8 Jul 2003 13:11:24 +0100, Nigel Goodwin   
   > > > >  wrote:   
   > > > Was there a simple relationship between the number of active lines for   
   > > > 405 and 625? Certainly 405/625 isn't a simple fraction. I'm not sure   
   > > > how many active lines the 405 system had.   
   > > Probably not all the 625 have picture on them.   
   > No, I realise that - which is why I said *active* lines. I've since heard   
   > that the number of active lines for the 405 system was 377. And I know   
   > that for the 625 system it's 575. But neither 405/625 nor 377/575 is a   
   > nice round fraction, which suggests that the averaging algorithm must have   
   > been horrendous.   
      
   I thought the ratio was a simple 3 to 2, with 575 active lines in, 383   
   active lines out, and the excess lines blanked. The loss of a few lines   
   would not have been noticeable and would have made for a simple 3 to 2   
   electronic line store converter. I guess they stored 2 lines, dropped every   
   3rd line, and dropped an extra line, about 10 times per frame, whenever the   
   store would have overflowed. There would have been only 372 active lines on   
   the output because 625 has longer field blanking than 405, so there would   
   not have been any picture information in the line stores to fill any more   
   lines than this on the output side.   
      
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