Origins of the 405 line system.

[Synchrodyne]

and USA changed to 525 to get closer to 16mm film quality I think, they decided 343 wasn't good enough.

The reasons behind the NTSC’s choice of 525 lines in 1941 is well documented in the book “Television Standards and Practice”, edited by Donald G. Fink, and published by McGraw-Hill in 1943.  This is available on-line (also in .pdf format) at:  https://archive.org/stream/televisionsta...h_djvu.txt.  See page 218ff.

 
NTSC had initially chosen 441 lines, but took a second look.  The justification for 525 lines – accepted by the committee – was written by Fink himself.  (In)visibility of the line structure and flatness of field were key factors considered in the choice.  One may infer that within reason, that adequately addressing these items took precedence over horizontal definition, given that NTSC was confined to working with a 6 MHz channel with 4 MHz vision bandwidth.
 
This book is I think essential reading for anyone interested the history of analogue TV transmission standards.  Also in that category, also edited by Fink, is “Color Television Standards”, McGraw-Hill 1955, with LCC #55-5684.  (I haven’t found it on-line, but then I haven’t searched too hard for it.)
 
Much, much later, Fink was on record as saying that the 6 MHz was too narrow, and that 8 MHz would have been ideal.
 
 
Cheers,
 
Steve