405 to 625 conversion

[ppppenguin]

It's a question of convenience when converting 405<>625 not to change the number of pixels along a line. For 625 line systems there are 702 visible pixels out of 720 active and 864 total. For 525 it's 704/720/852 respectively. 720 was chosen as part of the original CCIR 601 standard for digital video, along with a 13.5MHz pixel clock. This allowed 625 and 525 systems to have the same pixel clock and number of active pixels. It also allowed a margin for H blanking artefacts. What it didn't give was square pixels, something that became a minor nuisance in the computer age. HD standards all have square pixels.

For 405 we simply adapt the "601" standard by reducing the pixel clock by 405/625.