02-11-2022, 01:33 PM
Looking at how the introduction of 625 influenced the 405 parameters slightly it's instructive to look at how the digital CCIR 601 standard affected both 625 and 525 standards.
A critical decision was to use a common pixel clock (13.5MHz) for both 625 and 525. This was convenient at the time but resulted in non-squre pixels which had different aspect ratios on 625 and 405. This made interface with computer generated pictures more difficult. Later HD standards used square pixels but needed 2 different clocks to cope with all standards.
Choosing 720 pixels per active line encompassed all possible 625 and 525 signals with a margin for poor quality blanking and ringing due to sharp cutoff at the edges of the picture. This concept was carried forward to HD and has proven its worth.
When converting 625 to 405 it makes sense to use an output clock of 13.5MHz * (405/625). Hence there is a 1:1 correspondence along a line between input and output pixels. This ultimately sets the horizontal parameters of the 405 signal. There is no fundamental need to use a different clock. Designers could have used an interpolator to change the line length. AFAIK, none did.
A critical decision was to use a common pixel clock (13.5MHz) for both 625 and 525. This was convenient at the time but resulted in non-squre pixels which had different aspect ratios on 625 and 405. This made interface with computer generated pictures more difficult. Later HD standards used square pixels but needed 2 different clocks to cope with all standards.
Choosing 720 pixels per active line encompassed all possible 625 and 525 signals with a margin for poor quality blanking and ringing due to sharp cutoff at the edges of the picture. This concept was carried forward to HD and has proven its worth.
When converting 625 to 405 it makes sense to use an output clock of 13.5MHz * (405/625). Hence there is a 1:1 correspondence along a line between input and output pixels. This ultimately sets the horizontal parameters of the 405 signal. There is no fundamental need to use a different clock. Designers could have used an interpolator to change the line length. AFAIK, none did.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
