Quote:
Originally Posted by old_tv_nut
As for 720 pixels, it comes from the earliest studio digital video recorder standard, D1, which used a fortuitous common clock frequency for both 525 line/60 Hz video and 625 line/50 Hz video.
|
The 720 pixel standard predates the D1 recorders. The relevant standards documents are CCIR601, CCIR656 and SMPTE125M. These standards emerged from a lot of R&D work in several countries, mainly the US and UK but also France, Germany and Japan. The serial digital interface that became universal was subsequently invented by Sony. The original SDI in the standards documents was never used.
The choice of a common 13.5MHz pixel clock for both 525 and 625 was good at the time but has left us with some legacy problems. Non square pixels with different aspect ratios on 625 and 525 are a nuisance for a lot of processing.
All later HD standards have common image formats and square pixels. This gives different pixel clocks for 50Hz and 59.97Hz related standards. Also huge H blanking intervals on some standards such as 720/24p which are a sort of wasted space. Fortuantely they don't have to be transmitted.