05-11-2020, 10:06 AM
I think our antipodean friend, Synchrodyne, explored this in some detail a while ago.
There was an obvious need for an odd number of lines (for interlace) in the range 500-700 that is a product of small integers. This last criterion was for ease of sync pulse generation before modern logic. 625 meets this nicely with 5x5x5x5. 525 and 567 are OK too but were rather low points to aim at in the 50Hz world. 729 and 735 were aiming a bit high. I don't think there are any other obvious candidates apart from 675. You can arrive at 625 simply by aiming for the middle of the possible numbers.
As others have said, it roughly matches 16mm film capability, though under optimal conditions and modern emulsions 16mm can do better than 625 TV.
I know hindsight is a wonderful thing and in retrospect it's obvious but was it obvious at the time?
I think the argument about almost common line scan rates for 525 and 625 is spurious. It didn't matter at the sending end. Only place it matters is the receiver line scan. By then it was perfectly easy to design for any required rate up to at least 20kHz. The time it mattered was much later in standards conversion and setting the first "601" digital standard. For both of those the close line rate turned out to be a simplification.
There was an obvious need for an odd number of lines (for interlace) in the range 500-700 that is a product of small integers. This last criterion was for ease of sync pulse generation before modern logic. 625 meets this nicely with 5x5x5x5. 525 and 567 are OK too but were rather low points to aim at in the 50Hz world. 729 and 735 were aiming a bit high. I don't think there are any other obvious candidates apart from 675. You can arrive at 625 simply by aiming for the middle of the possible numbers.
As others have said, it roughly matches 16mm film capability, though under optimal conditions and modern emulsions 16mm can do better than 625 TV.
I know hindsight is a wonderful thing and in retrospect it's obvious but was it obvious at the time?
I think the argument about almost common line scan rates for 525 and 625 is spurious. It didn't matter at the sending end. Only place it matters is the receiver line scan. By then it was perfectly easy to design for any required rate up to at least 20kHz. The time it mattered was much later in standards conversion and setting the first "601" digital standard. For both of those the close line rate turned out to be a simplification.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
