Origins of the 405 line system.

[Geordie McBoyne]

There's been a lot of discussion about how the 625 line system came about but what about 405 lines? Initially, EMI considered a 243 interlaced system to compete with the Baird companies 240 line progressive 25Hz field rate system.
Simple calcs tell us that the line timebase frequencies are almost the same, 6075Hz EMI and 6000Hz Baird, the frame timbase frequencies differ being 50 and 25Hz respectively.
So it follows the 243 interlaced system would consist of two rather coarse 121.5 line pictures. Five divide by three counters are required for 243 lines and the story is well known that in 1935 the decision was made to change one the dividers to five so we get 3 X 3 X 3 X 3 x 5 = 405.

Geordie McBoyne.

[ppppenguin]

The usual story is that the decision to go from 243 lines to 405 lines was made one Saturday morning at Blumlein's house. However it's also worth noting that RCA's 343 line 60Hz system was near enough the same line scan frequency as our 405. Since there's evidence of EMI engineers visting RCA, mainly for exchange of iconoscope knowhow, it's not surprising that EMI went for 405.

Incidentally, in the early days of TV when frequency dividers were awkward things it was useful if the number of lines could be factored into small integers. 243, 343, 405, 441, 525, 625 and 819 all can.

[Mike Watterson]

I think you are right Jeffery, combination of similar line rate to 343/60 and divider ratios.

The 441 was used on the "Continent" and USA changed to 525 to get closer to 16mm film quality I think, they decided 343 wasn't good enough.
So really in 1936 to choice was either 405 or 441 (possibly 525 would have been too demanding in 1936) and I suppose 405 was a little easier and genuinely "HD" on a 9" screen and not far off on 12". The Americans by 1940 envisaged larger screens than 12" and even 12" was a bit large for 343.

[ppppenguin]

625/50 and 525/60 are also related by having near identical line scan rates.

[Murphyv310]

Hi.
I just wonder how 243i would look? Today I gave the TV22 a tweek and have been watching 240p on it, the flicker is really bad, instant headache
To be fair on a 9 inch screen other than the flicker it's actually not too bad.
I've done a YouTube video but the shutter speed is too slow to resolve the flicker, I've posted it though for anyone to see.
It's test card F but I can upload another video with programme material if anyone wants to see it.
https://youtu.be/553JnxbFJ4M

This set should have been used for last nights broadcast don't you think!!!!!

[Mike Watterson]

Hi.
I just wonder how 243i would look?

Well you can test any resolution with comparative special rescaling to same image resolution to simulate the lower resolutions when there is a static image. Though it's harder than you'd imagine.


The problem with YouTube is that
a) limited choice of frame rates /resolutions upload
b) viewing frame rate and resolution totally dependent on viewers hardware and software.
c) There will be codec artefacts and probably rescaling and frame rate conversion artefacts.

What you see on YouTube may bear no relationship to what others see, or indeed exactly what the original was.
Perhaps a 2K or 4K camera running at 100fps or 120fps progressive (depending on physical CRT) on YouTube would give a good idea if the viewer has same frame rate (100 or 120) and better than regular HD. (Computers surpassed regular TV HD in resolution and frame rate even on CRTs over 15 years ago).

[Murphyv310]

Hi Michael.
The problem here is not YouTube but the digital cameras I have access to, basically the shutter or could it be the CCD is too slow??? on both my cameras and on my phone it cannot replicate the flicker on 240p and on 405i and any interlaced system you get one or part of one frame, thus showing the line structure.
YouTube may make it worse but it's not any worse noticeably.

[Mark Hennessy]

Brilliant video, Trev - I can see you making a bit of pocket money from video blogging

(I also ended up watching the frame aerial video again)

It would be hard to accurately capture the flicker with any camera - it's a tricky problem. Perhaps the new high frame jobs would help, but even then, you've still got to work out how to display it. What you did capture was brilliant, even if it wasn't what you were seeing.

[Murphyv310]

Brilliant video, Trev - I can see you making a bit of pocket money from video blogging

(I also ended up watching the frame aerial video again)

It would be hard to accurately capture the flicker with any camera - it's a tricky problem. Perhaps the new high frame jobs would help, but even then, you've still got to work out how to display it. What you did capture was brilliant, even if it wasn't what you were seeing.

Many thanks Mark, your comments are really appreciated.
I did a shorter clip of the set on 405,  https://youtu.be/DMZfX1xJiHA
I just wonder how to really replicate the flicker though, I'm not into photography unlike my Dad but he is no longer here, 20 years in March

[Mike Watterson]

You need a high frame rate camera Trevor, if you are using regular 50 (or 60 for NTSC) you get shading unless the CCD is averaging over several fields to give 12 fps due to not being synchronised. Most consumer HD cameras do 50fps or 60fps progressive at best, which isn't fast enough. I can set my Sony Camera to a "fast" shutter speed for "sport" (it's still Digital equivalent of interlaced PAL) and then pointing it a CRT or TV or LCD is ghastly. It has a "slower" mode which gives shading free images. It also has an ultra slow monochrome "night shot" mode which will smooth out even 10fps animation on 60fps progressive screen, that mode requires a tripod as it works like daylight for moonlight!