by Steve Anderson » Sun Mar 23, 2014 7:33 pm
This is a break-off thread from what started in 'Video to NBTV converter' within the 'NBTV' section of this forum.
This is a 5" CRT monitor which with use of plug-in boards it is able to display a number of TV formats. Of prime concern here will be NBTV, but also SSTV (with and without frame-store), 120, 180 and Baird's 240-line standards will also be possible.
Of the NBTV projects planned are the standard 32/12.5 mode, a 32/50Hz mode (as per Graham Lewis's 'Frame Rate Convertor) and line doubling/interpolation. Others may appear.
The biggest problem is to modulate the CRT grid-cathode circuit with signals that run from DC up to some 2MHz whilst the grid-cathode circuit sits at around -1400V. Opto-isolators at first seemed the answer, but all those I am able to obtain here are too slow, even using some tricks.
So what do you do when you can't buy something? DIY...so here's my home-brew opto-isolator.
It uses a standard but fast red LED and a BPW34 as the optical elements, a few high-speed transistors and careful attention to reducing stray capacitance. To do this a lot of the circuit was 'air-flown'.
Below is a few photos of the prototype (messy but it works) and a couple of 'scope screen-scrapes showing the bandwidth. Judging from the PAL burst I would but the -3db point at around 4MHz. The old '0.35' Rule Of Thumb supports this, 0.35/71ns = 4.9MHz - a bit optomistic perhaps.
Pre-emphasis may improve this, but at this stage it's good enough.
Steve A.
- Attachments
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- HB Opto 1.jpg (49.48 KiB) Viewed 19709 times
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- HB Opto 2.jpg (57.4 KiB) Viewed 19709 times
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- HB Opto 3.jpg (67.59 KiB) Viewed 19709 times
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- Square Wave 1.gif (5.84 KiB) Viewed 19709 times
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- PAL Video 1.gif (7.51 KiB) Viewed 19709 times
Last edited by
Steve Anderson on Sun Mar 23, 2014 8:30 pm, edited 2 times in total.