TV line selector

[ppppenguin]

I built this in 1977 while I was still at university. It's still in everyday use in my lab. It works fine on 625 and 525 but doesn't do correct odd/even field separation on 405 which I needed yesterday. I think I can fix it by changing a monostable from 40us to 55us. It needs to be longer than a half line but shorter than a full line. Ideally on all standards. 55us seems about right. I'll try it today.

The basic design was from February 1977 Television magazine. I modified it to simplify and improve.
https://www.americanradiohistory.com/Arc...977-02.pdf

Looking at the parts used, at least some were "borrowed" from the BBC. I think it has suffered only 1 failure in over 40 years. The 5V regulator went duff many years ago. Fortunately it didn't take out any of the other circuitry.

If I was building a new one now it would use a small CPLD or FPGA to do all the logic. Line numbers would be correct on all standards. But it's not going to happen.

[ppppenguin]

The monostable turned out to be rather critical. The original 5n cap was very temperature stable so I replaced it wih a 4n7 polystyrene. After a few attempts to adjust values on test I resorted to fitting a 5K preset pot in series with the 15K resistor. The range over which is works for both 405 and 625 is very small but there seems to be some overlap.

I'll fit a lower value pot as it's right up at the low resistance end. If I'm not happy I'll have to drill the case and fit a 405/625 switch.

[`FRANK.C]

Hi Jeffrey
What a great useful piece of kit which has given excellent service.
I always believe that there is nothing like building your own where possible as you have control over quality.
And not a LM1881 in sight

Frank

[ppppenguin]

The LM1881 was but a distant dream in 1977.

While the LM1881 is a useful chip there are other sync separators that are better. More accurate timing and lower jitter. The LMH1981 is an example.
http://www.ti.com/product/LMH1981