Multi-Channel VHF Modulator Design

[flyback]

HI all,

David's KB Majestic looks well-kept, but can we see it on 405 lines! (and with real telly, not this set-top box muck!).

PNP, NPN, why not have a NPN on Tuesdays and Thursdays, PNP all other times!.

No but really, if you must have one (filter that is! and you don’t need it really) put the filter before the base with a stopper resistor like you have, maybe a series L/C type rather that a T filter, it (the MC44..) may be happier with a PNP as a source as the internals of the device (P31 of data sheet) shows a 'NPN emitter clamp' but have a look they show 500 ohm as a series feed!.

Jeff, remember the modulator is working on the whole of the video waveform, it 'clamps' the sync tips to a known level to suit its modulation so back porches and colour bursts are way up there from the sync tip level.

This goes back to what I said about the bandwidth of the video you feed to the modulator, it shouldn’t have anything near 3.5MHz in it!, make the output of the converter fit the 405 standard, then all is well.

The 'Gold Standard' Dinosaur converter has a 3.0MHz filter in its video output stage with a fancy feedback restorer in the modulator and that produces very good results.

Malcolm

[David_Robinson]

Originally Posted by flyback

David's KB Majestic ... can we see it on 405 lines!

Trust me Malcolm, that photo does show 405 lines! Thanks for the unwitting compliment to my standards converter / modulator.

Darius, you make some good points. I do like a real technical discussion!

You could add another emitter follower between the filter and the modulator chip, but as Malcolm has already pointed out, there is 500Ω in series with the video inside the MC44 chip anyway so I don't think it would make much difference.

So how significant are these 'high-ish pulse currents'? The video input current is quoted as 1µA maximum. To supply this current on average, the current during syncs (just taking line syncs for a ball-park figure) will be

1 * 98.8 / 9 = 11µA approx.

The drop across (180 + 68 + 500) Ω will be 11µ * 748 = 8.2mV which is 2.7% of the sync pulse amplitude, worst case (0.6% typical). There is no cause for concern!

As for the ability of my emitter follower to supply this current, the drop across its emitter resistor caused by this pulse current will be

11µ * 470 = 5.2mV

This will not come anywhere near to cutting the transistor off so the difference from a PNP emitter follower will be negligible.

Malcolm's suggestion of NPN on Tuesdays and Thursdays is therefore vindicated.

Phew - I think I need a lie down!

[oldeurope]

So how significant are these 'high-ish pulse currents'? The video input current is quoted as 1µA maximum. To supply this current on average, the current during syncs (just taking line syncs for a ball-park figure) will be

Hi David,
congrats to your converter picture.

If you watch a test pattern your calculation is correct.

The problems take place when you have a jump from a black to
a white picture. When the resitance of the driver is too high,
the sync pulses get smaller for a moment.

When the current (the 1uA) is to low or the cap is to big,
the sync tips lift up from the clamp.
When the cap is to low the picture gets dark from left to right.

You must have a pattern generator where you can switch directly
from a black to a white and vise versa picture!

With the 500 ohms in the IC is a big problem.
You'll never be satisfied.

The best is to generate a clamp current only at the sync tips.
You can see this in my first converter at the input of the cxl's.

In my new converter I had that problem with the input resistor
too.
In the M7403 ccd there is a resistor in front of the clamp too.
The only way to solve this problem is to make a seperate
DC restorer in front of it. And I made this.

Don't forget the cap is made for 625 at 405 you must
take 15nF at pin 9.
Look with the oscilloscope at 100% white
that the "line" does not fall from the beginning to the end of
the line. You first see this on the oscilloscope bevore you notice
it in the picture.


Kind regards
Darius

[David_Robinson]

Good afternoon Darius

Thanks for your suggestions. I have a Philips PM5570 generator that will do black to white jumps (0.25Hz), and many other functions. I will do a few experiments with this and then I will post again.

[oldeurope]

Hi David,
the problem is the changes of the sync takes place at the DC-restorer
and this is in the IC! So you must look for sync level changes at
the output of the vision demodulator in the TV set.
An eventually reduce of brightness from left to right can you measure
at pin 9 I think.

Kind regards
Darius

PS: The picture shows the input of the cxl's.
the clamp current comes from the LM1881 ony at the sync tips.
So the cap at the input will not be discharged during a line.

[flyback]

Me again!

David said,
'Trust me Malcolm, that photo does show 405 lines! Thanks for the unwitting compliment to my standards converter / modulator.'

Very good, but a 'real picture' would look better than that on screen message! and for a treat you could show us Test Card C.
(and what is on 6 music?, I cant get my DAB off 5 Live!)
I extended the blanking to chop of the converted line 23, gets rid of the WSS etc.

Not takers for the 'don’t have a filter' idea then?

And how about using a FET, maybe just on Sundays!...just teasing

Malcolm

Already lying down!

[ppppenguin]

Transistors? What are they? Don't people use opamps these days?

A client actually remarked to me that I'd used a transistor in one of my designs for him! You had to search for it between all the chips.

Use a cathode follower. It's more musical

If you've got a converter that really does have significant energy output above 3MHz then I suppose you ought to filter it to get rid of vision on sound. The acid test is listening to it on a good telly.

And I counted every single line and I claim a refund. I could only find about 380

[David_Robinson]

Originally Posted by flyback

... for a treat you could show us Test Card C.

Well I haven't got a full resolution copy of TCC so I hope TCF will do. I get big problems trying to take off screen photos - my camera keeps strobing out the interlace. After many attempts this is the best I can do.

http://homepage.ntlworld.com/david.r...20Card%20F.JPG

The whole picture is really interlaced, honest guv.

what is on 6 music?

<off topic> Well it's all a matter of taste, but for me it's the station I've been waiting for. Of course you have to avoid the breakfast show (why do music channels feel the need for a show that's all talk?) but after 10am it's fab. None of that rap they have on Radio 1 (don't panic mods, I said RAP ).</off topic>

I extended the blanking ...

Yeah I know, I said it wasn't finished...

No takers for the 'don’t have a filter' idea then?

Seems not!

Originally Posted by oldeurope

... you must look for sync level changes at the output of the vision demodulator in the TV set.

Of course it will have to be a set with proper gated AGC, to do the 0.25Hz test, none of that mean level AGC rubbish. Fortunately I have one such set. I have made the time constant on the base of the emitter follower nice and long, so the clamp is not presented with rapid changes in the sync tip voltage.

So how much white droop can we expect?

dV = I * dT / C

dV = 1µA * 80.55µs / 10nF = 8.1mV = 1.2% of (white - black) (worst case) (0.2% typical).

The result is attached with a 50Hz bar pattern from the Philips PM5570. As you can see any tilt on the white bar is negligible. I can't show you the 0.25Hz bounce test in a still photo, but I can tell you there was no problem

[oldeurope]

Good evening David,
Your test card looks really very good

dV = 1µA * 80.55µs / 10nF = 8.1mV = 1.2% of (white - black) (worst case) (0.2% typical).

I am not satisfied with 10nF.
You have more than 100 grey steps in a picture so I don't think
this is acceptable.
In your oscilloscope picture I can see that the
white level goes a little bit down. At the beginning is
the marking line of your oscilloscope in the middle and at the end at the top.
I think 15nF will make it better. I have a test pattern on DVD which makes this mistake visible.


so the clamp is not presented with rapid changes in the sync tip voltage.

I don't understand it. Does this mean the sync tips run away from the
clamp level with rapid changes? Where did you measure this?

Kind regards
Darius

[David_Robinson]

Originally Posted by oldeurope

Does this mean the sync tips run away from the clamp level with rapid changes?

I was referring to the time constant of C3 with R10 and R12. Following a sudden change in average picture level, this time constant causes a drift of the dc level which the clamp has to correct. For this to work, the clamp time constant has to be much shorter than the C3 one.

David

[oldeurope]

Hi David,
if you want to be as quick as my modulator in compensating
DC changes you must use a 2,5nF at pin 9.
And if you want a change in brightness over a line as low as in
my modulator you must take 47nF at pin 9.
This is the problem.
You will find no satisfieing compromise in your circuit.
The only way is to make a good DC restorer in front of pin 9.

In the old TV's they have a 0,1uF and 2M Ohms at the sync
seperator. You must make sure that this gets not in trouble.

Kind regards

Darius

[oldeurope]

Hi David,

http://hometown.aol.de/freddurst50/NW/Site2.45.htm

Is your DC restorer able to handle this signal?

Kind regards
Darius

[David_Robinson]

Originally Posted by oldeurope

Is your DC restorer able to handle this signal?

If I put it through my standards converter first, yes!

[oldeurope]

If I put it through my standards converter first, yes!

Ok David, but this is a 405 playback

I have a few vcr's and if you playback a recording of an other
vcr on this one, it looks like this. With his own recordings it is
ok.

Kind regards
Darius