Using "Z" Mod on a scope.

[Murphyv310]

Hi.

Today I moved the circuitry from the breadboarding stage to a better layout.
I've made some small changes to the circuit but fairly minor.
The only small issues are
1, You need phase reversal unless your scope can invert the signal, the picture will be upside down other wise.
2, There is a little non linearity at the bottom of the picture, worse on 25Hz field rate.

Solution is to make a phase inverter and do some waveform correction at the same time, I hope to do this during the week.

   

[Murphyv310]

Hi.
I got about an hour on the project today. I added an extra stage after the 555 to invert the waveform and hopefully cure the small amount of cramping on the bottom of the scan.
David Boynes suggested replacing the electrolytic charging cap with a non electrolytic type, I duly did this and there was a small improvement on 25i scan, 25p was no better though. Adding the stage certainly gave me phase reversal but the linearity is awful. My feeling is using a bipolar device is wrong so I'm going to use a FET instead, I think its all down to the impedance matching.
I'll have another fiddle tomorrow.

Incidentally all standards are excellent down to 180/25p, Ill post up pictures of the test cards and also some pictures with program material. Some of the fully mechanical systems can be resolved but I need to sort out the syncs on them with a different approach as the LM1881 isn't happy.

[Mike Watterson]

replace resistor charging cap on 555 with a constant current source somehow. Is there anything in the old Nat Semi 555 application notes, it was many pages?

[Geordie McBoyne]

Hi Trevor,
I notice that you have supplied the output from the LM1881 to pin 5 of the 555 timer. Pin 5 is connected directly to the three resistor potential divider network in the IC. I'm not sure if this is the correct pin for sync purposes, In the past I have used the reset pin 4 as a sync pulse injection point.
Normally pin 5 is connected to a decoupling capacitor of say 0.1microfarards. Pin 4 is normally connected to the VCC pin. A high to low transition resets the timer.

The LM1881: pin 3 is the vertical sync output. I assume that the pin goes low only during the vertical sync period, that is during the broad pulse train and not including the equalising pulse periods. The timing chart indicates that this is so.

I'm thinking if the V pulse from pin 3 of the LM1881 goes from VCC to ground this could be supplied to the reset pin of the 555.

Geordie McBoyne.

[Murphyv310]

It's certainly worth a try David but on a ramp generator pin 4 of the 555 should be tied to +B according to the specs.
TBH the circuit works perfectly as is, even the picture shape discrepancies are marginal, im just being picky.

[Geordie McBoyne]

Attachments show the in the internal circuits of the 555 timer and it's application as a astable multivibrator.

Time for the high period is given by  T1= 0.7 (Ra + Rb) Ct.

Geordie McBoyne.

[Murphyv310]

Hi David.
The circuit I'm using is a "Ramp" generator, the reset is tied to +B for free running, the BC557 supplies constant current. Negative going sync is fed to pin 5.
All electronic standards can be resolved as can some of the electronic/mechanical hybrid standards (240/25p & 180/25p)









As can be seen there is some distortion on the lower part of the scan, more so on 25p (25Hz) than on the interlaced systems.
Also there is a problem with the stored image on 240/25p, I'll look for a suitable test card of the time.
All a bit of fun but none the less successful.

[Murphyv310]

Hi.
Two further pictures from "off air" transmissions one 405 the other 240 lines.

[Geordie McBoyne]

Hi Trevor,
Impressive results. As soon as I finish off all the projects I'm presently doing I'll have a go at something similar.

Geordie McBoyne.

[Murphyv310]

Hi.
Give me a slap round the napper............. The linearity issue is not the 555 frame generator but the scope switched to AC coupling.
What a wally am I.
Perfect linearity on DC coupling.