NBTV Scope Evolution

[Steve Anderson]

...so you think those circuits using a single supply for their opamp should really be a dual supply ? So this should have been a dual supply ...this was the first 741 circuit i had seen with a single supply i always thought the only way to use a 741 was with a dual power supply.

Dual-supply op-amps can be used single supply but the inputs are not designed to go any closer than 2-3V more positive than the negative supply or in this case, ground/0V. Single supply op-amp inputs can usually go to ground/0V or often slightly negative (300mV or so) and the device functions correctly. Not always true for the positive supply though there are exceptions.

Single-supply op-amps can be used with dual supplies as well, but there's not much point. Often single-supply op-amps have poorer specifications than the dual-supply versions, but used in the manner we generally do here there's no practical difference. Many have a lower supply voltage rating, say 18V which is useless with +/-12V supplies.

As for using the 5534 in head-amps on a single supply, it all depends on the biasing, the voltage on the two input pins, if they are at least 2-3V above ground, and 2-3V away from the supply all should be well...much in the manner of that laser-link microphone amplifier.

Steve A.

[Harry Dalek]

Thanks on the explanation on the opamps Steve it is something i had not thought about and i wonder why there has not been a good range of single supply versions made perhaps it is much easier to design the dual supply versions .

I finished converting the oscillator to the 555 version didn't take to long worked first go so no mistakes ,i will see how it gos inputting it to the Lm311 tomorrow .

[M3DVQ]

I located my scribblings and scanned them in. I'm afraid they're a bit hard to read even with the contrast bumped up.

The first part shows the synchronous counter with the X and Y DAC ladders. I'm fairly confident that that part is correct and complete. I believe the counters are 74161 or 74163 (the difference is async vs sync clear which is unused in my circuit anyway)

nbtvscope1.jpg (714.42 KiB) Viewed 15049 times

The second page is rather a mess. I started off with a circuit based on my breadboarded prototype, ticking parts off in orange crayon are where I soldered a board to match the circuit drawn, but lots of modifications were made as I went along!

nbtvscope2.jpg (939.57 KiB) Viewed 15049 times

I certainly wouldn't recommend anyone try to build a copy from these circuits! That said, the synchronous counter and two resistor ladder DACs is proven. The 4060 based crystal oscillator and divider to drive it also works well. Some of the frame sync and z modulation circuitry is maybe a bit more dubious

I think these show fairly clearly how I take a high frequency "wobble clock" (I think it's 4 times the pixel clock) from the 4060 and mix an tweakable amount of this signal into the X deflection. Looking at it, I assume the little low pass RC network was intended to turn the square wave output of the 4060 into something closer to a triangle wave since mixing in a square wave clock just results in doubling up each line rather than thickening it.

[Harry Dalek]

I located my scribblings and scanned them in. I'm afraid they're a bit hard to read even with the contrast bumped up.

The first part shows the synchronous counter with the X and Y DAC ladders. I'm fairly confident that that part is correct and complete. I believe the counters are 74161 or 74163 (the difference is async vs sync clear which is unused in my circuit anyway)

Hi Alistair

The second page is rather a mess. I started off with a circuit based on my breadboarded prototype, ticking parts off in orange crayon are where I soldered a board to match the circuit drawn, but lots of modifications were made as I went along!

I like that system ticking off your soldering together version ,don't worry about the messy drawing the result was a thing of beauty !

Well changes i don't think any thing i start looks the same at the end so i have empathy!

I certainly wouldn't recommend anyone try to build a copy from these circuits! That said, the synchronous counter and two resistor ladder DACs is proven. The 4060 based crystal oscillator and divider to drive it also works well. Some of the frame sync and z modulation circuitry is maybe a bit more dubious

I am glad you found it posted it up some thing for me and others to study ,its certainly unique !

I think these show fairly clearly how I take a high frequency "wobble clock" (I think it's 4 times the pixel clock) from the 4060 and mix an tweakable amount of this signal into the X deflection. Looking at it, I assume the little low pass RC network was intended to turn the square wave output of the 4060 into something closer to a triangle wave since mixing in a square wave clock just results in doubling up each line rather than thickening it.

[/quote]

So this can do Spot wobble ? as i was thinking it was a z axis version .

I studied and i can see it can do this ...answering my question ! one big bit of help if possible i would love to see same picture one spot wobble one z axis to see the difference ...my results are pretty crappy to put it bluntly .......at the moment i have not tested the new PWM to see but the last was little fussy with what i could try to view letters were fine other stuff a little harder .

I am still working on mine ...which i will get back to now

[M3DVQ]

It's a bit too much effort to drag all the stuff out and set it up at the moment to take photos, but basically the difference looks something like this: (imagine a plain raster is being displayed)

wobble.png (6.79 KiB) Viewed 15036 times

That's a crude approximation drawn from memory but it gives some idea. What I've tried to show is that the lines themselves are less bright because the spot moves faster. With a square wave modulated in the spot spends most time on one side or the other so the edges of the band are bright with the spot spending barely any time in the gap between. A sine wave would lie somewhere between I think.

[Steve Anderson]

A couple of really hectic days, now I have some time for this...

I think perhaps we should make a differentiation between the two major themes within this thread, otherwise it's a bit obscure as to which one is being referred to. 'Spot-wobble' to me is where a small amplitude waveform (ideally triangular or sawtooth as per M3DVQ's example above) is added to the frame deflection, in the case of the NBTVA standard used here, horizontal. Conceptually the same as used in some 405-line TVs, except theirs was applied to the vertical deflection or used a totally separate yoke winding for this function alone.

The display method as devised by Jeremy Jago is spot-wobble in the extreme with a large amplitude PWM modulated square-wave added to the horizontal ramp waveform. Why not call it Split-Frame Modulation?...which is sort of what it is. Other suggestions? Horizontal Displacement Modulation?...Frame Displacement Modulation? Jago Modulation?...

...that's enough for now...

Harry, I'll get back on the case ASAP.

Steve A.

[AncientBrit]

I'd favour Displacement Modulation as a general term , then maybe qualify it with vertical or horizontal according to the displacement sense,

Cheers,

Graham

[Harry Dalek]

It's a bit too much effort to drag all the stuff out and set it up at the moment to take photos, but basically the difference looks something like this: (imagine a plain raster is being displayed)

Thats fine Alistair ! i didn't think all that would not be handy

That's a crude approximation drawn from memory but it gives some idea. What I've tried to show is that the lines themselves are less bright because the spot moves faster. With a square wave modulated in the spot spends most time on one side or the other so the edges of the band are bright with the spot spending barely any time in the gap between. A sine wave would lie somewhere between I think.

Thanks the explanation makes it clear,but i have not heard of it mixed also with those other modulated waveforms .

Got side tracked today clearing my shed once again ,don't know how many times i do it bench space soon disappears !

[Harry Dalek]

A couple of really hectic days, now I have some time for this...

I think perhaps we should make a differentiation between the two major themes within this thread, otherwise it's a bit obscure as to which one is being referred to. 'Spot-wobble' to me is where a small amplitude waveform (ideally triangular or sawtooth as per M3DVQ's example above) is added to the frame deflection, in the case of the NBTVA standard used here, horizontal. Conceptually the same as used in some 405-line TVs, except theirs was applied to the vertical deflection or used a totally separate yoke winding for this function alone.

The display method as devised by Jeremy Jago is spot-wobble in the extreme with a large amplitude PWM modulated square-wave added to the horizontal ramp waveform. Why not call it Split-Frame Modulation?...which is sort of what it is. Other suggestions? Horizontal Displacement Modulation?...Frame Displacement Modulation? Jago Modulation?...

...that's enough for now...

Harry, I'll get back on the case ASAP.
Steve A.

No problems here Steve I think Albert is pretty good at naming these things ,,,well i like the Split frame modulation as its descriptive.

It is of cause at Jeremy's whim ,i wonder what he calls hes invention..but without a name yes have to call it something.

I saw in an old newsletter they call it strange spot wobble ,that one was an early Allan Short try and he used a separate coil near the yoke.

The display from jermey's scope looked to me like the positive image could match a z axis modulation ?

[Steve Anderson]

I think you've gotten Albert and 'Ancient Brit' (Graham Lewis) confused Harry...but these things happen.

With this method you should end up with one positive image and one negative. The only difference in essence between the this method and conventional linear grid modulation will be the fact that the CRT has now become a linear device rather than the usual 2.2 gamma factor as mentioned by Klass some posts back. Depending on whether or not the source is gamma-encoded could mean you'll end up with 'crushed whites'. But for simply getting a picture of some sort on a screen it's a start - and quite a good one I think. The gamma issue can be dealt with later.

So one vote for Displacement Modulation so far, and one for Split Frame Modulation, preceded by Vertical or Horizontal as appropriate. I haven't made up my own mind yet...I used to be indecisive, but now I'm not so sure...

Steve A.

[Harry Dalek]

I think you've gotten Albert and 'Ancient Brit' (Graham Lewis) confused Harry...but these things happen.

OH i am getting senile but not that senile yet ! Albert is pretty good at naming things over the years on hes projects i just mentioned him has good project naming skills ,no offence to Graham or any one else .

With this method you should end up with one positive image and one negative. The only difference in essence between the this method and conventional linear grid modulation will be the fact that the CRT has now become a linear device rather than the usual 2.2 gamma factor as mentioned by Klass some posts back.

So missing the gamma adjustment you have no adjustment for the room lighting conditions then ..not really sure if its needed for any thing else ?

Depending on whether or not the source is gamma-encoded could mean you'll end up with 'crushed whites'. But for simply getting a picture of some sort on a screen it's a start - and quite a good one I think. The gamma issue can be dealt with later.

Crushed whites do you mean a narrower grey scale ?

So one vote for Displacement Modulation so far, and one for Split Frame Modulation, preceded by Vertical or Horizontal as appropriate. I haven't made up my own mind yet...I used to be indecisive, but now I'm not so sure...

Steve A.

[/quote]

I wish jermey was on the forum really he should be the one ,but again since we need a name i am happy with what ever most of us go for ....

Been busy as well last few days, i am getting my project out tonight see if my Pulse width modulation looks correct ..looked like a mix of AM and PWM last time mmmmm.

[Harry Dalek]

Changes here taking steves advice to swap to a 555 triangle wave oscillator hooked this up to the lm311 with the with now a ca3140 as the opamp.

This is what i am getting from the output of the lm311 few spikes on the waveform ,the 555's frequency gos up with what voltage i am using its around 38khz on 12 volts i ran it bit lower when i did this video .

I will hook it up to the scope tomorrow and see if this is any better.

[Steve Anderson]

Hmmm, it shouldn't look that messy really. Are you feeding the output of the '311 to anything else except the resistor to +12V? Also check your decoupling/bypass capacitors. It should really be a clean square-wave of variable duty-cycle. Also the 'scope appears to be AC-coupled or there's something in the path to the 'scope with a capacitor. I feel another circuit sketch is in order...

...also check your scope-probe compensation...better still, use a real 'scope. I know these small things are useful but they are no match for the real thing. You do have more than one!

By 'crushed whites' (or stretched blacks...much the same really) I mean less tonal graduation in the highlights and/or too much in the lowlights. But as I said previously, this can be dealt with later.

I'm tempted to go with Frame Displacement Modulation (FDM) preceded by Vertical or Horizontal as appropriate. It's a bit of a mouthful but it is descriptive.

Steve A.

[Harry Dalek]

Hmmm, it shouldn't look that messy really. Are you feeding the output of the '311 to anything else except the resistor to +12V? Also check your decoupling/bypass capacitors. It should really be a clean square-wave of variable duty-cycle. Also the 'scope appears to be AC-coupled or there's something in the path to the 'scope with a capacitor. I feel another circuit sketch is in order...

Yes i have a cap after the 311 as i wasn't getting any thing watchable direct coupling last time ,i will check the difference i forgot about that .I was thinking to
it should look a cleaner varying width square ware but thats what i am getting ...again i will check without the cap.

...also check your scope-probe compensation...better still, use a real 'scope. I know these small things are useful but they are no match for the real thing. You do have more than one!

I know its not a great scope i use it more for its frequncey meter ,i will double check it when i do my shed tests the Shed scopes !

By 'crushed whites' (or stretched blacks...much the same really) I mean less tonal graduation in the highlights and/or too much in the lowlights. But as I said previously, this can be dealt with later.

Well i would be happy Steve to make out any thing that isn't just large letters i am not to fussy on the highlights well not at the moment any way ,i think its pretty interesting this sort of modulation works ...i suppose would be possible to do a P7 SSTV version ,might not have ever been done i wonder .

I'm tempted to go with Frame Displacement Modulation (FDM) preceded by Vertical or Horizontal as appropriate. It's a bit of a mouthful but it is descriptive.

Steve A.

[/quote]

Descriptive is good i don't mind FDM.

Any case i will look into a cleaner PWM see if i can do better.

[Steve Anderson]

Harry, I've spent a little time studying Jeremy's design and I've sent him an e-mail asking if he used an additional negative (probably -12V) supply. If he didn't I'm starting to wonder how it functions. I'll await his reply and given some work-bench space maybe breadboard it.

The concept is definitely sound, I'm trying to simplify the last stage somewhat.

Steve A.