CRT Deflection circuit - sync pulses

Traditionally, TV sets have to be designed to handle noisy sync pulses. So they use some kind of locked oscillator, nowdays a full-blown PLL, where an integrator or capacitor smooths out the average frequency.

On a computer the sync pulses are cleaner than in a off-the-airwaves Tv signal, but it's still advantageous to use a PLL or some kind of stabilized and locked oscillator.

Tis is true of both vertical and horizontal syncs and sweeps.

The actual sweep generator has to deliver a linear ramp, in current, not voltage, to the CRT deflection coils. This requires an impossible voltage waveform (an infinte pulse at the beginning to instantly start the current rising, this is usually omitted and the start of the sweep is put a little bit off the left of the screen to not show this glitch). The rest of the waveform is a trapezoidal pulse (in voltage) which in an inductor generates a current ramp.

For the retrace, that requires a huge voltage pulse to sweep the current quickly back to the left or top edge.

These funny signals are always generated by a very clever resonant and diode-damped circuit, which is optimized to minimize the huge power dissipation it would take to generate the retrace using normal methods.

I would NOT suggest you try to do one of these circuits on your own, they're tricky and require parts that can take the high voltages and currents.

On a sunny day (31 Oct 2006 23:41:26 -0800) it happened sp snipped-for-privacy@yahoo.com wrote in :

When we talk CRT (not LCD), here you go: A local oscillator is phase locked to the H sync pulse. The oscillator drives a switch that basically puts a voltage across the H defection coils for the duration of the line. Because the coils are inductive, a linear rising current happens in the coils, the resulting magnetic field sweeps the electron beam from left to right. When the switch goes off at the end of the line, back EMF happens and the current reverses (using a diode). Use is made of resonance in this case. To make a correction for the angle a capacitor is is series with the H deflection coil, S correction, so angular speed is maximum in the middle.

In the V deflection the inductance and frequency is so low that the coils are driven by a normal totempole amp, the defection coil current is measured by a small series resistor, and compared to a reference linear sawtooth, also phase locked to the V sync.

In a VGA monitor there are many more corrections in the deflection system, and perhaps the same system for H is used as I described for V here. There is no 'staircase'.

When the frequency is higher, to get the same current in say half the time, you would need double the voltage on the H deflection coil.

It is possible they use low inductance H deflection coils and a similar drive as V, but I am not sure. The days of a little diagram that comes with TVs or monitors are past..

My Samsung CRT is still working after being on 12/24 7 days/week, now for

5 years, so I have not looked at the circuit. LCD is the future, but the picture is not as good as a good CRT. Especially for video. There are more better display systems in development.

I guess I'm still not 100% clear on the details of how the PLL is set up for this task. Part of the problem might be that I'm only just starting to learn about PLL's in general. But how would we take those short square-pulse sync signals, feed it into a PLL, and then eventually get a sawtooth output? And for the vertical deflection, how would we use a PLL to eventually get that "staircase-like" output?

Yes, I actually did realize that, but I guess I didn't word my post quite right. If you like, add a voltage-controlled current source to the output I described. Like you said down below, that might be very impractical (ie, power-hungry, and stressful on components). But my main interest is in generating the "shape" of the deflection waveform, even if it wouldn't be a smart way to build a real CRT.

Thanks for the reply.

-Sean

Perhaps there is some way to make a voltage-controlled relaxation sawtooth oscillator (where the threshold voltage for "resets" is somehow adjustable by a voltage) and use a Type 2 PLL ? ie, the type which only looks at the *edges* of the oscillator and reference input? Just a guess.

Apologize for the extra post, but I meant Type 2 phase detector

On a sunny day (1 Nov 2006 10:21:12 -0800) it happened sp snipped-for-privacy@yahoo.com wrote in :

It is relatively simple, charge a C, control the current source, use a voltage comparator to trigger a flip flop that then discharges the C immediately.

I actually published one here a week or so ago.

----------------------------------------- +12V | | | [ ] [ ] R1 [ ] | | | | |

No - the vertical scan is a smooth sawtooth - the horizontal scan lines are not truly horizontal - instead they slope downwards slightly as they cross the screen.

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Peter Bennett VE7CEI 
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On a sunny day (Wed, 01 Nov 2006 12:51:36 -0800) it happened Peter Bennett wrote in :

Yoa yoa, I put a piece of paper under the right side of the set.

Thanks, Jan, for the schematic you included, and the explanation from the previous post. That is what I was looking for. I had only been exposed to PLL's as used in FM demodulation (with an analog multiplier and a loop filter). After some more research and your posts, I think I understand what is going on (hopefully).

As an aside, for VGA (not broadcasted television), do we really even need the PLL? I'm not suggesting using a non-locked local oscillator, I'm just suggesting using the sync pulses directly to dischage the capacitor. I wouldn't think that there would be much noise on the sync lines. (Not that I would do this if I was a commercial VGA monitor producer, it's just another "what if" question).

Also, thanks to you and the other poster for clearing up my error about the vertical deflection (ie, the bit I wrote about it looking like a "staircase"). I hadn't noticed that when I first read my VGA literature.

Now to apply this information to what I'm doing with VGA:

So if a CRT was built using the type of circuit you described, the

*duration* of the h-sync pulses shouldn't matter, so long as they are long enough to bring the current in the inductor back to zero, right? (or bring it negative, or whatever corresponds to the left of the screen). If, for example, I continued to assert h-sync during the back porch, I would expect the image to simply by shifted so that it went *completely* to the left edge of the screen. (Exactly as if I didn't include a back porch at all, and shifted the video data ahead by the corresponding amount).

Perhaps that sounds tedious - something nobody would ever do. I guess a more practical circumstance might be if my VGA driving circuit ran at a frequency which wasn't an integer multiple of the desired dot clock. Depending on how slow the clock was, one might be presented with problems trying to keep within the timing specifications. It would be nice to know what behavior to expect from the CRT if h-sync or v-sync pulses were too long/short or at too high/low of a frequency. Hence my reason for digging into the "guts" of the deflection circuitry.

And for a strictly fixed-frequency monitor, if I wanted to just use the upper-left portion of the screen, could I just generate h-sync and v-sync pulses twice as fast? (This would, of course, mean that I'm sending the "wrong" sync frequencies to a fixed-frequency monitor. I'm not sure whether there any any components that could fail by doing this.)

Thanks again.

-Sean

On a sunny day (1 Nov 2006 16:13:34 -0800) it happened sp snipped-for-privacy@yahoo.com wrote in :

In theory at _one_ frequency, no PLL needed. If the oscillator needs to lock over a high frequency a PLL circuit as I did show has advantages. But most important is, that if your sync was to interrupt, the scanning of the CRT would stop, and the spot would burn in in the centre. You want a picture even without input signal, my monitor then displays a warning message about 'check signal cable'. A CRT can burn in in a fraction of a second when all the energy goes to one focussed spot on the screen.

The H sync time is specified in the spec, but it is _not_ the H sync time that drives the H switch directly, that would be very dangerous, some disturbance and the current in the horizontal output stage would kill it.

The time ratio (on / off) of the H stage is set in the drive circuit.

-------- ------------ ------- H sync pulse, leading edge is used. | | | | - -

on

-------- ----------- ----- Horizontal drive | | | | -- -- off

black ---- ---- | | | |

------- --------- ----- blanking to CRT

||||||| |||||||||| |||| video luminance signal

Normally the ;leading edge of the H sync is used for trigger, (phase aligned with a new scan), so the timebase circuit has some time to start. If you make H too long in a composite signal it gets into the picture as ultra-black. If it is external sync you may notice very little, as normally only the leading edge is used.

That depends on the PLL design. These days there is (hopefully!) a frequency discriminator that checks for a valid range (my monitor switches the HV and scanning off, if sync is out of range). Else it would blow up.

Simpler even, you use the same (correct) H and V frequency, but send the info at double speed (double pixel clock) in the first line, and wait the other half line time, and send the info in the second line you have at double speed, etc, until you run out of lines. This assumes your monitor can display those double high frequencies (detail). This also shows you need _half_ the number of lines in your original! On a 600 line height screen you have only 300 lines for half the picture.

So no need to do that.

That's only on TV sets that did not pay the licensing fee for the patent "straightening the picture by turning the yoke 1/525th of the way to the left".

This patent is useless here where the hot thing is to tilt the yoke by

1/625th.

--
Thanks,
Fred.

Philips USED TO have a pdf-format book of application notes "Power Semiconductor Applications" - but I cannot find it anymore amongst the corprat's "RAH RAH RAH: Look How Great WE Are" Garbage on the web site! Maybe you have more luck and/or patience.

In practice Yes, it is required to keep the horizontal and vertical blanking from wandering all over the screen.

Not at all, the horizontal and vertical oscillators would free run as designed. Moreover the anode high voltage is derived from the horizontal sweep and no sweep translates sooner or later to no anode voltage.

I have not had quick burn problems with my o'scopes. YMMV

They have to synchronize the horizontal oscillator, and for color TV gate the chroma burst PLL. The other issues are internal to the display.

Please consider, that inductor (yoke) voltage is e = -L * di/dt. Thus a very asymetrical current sawtooth is created by a voltage square wave that is a bit asymmetrical. After the flyback pulse drives the yoke current to the max -x position, the damper tube monotonically reduces the yoke current to about zero. Then the horizontal output turns on and continues the sweep from midscreen to the other edge. Then it cuts off and the resulting high voltage spike reverses the current in the yoke very quickly.

V-sync is not nearly so straight forward. Google for NTSC and look for a thorough explanation of vertical sync; to be worthwhile it will include "equalizing pulses". Now there is a can of worms for you.

No.

See within.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

On a sunny day (Sat, 04 Nov 2006 04:36:24 GMT) it happened joseph2k wrote in :

That is actually not correct, in old TV vertical was just a hard synced blocking oscillator, and V blanking was derived from the back EMF of the vertical out, and H blanking form the line output transformer. In such a case blanking 'cannot 'wander' if you are in sync. Ever had an old TV with H and V frequency control? I still have a portable that has it :-)

Maybe you should read again what the OP posted. He sort of suggested originally that you could derive the ramp from the sync pulse. This is possible, but there would be no scan when no sync, and burn in. There is enough energy when scanning stops in the HV to burn in the tube before the HV is also gone. In the very old days when you switched the set off, the picture would become smaller and smaller, and disappear in a point. These days there is a deliberate blanking circuit to prevent this.

We are talking TV, not scopes. I have had CRT burn in in less then a second (and expensive case, you need a new tube). There is a safety normally when you do niot connect the scan coils, but in the Philips set the safety did not work.... It is often just a bridge on the connector that allows voltage to the H part.

This is true, but incomplete. As only the leading edge of H is used, no problem making it wider if composite sync in. In comp video in FBAS (was not the OP question) you would indeed lose the burst.

Actually I had TV too on the scope :-) Not digital, Z input.

And how do you achieve that without the equivalent of phase locking?

Not in the sets that i have schematics to. Nor does that invalidate the fact that vertical blanking is transmitted as part of the NTSC, PAL and SECAM formats.

^ range

I remember those old sets, with a bright spot in the center as the anode voltage decayed, it also spread out as focus voltage decayed which is partly derived from anode voltage. They did not burn out either.

Nor did the old TVs from the late 1940s and early 1950s.

Of equal importance is how much wider? all the way to line time?

A subset of the width issue.

Please explain equalizing pulses.

See within.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

On a sunny day (Sun, 05 Nov 2006 21:53:39 GMT) it happened joseph2k wrote in :

Not sure what you mean here, normally the V osc would run slow, here is a (simplyfied) diagram that I used for example in a design.

----------------------------------------- +12V | | | [ ] [ ] R1 [ ] | | |-------------------------------- | || | === | | | | | | C1 | | | | | | |

------------------------------------------------------------------------------ 0V V hold poti

It _had_ to run slow, so it could be triggered before the restart, When the V sync causes T3 to conduct the UJT trigger voltage level is slightly lowers, and it triggers. The trigger will _coincide_ with the V pulse, so is also phase locked.

Yes, but only in composite video (FBAS). There is a very simple and obvious reason why it _must_ be so: as the set runs free (no station) you _still_ want the retrace of H (the flyback) and of V from bottom to top, suppressed! It is usually done by pulsing one of the anodes of the CRT with a signal derived from a tap on the H output transformer, and somewhere from the V, the same circuit will often also suppress the spot.

I forgot to point out that in modern KTVs the HV cascades (voltage multipliers) have considerable capacitance too, with plenty of energy to do damage to the phosphors.

Yes out of focus helps, normal BW sets had only about 15kV, color sets have

25kV.

Line burn in often happened, I have had a TV repair shop too, repaired thousands (no joking), and line burn in I have seen too. Even some old scopes (at about 4kV) had line burn in (remember DG57-34???).

Sure, more then half a line and you f*ck up the reverse H in the V in composite.

Glad you asked. We have to look a 3 different cases in case of sync:

1) Composite video (with composite sync). 2) Composite sync, 'S" input. 3) Separate H and V sync inputs (as in VGA 9 pole connector, the OP was talking about originally).

To start with 3 (simplest) most of the time the computer display is not interlaced, and H and V can just be simple pulses, there need not even be a frequency lock (both derived from the same clock etc). And in case of interlace you need no 'egalisation' in fact.

This brings me to your question in case '1' and '2'. The original reason for the egalisation pulses is dead simple, and almost never mentioned. The intention was to have the (interlaced) receiver sync separator circuit _as_simple_as_possible_.

The egalisation pulses are there to make sure the V integrator capacitor is charged in such a way that the vertical retrace will start at half a line (625 / 2 = 312 1/2 in European PAL).

This system was designed so you could just with a simple differentiator and integrator split the composite sync.

. . integrator . neg polarity R . ramp build up during v sync. comp sync ----------====-------------- V sync . | | | === C | | | /// | |\\ | differentiator | \\ | | | C ___ __| \\_____ -----| |---------------- H sync | / | | | | / | |/ [ ] R This edge is used (_also_ in egalisation pulses!) | ///

It must again be stressed that the comp sync does not need any egalisation pulses if the display is not interlaced. maybe some of you will remember the real start of computing, Motorola max-board with 6845 CRT controller, you just made comp sync by xoring a H pulse with a V pulse....... And the TV would be nicely locked. And that twas even a common clock. I designed a video camera (vidicon) with 2 free running UJTs (as above) as H and V oscillator, and xored the sync from these, then modulated on CH4 VHF in the sixties! WAY ahead of everybody :-) CCTV? I was first and wireless too! Had it in the garden, battery operated, lasted 20 minute on nicads....

Anyways, TV is a nice subject, have to do somehing now, we can deepen this out if you want to. Soon the old analog techniques will be forever lost for this generation. A pity perhaps, as those guys were very inventive really.

------------------------------------------------------------------------------

Hmmm, the equivalent of a PLL.

Generating a reasonable steady voltage from microsecond wide pulses does take some energy storage.

Anode voltage and mean operating current have little to do with it. While i have not serviced the sheer quantity of sets you have, i have had far more problems with computer monitors having burn in. This seems to match my understanding of the erosion model.

BTW i have a LCD monitor exhibiting the equivalent of burn in now.

I saw line burn in on a scope only once, and only heard reports of it on TV's.

^equalization See below.

No, it has to do with maintaining h-sync.

Equalization pulses bent the H sync so that it would be back in place at the end of V sync. They occur on blank lines both before and after the V sync series.

You forget it only displayed on even fields. It was not until the 6845A that you could even try to use both vertical scans and even then it did not work correctly.

I haven't said it would work, but it does not meet specification. It was a real PITA to meet spec in the tube days.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

On a sunny day (Tue, 07 Nov 2006 12:11:30 GMT) it happened joseph2k wrote in :

There was an interesting thread in comp.ibm.hardware.chips about the 'LCD" 'burn in' or storage effect, depending on the system and manufacturer it sometimes helps to display white for say a night.

First a correction, I think the tube was DG7-32

It is the small one next to the big radar tube..... This was my first oscilloscope 'design' (well I designed the power supply), it was a tube oscilloscope, probably 1MHz bandwidth, no trigger, just 'sync'. A year later I designed a transistor one, with video output transistors in the deflection, real trigger, double timebase, on a printed ciruit with RTL logic! But anyways, this DG7-32 burned in in the first hour or so...

Eh, yes and no. In the olden days the time constant in the H PLL was really big, so the set would just carry on at last freq if bad signal. Then VHS came, and it had big timing errors in H due to the head position changes, a big jump in H at the end of the frame (and missing lines at the head switch-over). So then sets got a much shorter timeconstant for H. There was a time TV sets had 2 time constants.

Yes I know where they are. Let's go a bit deeper: If you xor the H with a longer V you get reversed H, but now the H starts posive, and trhe differentiator in the sync separator deleivers a negatibve pulse, and as we were using the negative pulse it is about 4.7uS late. However the H PLL will adjust in few lines.

-- ------------ ------------ -- | | | | | | | | | | | | | | -- -- ------------- --- || || || || || || position of 'optional' egalisation pulses.

Actually in the above drawing the V ramp is negative of course.....

I would have to climb in the attic, but I had interlace on a Z80 system I designed with a 6845, but not sure it was 'A' or not.

Interlace is just a phase relationship between H and V, if you have a Lissajous (spelling) display of 2 sine waves on the scope, you can phase adjust so the lines do not overlap (pairing it is called). The extra pulses cause the V integrator cap to charge a bit different, and change the phase where the V triggers. That is what was in my TV study book (no idea who wrote it, but it was good).

I think the H frequency does not _normally_ 'jump' during V sync (as in VHS). It is just a matter of phase of V. You can generate a 15 kHz sawtooth, and scope it. Put the scope on a 20 ms trace, then you can change scope ms / division so you see overlapping periods, or the sawtooth periods next to each other. In one case you see 312.5 in the other 625 periods. Is this right? I think I tried this once, but maybe getting a bit rusty in this. What do you think?

Well, I worked in the TV studio as engineer in the sixties, the old sync generator had toobes, and very complicated, toobe flip flops.... yes it was full spec.... and a man height cabinet with forces air cooling. When color came (1967 here) we got nice transistor sync generators....

All this interlock color carrier, 25 Hz offset, 8 frame editing sequence for PAL, quadruplex.. amtec colortec color corrections.... But fact remains that you get color too with a 4,43MHz unlocked oscillator (like in video games). Just the interference pattern in the color areas then moves randomly on a BW set..... But the sets (old and new) would eat almost anything you threw at it.