Just did a video DC restorer function (in CMOS) for a UTC camera chip, and realized that it would be trivial to do the same function with bipolar discrete devices.
On the S.E.D/Schematics page of my website check out...
Video_Restorer_Discrete_NPNs.png ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
I have used a similar idea before, but its general usefulness is limited by the black level being dependent on the sync height, and to a small extent, the average video level.
I'm surprised, given this video signal is of known origin, why would you need such a circuit? Plus you can hardly integrate the 100nF on the CMOS.
-- Mike Perkins Video Solutions Ltd
Everything is ratiometric in "IEEE Units". In my particular case the video is fixed amplitude from sync tip to UTC (Coaxitron) peaks.
[UTC (Coaxitron) are the control bits for the camera.]Wrong
Video is commonly AC-coupled. A DC restorer "clamps" the sync tips to ground, from which all measurements are referenced.
Who said the 100nF was integrated? ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Nifty, but how bad would it be if Q3 was gone and Q1 / Q2 were just diodes?
In a full video circuit, the sync detectors are peak detectors so they don't care much about DC bias. The output of those can help fix gain and bias issues in circuits where it matters.
Not that it matters. I haven't seen an analog NTSC decoder in a LONG time.
On a sunny day (Wed, 02 Jul 2014 21:26:44 +0100) it happened Mike Perkins wrote in :
Right, the circuit is a DC resorter, but not a black level stabilizer.
A black level stabilizer will also restore DC level. The black level stabilizer is *required* to keep the black level in the display device constant, and usually all the way at the end of the 'chain' of amplifiers [1]. Sync amplitude changes, and cannot be used for that, and should not be dependent upon for display.
[1] It is done by clamping at the black level, just after the sync, and needs a special correctly timed pulse for that, as there is also the color burst there in a composite PAL or NTSC signal, so it should be done where there is no burst, just before the CRT or whatever, and that is where you want the black level constant anyway.You can make a good DC restorer by clamping on the sync tip to keep your signal in the linear amplifier range, but that is ALL it is good for. Not much accuracy is needed in such a case, as usually the amplifier range is much bigger than the composite signal, so a simple diode will do in most cases.
I'm not :-)
I see you have now removed the offending circuit from your website.
You "had" a 100k resistor down to ground to bias the circuit. The current flowing through this, and the clamp will be dependent on video level. The clamp will have an impedance and so there will be a small variation of clamp level with video level.
Ordinarily you would need a buffer after this to ensure the clamp is not affected by anything downstream.
Agreed, but why would your customer specifically introduce AC coupling?
-- Mike Perkins Video Solutions Ltd
Historically I would have used a parallel inductor and resistor to ensure the burst is not significantly affected by a clamp.
Agreed, and I have also used a simple diode arrangement before. It all depends on what you're trying to achieve.
-- Mike Perkins Video Solutions Ltd
Huh? It's still there.
Careful there, sonny. You're displaying your ignorance of circuit design.
I live in a hi-Z world... it's called CMOS. But it's not hard to have a "downstream" bipolar comparator to do the UTC slicing and NOT load the clamp.
Why do you have a hair up your butt?
...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Object is tight clamp to very close to zero, so threshold on UTC detection (the only video modulation shown in this example *) can be accurately set above white and not be aggravated by normal video.
You can easily do sync detection with, say, a comparator set at ~0.1V
Still being used by the security camera business... though recent inquiries suggest that they are going to digital.
-- | James E.Thompson | mens | | Analog Innovations | et |
The video subcircuit is already on the Device Models & Subcircuits page of my website.
So soon you forget... I did that subcircuit 19 years ago :-) ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
you want a "keyed clamp" that clamps the AC coupled waveform to a defined DC level at a specific point in time. (you need a timed keying pulse)
***If*** the sync pulse amplitude is known to be constant, then DC restoring (or keyed clamping) on the sync tip will work too.Mark
Yes - forgot it was in a lower directory.
It depends on what you want to do. If you want a crude video restore then fewer components could be used.
Most things have to interface to the real world and composite video is generally associated with 75 ohm impedance.
Is your abuse to cover being unable to answer a very simple question.
Most CMOS and integrated camera provide a DC output with no real need for DC restoration. Very old ones, yes.
At most a DC shift in output is all that would be needed, and that doesn't cause complications like line droop.
-- Mike Perkins Video Solutions Ltd
This hangs "to the side" and does not pass on anything other than voltages to hi-Z measuring/decoding equipment.
Nope. I just think you're struggling to be a horse's ass.
What can I say... my customer makes chips for the security and surveillance camera business. His customers want this function. I aims to please ;-)
And I have Sony circuit examples that do the same functions... my customer's competition. So I do form/fit/function differently enough to be patentable.
You're totally misunderstanding the function.
[snip] ...Jim Thompson-- | James E.Thompson | mens | | Analog Innovations | et |
Updated to...
Video_Restorer_Discrete_NPNs.pdf
to show input impedance/(near)ideal-diode behavior.
[Aside: Does anyone know if you can make 2+ pages as PNG's?] ...Jim Thompson-- | James E.Thompson | mens | | Analog Innovations | et |
Ah, that makes sense for old surveillance cameras.
Gently pushing the signal to 0V doesn't work for OTA video because the H-sync becomes a total wreck. That's where using the sync detectors for restoration does a much better job. (The sync detector gets help from a PLL in OTA decoders.)
Thankfully, surveillance cameras are going digital too. I just bought a nice one that has the entire surveillance control system running on a tiny Linux board inside it. I think it has more computing power than my old Mac Pro.
Not sure exactly what you want, but a PNG is just an image file... it can be any size you like.
I just recently scanned a 2-page article, saving each page as a separate PNG. Then fired up GIMP (Windows Paint would work as well) opened Page 2, selected All, copied to clipboard, opened Page 1, doubled the height of the work area, and pasted Page 2 at the bottom. Worked great, and was about 1/4 the file size of a PDF (but that may have been due to the cheesy free PDF converter I tested against).
You could paste the second page next to the first instead of below it, if you needed that for some reason.
Best regards,
Bob Masta DAQARTA v7.60 Data AcQuisition And Real-Time Analysis
I'm not familiar with that style of sync detector. Do have a link that can educate me?
...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Some old Philips chip would lock to the color burst, then insure it integrated over the color bust interval to get the DC level. Basically the circuit was strobed.
This is tech out of the 70s, and I haven't looked at it since the 80s, so I can't recall the exact details.
I remember when the first DSP decoders came on line. You could cancel out the color burst by adding alternate lines. Or maybe that was only for PAL.
Old TV circuits are art and magic - getting the most done with the fewest parts. I have no idea where to find a good schematic that's legible.
Clean H and V signals were absolutely critical. Not only were they needed to produce a steady picture on a weak signal, but it kept the TV from burning out or, in late models, triggering the X-ray protect circuit. 99% of the TV was powered off inductors driven by sync signals.
I did it in a very crude circuit when I was about 18 years old but I can't remember exactly how it worked. I think a 555 chip generated pulses slightly narrower than sync and used that as a mask on a filtered video signal that went back into 555 pins 2/6. HSYNC and VSYNC were pretty much the same except for polarity and frequency. HSYNC clamped the video going to the gun drive for DC restoration. I was using a CRT module that, thankfully, had X-ray protection.
On a sunny day (Sat, 05 Jul 2014 00:56:46 -0700) it happened Kevin McMurtrie wrote in :
There are some basic things about TV receivers (as of old analog). I have designed several ones, camera too. And that was in the late sixties. Nothing really changed, except more semiconductors.
1) gated AGC. When sync is maximum amplitude in the AM modulated video (the French did it in reverse to the rest of the world), you would, _after_ H lock by a PLL, only look at the signal amplitude during the H pulse for AGC control. 2) sync detection Impulse detection on peak of signal (that now is at a stable level due to AGC). 3) H sync separation CR differentiator 4) V sync separation RC integrator, although later models counted H pulses and what not. 5) H oscillator normally a PLL 6) V oscillator normally was an oscillator running slightly slower than the transmitter, and then hard synched. Later sets used PLL assistance for V too. 7) H deflection Almost always combined with HV generation, flyback, parabolic pulse into magnetic deflection coil. For shadow mask type color sets DYNAMIC geometry correction circuits were needed (east west, cushion, etc), sometimes used transductors to modulate H amplitude. 8) V deflection In the early sets all sorts of correction circuits were added to make a linear ramp in the 44 mH or so V coils. later (I think Philips started with that) a simple linear ramp was generated and the current in the deflection coils measured with a resistor in series with the coils, and compared to that ramp, no more adjustments. That was one of the biggest improvements ever. 9) Audio Old sets were mono, there were basically 2 systems: 1) separate IF for sound. 2) intercarrier sound here the video IF bandwidth was a bit bigger and the 5.5 MHz (Europe) FM component was taken from the video detector, so AFTER AM demodulation. then amplified and limited in a separate simpler 5.5 MHz IF, ratio detector for demodulation. When stereo came a second slightly higher in frequency FM carrier carried L-R, processed in the same way. There also was a sound in sync system, where the sound was in digital packets in the H sync, no experience with that.Video in general: You cannot depend on the sync amplitude being constant EVER. Even in a TV studio the sync is regenerated many times, new sync replacing old. All that counts is the BLACK LEVEL, and white level. Imagine an analog video mixer, getting input from different (synchronized) sources. Apart from the PAL or NTCS color phase requirements, the black levels must be the same. Now if you mastered all that, there are a zillion ways to design electronics that does that, some very simple and some very complicated, the simple often working just as well as the complicated ones, in NORMAL signal conditions.
But remember Einstein said: "Things should be as simple as possible, but not simpler than that." He was not always right (almost never actually), but probably right about that,
So after this long course in TEEFEE in now present to you the SCOPE TV
I did not bother about black level at all, because the scope has a capacitor in the input AFAIK, would have to modify it to clamp at the CRT grid, anyways that was just for fun.
The sync separator is the simplest possible AFAIK. See how it works?
Note the the linear V ramp generator, not even a V oscillator needed (scope wont burn in like a TV CRT).
If you want to be more precise:
This uses a PIC to grab black level during V sync, stores it in the 100 nF cap, and then, during interfering pulses, replaces the interfering pulses by that black level. The H and V decoding and front porch black level timing (sample pulse) generation is done in software. A simple diode clamp clamps bottom sync to make sure the video is in the PIC analog hardware comparator range, there are 2 diodes, running at different currents to make 2 slice levels of the sync pulse. Note the 680 Ohm 330 pF burst filter. An analog switch switches between signal and stored black level.
:-)
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