120Hz as a PC monitor - safe choice?? - Page 2

You have it backwards. TV sets have the powerline sync feature, it's implemented in the high voltage anode supply for the CRT and the technique was invented back in the 1950's, so maybe you don't remember the old B&W sets that did have hum bars before then. The H and V sync signals you are discussing are sent through the sweep amps, and those signals are NOT AT ALL RELATED to the flicker suppression feature. But the CRT anode supply uses an unfiltered 60Hz half-wave anode (because there is only one high voltage diode, or one vacuum tube rectifier in older designs) and thus the CRT intensity is modulated by the powerline frequency precisely in phase with the flicker from the flourescent lights. The two light sources (the screen and the flourescent tube) cannot get out of sync as long as they are powered from the same 60Hz generator.

The PC monitor bases the screen refresh off of the motherboard clock as delivered to the video processor on the video board. There is no synchronization with the powerline because th PC is DC-powered. That is why when you set the monitor screen refresh for a 60Hz refresh, you DO SEE a throbbing on the screen which depending upon just how different the PC's 60Hz is from the powerline, could be described accurately as:

1) Flicker.
2) A massive hum bar.
3) A heterodyne difference frequency with the powerline.

virtu is also correct about phosphor persistence, because TV phosphors are designed to optically blend togather the Odd and Even interlacing fields into one image. But the glow from the screen is still modulated by the anode voltage as long as the set is powered.

Now roll forward to LCD designs with CCFL backlighting. Yes it is true that the inverters run at high frequencies. It is also true that a 60Hz sine wave is imposed over the high voltage for precisely the same reason as with the old CRTs - because modulating the backlighting in precise phase with room lighting reduces screen flicker. Same deal with plasma displays - the plasma high voltages must be modulated with 60Hz to avoid flicker.

The modern sets require far more components for flicker reduction than do the old NTSC televisions. In an old NTSC set there was a diode in the fat anode wire that attached to the side of the CRT. The only anode filtering was the coating applied both inside and outside of the CRT, called "Aquadag" or something like that. The glass envelope of the CRT was the dielectric insulation for a capacitor that was used to filter out the high voltage sweep frequencies from the flyback transformer in the anode supply. This small (value) capacitor did little to suppress 60Hz ripple, thus providing effective flicker reduction at little added cost and zero added parts.

Sorry for the history lesson, but I had my face rubbed in flicker reduction technology in my first job in the computer industry, back when we used large mainframes with attached "green screen" CRT terminals. Flicker reduction circuits on those required manual adjustments, unlike television sets.

Do you understand what this means?

The anode voltage is developed by the flyback transformer, which operates at the horizontal rate (15.734 KHz for NTSC)...not 60Hz, it is rectified and filtered to about 25KV DC.


The CRT is refreshing at 59.94 Hz, not 60.



there is no 60Hz modulation of a CCFL backlight...where are you getting this from?

You said the opposite in a previous post, but you are correct now. There has never been powerline sync in a computer monitor, or a colour TV.


Plasma displays don't need, nor do they use high voltages.


Yes, it's still there, it's called a rectifier and is required to turn the high voltage 15KHz AC from the flyback transformer into DC.


Aquadag is a conductive coating used to electrostatically shield the tube, it's connected to ground in all Tv's and computer monitors. It doesn't filter anything and is not connected to the anode at all.


it still is, the capacitance doesn't need to be high since the frequency is so high.


This doesn't make sense. If the capacitance is too small to "supress" 60Hz ripple, then it's not going to be effective at reducing flicker. But it doesn't matter since this isn't based in reality.


I'm sorry that you think any of this is history...it isn't. you really don't have a very good understanding of CRT's.

duvetyne, you seem to be woefully lacking in your knowledge of the history of CRT tech.

I'll explain one little part because it's an elegant example of EE design.

Flicker is experienced in an old vacuum tube television because there is a difference in powerline frequencies between the TV studio and the viewer's receiver. The EE realizes WHY this is happening, and the solution is:

1) Redesign the high voltage power supply to use a cheaper auto-transformer in place of the more expensive high-frequency ferrite transformer originally used. The bottomside is not grounded, it is instead attached to the 250v anode on top of the tube driving the flyback autotransformer. Money is saved in the cheaper transformer. No safety issue here since the whole flyback circuit is shielded in sheetmetal.

2) Eliminate the expensive electrolytic filter capacitor that suppresses 60Hz ripple in the B+ supply for the driver tube. Allow 60Hz ripple to be imposed upon the high voltage oscillator output from the flyback circuit used to generate the (approximately) 25K-volt CRT anode voltage.

3) Aluminize the inside of the CRT envelope using vapor deposition. Create a (small capacitance, high voltage) capacitor between the aluminum coating inside the CRT (which is at CRT anode voltage) and the Aquadag coating on the outside of the tube, which is as you said, grounded. The CRT glass envelope is the dielectric and effectively becomes a physically large, but small value conventional non-polarized capacitor. The value of the cap can be adjusted by varying the area covered by conductive Aquadag. Much money is saved in a heat-resistent high voltage electrolytic capacitor, and relaibility is increased by a simpler high voltage supply with a lower parts count.

4) Now tweek the capacitor value to suppress the high frequency (my terminology, you spelled it out as 15.734 KHz) oscillator frequency, without much effect on the 60Hz power supply ripple (because the very small capacitance does not do much at 60Hz). The net result is a CRT anode modulated with 60Hz ripple. The CRT intensity and the room lighting now vary in intensity exactly in phase because they are powered from the same 60Hz source. A major source of flicker is eliminated by cheapening the receiver design.

==> This is a classic EE design example found in many textbooks on circuit design. As to WHY the details of the design got pounded into our heads, the glass CRT envelope is a very high quality capacitor, with effectively zero leakage. The full anode voltage of the CRT remained present in this cap after the set was turned off, ready to shock an unwary technician who did not discharge it to the chassis before unplugging the connector on back of the CRT neck. That happened to me 40-odd years ago and I still remember why. Evidently you were spared this experience or you would also remember.

The context involved a colour TV...which has a fixed refresh rate, not related to the powerline frequency in any way, and the decay time of the phosphors was chosen to reduce flicker....see interlacing for more information.


I know, but it contains very little fact.

There is already a thread that clearly describes why CRT, PDP, LCD do or do not flicker.

Also, Plasma displays use ~200-300 volts to operate. I guess it is relative if that is considered high voltage or not.

See, you missed my point entirely. I was not talking about any sync frequencies embedded in the video. I was talking about how a 60Hz power supply ripple imposed on the CRT anode causes the CRT brightness to vary at 60Hz - exactly in phase with the room lighting. You are off in a different direction, talking about the H refresh rate and other flicker mechanisms associated with that.

Heterodyne is also real whether one is talking about airplane engines or light sources pulsing at slightly different rates. The difference frequencies that result from the two sources are also real.

Many people study Physics, few ever use it to understand the world around them.

Spock to Kirk: "The prefix code?"
Kirk to Spock: "It's all we have."
Lt. Saavik to Kirk: "I don't understand."
Kirk to Lt. Saavik: "You must learn how things work, lieutenant."

got a link for that one? Please paste it here.

The problem with your theory is that it isn't true. There is no 60Hz ripple "imposed" on the anode voltage.

I can tell you haven't spent much time around old NTSC vacuum tube televisions. I trust my EE-378 professor who taught us about this as a literal "textbook example" of good electrical engineering design from the 1940's (he was there). But I'm not going to spend any more time trying to explain it to you. You have a nice day now.