Why weren't electrostatic deflection CRT's more popular for TVs and monitors? - Page 1
Forum section: electronics / general

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 26, 2023, 07:53:03 pm
content: Isn't it cheaper, more compact, and ever so slightly more energy efficient to use electrostatic deflection?Why didn't more TV's use electrostatic deflection? was it problematic to get enough deflection as the screen size grew?

author: Berni
author_rank: Super Contributor
timestamp: August 26, 2023, 08:12:04 pm
content: Electrostatic deflection is weaker. So it involves a lot of voltage to drive the deflection plates while not providing as much deflection, so the CRT ends up being pretty long. Not a problem for a scope but a big problem if you are making a 35 inch TV.Magnetic deflection is plenty fast enough too. You can do 1080p like resolutions on a CRT just fine. We didn't have TV broadcasts that high resolution back then and computers typically ran at around 1024x768 back then. So we didn't need more speed anyway.The higher acceleration voltages of color CRTs also appeal to magnetic deflection, since electrostatic plates have less time to do the deflection with faster electrons.EDIT: Oh and you are getting your electrostatic deflection back in the form of DLP chips and micromirror scanned laser projectors

author: TimFox
author_rank: Super Contributor
timestamp: August 26, 2023, 08:23:18 pm
content: Yes, deflection angle.Consider scaling up a 5CP1 CRO CRT from a 5-inch envelope up to even a 21-inch envelope.Early TVs mounted the CRT vertically with a viewing mirror for the relatively small screen.

author: N2IXK
author_rank: Frequent Contributor
timestamp: August 26, 2023, 08:39:47 pm
content: Because as screen size/deflection angle increases, so does the voltage needed for full deflection.The early electrostatic deflection TV sets were mostly 7" CRTs, and they needed 3-400V of drive to sweep the beam across the screen, The amount of voltage to get full deflection on a 20"-25" CRT would be MUCH higher. Magnetic deflection is used to eliminate the need for high voltage deflection amplifiers.The main advantage of electrostatic deflection is high deflection bandwidth, which is important in an oscilloscope CRT, but MUCH less important in a television application, where raster scanning is done at relatively low, fixed frequencies. The deflection amplifiers to drive a magnetic deflection yoke are simpler and lower cost, and the horizontal amplifier can be made to double as the driver for the CRT accelerating voltage power supply. The early electrostatic TVs needed a separate HV power supply to provide the accelerating voltage for the CRT.

author: TimFox
author_rank: Super Contributor
timestamp: August 26, 2023, 08:52:02 pm
content: By the time of my RCA RC-24 Receiving Tube Manual (1965), magnetic-deflection monochrome CRTs could do 110 to 114 degrees total deflection.Note that the energy used for the higher-speed horizontal deflection was not wasted:The linear increase in current during the horizontal scan was done by switching a low-resistance tube in series with the horizontal coil and the DC supply: to first order, dI/dt=V/L.During re-trace, the current collapsed into a "damping diode", and the induced voltage went into the high-voltage rectifier tube from a higher-voltage tap on the deflection transformer to generate the CRT voltage.Hence, the term "flyback" supply, still used for other purposes.Later, as transistors entered TV sets, the flyback supply was used to generate the low voltage required for the solid-state circuits, avoiding a heavy power transformer.Early TVs used plate-capped 6L6 variants (807, 6BG6), but later "sweep tubes" such as the 6DQ6 were designed for that service.For both horizontal and vertical deflection in a periodic raster, the DC component of the deflection coil current is zero, so the waveforms can be transformer coupled.The slower vertical waveform used an actual sawtooth power oscillator; the controls for both horizontal and vertical "hold" and "linearity" required manual adjustment, and the drives for both were derived from the sync pulses extracted from the composite video waveform.

author: TimFox
author_rank: Super Contributor
timestamp: August 26, 2023, 09:28:01 pm
content: Magnetic deflection is used in other applications, such as electron microscopes and particle accelerators.I believe there is no real frequency limit, so long as you can drive the inductance of the coil.Of course, RS-170 (US monochrome TV) uses 60 Hz vertical and 15,750 Hz horizontal for 525 line interlaced.1080p HDTV with color CRTs needs 64.8 kHz horizontal for 60 Hz vertical.Since rasters are produced with two fixed-frequency periodic waveforms, "bandwidth" applies to the "video" signal, modulating the beam intensity, done with a grid in the usual vacuum-tube manner.

author: bdunham7
author_rank: Super Contributor
timestamp: August 26, 2023, 09:31:46 pm
content: Quote from: ELS122 on August 26, 2023, 09:20:19 pmA 1600 x 1200 CRT monitor at 120Hz would be 144kHz and that would be a pretty ambitious specification. I suppose you could go quite a bit faster with a smaller tube if you didn't care about efficiency or precise geometry.

author: TimFox
author_rank: Super Contributor
timestamp: August 26, 2023, 10:10:37 pm
content: Quote from: ELS122 on August 26, 2023, 09:49:26 pmYou can design a lower inductance deflection coil, requiring more current for the same B field, but keeping the voltage down.Yet another quantitative question.In very high energy electron accelerators, magnetic deflection is always used, since electrostatic deflection would require excessive voltage.Yes, normally the video signal is connected to the cathode, but that modulates the grid-cathode voltage against a fixed grid-1 voltage.The circuit shown in my RCA RC-24 has the brightness control driven by the supply of the grid-2 of the horizontal-output tube, and then coupled to the cathode to set the DC level there.Better TV systems included a proper "DC restorer" (misnomer) circuit instead of just C-R coupling, to keep the black level constant against changes in the brightness range of the video.

author: TimFox
author_rank: Super Contributor
timestamp: August 26, 2023, 10:24:46 pm
content: Yes. The effect depends on the g2:g1 mu factor in a tetrode.Also, the polarity of cathode drive vs. grid drive is opposite in current changing.

author: TimFox
author_rank: Super Contributor
timestamp: August 26, 2023, 11:01:39 pm
content: Back to the question about required voltage to drive magnetic deflection at high rates.Magnetic deflection "yokes" are multi-turn coils with a shape designed for the desired geometry of theBfield induced therein.The number of turnsN(at least one) is the choice of the designer.Now, for a givenBfield, the currentIis proportional to 1/N, sinceB=aNxI, whereais some constant.For a given waveform, the derivative dI/dtis proportional toI, and therefore also proportional to 1/N.However, the inductanceLfor a given geometry is proportional toN2.Therefore, the required drive voltageV=LdI/dtis proportional toN2x (1/N) =N.We see that a givenBfield, under these constraints, requires a voltage drive proportional to the number of turns, and we can trade off drive voltage for drive current by changing the number of turns within a given winding geometry.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2023, 03:37:26 am
content: I respectively mostly disagree with every reason given above. The reason is because of the voltage difference between the electron gun(s) and electrostatic deflection plates.In an oscilloscope CRT, the electron gun operates at about -2000 volts so that the deflection plates can operate at 0 volts and the deflection amplifiers can be direct coupled. This requires the z-axis amplifier to be coupled into the grid with a DC restorer circuit because it has to drive the grid at a potential of about -2000 volts, which would be unacceptable with a television CRT, although it could be done.Alternatively if the oscilloscope CRT operated the electron gun at zero volts, which would considerably simply the z-axis circuits, then the deflection plates would be at 2000 volts which is completely unacceptable, although some very early designs did this.With a television CRT, magnetic deflection provides acceptable performance and it allows the electron guns to operate at zero volts for easy coupling to the red, green, and blue drivers without DC restoring circuits. I suspect magnetic deflection was more effective in this case simply because the pre-deflection acceleration voltage is lower than with an oscilloscope CRT, however I did not find any service manuals showing exactly what was going on and the online references I found did not discuss the most common type of CRT.

author: Berni
author_rank: Super Contributor
timestamp: August 27, 2023, 06:01:20 am
content: Quote from: bdunham7 on August 26, 2023, 09:31:46 pmYep my last CRT was running at 1600x1200 at 80Hz.It was one of the better CRT monitors both in terms of specs and image quality.It was pretty common for computer CRTs to support resolutions this high and fast as long as you didn't get the cheap ones.The limit is simply how hard you can drive the deflection coils. More drive voltage = more sweep speed. Tho i suppose at some point the windings might arc over, but it would have a pretty monsterus drive amplifier at that point. It was more that we didn't really need more resolution back then.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2023, 08:31:17 am
content: Quote from: ELS122 on August 27, 2023, 07:03:00 amColor CRTs always seemed to be higher resolution than B&W CRTs.It might have had to do with acceleration voltage and secondary emission. Without a shadow mask, the high acceleration needed for a small enough spot size on a B&W CRT will produce secondary emission from the phosphor resulting in a circular "spot" around the point where the electron beam hits. This will not be visible as such, but it will severely reduce the contrast like it does with a bright oscilloscope CRT.I think a shadow mask captures the secondary emission so the contrast is higher.

author: Berni
author_rank: Super Contributor
timestamp: August 27, 2023, 10:00:52 am
content: Shadow mask spot size =/= resolution.We figured out how to make really fine pitch shadow masks, so for those the spot size of the electron beam would tend to be bigger than the pitch of the shadow mask. But TVs did indeed have pretty huge pitch shadow masks because you don't need anything finer when displaying such a low PAL/NTSC resolution on a huge screen. The big masks help with brightness, but they are always a fair bit finer than the CRTs designed resolution.Some cheap PC monitors also skimp on the shadow mask pitch, but the good one have one so fine you can't see it with your bare eyes even when pushing your face right up to it. More of a problem is that CRT computer monitors didn't tend to come in sizes big enough to be useful at 4K resolution (remember that UI scaling was not a thing in the year 2000). But if you are to find a large enough CRT tube with the nice fine shadow mask, it could certainly do 4K. Tho i wouldn't expect the geometry to be within anything better than a dozzen pixels across the whole area without employing some fancy calibration methods.Don't think you could even get that much resolution from any normal consumer graphics card back then or even had a standard video connector that can handle it. The early digital video standards didn't have the bitrate to handle 4K while the analog standards would need a pretty huge amount of bandwith in the video DAC to pump out so many pixels. Heck even reasonably modern standards struggled with 4K when it arrived in the consumer world, HDMI could only do it at 30Hz

author: Berni
author_rank: Super Contributor
timestamp: August 27, 2023, 10:40:33 am
content: Quote from: ELS122 on August 27, 2023, 10:18:05 amBut you you still have the diameter of the electron beam.There are limitations to how tight of a beam you can make with the available voltages and distances in the electron gun, while still shooting enough electrons for a bright image. So you don't get 'infinite resolution' just by going to a BW tube. Sure you can sweep it infinitely finely, but any detail smaller than the diameter of the electron beam just blends together into a blurry blob.

author: TimFox
author_rank: Super Contributor
timestamp: August 27, 2023, 01:39:47 pm
content: Quote from: ELS122 on August 27, 2023, 07:03:00 amBack in the 1970s, there was a 10,000 line resolution b/w CRT, I believe the vendor was Celco.An old paper from them:http://celco-nj.com/SID_Paper01.htmIt was used with a good film camera for high-resolution imaging.As discussed in that paper, they used magnetic deflection: since magnetic deflection does not change the kinetic energy of the electrons, the electron optics to maintain a small spot size are better.