How did CRT TV sweep down 1 row at a time ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 28, 2018, 12:44:44 am
content: For the horizontal sweep I'm thinking the horizontal deflection plate voltage is like a ramp generator, with a smooth continuous linear slope, not stepped. The firing of gun is step-wise tho (on/off depending on image).But what about the vertical. Wouldn't the voltage on the vertical def. plates look like a big staircase.So how did early TV's make a big staircase like ramp voltage ?Or is it also a smooth, continuous, linear slope, which should mean the 1st pixel of a row would be higher than the last pixel, even if we don't notice it?

author: sokoloff
author_rank: Super Contributor
timestamp: July 28, 2018, 12:48:10 am
content: I’ve always assumed it was the latter.And then, you could rotate the whole setup counter-clockwise slightly to remove some (not all) of the skew.

author: rs20
author_rank: Super Contributor
timestamp: July 28, 2018, 12:52:29 am
content: Smooth and continuous, depending on your philosophy/point of view. Even if the small skew that would result would be visible to the human eye, which I doubt; it can also be corrected for by putting the opposite tiny skew into the construction of the coils, or subtracting a tiny version of the horizontal plate voltage from the vertical voltage in order to undo the skew (or to put it another way, generate a suitable approximation to the staircase waveform that you describe).CRT computer monitors have explicit settings which allows you to mix the deflection voltages in any way you want to achieve arbitrary skews and rotations; removing one pixel's worth of skew is a tiny and trivial adjustment comparatively.

author: BrianHG
author_rank: Super Contributor
timestamp: July 28, 2018, 01:06:42 am
content: Vertical deflection is usually smooth.The yoke is just rotated ever so slightly to correct the horizontal lines. This is usually easy with monitors which only support a small range of scan rates. The ramp oscillators were usually almost all analog circuits. Another fix is to tie a high frequency pass filter, IE series cap+resistor, from the H-yoke to the V-yoke to bend the beam a little.As for advanced very advanced multiscan monitors with a wide frequency range for horizontal and vertical, with all OSD controls, they have 1 or 2 software DSP IC synths, usually both identical, 1 for vert, 1 for horiz, (DONT ASK WHY it was like this, these were patented IC and all my studio grade 21 inch screens had them except for the earliest which had a massive shielded huge DSP/FPGA brick with 2 dacs in it...) they synthesized the horizontal and vertical separately driven to 2 amps to drive the yoke.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: July 28, 2018, 03:44:14 am
content: Quote from: lordvader88 on July 28, 2018, 12:44:44 amYou are reading current technology into the past when it is just not there. Analog TV did not have "pixels". Each "horizontal" line of video was a continuous waveform which represented darker or lighter areas. After demodulation, a higher voltage in the signal would drive the electron gun to emit more electrons to hit the screen. A lesser voltage would do the opposite. If a brighter patch of a pictures line was 10 uSec long or 1 uSec long or .12345 uSec long, that's what it was. There was no transition from one time increment or "pixel" to the next. The gun was not "fired" in a series of bursts, it was just on with the smoothly varying brightness voltage that was the video signal.As you say the horizontal sweep voltage applied to the CRT was smooth ramp at 15,734.3 Hz (in North America after 1953) and moved the electron beam smoothly from left to right. The ramp ended at the right side of the screen and was reset as fast as could be done to move the beam back to the left side of the screen. The beam was usually blanked blacker-than-black during the reset so that the retrace would not be seen on the screen.The vertical travel was similarly controlled by a smooth ramp from top to bottom at 59.94 Hz. Because neither horizontal nor vertical sweep were "stepped" each horizontal line was just slightly slanted downwards as the beam traveled from left to right. The slant was not seen for two reasons. First because the camera was doing exactly the same thing at the same time so ITS scanning was just slightly slanted to match. Secondly, the slant was so small as to be invisible, anyway.In an attempt to scan the picture at close to 60 Hz (to reduce flicker) without having to actually use the video bandwidth necessary for 60 Hz scanning of all lines, a compromise was used. That is, that the video frame was created and transmitted as two "fields" of video that made up the whole frame. Frames were scanned only 30 fps and made up of two fields. The lines of one field would be interlaced with the lines of the other. The horizontal scanning of the CRT was arranged so that that would happen in both the camera and the receiver. The lines on the screen were then counted down from the top of the screen as 1, 263, 2, 264, 3, 265,... down to 262, 525 at the bottom (at least in North Am.). This arrangement put some very interesting complications into the synchronizing waveforms that were part of the transmitted signal.I just scanned the Wikipedia entry on NTSC and see that it could be very confusing if you didn't understand the above basics, but good luck if you want to look it up because it's all there.

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 28, 2018, 06:23:21 am
content: Ahh interesting, thanks folks. I watched a video of making old style CRTs, and it never occurred to me that there were no separate pixels.As a kid I had a B/W vacuum tube TV. Must have been 10-12". I wish I wish I wish that was never thrown out.1 of these days I will get an old broken v.t. TV from ebay to play with. Lots to learn yet.

author: Brumby
author_rank: Unknown
timestamp: July 28, 2018, 06:37:06 am
content: Monochrome CRTs were a single phosphor surface - but colour screens were different.The style I have known had a very carefully arranged array of 3 different phosphors and a shadow mask located inside the picture tube just behind the front face, aligned between the electron guns and the phosphor 'dots'. Each of those dots was a sub-pixel.The scanning process was essentially the same, but with the screen now masked, higher EHT voltages were required to get the brightness back up.

author: CJay
author_rank: Super Contributor
timestamp: July 28, 2018, 08:01:18 am
content: Quote from: Brumby on July 28, 2018, 06:37:06 amExactly that, monochrome CRTs are also capable of much higher resolution than colour CRTs and LCDs, both of which have a defined maximum horizontal resolution.Both horizontal and vertical deflection were a sawtooth waveform because the energy required to deflect the beam over the CRT face varied with beam angle.

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 28, 2018, 08:17:47 am
content: I've scrapped probably 10 CRT TVs for parts. I had a working 12" that I was going to poke around on, but some roommate took it from the basement. Time to find another 1.I have caps/resistors to make a HV probe like EEVblog Dave and his buddy made. Time to make that too.

author: CJay
author_rank: Super Contributor
timestamp: July 28, 2018, 10:01:22 am
content: Be very careful with HV probes, buy named manufacturer parts from a reputable, traceable supplier and make sure you have a datasheet for them that specifically states the voltage rating.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: July 28, 2018, 06:09:23 pm
content: Quote from: lordvader88 on July 28, 2018, 08:17:47 amTime to get another roommate, too. A more honest one.

author: strawberry
author_rank: Super Contributor
timestamp: July 28, 2018, 07:01:52 pm
content: http://obsoletetellyemuseum.blogspot.com/2010/06/cathode-ray-tube-displays-case-study.htmlThey are not dangerous, simply don't poke around with both hands. CRT tube is like large capacitor, and glass surface(dirt , moist..) can conduct electricity about 20..~50mm apart from anode plateVertical deflection signal is sinus like shape. Horizontal deflection have sawtooth shape, at the end there is beam roll back signal....

author: BrianHG
author_rank: Super Contributor
timestamp: July 28, 2018, 07:27:15 pm
content: I didn't realize that the OP was at the absolute basic level on CRT tech and CRT color tech. I forgive that my earlier response was intended for someone who was way more advanced and wanted to know about sophisticated super high end computer CRT tech.Here, watch these videos and you will see everything you need to know about CRT driving, that there are no pixels and why color CRTs also truly have no color pixels...If you need more, let me know...

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: July 28, 2018, 07:33:40 pm
content: The deflection was magnetic, not electrostatic because of the wide deflection angle needed.The vertical (field) deflection was done with something much like an audio amp driven with a sawtooth signal, except that the feedback was taken from a current sensing resistor in series with the deflection yoke. At this low frequency the coils behaved much like a speaker, having mostly resistance but some reactance. Hence the current in the coils was an accurate sawtooth, but the voltage across them was a somewhat distorted sawtooth due to the inductance.The horizontal (line) scan at 15kHz could not be done that way because the coils had too large a reactance, in fact they behaved like a near-perfect inductor with minimal resistive loss. To achieve flyback in a reasonable time called for a large voltage, typically 1200v, to be applied for a few microseconds.The way this was achieved was similar to IC engine ignition - the primary of the flyback transformer (or choke, as far as the scan circuit was concerned) was connected across the DC supply by a special high voltage transistor such as a BU508, the current allowed to build up, then the circuit abruptly opened. The sudden collapse of the magnetic field in the transformer created a large voltage spike, the flyback voltage. This was applied to the scan coils via a capacitor, and caused the spot to rapidly return to the LHS of the screen.The forward scan drive required nothing more than a fixed voltage to be applied to the coils. Remember they behave as a near perfect inductor, hence with a fixed voltage applied the current, and hence deflection, would rise at a near linear rate. This forward voltage was provided by the charge induced on the series capacitor during flyback.Consequently the scan waveform looked like a half-sine pulse a few microseconds long, of ~1200v amplitude at the end of each scan. You needed a x100 probe to scope it - a x10 probe could fail as they are typically not rated for that voltage. The DC scan voltage was not greatly noticeable due to its much smaller value.-and no, the BU508 was *NOT* driven with a sawtooth waveform. It was driven with a square wave of slightly more than 50% mark to space ratio.Why >50%? Because it also served as a return path for the scan current on the second half of the scan. On the first half the current was in a negative direction and was carried by the reversed C-B junction. Or, by a separate diode. The second, positive, half of the scan current required the transistor to be already on when it commenced.As with SMPS practice today, the BU508 had to have a fully saturated 'on' interval and a clean, sharp switch-off otherwise it would dissipate substantial amounts of heat due to resistive loss. When the transistor failed repeatedly (in spite of no obvious picture symptoms) it was usually because of a driver stage fault giving weak base drive.The scan voltage was the one to really watch out for, safetywise. 25kV EHT could give you a painful zap, but it was usually just that. A kilovolt at a substantial current could terminate your existence. Period.