CRT / VGA - Page 1
Forum section: electronics / beginners

author: 0x007
author_rank: Contributor
timestamp: August 31, 2015, 09:11:54 am
content: Hi ,I am trying to build (playing with) a audio visualiser from an old pc crt screen.I have some general questions about the hardware of a pc monitor:first of all , can I some how control the frequency (wave length) of the horizontal sweep using the vga port ?I've seen a picture of the pinout it has V and H sync, I know that the driver thingy require a synchronization to generate the saw tooth wave so if I'll feed the H sync pin a slower frequency will it sweep slower ?or is it just me misunderstanding TV anatomy?And in case I cant use the VGA input ,can some one point me to some datasheet or a webpage in which I can find the pinout of the crt it self and the voltages required to operate it ?I studied the subject in school but it was all general and I don't know the terminology in English.

author: tszaboo
author_rank: Super Contributor
timestamp: August 31, 2015, 09:29:05 am
content: Yes, it is called refresh rate. The monitor has some specific refresh rate it can work on.https://learn.digilentinc.com/Documents/269This is a good starting tutorial, at least to understand the basics.

author: 0x007
author_rank: Contributor
timestamp: August 31, 2015, 01:07:56 pm
content: I see, I wont be able to force it to refresh at the frequency I intended to, using the VGA port .but I can control the color and the brightness, I'll have to build an external oscillator and a current amplifier to drive the coils directly.

author: 0x007
author_rank: Contributor
timestamp: August 31, 2015, 01:11:30 pm
content: Does it have to get a synchronization signal to keep the screen from going to sleep ?

author: helius
author_rank: Super Contributor
timestamp: August 31, 2015, 01:39:06 pm
content: The monitor is more than just signals going directly into the CRT. It has several boards inside it that control signal amplification, horizontal and vertical scan, and sometimes dynamic focus or color convergence.Most CRT monitors are multiscan, which means they have a series of resolutions/refresh rates, and attempt to lock on to the signal that most closely matches one of those settings. This will be detailed in the monitor's operation manual. There were monitors that were not multiscan (called "fixed frequency" monitors), but those were mostly not used with PCs.If the vertical or (especially) horizontal sync frequency is outside the monitor's compatible range, it will not lock and instead display a pattern of moving noise. A few had "V-hold" or "H-hold" knobs so you could attempt to re-lock the signal, but this ended when they began using digital microcontrollers."Sleep" was a feature on some late-1990s displays that were made to save energy (they will have EnergyStar labels). It just means the CRT circuits will be disabled when just 1 of the H-sync or V-sync signals is missing.Each CRT tube will have a different pinout, there were dozens of arrangements in use. Driving a tube directly is no simple matter, there were special circuits and components designed for a reason.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2015, 03:34:12 pm
content: Old (VGA and maybe SVGA) monitors only work over a limited range of frequencies. Outside those frequencies, they might not lock (internal oscillator circuits go on at their own rate, essentially ignoring your input), or they might catch fire (there were instances of hardware from the 80s that could be damaged by setting incorrect timing values in the display controller chip).Modern "multisync" monitors (high resolution SVGA, Trinitrons, compatible LCD panels) detect the rate and adjust their internal settings as necessary. Though if you aren't driving them at known standard rates (there is a formula to calculate the scan timings needed for a given resolution), they might complain ("input out of sync range") or end up garbled (but probably not catch fire).Something that may not be obvious at a glance: your HSYNC and VSYNC signals need to be synchronized with each other. Back in the day, this was done with a series of frequency dividers; now it's done with digital counters and a master clock. It won't suffice to set up a 555 timer at 31.25kHz and another at 60Hz -- one needs to be triggered by the other, but with a large (and stable) ratio between them. This is the kind of thing you might as well generate from a microcontroller, or even a proper CRT controller IC.Tim

author: Chris C
author_rank: Frequent Contributor
timestamp: August 31, 2015, 08:58:56 pm
content: For this, the older and more simple the monitor, the better.I did this in the early 90's with an old monochrome screen. No sleep mode. No multisync. No self-diagnostics to shut it down when I disconnected both deflection coils, and hooked them to the left and right channels of a stereo.Of course, that wasn't very smart. I wasn't changing the brightness according to how fast it was scanning. So when there was no audio, the electron gun remained centered, focusing sufficient energy to light the whole screen into a single point. It instantly burned out the phosphor there, leaving a permanently dark spot, and a smoke trail rising up the inside of the screen.At the time I wasn't sure how to make an automatic brightness circuit, so I ended up reconnecting the vertical coil to the internal circuit. That way the gun was scanning, at least a bit, at all times. Then I set the brightness so that it wouldn't cause any damage when there was no horizontal signal.

author: 0x007
author_rank: Contributor
timestamp: September 01, 2015, 06:43:01 am
content: Yeah older more simple is better for sure, I had this idea for a long time and just now I managed to find an intact monitor.we have scavengers that smash them to pieces and pull out the copper coils, so this kind of things doesn't live long when thrown out, and other then dumpster diving I dont have a way to get my hands on a crt.now I am having a different problem , the coil driver seems to overheat or detect that the horizontal coil is not attached.it makes the screen blink (the vertical line).I didn't managed to fool the driver it requires a coil , so I think I have to bypass the electronics and go directly to the crt leads , I know that the high voltage circuit works and only the RGB signal is switching on and off so if I'll hook a voltage directly to this leads it will work, if I recall correctly the voltage supposed to be 8 or 15 volt .

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2015, 02:46:17 pm
content: A fixed scan CRT requires that the deflection coils be connected, otherwise the high voltage is all out of whack too.You could get two sets and leave one coil plugged in, off to the side.Good monitors have built in blanking which disables the beam when deflection is lost; this is what a partial failure looks like on a Trinitron set.http://seventransistorlabs.com/Monitor/Images/Symptom.jpgNote the narrow scan (deflection failure), dim picture and odd colors (seems it's just crudely cutting off all colors, with no particular care for what order; which makes sense, it's only protection).This would be harder to implement using external deflection coils.Tim