The other question is “why don’t you see that noise when the TV is tuned in?”
The TV has automatic gain control. When the signal is weak, it will amplify it up to the right level. If the signal is just background noise, then it will amplify that background noise until it’s at the “right level” for a proper TV signal. So, the gain is lower for stronger signals, and very high when there is no signal at all.
Fun fact: B&W TVs are more suitable as computer terminals because they have more bandwidth for the luma signal. Color TVs have to filter out the chroma signal, which reduces the bandwidth for the luma. If you are stuck with a color TV for your terminal, consider modifying it to accept a signal after the low-pass filter. This is not too hard for someone comfortable with a soldering iron, someone who isn’t scared of drilling a hole in their TV.
For those who are interested, automatic gain control on VHS players was exploited by Macrovision to prevent the copying of commercial tapes. Analog video signals contain synchronizing pulses, which are necessary for correct sweeps. These pulses are not part of the visible lines shown by the TV. A Macrovision tape contains spikes of varying amplitudes within the synchronization pulses. The spikes are placed far enough from the visible lines (usually the first 40ms of a frame), so that the TV's AGC is not affected, and the picture remains stable.
When a VCR is receiving a Macrovision signal to record, the AGC (which unlike the TV does care about the first 40ms of a frame) amplifies the signal, causing the visible lines to be heavily altered in an erratic fashion. This sometimes appears as "snow" on the duplicated tape.
Some modern "VHS-To-PC" device drivers/conversion software will detect a Macrovision signal and refuse to accept it.
It was defeated in numerous ways, but the hassle free way to duplicate a Macrovision tape was simply to use an older VCR without an AGC circuit.
> If you are stuck with a color TV for your terminal, consider modifying it to accept a signal after the low-pass filter. This is not too hard for someone comfortable with a soldering iron, someone who isn’t scared of drilling a hole in their TV.
Even better, if you want a color terminal, feed the original RGB signal (or its approximation) directly to your TV, don't use RF modulators or a composite input, so there's minimum degradation of video quality. It should be easy on a relatively modern TV. Follow this flowchart.
1. Is it an European TV with a SCART connector? If yes, it already has RGB input. [0]
2. Does the TV include a YPbPr component input? If yes, read Linear Technology Application Note #57 [1], which shows you a circuit to convert RGB to YPbPr using LT6550 chip (or find a similar commercial converter box).
3. Does the TV have an S-video input? If yes, read Analog AD725 datasheet [2] and use this chip to convert RGB to Chroma-Luma component signal for S-Video (or find a similar commercial converter box).
4. Does the TV have onscreen display (i.e. menus overlaid on top of the image)? If yes, there already exists an RGB signal in the TV for feeding the OSD. You can hack your TV and expose the RGB signal input for your own use [3]. Caution: high voltage, discharge the tube, and never work on it with power on.
5. If everything else fails, don't forget the fact a TV's electron gun is ultimately driven by an RGB signal. You can design a simple driving amplifier to feed the RGB signal directly to the electron guns. Unfortunately, I cannot find a reference design for the moment. But it's doable.
My advice was aimed at people living in 1980 who had a spare TV they could use for the computer. I suppose your advice is good for people living in 1990 who don’t have a computer monitor yet.
Well timed. The appnote [1] I referred to, is a collection video circuits published in January 1994. Linear's effort to republish its old application notes at the beginning of the 21st century is respectable, lots of valuable documentation and tutorials from the 80s are still available in high-quality digital formats.
I get you're excited, but old school CRTs can easily kill a person with the discharge off a capacitor, even when they've been unplugged for a while. It may not be a good idea to casually encourage people to go start hacking on them.
Note that suggestion #1, #2 and #3 do not involve modifications of the TV set (and the voltage is a small video signal), so it's risk-free, and should already covered the vast majority of the cases. As for #4, and #5, are more of a possibility and requires knowledge of the internal TV circuitry, those who already have the knowledge to do so, should already be well aware of the dangers involved.
Yeah CRT anything is bad news to go tinkering with. High voltage components and the tubes can violently explode if you compromise them, I tossed a monitor into an empty dumpster once and very nearly soiled myself as it happened to fail just right. If you have one open and the CRT tube fails you could very well be picking tiny shards of glass out of your person.
Actually the high voltage anode lead is very well insulated; if it isn't any more, you'll be sure to notice it quickly (smell, arcing noises and lights, TV stops working eventually). However, in a typical CRT drive circuit you still may find somewhat high-power 100-200 V DC supplies (for deflection drivers and also what powers the LOPT) and low power ~1-3 kV supplies for focus control and such.
A decent TV/CRT monitor should have high-value bleed resistors to discharge the capacitors, but since high-voltage resistors cost money they probably stopped fitting them about the same time as TV's stopped coming with circuit diagrams.
Even if they are installed you can't know if the resister has failed open until you try it. Always assume the tube is energized until you've grounded it.
In the later apple][/C64/etc era most of the color composite monitors also had that B&W switch built in. That way you flip it when editing text/etc vs playing games. The one I had also had a green screen button, which turned it into a green/black monitor.
The 8-Bit guy did a great piece on CGA that goes into detail about how old developers abused the fuzzy smeary nature of composite signals to output somewhat decent graphics. The tricks however didn't work when people upgraded to proper monitors and as a result a lot of CGA games look much worse today than they did on the intended equipment of the day.
I didn't understand your explanation but I think there is something (similar to car radios today) that detects if you are on a signal or not. And if you are not on a signal or it's too weak, it turns the screen black automatically (or mutes the radio). I have seen this happen on a TV when a bad storm weakens the signal from an antenna, for example.
The TV has automatic gain control. When the signal is weak, it will amplify it up to the right level. If the signal is just background noise, then it will amplify that background noise until it’s at the “right level” for a proper TV signal. So, the gain is lower for stronger signals, and very high when there is no signal at all.
Fun fact: B&W TVs are more suitable as computer terminals because they have more bandwidth for the luma signal. Color TVs have to filter out the chroma signal, which reduces the bandwidth for the luma. If you are stuck with a color TV for your terminal, consider modifying it to accept a signal after the low-pass filter. This is not too hard for someone comfortable with a soldering iron, someone who isn’t scared of drilling a hole in their TV.
Thank you for coming to my TED talk.