High-level Explanation Of How This Mod Works
Spoiler
Cathode - G1 voltage acts as an electromagnetic iris. Increasing cathode - G1 voltage shrinks the diameter of the aperture of the iris, which more tightly packs the electrons into a thinner, sharper cathode ray, which creates a smaller illuminated spot size on the screen. However, this also changes the black levels, so G2 - G1 voltage needs to be increased to calibrate black levels, and the G2 - G1 voltage needs to be increased by more than the increase in cathode - G1 voltage. In addition, a higher G2 - G1 voltage also serves to increase the acceleration of the electrons in the cathode ray, which further improves spot size because the electrons have less time to spread out. A much more detailed explanation is in the next spoiler section.
Spoiler
It is well known that many consumer CRT TVs can be modded to make them support RGB input, but apparently nobody in the CRT gaming scene has looked into what it would take to modify a consumer CRT TV to make it high TVL. This post provides an overview of the major factors that influence CRT TVL, and calls out one of the factors that can be modified in any CRT: spot size. The main factors that influence the effective TVL of a CRT display are:
Input signal quality
Composite video bottlenecks the effective TVL to around 450, s-video, component, and RGB allow for 800 to 1000 TVL. This bottleneck is well known in the CRT gaming community and RGB mods exist to remove the bottleneck from consumer CRTs.
Supported input resolutions aka "Addressability"
In this post, I am limiting my discussion to 15khz CRTs, which allow for 240p/480i resolution. Obviously this limits the effective resolution of the display, but for retrogaming, the 240p aesthetic with "thick scanlines" is the goal we are after. CRT resolution technically considers "addressability" to be distinct from the resolution of the CRT itself, because addressability is a limitation of the circuitry that drives the CRT, not the CRT itself. Detailed discussions of CRT resolution versus addressability are in (this reference).
Spot size
"Resolution is primarily a function of CRT spot size" (see reference). When the electron gun beam hits the phosphor of the CRT screen, it creates a spot of light. The spot size refers to the diameter of the spot. A CRT moves this spot very quickly, left to right, across the screen for each illuminated scanline of video. The smaller the spot size, the thinner the illuminated scanlines and the thicker the black, unilluminated scanlines. A larger spot size causes the illuminated scanlines to bleed into each other, causing the unilluminated scanlines to be very thin to the point where two illuminated scanlines can bleed into each other, decreasing vertical resolution. Similarly, a larger spot size decreases horizontal resolution.
For 240p gaming, addressability does not increase or decrease when the TV has a high TVL or low TVL. What makes the games look better is the high resolution of the CRT.
To stress the difference between "resolution" and "addressability" for CRTs, which many people seem to get hung-up on, here is a screenshot of the most important section in this reference:
Mask pitch aka Dot pitch
Dot mask, slot mask, and aperture grille mask the phosphors on a color CRT. A smaller pitch allows for a higher TVL. This is clearly not moddable, as the mask is bonded to the inside of the CRT. However, consumer CRTs 24-inches and larger have a spot size that is at least twice the size of the mask pitch. A deeper discussion of the interplay between mask pitch (sometimes called dot pitch) and spot size is in this reference in section "A Discussion of Issues Relating to Monitor and CRT Resolution". This means that the effective TVL of a CRT can be doubled before the mask pitch becomes the bottleneck. In fact, the aforementioned reference indicates that spot size can be smaller than the dot pitch and yield a pleasing sharp look.
Large Spot Size
Small Spot Size
Cathode Ray Tube Theory
Before I get to how the spot size can be modified in a consumer CRT, let's first overview the theory of how CRT works, as it will set the context for the description of the mod. I am going to skip lots of details and start with what happens in the CRT's neckboard printed circuit board. The pre-amplified RGB video signals run through high voltage RGB amplifiers that dramatically increase the voltage of the RGB video signals so as to drive the electron guns in the CRT. There is a separate cathode K for each color R, G, and B. For simplicity of this explanation, the picture below is a diagram of a single color. Convergence magnet rings around the CRT's electron gun ensure that all 3 colors converge at the same spot.
The amplified voltage waveform that drives the cathode K, determines how much that color component of the electron beam will vary as the beam scans the screen of the CRT. The G2 anode also known as the "screen" or "accelerating anode" is a hollow metal cylinder that the neckboard electrifies with high positive voltage. Electrons are negatively charged, so the positive voltage of the G2 anode attracts the electrons from the cathode K. The G1 anode, also called the "control grid", has a voltage that is always lower than the voltage of the electron emitting cathode K. The G1 serves two purposes: it sets a voltage floor that allows for "cut off" voltage for the cathode, so that no electrons from the cathode are allowed to pass the G1 anode and enter the electron beam that reaches the screen. This is needed for true black levels. The G1 also starts shaping the electron beam because it has a small hole causing electrons that pass through the G1 to "squeeze" through the hole. The G3 for "focus" anode allows for further focusing the electron beam so that it is in focus at all positions on the screen. There is a forth anode, G4, which has an extremely high voltage of around 30,000 volts. This causes the electrons to rapidly accelerate to the from of the CRT. The dangerous suction cup shaped plug that people discharge on a CRT, as a safety measure, is the G4. The G1, G2, G3, and G4 are often called "grids".
More detailed accounts of how a CRT works are at the following links:
Link 1
Link 2
Modifying Spot Size
There is an excellent 3-part slide deck on SlideShare that provides all of the details. Part 2 here has the most relevant parts. The most important slides are inlined in this post.
The electrons from the cathode K are negative charged and repelled by the negative G1 voltage. However, the G1 anode is a cylinder with a hole at the end that the electrons can pass through. As the G1 voltage is made more negative, the electromagnetic aperture that electrons can pass through is made smaller, causing the electrons that pass through to be more focused into a smaller spot size. The same technique is used by cameras with a physical aperture.
Consumer CRTs have the G1 voltage tied to ground, that is, a voltage of zero. Why do they do this? It is inexpensive. The electron gun only needs the G1 to be a voltage that is lower than the voltage of the cathode. Sure it limits TVL, but since many consumers used their CRT TVs with RF or composite video... low costs matter more than anything else. Consumer CRTs are full of parts that create positive voltages, but additional parts are needed to create a negative voltage and that costs money. It doesn't get any cheaper to just wire G1 to ground.
The modification to decrease spot size is 2 parts: Tie the G1 to a negative voltage power supply so that the G1 is set to a constant value between 0 volts and -100 volts. The spot size decreases as the voltage is more and more negative. Since the electromagnet aperture created by the G1 anode is half the size at -100 volts, the electron beam that can pass through the G1 is more focused, but it also means a higher positive G2 screen voltage is required to increase the pull of electrons through the smaller electromagnetic aperture. The G2 screen voltage can be tuned as if you are setting the black level of the CRT by tuning the “screen” knob on the flyback.
Input signal quality
Composite video bottlenecks the effective TVL to around 450, s-video, component, and RGB allow for 800 to 1000 TVL. This bottleneck is well known in the CRT gaming community and RGB mods exist to remove the bottleneck from consumer CRTs.
Supported input resolutions aka "Addressability"
In this post, I am limiting my discussion to 15khz CRTs, which allow for 240p/480i resolution. Obviously this limits the effective resolution of the display, but for retrogaming, the 240p aesthetic with "thick scanlines" is the goal we are after. CRT resolution technically considers "addressability" to be distinct from the resolution of the CRT itself, because addressability is a limitation of the circuitry that drives the CRT, not the CRT itself. Detailed discussions of CRT resolution versus addressability are in (this reference).
Spot size
"Resolution is primarily a function of CRT spot size" (see reference). When the electron gun beam hits the phosphor of the CRT screen, it creates a spot of light. The spot size refers to the diameter of the spot. A CRT moves this spot very quickly, left to right, across the screen for each illuminated scanline of video. The smaller the spot size, the thinner the illuminated scanlines and the thicker the black, unilluminated scanlines. A larger spot size causes the illuminated scanlines to bleed into each other, causing the unilluminated scanlines to be very thin to the point where two illuminated scanlines can bleed into each other, decreasing vertical resolution. Similarly, a larger spot size decreases horizontal resolution.
For 240p gaming, addressability does not increase or decrease when the TV has a high TVL or low TVL. What makes the games look better is the high resolution of the CRT.
To stress the difference between "resolution" and "addressability" for CRTs, which many people seem to get hung-up on, here is a screenshot of the most important section in this reference:
Mask pitch aka Dot pitch
Dot mask, slot mask, and aperture grille mask the phosphors on a color CRT. A smaller pitch allows for a higher TVL. This is clearly not moddable, as the mask is bonded to the inside of the CRT. However, consumer CRTs 24-inches and larger have a spot size that is at least twice the size of the mask pitch. A deeper discussion of the interplay between mask pitch (sometimes called dot pitch) and spot size is in this reference in section "A Discussion of Issues Relating to Monitor and CRT Resolution". This means that the effective TVL of a CRT can be doubled before the mask pitch becomes the bottleneck. In fact, the aforementioned reference indicates that spot size can be smaller than the dot pitch and yield a pleasing sharp look.
Large Spot Size
Small Spot Size
Cathode Ray Tube Theory
Before I get to how the spot size can be modified in a consumer CRT, let's first overview the theory of how CRT works, as it will set the context for the description of the mod. I am going to skip lots of details and start with what happens in the CRT's neckboard printed circuit board. The pre-amplified RGB video signals run through high voltage RGB amplifiers that dramatically increase the voltage of the RGB video signals so as to drive the electron guns in the CRT. There is a separate cathode K for each color R, G, and B. For simplicity of this explanation, the picture below is a diagram of a single color. Convergence magnet rings around the CRT's electron gun ensure that all 3 colors converge at the same spot.
The amplified voltage waveform that drives the cathode K, determines how much that color component of the electron beam will vary as the beam scans the screen of the CRT. The G2 anode also known as the "screen" or "accelerating anode" is a hollow metal cylinder that the neckboard electrifies with high positive voltage. Electrons are negatively charged, so the positive voltage of the G2 anode attracts the electrons from the cathode K. The G1 anode, also called the "control grid", has a voltage that is always lower than the voltage of the electron emitting cathode K. The G1 serves two purposes: it sets a voltage floor that allows for "cut off" voltage for the cathode, so that no electrons from the cathode are allowed to pass the G1 anode and enter the electron beam that reaches the screen. This is needed for true black levels. The G1 also starts shaping the electron beam because it has a small hole causing electrons that pass through the G1 to "squeeze" through the hole. The G3 for "focus" anode allows for further focusing the electron beam so that it is in focus at all positions on the screen. There is a forth anode, G4, which has an extremely high voltage of around 30,000 volts. This causes the electrons to rapidly accelerate to the from of the CRT. The dangerous suction cup shaped plug that people discharge on a CRT, as a safety measure, is the G4. The G1, G2, G3, and G4 are often called "grids".
More detailed accounts of how a CRT works are at the following links:
Link 1
Link 2
Modifying Spot Size
There is an excellent 3-part slide deck on SlideShare that provides all of the details. Part 2 here has the most relevant parts. The most important slides are inlined in this post.
The electrons from the cathode K are negative charged and repelled by the negative G1 voltage. However, the G1 anode is a cylinder with a hole at the end that the electrons can pass through. As the G1 voltage is made more negative, the electromagnetic aperture that electrons can pass through is made smaller, causing the electrons that pass through to be more focused into a smaller spot size. The same technique is used by cameras with a physical aperture.
Consumer CRTs have the G1 voltage tied to ground, that is, a voltage of zero. Why do they do this? It is inexpensive. The electron gun only needs the G1 to be a voltage that is lower than the voltage of the cathode. Sure it limits TVL, but since many consumers used their CRT TVs with RF or composite video... low costs matter more than anything else. Consumer CRTs are full of parts that create positive voltages, but additional parts are needed to create a negative voltage and that costs money. It doesn't get any cheaper to just wire G1 to ground.
The modification to decrease spot size is 2 parts: Tie the G1 to a negative voltage power supply so that the G1 is set to a constant value between 0 volts and -100 volts. The spot size decreases as the voltage is more and more negative. Since the electromagnet aperture created by the G1 anode is half the size at -100 volts, the electron beam that can pass through the G1 is more focused, but it also means a higher positive G2 screen voltage is required to increase the pull of electrons through the smaller electromagnetic aperture. The G2 screen voltage can be tuned as if you are setting the black level of the CRT by tuning the “screen” knob on the flyback.
These before and after photos were taken on a hand held iphone. So the exact angle and position of the photos is a bit off because my hand moved between shots. Also, I did the prototyping work in my garage, so the CRT screen is dirty and there is sun glare on the glass in the photos. Sorry, I am not a photographer. The prototype in these photos is an old used 27-inch curved slot mask consumer Panasonic TV, model number CT-27L8G. This TV's un-modded TVL is roughly 450TVL.
2-chip SNES 240p Test Suite Displaying a Solid Green Screen Over Component Video (left before, right after):
Spoiler
SNES Super Mario World (left before, right after) (credit goes to ElBartoME):
Click here for high resolution: https://i.imgur.com/LTCwG3e.jpg
SNES Super Metroid (left before, right after) (credit goes to ElBartoME):
Click here for high resolution: https://i.imgur.com/W2oilYX.jpg
SNES Super Metroid (left before, right after):
Minimum CRT Requirements For Mod To Achieve 1000 TVL
Spoiler
Even if your CRT does not meet these minimum requirements, the mod will still increase your CRT's TVL, but it will not reach 1000 TVL unless it meets these minimum requirements. Looking at my consumer CRT collection, most of the TVs 27-inch and above meet these requirements.
When measured in units of TVL, a 15khz CRT’s resolution (as opposed to addressability), can be calculated using the following formula:
When measured in units of TVL, a 15khz CRT’s resolution (as opposed to addressability), can be calculated using the following formula:
Code: Select all
CRTResolutionTVL = minimum( CathodeTVL, PhosphorTVL, SpotTVL )
...where...
CathodeTVL = (RGB drive amp bandwidth in megahertz) * 39.45
PhosphorTVL = (3/4) * (CRT width) / (phosphor triad width)
SpotTVL = (3/4) * (CRT width) / (cathode ray spot width)Spoiler
The changes are all made on the CRT neckboard, and these steps assume you already have a 250v power supply. The next spoiler section describes how to make such a supply for about $1:
1. Cut trace between ground and the trace to the G1 anode. My TV had a jumper between ground and G1, so desoldering that jumper was all I needed to change. Another option is to pull the CRT neck socket pin for G1.
2. Connect an 1uF and approximately 350v film capacitor between ground and the trace to the G1 anode. This keeps the G1 voltage from jittering. It is optional if the power supply you use in the next step has a good smoothing capacitor built in.
3. Connect the positive rail of an approximately 250v power supply to the ground leg of the capacitor from step 2. A higher voltage will make the spot size even smaller, but if you go too high, you will get arcing between the G1 and cathodes.
4. Connect the negative rail of the 250v power supply to the G1 leg of the capacitor from step 2.
5. Using the 240p test patterns (or equivalent), turn clockwise, the screen potentiometer on the flyback to increase the screen voltage until proper black levels are restored.
1. Cut trace between ground and the trace to the G1 anode. My TV had a jumper between ground and G1, so desoldering that jumper was all I needed to change. Another option is to pull the CRT neck socket pin for G1.
2. Connect an 1uF and approximately 350v film capacitor between ground and the trace to the G1 anode. This keeps the G1 voltage from jittering. It is optional if the power supply you use in the next step has a good smoothing capacitor built in.
3. Connect the positive rail of an approximately 250v power supply to the ground leg of the capacitor from step 2. A higher voltage will make the spot size even smaller, but if you go too high, you will get arcing between the G1 and cathodes.
4. Connect the negative rail of the 250v power supply to the G1 leg of the capacitor from step 2.
5. Using the 240p test patterns (or equivalent), turn clockwise, the screen potentiometer on the flyback to increase the screen voltage until proper black levels are restored.
