Did old 15Khz CRT arcade monitors just have 240 vertical lines?

[elpablo]

I noticed that the specs for old CRT arcade monitors note they have 240 vertical lines. Did the CRTs do 240P with just 240 progressive vertical lines or were they like consumer CRT TVs with 525 vertical interlaced lines? In this case I know that special timing signals would be used as to not alternate between even and odd lines to display 240P (what Nintendo referred to as “Double Strike”)

Any info is appreciated!

 

[the Goat]

The actual CRTs do not have a discrete number of lines like you are asking. The circuitry around the CRT is what limits which signals can be rasterized. In general the horizontal sync rate of the monitor it is the critical factor. Usually a 15kHz horizontal sync monitor will display 240p or 480i without issue.

A CRT would never display 525 lines from an NTSC signal. 525 scanlines refers to the number of scanlines in one NTSC frame. That includes the vertical blanking interval where there is no picture. By the specification the visible picture occupies 483 lines out of the 525 total.

 

[jassin000]

Did the CRTs do 240P with just 240 progressive vertical lines or were they like consumer CRT TVs

The tubes were rated higher than consumer grade, however at the tube level they function the same way.
240p is a "hack" in that the alpha field is double striking, meaning the beta field never strikes.
ie Tubes were designed to interlace/alternate alpha/beta fields, but by disabling the 2nd beta field they could create a progressive image by displaying/striking the alpha field twice.

This is also why modern TVs don't understand 240p and label it as 480i.
Because it IS actually a 480i class signal, but with the beta field disabled and the alpha striking twice (a hack of how it was originally designed to work).

And and why we get that gap between lines in 240p on a CRT, the beta field is totally gone/missing and this is the gap you are seeing.

 

[elpablo]

Thanks for your responses. So 15Khz arcade CRTs displayed just 240 progressive lines unlike consumer CRT TVs since the arcade CRTs had different circuitry to output different signals?

Arcade CRTs didn’t need to worry about NTSC spec so could just display 240 Progressive lines opposed to being interlaced and having to “Double Strike” to display 240P like TVs?

 

[elpablo]

I can see I’m wrong and that the 15Khz arcade CRTs were displaying 240P over 480i (which is what you guys were telling me lol)

So the arcade monitors used the same “Double Strike” signaling to achieve 240P alike consumer CRT TVs?

 

[the Goat]

the 15Khz arcade CRTs were displaying 240P over 480i

I'm not sure what you mean by, "displaying 240p over 480i". Can you elaborate?

So the arcade monitors used the same “Double Strike” signaling to achieve 240P alike consumer CRT TVs?

Yes, of coarse they do. That is what 240p is. Remember the horizontal sync rate is locked at 15kHz. So each 240 line frame takes 1/60 of a second to draw.

 

[elpablo]

I was just referring to how 15Khz CRTs can do 480i or 240P. Thanks for confirming that the arcade CRTs used the same signaling as CRT TVs to achieve this.

Someone on Reddit was trying to tell me that 240P arcade games employed different signaling then “Double Strike” because they didn’t Have to follow NTSC standards like TVs had to.

 

[skate323k137]

Vertical lines are measured as TV lines (TVL). The number of vertical lines the CRT display can draw without them running together is the TVL rating. Consumer TVs back in the day were 500ish TVL, PVMs are 600 TVL up to 800 TVL. So a picture of 600 vertical lines alternating white and black would display as jailbars on a PVM but would be a solid block on a consumer TV.

https://en.wikipedia.org/wiki/Television_lines

 

[the Goat]

Vertical lines are measured as TV lines (TVL). The number of vertical lines the CRT display can draw without them running together is the TVL rating. Consumer TVs back in the day were 500ish TVL, PVMs are 600 TVL up to 800 TVL. So a picture of 600 vertical lines alternating white and black would display as jailbars on a PVM but would be a solid block on a consumer TV.

https://en.wikipedia.org/wiki/Television_lines

I'm pretty sure when @elpablo says '240 vertical lines' he means horizontal lines crossed while moving vertically.

 

[elpablo]

Yes, my bad. I should have said vertical resolution (number of horizontal lines counted from top to bottom) which is different than TV lines. Thanks for your input guys.

 

[Zebra]

I can see I’m wrong and that the 15Khz arcade CRTs were displaying 240P over 480i (which is what you guys were telling me lol)

So the arcade monitors used the same “Double Strike” signaling to achieve 240P alike consumer CRT TVs?

Both CGA crt arcade monitors and regular SD consumer CRT TVs can display either 480i or 240p. They both work in exactly the same way in this respect.

Their limitation was down to bandwidth. A cga monitor (or SD TV) is limited to a max horizontal refresh rate somewhere around 15.7 kHz. At a vertical refresh rate of 60hz, this allows it to display 240p or 480i. At 30fps, they can do 480p too. This limitation was down to the chassis, not the tube.

No arcade monitor manufacturer made their own tubes. Arcade monitors used the same (Toshiba, Sharp, LG, Samsung etc) tubes as in consumer TVs. What makes it a 15khz Wells Gardner monitor or a Kortek etc was the chassis. You could convert a 15khz monitor into a 24khz or 31khz one buy replacing the cga chassis with a compatible EGA or VGA one.

CRT displays have no native resolution though because they don't have a fixed pixel pitch like modern flat screens. They just draw the required number of lines up to the monitor's bandwidth limit. CRT displays can effectively switch native resolutions with no scaling required which is what makes them so awesome.

 

[Hackcell]

Very interesting thread.

 

[dewmansnk]

The deflection yoke that is mounted to the tube will be the limiting factor in going to a higher refresh rate, specifically the horizontal deflection coil is of most concern. In order to go from 240p to 480p you need to double the scanning frequency, which roughly doubles the rate-of-change of current (di/dt) in the coil. The 2 biggest factors in the di/dt of a coil is inductance and applied voltage, so to increase the di/dt in a coil with large inductance would require a lot of voltage. Many horizontal deflection coils from standard definition TVs will be in the 1-2mH range for inductance, whereas deflection coils found in high def TVs and tri-sync arcade monitors can be down in the 300uH range. This allows a more reasonable voltage across the horizontal defection circuit (still 100's of Vpp) to achieve the higher scanning frequency.