Tandy 640x400 interlaced mode

[NewRisingSun]

I have dug deeper into the Tandy 1000's "Extended Addressing Mode" and noticed that the Tandys with the "Big Blue" video chip (EX, SX, HX, TX) also support interlaced true scanning. (The original IBM CGA aligned the synchronization signals always so that scanning would be progressive no matter the CRT controller's setting. I don't know if this is also true for the PCjr and original Tandy 1000.) Together with the 2x32K banks per page feature in the Extended Addressing Mode, this enabled me to create true 320x400x16 and 640x400x4 video modes on the TX. They look quite nice, if one is willing to accept the flicker and, for 640x400 mode, the four color limitation.

It would be great if someone could try this on an RL, SL or TL. The TL reference manual lists CRT controller register 8, which normally is the interlace register, as "Reserved", so it may be that Tandy removed the interlace mode from the "Tandy Video II" chip, which would be quite unfortunate.

These are two images: one with 640x400 text (8x16, from VGA), and one showing the Pinball Fantasies title screen (reduced to four colors). As with the modified IntroJR, this will not work on the original Tandy 1000/A/HD since they do not have the Extended Addressing Mode.

 

[commodorejohn]

I'll give 'em a try on my RL when I get a chance.

 

[Great Hierophant]

It does not work as intended on my TL, there is no characteristic interlace flicker and the graphics appear very incorrect compared to how they should look (on my SX). Here are photos :

https://www.dropbox.com/s/q4c4al5qc7ghncy/20160121_111615.jpg?dl=0

https://www.dropbox.com/s/ykg7sevafktwqqn/20160121_111631.jpg?dl=0

 

[NewRisingSun]

I hate it when manuals are correct like that. Oh well, at least it works on the SX, if I understand you correctly. How annoying is the flicker on the CM-5 (or what have you connected to your SX)?

 

[Great Hierophant]

I hate it when manuals are correct like that. Oh well, at least it works on the SX, if I understand you correctly. How annoying is the flicker on the CM-5 (or what have you connected to your SX)?

It's nothing I would want to look at for any length of time, whether on a CM-5 or a TV. I tried taking a video of it and it showed a deceptively stable image.

Edit : Try as a might, my camera will not show the flicker inherent in NRS's display method.

 

[NewRisingSun]

I agree that WC2TEXT would be unbearable for word processing use. (Motorola themselves recommend in their MC6845 data sheet that for interlaced video, a long persistence phosphor monitor [such as the IBM 5151] should be used. But for a title screen that is shown for five seconds, I think interlaced video sure looks nifty.

Try as a might, my camera will not show the flicker inherent in NRS's display method.

My cellphone camera will to some extent during WC2TEXT, if (and only if) you watch this video in 60p mode, at least when it's not confused by the direct light coming from the CRT.

 

[Trixter]

I think interlaced video sure looks nifty.

I agree. What a shame nothing used this back then.