I like this project! The PCB looks clean, and has a lean, but complete set of components.
160x200 video is a mistake however. Those pixels are twice as wide as they are tall, in NTSC signaling to be perfectly fair. And to be a bit more fair, using that resolution can mean eliminating color "dot crawl" common to devices outputting a composite color video signal, which this little computer appears to do.
Now I have written quite a bit of video related code for the Propeller 1 chip featured in this project. It is possible to compromise and double the video resolution to 320x200 and still maintain that crisp look seen in the product photos.
The secret is to avoid the color signal interlacing, and or output s-video, which is separated Luma and chroma signals similar to the C64 output signal. Additionally, one can get these little S-video to composite adapters that work pretty well on a non s-video capable display, though the Propeller can offer either video signal just for the asking, and the same video circuit is used for both.
With NTSC, anything over 160 pixels in the active video area, which is the signal inside the borders seen in the product photos on the project page, tends to leave fringing between high contrast pixels. Contrast being say, red and blue, which are far apart on the color wheel. This is caused by the low resolution of the NTSC color signal itself being superimposed onto the monochrome Luma signal in the composite, single RCA jack output case.
What I did for my video drivers, which can be found on the Parallax site with usually an MIT license, was to simply not interlace the color signal. The spec calls for the color timing to be such that every other scan line is out of phase relative to the prior scan line. This allows for 320 pixel color resolution with the cost being the fringing at high contrast pixel transitions.
If the driver simply does not do that, the result is a bit less sharp color, but also no shimmering in the text. That works pretty well with composite on most displays.
And when S-video is used, doing the interlace simply improves on the non interlaced color, making it sharper.
A propeller chip video generation is almost entirely software driven, which means one can do just about anything with the output signal. It is really a fun device.
In my opinion, a 40x25 text display with 7 bits per character, rather than the 4 bits seen on the CODY, would be a whole lot easier to read.
The good news is anyone can do that due to how the Propeller is a software driven video device.
All that said, the project author may have intended a specific look and feel, and if that is the case, the project looks like a home run!
I also wonder why use NTSC for outputting video. VGA is easy enough to bitbang and pretty ubiquitous. HDMI is probably more available at this point, but has unfriendly licensing issues. DisplayPort seems better in all aspects, but it’s the least user friendly and requires a monitor rather than a small TV.
> All that said, the project author may have intended a specific look and feel, and if that is the case, the project looks like a home run!
I’m assuming that. The C64 palette is a good indication.
> I also wonder why use NTSC for outputting video. VGA is easy enough to bitbang and pretty ubiquitous. HDMI is probably more available at this point, but has unfriendly licensing issues. DisplayPort seems better in all aspects, but it’s the least user friendly and requires a monitor rather than a small TV.
Period authenticity, color palette, and pin count, among other reasons:
The idea was to make something that really felt like an old 8-bit you could build, and composite video output was a big part of that.
The NTSC colors were also a lot closer to the "feel" I wanted the computer to have, and unlike VGA output on the Propeller, you don't have to increase the number of pins to increase the number of output colors; the color information is conveyed by the phase of the NTSC signal. On the Propeller you can get a rather large NTSC (or PAL) palette with a 3-pin DAC, but with VGA, you'd want a six-pin DAC plus H and V signals for VGA (and likely not find color matches as close to the C64 palette anyway).
You also need to take into account the timing limitations for "racing the beam" and generating the video on the fly as the Propeller firmware is doing internally; TV scan rates are comparatively slow and give you more time to assemble a line of pixels.
Lastly, as far as HDMI or DisplayPort, that was well out of scope for what I was trying to do, and likely out of range of what the Propeller could bit-bang even with its shift register hardware. I know people have done something similar with DVI output from the Pi Pico, but I'm still not sure that even the Pico could do as much as the Propeller is doing here overall, and probably not as cleanly.
One of the things about engineering, or really anything, is that's more often than not a matter of finding the least bad option given competing constraints; in this case what I thought was the best technical fit was also the one that fit the 1980s aesthetic.
I did some cursory reading on Propeller and Propeller 2 and they seem really geared towards TV timings, and the perfect choice for a 200x320 screen with composite output. One of the reasons I mentioned DisplayPort is that, AFAIK, you don't need to race an imaginary beam with it.
> fit the 1980s aesthetic.
Can't blame you for that - the 80's is where I grew up and that feel is unbeatable (even though I liked the higher-end gear from DEC and IBM as well).
Both are really easy on the Propeller, but NTSC is actually easiest, and it uses the least pins and discrete components.
The Propeller has hardware assisted video signal generation. It is a lot of fun to program and one can basically duplicate a lot of old computers video signals.
One advantage NTSC has over VGA is when using a component video capable television. The Luma "Y" output can drive an HDTV in monochrome all the way up to 1080i using one single pin!
The Propeller chip used in this project may not be capable of 1080i, but it can do 720i.
I'm the person behind the project (and I have a funny feeling I used your 8x8 text driver back when I was trying to learn how the Propeller worked). Anyway, there were actually a few reasons behind the 160x200 video mode:
One was simplicity for the user. When this was all just an idea, I often compared it to a "Commodore Junior" in terms of functionality, and I wanted to keep it simple to learn from and have fun with. I thought something like the C64 or Plus/4's multicolor graphics mode would be the best compromise in terms of letting you make graphics/games but also having readable text output. A lot of old C64 games did just fine with 4x8 multicolor text, and even the VIC-20 did passably well with some of the 40-column extensions people wrote for it.
Another was simplicity from the standpoint of designing the thing. I actually looked into trying to mimic most of the VIC-II's functionality, including being able to mix low-res and high-res on the same line. I just couldn't convince myself there would be enough cycles to pull that off, or at least I'm not sharp enough to manage the feat, particularly with sprites also involved. You could do it with more cogs (Propeller cores) but since the Propeller is handling so many jobs (kind of like the TED on the Plus/4 series), there were constraints there as well. From there I went back to thinking about separate high-res and low-res modes, but that circles back to the above point about complexity.
Yet another reason was that Cody actually liked the multicolor graphics modes (and I do realize how that sounds). When I played around with different video graphics he got really big eyes and a smile watching multicolor tests on the TV. He also really enjoyed YouTube videos of Commodore Plus/4 games done in the multicolor mode it had when I was reading up on the TED. If it got his attention I found a way to work it in, and I also think a really smart Pomeranian isn't necessarily that much different from a really young boy in terms of researching your audience for this kind of thing.
(True story: At one point when I was showing Cody the Propeller breadboard I used for experimenting, he started slapping the RCA jack with his paw with a curious look like it was going to do something. I think he saw me plug that into the cable so many times that he concluded the RCA jack is what made it run or something.)
Excellent choices, and you probably did! Potatotext was one of my first Propeller projects. I wanted a nice text display for future learning.
I think your project is great, and I am thankful I made the final comment I did, because you did aim for specifics and appear to have nailed them.
Great work! I am likely to build one.
I know the developer of that chip personally and will make him aware of your project. He will likely think it is really neat.
Another true story:
I stepped away from electronics and microcontrollers, until a friend sent me the datasheet. I ordered one immediately and had a blast! Got caught right back up. Met the developer, who said I* was who he made the chip for. We have since had a running conversation about this stuff and a lot of fun.
*That being anyone looking to get into this stuff no matter what age, or experience level.
160x200 video is a mistake however. Those pixels are twice as wide as they are tall, in NTSC signaling to be perfectly fair. And to be a bit more fair, using that resolution can mean eliminating color "dot crawl" common to devices outputting a composite color video signal, which this little computer appears to do.
Now I have written quite a bit of video related code for the Propeller 1 chip featured in this project. It is possible to compromise and double the video resolution to 320x200 and still maintain that crisp look seen in the product photos.
The secret is to avoid the color signal interlacing, and or output s-video, which is separated Luma and chroma signals similar to the C64 output signal. Additionally, one can get these little S-video to composite adapters that work pretty well on a non s-video capable display, though the Propeller can offer either video signal just for the asking, and the same video circuit is used for both.
With NTSC, anything over 160 pixels in the active video area, which is the signal inside the borders seen in the product photos on the project page, tends to leave fringing between high contrast pixels. Contrast being say, red and blue, which are far apart on the color wheel. This is caused by the low resolution of the NTSC color signal itself being superimposed onto the monochrome Luma signal in the composite, single RCA jack output case.
What I did for my video drivers, which can be found on the Parallax site with usually an MIT license, was to simply not interlace the color signal. The spec calls for the color timing to be such that every other scan line is out of phase relative to the prior scan line. This allows for 320 pixel color resolution with the cost being the fringing at high contrast pixel transitions.
If the driver simply does not do that, the result is a bit less sharp color, but also no shimmering in the text. That works pretty well with composite on most displays.
And when S-video is used, doing the interlace simply improves on the non interlaced color, making it sharper.
A propeller chip video generation is almost entirely software driven, which means one can do just about anything with the output signal. It is really a fun device.
In my opinion, a 40x25 text display with 7 bits per character, rather than the 4 bits seen on the CODY, would be a whole lot easier to read.
The good news is anyone can do that due to how the Propeller is a software driven video device.
All that said, the project author may have intended a specific look and feel, and if that is the case, the project looks like a home run!