ZX Spectrum+ Toastrack Part 2

This is a quick update from my Restoring a ZX Spectrum+ Toastrack blog.

RGB is not analog, we need more colors

First, I found that the 6-way IDC analog RGB extender that I used with the RGB2HDMI adapter wasn’t detecting all the colors. Specifically I wasn’t seeing the half bright variant of the 3-bit 8 color palette.

6-way IDC analog RGB extender

As you can see I only have 8 colors.

The half bright variants are missing

The Spectrum uses 3-bit + half bright variant that is similar to 4-bit RGBI. The problem is that the Spectrum 128K DIN connector RGB signals are TTL, not analog, they don’t have the bright signal combined. Instead there’s a separate signal for bright. See the internal circuit:

Side note: even if the RGB signals were analog and had bright combined, the 6-way IDC analog RGB extender still wouldn’t be able to detect the half bright variants because it doesn’t have the necessary voltage level comparators.

This is a great document explaining everything there is to know about RGB for ZX Spectrum 128, +2, +2A and +3 if you’re curious.

So, what we need is a 12-Bit RGBI2HDMI board that connects to the Spectrum’s RGB and bright TTL signals. green2 is used for bright. You can use the 12 Bit Extender 16 way IDC extender for convenience if you want to (I did), or skip the extender and connect directly to the internal 12 way IDC.

New extender and new DIN to IDC cable using the bright signal

And voilà, we now have all our 16 colors, including the half bright variants.

We can now see the 4-bit 16 color palette

What about loading tapes?

I’ve been using my multisystem Backbit cartridge on the expansion port to load games and demos, but I wanted to load a game the old fashioned way, from a cassette tape.

Well, sort of.

I have this little device that I bought when I was invited to an event from the LOAD ZX museum a few months ago. It’s a microcontroller that can read .tap files from the SD card and play them through the 3.5mm jack connector (similar to the TzxDuino).

Well lets connect the stereo to mono cable from the device’s audio output to the EAR jack on the Spectrum and press play.

There you go, loaded Bubble Bobble the good old way.

What else should I do with this Spectrum?