I spent some time researching graphics and sound options for BoxLambda. Here’s what I came up with.

Graphics

If you’re reading this, you must be into the build-your-own-computer thing, which probably means you’re aware of the super cool Commander X16 project. Frank van de Hoef created the very elegant VERA (Video Embedded Retro Adapter) module for the X16. Here’s a high-level specification, taken from the Commander X16 website:

VERA module specifications:

• Video generator featuring:
  • Multiple output formats (VGA, NTSC Composite, NTSC S-Video, RGB video) at a fixed resolution of 640x480@60Hz
  • Support for 2 layers, both supporting:
    • 1/2/4/8 bpp tile and bitmap modes
    • Support for up to 128 sprites (with inter-sprite collision detection).
  • Embedded video RAM of 128 KB.
  • Palette with 256 colors selected from a total range of 4096 colors.
• 16-channel stereo Programmable Sound Generator with multiple waveforms (Pulse, Sawtooth, Triangle, Noise)
• High-quality PCM audio playback from a 4 KB FIFO buffer featuring up to 48kHz 16-bit stereo sound.
• SecureDigital storage.

Other features, not mentioned in the blurb, include:

• Fractional display scaling (scaling lower resolutions up to the 640x480 display resolution).
• Horizontal and Vertical smooth scrolling

Lucky for us, Frank recently released the VERA verilog code under the generous MIT license. You can find the code here:

https://github.com/fvdhoef/vera-module

I’m not particularly interested in VERA’s PSG (Programmable Sound Generator), or the non-VGA output formats, so I might remove those from the build.

The 128KB of video RAM will take a big chunk out of our available Block RAM resources, but it’ll be worth it. We’re getting a lot of bang for our buck.

Note that the VERA is designed as a separate FPGA with a SPI slave interface. Some modifications will be required to integrate it into our SoC.

Xosera

I also considered, but eventually dismissed, Xosera:

https://hackaday.io/project/173731-xosera-fpga-based-retro-video-graphics.

Xosera is a VERA-inspired video controller, but it is being developed independently by Xarc. I like the Amiga-style Copper processor that they added. Unfortunately, Xosera doesn’t have hardware sprites. That’s a showstopper for me. I’ll keep my eye on this project though. It’s an active project and features are still being added.

Sound

A sound core is a perfect candidate for Partial FPGA Reconfiguration. There are a lot of options (Wave-Table synthesis, FM synthesis, PSG…) and a lot of open-source cores available. It would be pretty cool if the software application can just download its synthesizer of choice as part of the program.

Pretty much any core developed by Jotego sounds like a great idea.

Technically, I don’t have to select a sound core. We already have sound through VERA’s PCM audio playback. I’m going to select a sound core anyway because I like retro sounds and I’d like to mess around a bit with one of the old-school PSG chips.

I think I’ll go for a dual YM2149, one for music, one for sound FX, in a game context. The YM2149 was the Atari ST’s sound chip, so we’ll have a large music and sound FX archive at our disposal. Jotego developed an FPGA clone of the YM2149, the JT49:

https://github.com/jotego/jt49

Why not VERA PSG?

The only reason I’m not going for VERA PSG is that, as of yet, very little music has been written for it. I’m sure it is a perfectly adequate PSG implementation.

Why not SID?

The SID chip is partially analog, making it much harder to emulate correctly on an FPGA. Also, while I like SID, I’ve probably heard enough SID music to last me a lifetime. I’m currently more interested in finding out what other retro sound chips have to offer.

Interesting Links

• https://misterfpga.org/ : Jotego is one of the key developers behind the MiSTer FGPA project.
• https://www.youtube.com/watch?v=u5Mi3FkZgFI : a compilation of some excellent YM2149 music made for the Atari ST by genius composer Scavenger (Joris de Man).
• https://en.wikipedia.org/wiki/List_of_sound_chips : A list of sound chips