Ok, so this here is a circuit that can be used to lower the background noise generated by a SID chip quite dramatically. It works for both SID models. The basic idea is to separate the SID from the data/address buses of the C64 when the chip is not accessed. As the digital and analog sides use the same ground, the more rapidly changing digital signal bleeds to the analog side through the common ground (hence the background noise).
(Click the image to download the pdf)
Whilst working on the Sega Megadrive cart, I ended up searching the net for other cart projects for various game consoles and eventually stumbled across Arto Hatanpää’s Funky Flash Cart (FFC) for the 8-bit Nintendo Entertainment System. I contacted Arto about the cart a few weeks ago; as he still had some boards from the initial prototype batch on his hands he was willing to sell one to recoup some of the manufacturing costs. Parts weren’t hard to source either; lucky for me Arto had extras of some of the chips and was kind enough to sell those to me along with the PCB. The NES lockout chip and the cartridge case I recycled from a damaged cartridge bought second-hand for a few euros. So, I had the board assembled almost in no-time:
After a bit of side-tracking with other projects, it was about time to finally tackle this console. I want to release a Atari 2600 tune for the executable music compo at Breakpoint 2006 so I need to have the hardware ready by then. Specifically, video and audio out mods are needed.
After looking around a bit, I came across Nathan Strumm’s excellent Atari 2600 video mod comparison page and decided to implement the mod mentioned from atariage.com. As for the audio, I wanted to have the two channels of TIA mono/stereo selectable. This is where Rob Mitchell’s page came in handy. As the video mod makes the RF circuit useless I figured I can remove it altogether and mod the RF switch to be used for the mono/stereo selection. The pics:
Why the Scart socket? Well, I have a switcher and plenty of fully coupled Scart cables so figured I’ll use this one instead. The resistor attached on pin 16 (tied to ground on pin 18) is used to signal the television (or whatever the receiver) that a composite signal is fed instead of RGB.
As for the quality of this mod? Bollocks. I’m only getting a really faint distorted signal, so it definitely needs some inspection. However, as the audio bit is in order I’m going to delay it to a later date and assume it’ll be enough for demonstration purposes at Breakpoint 2006.
Yet another case of projects that have been stuck to my head for quite some time, I decided to take a step towards Paul Slocum’s awesome Sequencer Kit for the Atari 2600. In order to run the software on real hardware, I needed to make a cart for EPROMs. Given how old this console is (first introduced in 1977), it’s not like the cart is anything complex. So, with the help of some schematics and pictures from the net I drew up a schematic and a board layout and etched/assembled the cart.
I also have a couple of second-hand Atari 2600’s waiting, next up it’s time to modify them a bit.