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Post by Pyrofer on Nov 22, 2016 11:46:52 GMT
Not happy with the price/features of existing stereo SID boards I designed my own. Brief feature rundown. Both SIDs share normal address no extra cabling to connect to address bus Audio out goes via normal AV DIN plug second SID audio out goes via the audio IN on the AV DIN. No cutting/soldering or modding of the machine in any way. It's designed to be a simply plug and play minimum hassle install to get sound out of both speakers. Here is the board design. I basically copied the 128s existing audio out circuit for the second SID. I might need to tweak component values to get it sounding exactly like the existing one but it should work. For Stereo you just connect the second channel to the previous audio IN pin on the AV connector, this means no switches or sockets or jacks to fit into the machines case. s5.postimg.org/hbqxr0auv/pseudo_SID.pngIt's a no-frills board designed to be non-damaging to the host machine and present the cleanest solution in regards to cabling. Comments? (I would appreciate any serious input from people who understand electronics and can follow the PCB. I guess I should do a Schematic!)
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Post by Pyrofer on Nov 15, 2016 8:32:39 GMT
All I can say on the "mask rom" thing is that my early 128 actually had an EPROM in it from the factory. But yeah, Commodore certainly shaved those pennies off where they could! I think almost every 128 shipped has a bodge wire fix because that worked and there was no point spending the time/money to redesign the PCB to remove it
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Post by Pyrofer on Nov 14, 2016 7:10:56 GMT
They did use the same physical part. All C128s report 16k as far as I know, even the 64k ones. Anyway, they did different ROMS over several machines. It's not that hard to program them with a slightly different image. The 64 needed a different one to the 128, so having 2 models of 128 wasn't that hard.
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Post by Pyrofer on Nov 14, 2016 7:07:29 GMT
You need to present different colour values to the DAC than would be present with the inputs. This means logic. You CAN do it with standard logic chips. There are a few circuits out there and indeed this was my first method used when I tried this. Sadly due to a little logic error on my part I was missing colour 13 completely (came out black). Now I managed to turn the whole thing into a fairly simple sequence of logic. I could try and replicate that with a load of chips and hope I got it right this time or do it in one programmable chip and know I could fix it in software later So the questions, is one PLD overkill compared to a handful of normal logic chips. A friend did the math on the outputs and came up with a plan that used only 2 bits per colour and gave almost perfect results without messing around with 3 or 4 bits per colour. My board is quite minimalist compared to others and I don't consider it overkill at all. The passive only circuits look terrible in comparison. The colours are washed out and blurry. Even with the diode/transistor circuits etc you don't get as good a picture with the colour separation from a logic driven system. I also don't know of any way to do the brown fix on them short of adding in a logic chip. The PLD was simply the correct choice for me. digital in -> logic -> digital out -> simple 6bit DAC -> done.
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Post by Pyrofer on Nov 11, 2016 13:18:52 GMT
I only ever had a monochrome green monitor for 80 column back in the day. This is in part what drove me to design a converter to make it easy to use. Something I wish I had back then! We had the 128 hooked up via an composite cable to a colour monitor (previously just RF to a small TV) and had the mono only cable from the 80 col output to the mono monitor. I remember being severely disappointed with the total lack of graphics on the 80 col output and we never really made much use of it. I believe both my adapter and the one by rbm take into account the brown fix which is the main reason we ended up with programmable logic chips.
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Post by Pyrofer on Nov 10, 2016 20:46:59 GMT
Yes, I remember comparing mine to John's one, the colours were quite close. I think the misunderstanding is talking about "colours" instead of "shades". I am pretty sure the names of each colour are the same for 128 and CGA, they are just slightly different shades of each colour. So the question comes how well calibrated the CGA monitors back in the day were? I bet if we took all three adapters, mine, this one and John's the colours would be quite close molebrain - Mine was on sale on Amibay. If you registered your interest you will be in the queue. Sadly the PCBs are taking aaages to arrive. I might even consider just ordering a new batch instead of waiting. There are more changes I want to make now after seeing this board anyway I didn't expect to be making a V3 but I just might have to.
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Post by Pyrofer on Nov 10, 2016 19:20:44 GMT
which one are you talking about? If it's mine, I am waiting for PCBs that appear to have been sent on the slowest boat from China ever.
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Post by Pyrofer on Nov 10, 2016 16:47:58 GMT
Nice! You went for a couple things I considered then scrapped in favour of price/size. I was originally using the 16v8 too and had planned to use 2 of them just as you have. I decided on balance that I only needed 2 bits per channel on the output for completely reasonable colours. I later swapped to PLCC (I originally used DIP) GALs with more pins, but didn't bother adding any more bits for colour as I didn't see a need. I had success without the need for either an input buffer or output amp so dropped those too to reduce the size of the board. On Balance I should probably have put some caps on the audio and video output. You've done some lovely work there. Are you planning to sell them or was it for personal use only?
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Post by Pyrofer on Nov 10, 2016 12:42:47 GMT
I am glad that there is more interest in the 128 and specifically the VDC output. That was my goal when I started making my first adapter. I want everybody to be using the 128 in 128 mode with 80 col software. Your design looks extremely well done! It would be great if you could share some details about the circuit design and what you put in and why.
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Post by Pyrofer on Nov 10, 2016 11:56:15 GMT
Ok, so I can confirm that it IS possible to scroll smoothly in both up and down directions.
As for side to side? I don't think so!
I tried everything I can think of. I have perfect smooth scrolling in both vertical directions but I just can't get Horizontal smooth scrolling to work without a flicker when you reset the pixel register.
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