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Do you have a CRT to test with? I've run into similar issues using a deadbug style circuit with a ground plane. CRT works perfectly, but LCD has jailbars. I'm pretty sure it's caused by the LCD sampling the signal in a lazy way causing a lot of aliasing. That's not to say the DAC isn't at fault(any noise on the power supply will flow through to the output), but the aliasing is way worse than it should be. I'd be curious to see what the output of the DAC looks like on the scope. Also as other people have mentioned star grounding is the opposite of what you want. It's the area inside a current loop that determines inductance so you want the signal/power and ground wires to be close together as possible.

manic miner is probably jumping because you have not FF value in kempston port #1F after game load, the game detects presence of KJ by checking bit 5(or 6, or 7) of kempston port

Removing Jail Bars is not difficult. Ensure that your PDN (Power Delivery Network) is designed with low impedance to ensure that high frequency components are attenuated and routed away from the analog area. With proper PDN design, there's actually no need to have separate power supplies for the analog and digital sections. Avoid using chokes if possible - the use of them is an indication of poor design (but are often used!). A possible reason why the full intensity colours don't have jailbars is because the signals are above the maximum 0.7V peak thus any ripple is ignored. For the prototype, I would just use a galvanically isolated DAC to have separate clean voltages to drive the monitor.

Great intro to binary numbers, I'm watching the whole series! I'm a lawyer, but always had interest in computers and games. You are a great teacher, thank you!

How fast can it run at full speed?

A great project and well explained as all ways. I would like to try some of your ideas out on my RC2014. I am very interested in the Arduino Mega reading the video RAM, Will you share the your code and or make a video on your techniques. You always seem to skip over the finer details which is the most interesting parts for me.

A ground grid works better than a tree. The whole ground loop is only for the point between analog and the digital. For each section connect all the grounds together in all different directions. This makes a mesh and it will act like a ground plane up to a frequency based on the largest loop.

Great job, it's looking amazing through VGA! Incredible you managed to replace the ULA with an EPROM! If it's not correct timing wise against how the original ULA works, that will be why some games aren't working! Does it halt the CPU when the ULA is doing display stuff? .sna are a bit hit and miss - sometimes they are from a 128K machine (freeze or just crash on 48K), other times they are using a specific controller input, eg. kempston, cursor, SJS etc.

Great job! The ULA will stop the CPU clock when it needs to draw the screen contents. So that will change the timing of the code you run. And timing us critical in many games.

Very nice work, about the jailbar, maybe an rc filter isolating the power of the video circuit from de rest of the circuit could help, something like 10R and 10uF +100nF

I really love this series! Can't wait to see how the jail bars issue is resolved

It works very well. Nice one.

Wonderful work, particularly the interlaced memory access between the CPU and video state machine. I cannot see any diodes, only resistors in the VGA output circuit on your board. The intensity signal should sum with RGB through a resistor and a separate diode for each each R, G and B signals going into the VGA connector. Also, 220 pF to ground from each of the RGBI lines of the VGA output may also help with the jailbars.

Does colour clash display exactly as before or are there unexpected effects?

Superb! The fact that this works without an ULA is the real kicker. Even with the minor problems (which are likely to be ironed out) the picture is miles away from the TV (or composite) on a real Spectrum. Way better. Clean up the input to your DAC, some buffer gates (or a bunch of FFs if its really a timing issue) and plently decoupling close by should help. Besides, Jet Pac and Bruce Lee bring back many memories.

I had the same jumping issue with the ZXPicoIF2ROM on my 48K. I spoke to the author of the board, Tom Dalby, and he said that this happens because the snapshot was taken on an emulator with a Kempston joystick interface enabled, and as my Speccy has no such interface, it gets confused. He suggested that I create my own snapshots in an emulator, with Kempston disabled, and use those instead. I haven’t got around to trying this yet, but I thought I would share, as it seems it might help in your situation too.

Hi, regarding the jail-bars they might appear because of the way you physically implemented the intensity bit comming into the video dac. I noticed in one of my builds if I buffer all the color and intensity bits and the pixel clock and bitmap data so all „flows“ synchronous through the output state all of the clock related artifacts disappear leaving only power related stuff which is more easy to debug. This I could get away with ample decoupling and shorter slightly thicker wires (also using star grounding and power but separate digital and analog circuits power) I did however also buffer the analogue video out with a fast 3ch opamp - the same that is used on some console RGB-mods.

Outstanding!

Great video! To clarify most of the pseudo hi res graphics games that don’t require any additional hardware such as Rocketman , Forty Niner etc all use the same technique .. change the I register to another random set of ROM data - values of 12 and 8 are the most common… this gives access to pseudo random bit data for each “character”. The video buffer in memory is 182 lines of 32 characters each of which ends in a Ret not Halt opcode (both with bit 6 set in the opcode of course). A custom video driver jumps into this video buffer and the trick is that the display driver resets the Line Cntr for each line so it never increments !! Tada pseudo high res images …

Has anyone tried building a TI-99/4A with 512K of RAM and ROM so it will work as intended and not throw the out of memory error

Should be, some guys decoded fully die shot and done full schematic and even build one from transitors with blinking diodes

Good video. I like how you showed how yhe D flip flop is triggered on the rising edge. Are you going to post thar schematic diagram somewhere? If so a link in the description would be great.

Great explanation.

Another good video! (5:51) Where's the data line? (9:04) Ah. You showed the toggle flip-flop at 5:51, not the data flip-flop. (9:21-9:40) The top line is still set and the bottom line is still reset on the internal latches, but the relays used for set and reset are swapped compared to the previous video. This does mean the data flow matches the design you showed with gates better. However, it also means the internal D-type latches have _D-bar_ inputs, not D inputs. (There are other ways to rectify this, but I think this way requires the fewest changes.) And now for some fun facts/over-analysis: As for changes between the logic-gate design and relay design, you still have the outputs of the latches between the OR gates and their inverters; the outputs are just buffered this time. Additionally, the gate design uses an SR-type latch as the follower, whereas the relay design uses a D-type latch as the follower.

хорошая работа, отличный контент, спасибо.

so that's why they're called ripple counters. I thought they might be used to counter the ripple on a signal (like AC on top of DC), but that never made any sense

I really liked the explanation! Makes understanding these circuits so much easier

Make more videos

really good

Very interesting, and it occurred to me after watching this part, that you could add another latch or two to enable you to bank switch part of the RAM (the part which doesn't hold the video memory) to allow the computer to be expanded to run either multiple programs or programs with large amounts of data. Edit: Maybe even simulate a RAM drive for fast loading of programs.

great great series, probably I've already commented when I saw first time O:-) ... But a question: will the proposed circuit handle true hi-res? maybe there is the need for an additional episode to finish or to explain it?

nice project

Witch Wires are this? Im building a z80 handwired computer and im using much silicon wires that i need to decap all te sides before soldering then

Brilliant! Subbed!

I had no idea about mirrored memory map...so zx81 can have max 32KB? Is ROM replacement possible to enable more without banking?

Would it be possible to program an eeprom to replace the real ula in a speccy as replacing them is silly prices or maybe use a pico

I can't stop thinking of loosed cycles of CPU when line is short.... but it is still a crazy genius... i still rember that HALT was NEW LINE code ;)

I remberd that HALT was code for NEW LINE (it was ZX81 thing ?)

@0:53 It has 65536 I/O adresses but software and hardware must use it properly.

No FPGA? Why not? Because you don't know how to use them? The original Spectrum contained an ULA which is the masked programmed equivalent of an FPGA. So using an FPGA is actually closer to the original Spectrum. There are no points awarded for making a design more complicated. I would personally give points for someone that does use an FPGA as it demonstrates they can design the circuit for it. It demonstrates they have more knowledge and skill than just using discrete logic.

In the Spectrum commercial from Micro Men they state CPU frequency to be 3.54Mhz, which is true for 128K and later models, but the original Speccy ran at 3.5Mhz 😁

Impressive work. Is this going to be 640x480 with 256 colors which is the spec for a vga signal? Thank you cant wait until next video!

A spectrum with a VGA display is pretty sweet. As an aside I love the VGA display standard. It's probably the most home brew electronics friendly video format.

Things are tough and getting harder by the minute, so you're just staying ahead of the curve. You've got this!

Great stuff! Will you be releasing your design once it's finished so that others can build it?

2:25 original ZX81 1k is capable of true HiRes! (obviously limited by the RAM) ... and in fact since the possibility of software driving hw much better than in the Spectrum, the only real limit of ZX81 are the colors!

Nice, very nice. I am not sure of your goals, this idea may ruin the purity. Maybe you could share the address/data bus with an RP2040, use that to generate the VGA output and also use that as a USB interface and flash storage for the Z80 images to load/save. I've seen the RP2040 used to emulate ROM, I am not sure if the timing would be good enough for your project though.

Now add soundblaster support, LOL 😂!

Audio is either a synth or really really compressed somewhere in the chain. Hard to listen to.

Weirdly I was wondering if you had a new video up with this project today as it has been a while since I checked - and here it is! Thanks for the update Dr Regan!