ZX Spectrum build video circuit.

Bring up for the video circuit.

Part 2: This video

ZX80/ZX81 build

[pdrg]

I love how these videos demystify what I could never understand back in my zx81 childhood! There was always a gap between combinational logic and microprocessors and memory and being a computer, so this pulls them all together.

[talideon]

My memory is rusty, but my understanding of why the Spectrums screen buffer has such an odd address layout is down to simplifying the work the ULA needed to address both the pixel data and colour data at the same time.

I decided to double-check, and here's what was on "Break Into Program":

"When the ULA is drawing the screen, it needs to make two fetches from DRAM, for the pixel data and the attribute data. In order to make this more efficient, the ULA uses DRAM page read mode. The advantage of this mode is that the bytes can be read quicker, but there is a limitation: the least significant 7 bits of the address being read must be the same."

[darkstatehk]

It is so satisfying watching these videos. I love the way you explain everything so clearly as well as the diagrams you provide.

[RayR]

Dr Regan you have a talent for educating. Thank you for sharing your skill with us all.

[supahfly_uk]

I love your videos TTL has always fascinated me.

[paul.c.gregory]

Absolutely amazing, I wish I understood half of this, but fascinating all the same.

[adamw.]

Good idea, using static RAM allow to discard all refresh logic (ULA) and earn more time to operate with video functions (no refresh cycles). In some solutions video output acts as refresh due cyclic access to memory addresses.

[drgusman]

This is really impressive.
Do you plan to publish the documentation of the project once you finish it?

[mheermance]

I finally found the time to watch this. Some really clever ideas for multiplexing the RAM and using the EEPROM for raster generation. When I watch videos using digital logic to generate raster scans, I also am amazed that the original TV engineers did it using vacuum tubes and analog circuits. Analog circuits still seem like magic to me.

[yabbaso]

Incredible, so glad the YouTube algorithms highlighted this. Beautiful explanations.

[artoheino]

Excellent adaptation, this could be used for many other retro video system, I am thinking of the TMS9929/18 TI chip, inspiration is only one Eprom away, Great explanation and concise timing, thanks for posting.

[notCalle]

15:52 looking at the address bits for raster and attribute like this, or rather in reverse, with focus on the low 8 bits, the V5:3 shuffle actually makes sense, because the low 8 bits are identical and stays the same for a full video read cycle, saving precious logic cells and timing constraints in the ULA. raster-V7:6 has to go where it is to avoid overlapping the raster and attribute buffers, and V2:0 goes where there's space left, in the middle. This also enables the RAS-CAS-CAS read pattern to fetch 16 bits of raster/attribute in 3 cycles, and still refreshes the video RAM fast enough while the Z80 is running uninterrupted in high RAM.

[daveturner]

In the late 70s (pre zx80) in the UK there was a company that produced eurocard sized boards for homebuilt computers - Reading based if I recall correctly. One card was a clever video card which used a program on a 2048 bit ROM to cause a Z80 MPU to generate the required signals for a video output. Sadly I had to dispose of the computer that I built around these boards many years ago.

[charlesdorval]

lmao glad to see I'm not the only one using that song
I also suggest "The darkness - Seemed Like a Good Idea at the Time" when appropriate ;)

[STONEman]

I advise you to look at the design of the Soviet Spectrum, Zonov’s version - although of course all our Spectrums had 14 MHz quartz - but it was Zonov who made the minimal version using TTL logic and a single memory field where the video adapter did not conflict with the processor

[chpsilva]

Just got you channel recommended by YouTube, loving all this series! Now, I am intrigued: how does this SRAM sharing approach affects the overall performance? I understand this circuit leaves the CPU pretty much free of wait states (unless I missed some important caveat). Can you make some comparative benchmark with an original ZX Spectrum ?

[Milkybar]

You could be talking in Klingon as I understand about 1%, but it’s so fascinating and addictive. Any recommendations on where to start to learn your Klingon please.

[MeisterTheFunk]

I'm really interested in the use of look up tables to emulate logic, in a similar way to using an eeprom. I'm thinking that a simple table look up is quicker than implementing a series of gates in code, particularly something like a counter or a flip-flop. I have started work on an Atari Pong emulated as look up tables which maybe one day can output a composite signal using a microcontroller. Only issue I can see is that this method won't take into account gate propagation delays which are integral to Pong.

[polarstar]

I was stationed in England and had both the Spectrum and the ZX81. I brought my Spectrum back to the States and tried like hell to get it working. I even wrote Timex to see if somebody could help. ::sigh:: I wish I still had that.

[dimasmirnov]

Очень хорошо всё объяснено, спасибо. Только цвета чернил и бумаги перепутаны. Белый фон, черные буквы