Join us on Discord: discord.gg/jmf6M3z7XS Support the channel on Patreon: www.patreon.com/JamesSharman I did realize during editing that I got the gradient range wrong, the bars only go up to 224/255 but the test is just as valid. Should have made it 4 bars with an offset of 58 each line to cover the full range with a bigger overlap.

I absolutely love your videos! Thanks for making them! I also really enjoy the fact that you're open to feedback, consider options and experiment.

I also felt the R2R approach would be fine since the spread within a single batch of resistors is often small. You just have to keep an eye on the time constants if the pixel rate is high enough. That stencil was just about the funniest thing I've seen on any of your videos, James. You had me roaring with laughter. But I was also shouting at the TV - "You need more signal grounds!" Of course, it worked out OK, but with eight data bits at pixel speeds, a pixel clock and another control signal, I was surprised that the two grounds that you had were sufficient. When you put three DACs onto a single board that won't be such an issue depending on how it's all connected together. A filtered supply to each of the DAC driver latches might help clean things up a bit too by blocking some of the general system noise on the supply. Great progress, and looking forward to the next step

Every single time I get a notification that you've posted a new video I get all kinds of giddy. Thank you so much for these!

Hi James. Another awesome video. :) Regarding your R2R ladder and resistor values, note that resistors come in Preferred Value series. The preferred values are a percentage relationship to each other. e.g. The E96 series has each value +/-1% of it’s neighbouring value (E48 is +/-2%, E24 is +/-5% etc.). The manufacturer then just assigns an individual resistor’s marked value by dropping it in the Preferred Value bucket that it fits (hence the percentage relationship between Preferred Values in each range). This means two things: 1). The error in an individual resistor’s value doesn’t fit a bell curve. It is simply a random value within the +/-% tolerance Preferred Value range. 2). If you want specific matched values it’s common to just test & select some closely matching value resistors from your parts drawer. Obviously, choosing resistors in the E96 (or even E128) series makes finding matching resistors, within a fine tolerance, much easier. Obviously, in use, there is also individual value variance due to temperature drift etc. But for a simple R2R ladder the relative effect should be negligible. In summary, you simply need to measure and select appropriate matching value resistors to use in your R2R ladder. Hopefully this information is of some help?

Always a pleasure to see your videos in my feed. Thank you James!

I laughed hard when I saw the size of the stencil. I didn't knew you made jokes unintentionally. That's a big pan. At least that doesn't require painful debugging. Loving this series.

Awesome! Glad to see it worked out so well! I'm excited to see your full-color output soon

Always great to see a well thought out test plan & execution.

Watching you solder with the stencil and hotplate has spurred me on to use my hotplate for my next PCB. Up until now, I've been soldering all my SMDs by hand or hot air gun.

Excellent video as usual James. Your videos are always very well edited and your presentation of your work is both enjoyable and refreshing when compared to so many other channels..

Genuinely amazed that I had no idea your channel existed until yesterday. Your videos have a great flow and the right balance between information and demonstration.

This knd of proof of concept demo is very helpful to understand the details, very insteresing! And I appreciate the eye candy of seeing it working on the monitor at the end :). Great content as always, and it is still getting better, thanks!

I found your videos not to long ago and I am so glad I did. I've always loved the idea of building a computer at a low level and you do a great job at being informative. Thank you for the great content.

This is fascinating. Thank you very much for sharing your progress on this.

Though I didn't think the R2R ladder would cause you any real world issues, I didn't expect it to be quite that good and linear. Nicely proven out!

Bourns makes r2r ladders. You could use colour lut with 256 indexes at 8bits per colour - this would possibly be the best option with your 8 bit per transfer limitation.

Amazing video as always ! I wish I understood electronics that well, but seeing you doing what you do is just incredibly satisfying seeing this entire setup being built just - because ! (this will probably never see any kind of real world application, so it is basically - art !)

Lovely to see that done in detail like that, thanks. Over the years i've worked with r2r and DAC ICs, I've not noticed huge issues with either but i've never tried to use r2r with more than 8 bits for the reasons you outline. I've found even 5% is fine as long as all bits pass via a single pack at a time, otherwise all bets are off as that cross over notch becomes real. I have seen issues with boards where temperature is uneven, not sure if that is all a resistor issue or if the boards I used had a buffer amplifier of some sort... That is going back many years so boards tended to be much larger, and hotter!

that stencil is amazing

Always impressed. Carry on the good work.

I feel this one video is exceptionally excellent and useful standalone. Your CPU and VGA and UART and audio work may be great, but they're all large projects. This is a worthy project on its own, yet fits within a single video and doesn't significantly depend on any of the others. It demonstrates much of the technique and knowhow underpinning a good chunk of your circuit work. I will be recommending it to friends and family.

Enjoying this journey. :-)

As always, fascinating, thank you :)

Another consideration is that the human eye does not have a linear response to the primary color sources in its primary sensor, the retinal cone cell. Hence the gamma correction function comon to color display output controllers. So either the analog voltage of source prior to the dac or the resistor values themselves can encapsulate the gamma function for each primary colors before the output will center the response in the human color space. If I remember correctly the red curve had a .58 adjustment while the green had .39 and blue had .16. That means that the blue would need an effective amplifier gain of 1/.16 to center, or 6.25.

It's a good thing the old 8-bit micros aren't sentient, because if they were they would be *seething* with jealousy. Oh, look at Mr. Fancy Parts with his 8 bits *per channel*. What does he think he is, a 386 with VGA?! Eagerly looking forward to the design for the other parts of the VGA subsystem.

That was inspiring. I'm not a fan of analog devices (or converting to analog), but I really enjoyed the simplicity of this design. It makes me feel like going back to electronics as a hobby... :)

You are going to get some error from the driver chip output drivers not all being the same. Also I'd try 0-100-0-100% checkerboard pixel test to see if you can slew fast enough.

With two 470 ohm resisters up from the ground rail, that's 2x the 1% error. I presume that's accounted for in your spreadsheet. I only mention it here because it's the sort of thing I'd forget, not that you would. (Edit: loved that DAC performance and layout, the enormous stencil, and once again very clean resistor array soldering)

Absolutely excellent results. And I think this proves why IBM chose to go with the same R2R ladder design for their first VGA designs. Can't wait to see the parrot in 24-bits. Thanks for an excellent video.

That stencil! Oh dear... At least it was usable.

The likely hood of the error being even terribly noticeable is minimal. I think this is more than a reasonable circuit to do 24 bits with.

Love that crisp slope coming out the DAC :D Ya love to see it! The acid test is gonna be plotting a colour gradient on the screen and seeing if there's any banding :) As for mixing the colours to get monochrome, you need to mix the RGB channels in these proportions respectfully: 29.9%, 58.7%, and 11.4% (I *think*. Don't quote me, play with the numbers and the orders :D) I think you can use opamps to weight the channels and mix them in the correct proportions. Can't wait to see more! :D Edit to add: It might be the case that the weighting and colour mixing is done inside the monitor, and you can just send the full signal to each channel. I guess make the parrot text card image monochrome in Photoshop or similar, and see if it looks the same xD

Great vid James :)

During the early design phase I was thinking you could use a meter to rank the resistors by deviation from the average and then use the most deviant ones as LSB, but with chips and PCBs that's a bit onerous. Incidentally, doing a project like this is my dream, but I haven't got anywhere near the time or money, or privacy to do such delicate work. I shall enjoy it vicariously through you, sir!

Such cutie of a PCB

Excited! 👌👌👌

The criticisms of using r2r ladders using discrete resistors are both completely valid and way off base. The 5% (or 1%) variability, is a max value, with today's manufacturing techniques you are getting way better than the stated amount. A nice lite diversion none-the-less.

Marvellous and I understood it !....cheers.

Great analysis to get to a good enough solution. This is what engineering is all about, to be fair you could check the resistor arrays before you start to make sure there aren't any outliers. Other than that for a home made video DAC this is a pretty good design. The only thing I would add would be the ability to add some bias voltage to each channel so you can tweak colour balance.

A great video but I feel the outcome was very much to be expected. Resistor ladders are very common in video output circuity. If they really had such big problems that would have never happened, everybody would use DACs or the would have specialized resistor ladder packages.

I’m building a VGA output for an FPGA and this video gave me some good things to think about… thanks!

I'm looking forward to see if you use a RAMDAC look-up table and how it is implemented. Regarding tolerance, I'm sure your choice of resistor packs will be absolutely fine. I would be inclined to use higher value resistors to reduce current consumption, but its all just a personal choice. You might notice a slight (lack of) gamma correction issues, but it isn't as if you are doing any Photoshop work on it! Definitely add some appropriate power supply capacitors to the chip driving the R2R ladder etc, and some bulk electrolytic to any output drive amplifiers. Don't forget the output may need a 75R impedance matching resistor to stop any 'ringing' or echoes in the video cable.

It would be interesting to see how a regular 8 bit dac compares to your home brew. Great work

Thanks for the reference to the TLC5602CDW. Although I also use a R-2R ladder for my stuff I've been looking for a more self contained packaged DAC solution.

Creative video, thanks :)

If you're going to design and build a new board for the final output as an absolute minimum I'd recommend adding those op-amp output buffers to minimise the loading effect of the cable and monitor on the digital chips.

Very nice. I don't know how you have the patience to wait for the PCB's. I'd be figuring out a way to manufacture those things before the sun goes down 🤣

Omg that stencil size xD

Couple of questions. Is it worth using higher value resistors to minimise the current you are sourcing/sinking from/to the HCT? Then use an opamp to buffer the output and of course you can then put the series termination resistor in to impedance match the VGA cable? Note that the more current you pull from the latch, the more vcc noise you will have and that will couple through to all logic 1 bits. Given your user of 2 layer PCBs, that noise might actually trigger some ringing in the 100khz-10Mhz range. That ringing unlikely to break your logic level stuff but might well be visible on your display as waves that ripple across the page or down the page depending on the frequency. P.s. opamp slew rate might be limiting factor, but I am sure you can buffer the voltage somehow.

I look forward to seeing how you reconcile the 24 bits of pixel data with the 8-bit processor. Perhaps something similar to what you did with the registers on the audio circuit? Alternatively, perhaps you could drive something from the 16 bit registers and still get lots of colors. I think the 16-bit RGB "standard" is 5 bits red, 6 bits green, and 5 bits blue, in that MSB-LSB order. Depending on your target screen resolution, that might be a fine... don't want too many more colors than pixels. 😁Then again, maybe that opens up palette opportunities? 🤷‍♂👍

If you want to improve smoothness of sawtooth, you can order more resistors than needed and manually select ones, which fits better together. I'm not sure how to do measurements for such tiny devices, but it is possible for sure.

Would it be worth to bake the resistors in a oven for say 24-48 hours and then measure the resulting resistance? Then you can pick the most accurate resistors for the top 3 bits in each ladder.

Yaay!

Buy more than you need and measure them all out to get matching sets. That's what I used to do on audio circuits to guarantee the left and right channels are matched.

Not sure if it's true for resistors but for capacitors the error is rarely close to zero (i.e. the bell curve has a hole in the middle) because the parts with good tolerance get binned out and sold for more money... So a +/- 20% capacitor will probably have an error of between - 20 and - 10 or + 10 and + 20. If it's below +/- 10% then it will get sold as a 10% part.

Nice sir

Might have missed it but shouldn't the unselected bits of the '574 be tristate (which you can't do for individual pins) or you're sinking current back through the R2R DAC and skewing the output? Simple as a signal diode per output perhaps?

You can also increase precision by putting resistors in parallel, 10 4700ohm 1% resistors in parallel will act as 1 470ohm 0.1% resistor, (this naturally depends on the normal distribution of the resistors you use ofc so beware of that, Dave from EEVBlog has a nice video on it)

I didn't try it, but maybe with a good sturdy paper cutter it might be possible to (cleanly) cut the stencil to size and the material is quite nice for shields and such.

An option that trades time for very low cost: get 5% (or even 10%) standard resistors, buy a box of 1000 (

yeah excellent. loving the preheater. very smooth result. *cough* 24 bit audio? *cough* hehehe

You could also measure the resistors and hand select them.

One suggested change. Make three boards. One RED, one GREEN and one BLUE!!

What about an interface that converts the values into a data stream that can feed a HDMI input on a modern TV together with the SPDIF audio stream for the audio meant to emanate from the speakers of the TV.

re: R2R ladder - you could have used 0.1% resistors for the 2-4 most significant bits and 0.5%-1% for the rest - you can spend 10$ on 100+ 0.1% resistors and bin them with a 10k+ count multimeter and use the ones closest together. Standalone resistors aren't that expensive, Digikey has 0.1% 0603 resistors at 11 cents each if you buy 100. 0.5% resistors are 4 cents each if you buy 100.

I would say, at least 6 bits per color component is what I like, gives that retro feel for a VGA display! I am working with (non-palette) 15 bit color (5 bits per channel RGB) frame buffer, and I don't get that retro feel.

If it was a problem, when doing one-offs, characterising the resistors and putting the worst at the least significant end should be fine. I like PWM in these cases, but that has its own problems... and some fun dithering tricks

Metal film resistors might have a nice tight tolerance to negate the problem.

James... I hope everyone says they found this interesting... I know I certainly did

have you thought of using a proper voltage reference in your DAC?

Regarding your slide at 8:03, that's not strictly true. While the distribution of manufactured parts is a bell curve, the higher accuracy parts are removed from the centre of the curve about the mean, leaving you with a bell curve missing the centre most part (since that's sold at the more expensive rate for the higher tolerance parts). It has a fancy statistical name (which I've forgotten). But in essence, you're very unlikely to get parts

The tunable MSB with a micro pot could be a quick (and almost elegant) way to make sure the DAC works well for years. Reminds me of the CD reader of a Gamecube. When you thought it was finally over for the damn thing after so many years... Some beloved engineer exposed a resistor value to change the voltage to the laser.

The FET circuit at 12:33 doesn't make any sense to me! You don't need a gate resistor (it just slows the FET down since you're making an RC delay with the R and the C of the FET gate). And resistance in the source doesn't make much sense in this digital application. However; looks like it works, so it's good enough :)

I would now have built the 8bit DAC 3 times and then plugged it into another board.

16:17 - kitchen scissors will cut the steel sheet, no problem.

I hoped you will test 25 MHz clock

At least you won't have to buy any stainless shim stock for a while

The sum of the errors is always accumulative.

The MiSter IO board uses a R2R ladder. I've built a few and I can't see any difference between 5% and 1%.

Electrically it might not make any difference but it keeps us "right way round" nazis happy. ;) I'd have bought an off-the-shelf DAC - spent more money, got the same results and not learned as much. :(

OMG... what a huge stencil. Sorry 😊😁