Robert Morrison

I suppose it’s quite nice sometimes maybe to switch the computer off and go outside for a bit. We saw dolphins and it was lovely (not pictured because I was looking at them and enjoying it and not pointing my pocket computer at them).

Goin’ surfin’ 😎 🏄 #amstrad #nc100

I experimented a bit more with creating an image to deliberately exploit the effect that a 50Hz alternating image has on my TV (and other upscaler chipsets).

Since the TV seems to display all the even lines of one frame, and all the odd lines of the other I thought I’d create a double-height image of 256x384. The source image is scaled down horizontally (so that it looks stretched) but that will be corrected when displayed at 256x192 because the de-interlacing effect scales down the image by half vertically (each row of the display is half a pixel high).

I exported the image as an (8-bit) 16-colour PNG, then ran it through my 8-bit to 4-bit conversion script that I’ve been using to convert sprites. I modified the script to export all the even lines to one file, and the odd lines to another file. (Each 128 bytes in the file represents one screen line, and the lines are ordered top to bottom).

I thought about just using one contiguous file and doing some processing on SAM but then I’d have to deal with a 48kb image file. The resulting files are basically a normal Mode 4 screen$ file, (just missing the palette data) so that keeps things straightforward.

To help further with moving the image around SAM’s memory I compressed the data using zx0. In this case it took the 24kb file down to 7113 bytes. The Z80 implementation of the decompressor is really fast and I use it to dump the two images directly into the respective frame buffers when the program starts up.

I don’t yet have a way of automatically mapping the palette from the PNG index to SAM’s format, so I just opened up the 16-colour PNG in SCADM’s handy image importer and copied the palette entries from there. (The order was wrong; I’m not sure how my image editor assigns the colour indexes so I just reordered them by hand).

I’ve started using Simon Owen’s pyz80 extension for VS Code which provides a wrapper around Andrew Collier’s pyz80 assembler as well as Simon’s samdisk tool for manipulating disk images. So now with a keyboard shortcut the code is assembled with pyz80, and samdisk creates an auto-booting disk image as well as broadcasting the assembled binary to my SAM listening on the local network. The TrinLoad softaware running on SAM recieves the code and launches it. Here’s how the resulting image looks on the TV:

It definitely works to smooth out certain things—like the curvature of the earth, the top of the space helmet or Sonic’s quills (compare that to the stair-stepping inside Sonic’s ear for example):

I noticed I had to load the even lines in to the first buffer; the other way around incorrectly interlaces the images and results in visible comb artefacts:

But otherwise I think it’s a neat effect. What do you think?

Here’s a disk image if you’d like to try it out: image-fun.mgt

Denise #KeepingUp #Commodore

I had some success removing the yellowing from my SAM with “vapour brite” (works just like Retr0brite but the plastics don’t go directly in the solution). Compare the drive facias (before treatment) to the SAM top case (after half a day in the sun). 😎

Tonight I’m mostly using @shieladixon’s wonderful MIDI interface for RC2014 to take a closer look at how serial Networking is implemented on the SAM Coupé. (TL;DR it’s MIDI).

SAM uses DIN-7 connectors but the sockets are compatible with DIN-5 (MIDI).

The RC2014 MIDI Interface Z80 Framework gives straightforward access to the raw bytes. I’ve noticed that my USB-MIDI adapter (also pictured but not in use here) automatically translates the bytes into “standard” two- or three-byte MIDI messages (usually by injecting extra control bytes in an attempt to try to make musical sense out of non-musical data).

The data structure is similar to how ZX Spectrum BASIC programs are stored. Header bytes give file type and file name etc, the BASIC keywords are tokenised and variable names and values are tacked on at the end.

This week I’ve been getting to grips with SAM’s VMPR (Video Memory Page Register) and using it for double-buffering the screen. Into that framework I dropped the sprite drawing routines I used on Spectrum, and graphics I made a while ago for a web-based game. #GameDev #SAMCoupé

Those low-colour pixel-art PNGs converted quite nicely to SAM’s 16-colour 4bpp screen mode. I haven’t got round to making a unified/optimised palette yet (the squirrel character isn’t supposed to be blue!)

I truly love paper magazines but is using OCR for code listings…cheating? (Asking for a friend). #z80

Now playing: SAM Coupé Shanghai by Fuxoft/Franxoft for Pete Prodge’s Covertape wars (YS vs SU - Dec 1992)

The colourful tiles are easier to read than the typically monochrome tiles in Spectrum mahjong games, but overall it’s not as featureful. A nice straightforward solitaire game though.

…and in with the…Old! I replaced the 1984 Z80 with one produced in 2018 but it caused some weird screen corruption. I swapped it for another from 1984 (produced the same week!) and it works perfectly. Meanwhile the 2018 Z80 is perfectly happy in my Harlequin; CMOS/NMOS issue??

Out with the old! My desoldering gun is still not Quite Right so this was mostly done the old fashioned way: a manual pump, braided solder wick some flux and lots of patience. (If the chip is know to be totally gubbed it’s easier to just snip the legs but I wanted to save it).

Well it looks like we have a busted M1 signal. It’s not used internally so the Spectrum itself will function without it, but it’s needed for peripherals like the Interface 1 and modern devices like the DivMMC.

This evening I’ve been fixing a ZX Spectrum+ for a friend. Two bad RAM ICs and a 3rd (previously repaired way back) had gone a bit crusty. Wrong polarity power was maybe used, but the DC circuit measures okay. Needs a new membrane but I’ve given the keys a bath in the meantime.

I ordered some circuit boards from PCBWay using MerlinKV’s PlusD Clone design (CC BY NC SA). Now I’ll need to find all the parts (and work out how to blow a GAL).

“Oh”, they said, “so that’s what you wanted a serial protocol mouse for!”

#SAMCoupé #Lemmings

Update! Thanks to Colin from Quazar for clearing up my disinformation and abject nonsense.

I removed the old crusty battery and replaced it with an equivalent.

I also removed the corroded power socket. Apparently it was never, from a strictly electrical point of view, “entirely safe”, and so the official power supply never materialised. The 3.5mm audio jack would toggle between internal and external power when the jack was inserted or removed. Safe enough so long as all the power is off!

A wire link in place of the socket ensures the power supplied by SAM can still propagate to the connected devices on the four expansion ports.

A good scrub to get rid of the grime and battery juice, and it’s all working properly again.

What about those flat-head screws though, eh? Those MGT boys sure were goofballs! The cases were sent to customers separately after the bare boards were issued. So the original owner (or someone else) may have used any old screws.

Phew! Sorry for all the mistakes!

Yum! This is the special sugar-frosted MGT SamBus. (a.k.a. The SAM Card Cage). It adds extra expansion ports, a real time clock, and special secret coating that will keep you full until lunchtime. #SAMCoupé

Good news, everyone. It’s mousin’ time!

Oh no! I’ve been working on a fun little ZX Spectrum project using Microdrives and serial networking. That’s the end for this cartridge though. 😬

Come out to the coast… we’ll get together, have a few laughs.

I was playing the SAM version of Prince of Persia for about 20 minutes before realising I was actually playing the demo! After looping around the same 6 screens a dozen times I managed to glitch through a gate and it turned into Frogger (well sort of)

https://youtu.be/Ee_oX5J6_80

Low voltage 🔋🧲

High voltage ⚠️

I noticed my SAM was prone to tape-loading errors, and particularly when the cable gets jostled. Well that was because the socket was cracked, so I replaced it with one harvested from a broken cassette recorder.

The user manual is a bit vague about this, but SAM actually uses a stereo socket although only the ring (normally the right channel in stereo audio) is used for both input and output. The tip is not connected to anything. The idea is you would leave the SAM end alone and only swap the cable between the cassette recorder’s MIC or EAR socket depending on whether you were SAVEing or LOADing.

The replacement socket is an old-school mono design but it makes a rock solid connection so loading is now much more reliable.

Most SAM software is on disk of course, but quite a few demos were put out on Spectrum magazine cover tapes (and originally the machine came with a demo tape from MGT). I don’t have any official SAM tapes, but it works well with the TZXDuino digital tape player that I use for Spectrum software. I do have a few of those magazine cover tapes though and they also load really reliably now.

And, of course, SAM can emulate a 48K Spectrum so it’s possible to load those via tape (or TZX file) as well.

I’d been hesitant to take a soldering iron to my SAM Coupé, despite its display output suffering badly from reflection (i.e. ghosting) artefacts. But the recommended fix (by Quazar, see SAM Revival #1) is so simple I couldn’t put it off any longer! A huge improvement. 🧑‍🔧👻