Fun things you learn from old computers â€¦

Microsoft used to supply two versions of its BASIC for Macintosh. One used decimal mathematics for precise tallying of small amounts. The other used the more familiar floating point mathematics, rounding errors and all. I don’t know what floating point library Microsoft used for sure â€” perhaps Motorola’s 32-bit Fast Floating Point system â€” but it introduces rounding errors pretty quickly. Modern routines don’t start displaying oddly until after 15 decimal places.

Consider this simple program:

```10 LET x=36/10
20 LET a\$="## #.#"
30 FOR n%=1 TO 18
40 PRINT USING a\$; n%; x
50 LET a\$=a\$+"#"
60 NEXT n%
70 END```

Along with the number of decimal places, it should print 3.6, 3.60, 3.600, 3.6000, â€¦ with an increasing line of zeroes after the 3.6. Bas makes a valiant but typical attempt:

``` 1 3.6
2 3.60
3 3.600
4 3.6000
5 3.60000
6 3.600000
7 3.6000000
8 3.60000000
9 3.600000000
10 3.6000000000
11 3.60000000000
12 3.600000000000
13 3.6000000000000
14 3.60000000000000
15 3.600000000000000
16 3.6000000000000001
17 3.60000000000000009
18 3.600000000000000089
```

Oddly enough, good old Locomotive BASIC on the Amstrad CPC does it more correctly somehow:

So the variables, they vary.

Spectrum is Green!

My previous adventures with my Sinclair ZX Spectrum 48K in Canada were not resounding successes. I couldn’t get the display to work, and tapes wouldn’t load well, so I’d been using Fuse while the hardware sulked in a cupboard.

I’d previously got a proper power supply (9 V DC, â‰¥ 1.4 A, centre negative) and bypassed the PAL UHF modulator to give composite video. No television, monitor or converter box that I had tried seemed to give a useful display.

Back in May, Walter Miraglia brought a tiny 7″ composite colour monitor to TPUG‘s Retrocomputing Night. He let me try it with the Spectrum, and it worked very well. Walter said it was an extension monitor for a car DVD player.

I dug around, and found that local surplus clearout store Tech Source Canada had the Philips 7″ portable DVD Player PD7016/37 for \$60. This gives you two identical DVD players with composite input. I think my other one will be destined for a Raspberry Pi project somewhere.

To get these monitors running, you’ll need:

• a 9â€“12 V DC power supply able to give â‰¥ 1 A. I use a regulated supply that gives 9.1 V open circuit and is rated at 2 A. Note that the power connector is slightly smaller than the common 2.1 mm barrel, so you may have to order this one, unless you can solder something up.
• A cable like this 3.5mm Stereo to Composite Video + Audio Cable (3 RCA). These are sometimes just called camcorder cables. They use a 3Â½ mm TRRS jack, and can also â€” if you don’t mind not quite having the connectors in the right order â€” work with the composite/audio output of more recent Raspberry Pis. Tech Source had these for under \$5.

Connect it up , and â€” success! Well, slightly qualified success. The screens do not have the greatest resolution, so pixels are slightly smeared together. The screens do have a decently fast refresh, and the whole look is just right. With its colour clash and dot crawl, nobody ever expected great video from the Speccy anyway.

Here are some screen shots taken with my phone, and a couple of pixel-sharp screenshots from Fuse to compare:

So I can now definitely view the screens. Huge thanks to Walter for tipping me off to these DVD players.

[Incidentally, the screens are designed for car use, so don’t stand up properly unless you get creative with some supports. I laser-cut these out of 3 mm plywood:

Glue the little sticks on to the flat ends, and they’ll fit into the slots in the back of the monitor. Here are the feet with the sticks fitted:

There are better-designed feet than these, but they work, mostly.]

I was still having game loading problems. Try as I might, I couldn’t get anything to load reliably. Retrocomputing Stack Exchange came to the rescue, in the shape of mcleod_ideafix’s very helpful answer.Â  If your audio player is running from batteries and you can use a stereo cable, you can convert the normal mono loading audio into stereo with one channel inverted. This gives you effectively double the volume, and works quite well with my audio player, an old Edirol R-1*.

This audio will not load into an emulator, or work with a mono cable. Some audio players even render it as silence.

Here’s a script to take monophonic loading data and convert it to this faux-differential stereo format using Sox:

```#!/bin/bash
# wav2differential.sh - convert mono game tape audio to 2Ã— stereo
# usage:Â Â  wav2differential.sh infile.wav
#Â Â Â Â Â Â Â Â Â  (createsÂ  infile-differential.wav)
# scruss - 2016-06-07
# method by â€˜mcleod_ideafixâ€™; many thanks / greetz
#Â Â  â€” https://retrocomputing.stackexchange.com/a/774/439

base="\${1%.wav}"
sox -q --norm=-3 "\$1" -b 16 "\${base}-n.wav"
sox -qÂ  "\${base}-n.wav" "\${base}-ni.wav" vol -1.0
sox -qÂ  --norm=-0.1 -M -c 1 "\${base}-n.wav" -c 1 "\${base}-ni.wav" -c 2 -b 8 "\${base}-differential.wav"
rm -fÂ  "\${base}-n.wav" "\${base}-ni.wav"```

If you want to check your audio levels, sox can also create the 800 Hz header tone used by the Spectrum. Run the output of the command below through the script above, load it onto your audio player and fiddle with the volume until the border flickers steadily:

`sox -n -b 8 -r 8000 800hz-header.wav synth 30 square 800`

I was also looking for the games to load fairly quickly. Tapes used to take over three minutes to load, and while retrogaming all is about the experience, I haven’t got time for that. Fuse has some utility programs which will convert a .Z80 game snapshot into an audio file that loads in about 1Â¼ minutes.

To convert the snapshot to a speed-load TZX tape image:

`snap2tzx -o game.tzx -s 3 game.z80`

To convert that virtual tape image into audio:

`tape2wav -r 16000 game.tzx game.wav`

You can then run that WAV file through the stereo/differential script I listed above. Have fun!

Building the Stick of Joy

Update, 2017-06: I’ve updated the plans so you shouldn’t need to spendÂ  time sanding things to fit.

Tracking down old Atari-style joysticks for retrogaming can expensive, and it’s hard to tell if you’ll get something reliable. So I made one for less than the cost of a used stick on eBay.

To build this, you will need:

• 8-way joystick , or any stick compatible with the industry standard Sanwa JLF-P1 mounting plate. This has M4 holes at 84 Ã— 40 mm.
• Two concave momentary arcade push buttons. In my built, I used an older design that’s much taller. You could make the joystick box shorter if you used these snap-in buttons.
• DE-9 (DB-9) Female Socket Connector
• Terminal block, with at least seven connectors. You’ll likely want more, so this 12 position screw terminal block should work.
• 4Ã— M4 countersunk (oval head) machine screws with nuts and lots of washers. You’ll need washers to act as spacers between the box and the joystick mounting plate. This allows the joystick’s dust washer to move freely.
• a couple of metres of 8-core stranded signal cable
• hookup wire and spade connectors for building the button harness.

The case is made from 6.4 mm high quality plywood, using a template generated by BoxMaker.Â  The external dimensions of the box are 163 mm x 143 mm x 83 mm. I haven’t included any kerf width in the design, so the edges should fit together easily for gluing.

Joystick box plan for download: joystick-box-201706.svg (SVG: best in Inkscape); joystick-box-201706.pdf (PDF).

If you want to make your own design, here’s the top plate plan: joystick-box-top-201706.svg (SVG); joystick-box-top-201706.pdf (PDF).

The basic DE-9 pin wiring for Atari-style joysticks goes like this:

1 â€” Up
2 â€” Down
3 â€” Left
4 â€” Right
6 â€” Button
8 â€” Ground

There are many variants that add features to this scheme, however. If you’re building for a specific computer, Tomi Engdahl’s Joystick information page has the details.

Many thanks to Andrew Horsburgh for the use of Protolab‘s laser cutter.

LibreOffice brings the old

Twenty-two years ago, I wrote a thesis. It wasn’t a very good thesis, but it did what it needed to do. For years, its model files have been unreadable, because the spreadsheets were written in a ~1992 version of Microsoft Works. These are old files:

```1993-04-21 03:19 newmodel.wks
1993-04-21 03:19 newmodel.wk1
1993-04-08 06:29 pr_fa.wks```

Quite recently, LibreOffice realized that there are old files out there that (unlike my thesis models) could still be useful. As they have no commercial requirement to only support the latest and greatest, LibreOffice added the ability to read these ancient works. So my old stuff lives again:

I found a screen dump that I used back in ’93 to illustrate the layout. The display was colour, but here it is brought back to life with a little bit of antialiasing:

LibreOffice can also read old AppleWorks files. Although Works 6 still runs on Catherine‘s Mac, it looks a bit â€¦ dated:

Thanks, LibreOffice! It’s sometimes easy to forget (like right after updated to Ubuntu 15.04, which decided that BlueTooth support was kinda optional unless you jumped through hoops â€¦) that people do write software just to be more useful.

The shell of dead media lives on

You have to be of a certain age to recognize this:

… not just as an artist’s travel palette, but as a repurposed case for a 9-track tape spool. While tape drives were iconic for mainframe computers (so much so, there’s a Unicode glyph for them: âœ‡), the last drives and tapes came off the line a decade ago. They’re not truly dead until everybody forgets what these cases were originally for.