Andrew Beresford's Blog

I just discovered a nice side effect of being at the top of Google‘s ranking when you search for me. Typing “Beezly” into the address bar in Firefox takes you straight to my home page!

Today was a slow day in “Bob The Builder Props” so me and my colleague edited ourselves onto Wikipedia (look at CREW.)

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JonnyK.

Whilst in London last week, I went to Viva Viva in Hornsey and saw Brody and Quint, a three piece Alt-folk-pop group – who not only sounded pretty darned good, but they’re also holding the torch of northern-rooted music in London (Bolton, Harrogate and Liverpool) and as an added bonus, they’re very nice people too! Anyway, if you get the chance to go and see them, do so. I’m sure you’ll enjoy it.

Traction Engine Man

I’m in Woodbridge at the moment. Nick, Manda, Jon, and I went to a Steam Rally yesterday afternoon and saw some neat stuff. I’m going back up today, although I’d much rather stay down here. Sadly, work beckons

Hi, my name’s Martin and I’m an old-skool-aholic. Here’s something that has been fascinating me recently.

One of the few things I can remember about being 10 in 1990 was the absolute and easily demonstrated superiority of the Amiga for demos and gaming over the PC. I remember the family Amstrad 286 slowly gathering dust (save for the odd game of Wolfenstein 3D) while my poor A500 gradually turned yellow from relentless use by myself and 4 younger siblings.

Recently I’ve developed a yearning for those early days of metal-bashing simple, documented hardware. I’ve been looking into what it would take to write an impressive demo for a typical affordable 1990 PC, say an 8086 with VGA graphics and some sort of audio hardware.

During my initial research I’ve discovered several interesting things about these old PCs in general. First of all, the original 4.77mhz 8088 isn’t as slow as you might think. It achieves 0.22 dhrystone mips, which compares favourably to the Amiga 500, which gets about 0.55 on the same benchmark. Of course, you may argue a 68000 has a better “flat” memory model, and I would agree. The 8086 isn’t totally hobbled by segmentation like a Z80 or 6502, though. A program in one segment can read and write data in another, after all.

An average of 220 thousand instructions per second is actually pretty respectable. There’s certainly plenty of CPU-time per frame to do work, provided you don’t have to push individual pixels do audio mixing. Which is normally where you expect our “old-skool” PC falls down compared to the Amiga. The lack of hardware-offloading. But things are really not as bad as you think.

I’ve been looking at the VGA and I’m surprised at it’s capabilities. On paper it is superior to the Amigas graphics hardware (even the AGA) in many ways. The most common VGA mode for games and demos was 320×200 with 256 colours from a palette of 262144, although 400×300 was possible with a bit of register magic.

All the registers are there to synchronize graphics to the horizontal and vertical sync for raster-effects and smooth graphics. Tear-free double-buffering and pixel-perfect hardware scrolling (both horizontal and vertical!) is possible using exactly the same mechanism as is used on the Amiga.

The VGA has “chunky” graphics, meaning one byte per pixel. It’s what made Wolfenstien 3D and Doom possible, and possibly contributed to the A1200s early demise as a gaming platform. The Amiga has “planar” graphics, which is a whacky method of laying out graphics using 1 bit per pixel frames. Changing the colour of a single pixel is therefore hard work.

The VGA lacks a copper (a piece of hardware that can be programmed to push values into registers on certain scanlines), so the cpu has to poll the beam registers (but that doesn’t take much cpu time to do). Also the VGA has no sprites or blitter, but the chunky mode means that a lot of things you need to use the blitter for on the Amiga are simply not needed with VGA.

I can’t see any real reason why all those early PC demos seemed to look so bad. I guess the Amiga attracted all the good coders away from it!

Now, the sound. A 1990 PC with sound would have either an Adlib card or an original SoundBlaster (which is Adlib compatible). The Adlib is a 2-operator FM synthesizer with 9 voices. Each “operator” is a sine wave generator with an ADSR (attack,decay,sustain,release) amplitude envelope. The operators can be stacked so that one modulates the frequency of the other, which can give square-wave-like and sawtooth-like timbres.

This is actually a powerful synthesizer, and comparable to the Yamaha DX7 (a 6-operator FM synth), which was used by many mid-80s synthpop bands. After listening to some adlib tunes from www.chiptune.com I came to the conclusion that it “rocks”. Your mileage may vary, of course.

I prefer digital sound however, so I’m more interested in the soundblaster. The soundblaster has a single 8-bit DAC with a maximum frequency of 22050hz. Not brilliant, but with care I reckon you can get useful results. First of all, if you assume a 4.77mhz processor, full sample mixing is right out of the window. To play a MOD, each sample would require several adds, shifts and table lookups and the CPU simply doesn’t have the power (at least at a listenable sample-rate).

If you throw away volume and frequency scaling, however, and simply add each channel together, the mixing process is vastly simplified. Playing different volumes and frequencies would require different samples, but this could be implemented using a wavetable of very short chip-like samples. So you basically get the ability to use sampled drums and loops with synth-sounds.

If you assume 220 thousand samples per second throughput on our 8086, it would be possible to mix nearly 10 channels at 22050hz in this way. This theoretical performance may be way too high, but given the 8 bit sample resolution, you probably wouldn’t want to mix more than 4 channels in this way anyway.

Unlike an Amiga, both sound systems are mono, although the Amiga’s hard-panned stereo isn’t exactly ideal.

Well, that’s my current take on the situation. I’m looking forward to discovering what really was and wasn’t possible.

I ain’t saying nuttink else Good luck buddy old pal.

What I want to know is… why does Karl Kennedy run the doctors surgery AND works in the hospital looking after folks with multiple injuries? Eh? EH? EH?!

I have a week off work! Hooray. Next weekend I am off to visit Amanda and Jon and during the week I spending some time in London, staying with Harry on Wednesday and Thursday night. I hope to see a few people around London during the week, so if you are about, give me a shout.

Leo

I went to Vox last night to see Leo playing – an excellent set and quite a few people turned up to hear Leo play. Good stuff!

Do you have some ports blocked outbound on the network you are connected to. My employer blocks IRC outbound because of trojan problems, but now you can get around it with Tor.

There is also an extra benefit of anonymity too – useful where you don’t want people to know what services you are connecting to. You can also use Tor to run any service over TCP anonymously.