1200-Baud Archeology

jamie found this singularly geeky article on reconstructing Apple I BASIC from a cassette tape. It claims to offer the first confirmed perfect dump (BIN) of the 4096 bytes of this venerable interpreter. Terrific fun for the whole family. "The Apple I is extremely rare. Only 200 were built, and less than 100 are believed to be in existence. Neither Steve nor Woz own an Apple I any more, and neither does Apple Inc. The cassettes are even rarer, as not every Apple I came with one... So here is how to decode the signal. Let us first open the audio file in Audacity and look at the waveform... It is now time to write a small program to measure and dump the width of the pulses."

Re:Electron ROM Ripping, ol' school

[Anonymous Coward]

There is now a Free Software Acorn Electron emulator called ElectrEm:
http://electrem.emuunlim.com/ [emuunlim.com]

Unfortunately a GNU/Linux port of the new codebase version does not exist yet. Someone please contribute that the project. The graphics code is written for Simple Directmedia Layer (SDL), so this emulator should be quite straightforward to port.

Re:this makes me want to take a dump of my own

[eclectro]

be sure to check the logs

There are no logs with a cload.

MP3 ?

[XNormal]

The psychoacoustic models of MP3 compression must have done wonders for the ancient recording.

It's like compressing a bitmap of line art with JPEG.

Pom1

[anarkavre]

I wrote, ahem, ported a Java Apple 1 emulator about a year ago to SDL and added a few of my own features. Haven't done much more to it since then. But for those nostalgic geeks out there, you can find it at the following link.

http://pom1.sourceforge.net/ [sourceforge.net]

There is a MUCH Easier Way...

[macs4all]

I own an Apple 1. ...And a copy of Apple 1 BASIC on cassette, and Woz's Mini-Assembler that is "origin-ed" for the Apple 1. (This is the same Mini-Assembler that was in the Apple ][ ROMs, at $F666). And a few other Apple 1 goodies.

Do you realize that the cassette interface for the Apple 1 and the Apple ][ are identical?

Yep, you can read an Apple 1 audio cassette with any old, easy-to-find Apple ][. And from there, you can use any one of a million methods to get the data out of memory and onto another medium.

Also, you can simply use the Apple ][ to create a NEW cassette for your Apple 1 (if you happen to be lucky enough to have one).

BTW, I think mine is "serial number" 0064. At least that's what I think the "0064 that is written in Sharpie on the PC board means...

Re:Those were the days...

[Gordonjcp]

That's because they're electric trains with big spinning magnets ;-)

The BBC used to get people to take taxis rather than use the Underground, for the same reason.

Re:Electron ROM Ripping, ol' school

[Megane]

It was 500 baud. I've done a similar thing trying to read some of my own Model I tapes, but noise makes the simple decoding mechanism in TFA not work so well. (The Model III used 1500 baud)

I've also done some experimenting with Apple II and C-64 GCR decoding with a Catweasel board (FM/MFM already having been handled quite well), and the C-64 GCR is a lot harder to keep in sync because it doesn't have address-mark-only nibbles.

Re:Kansas City Standard

[macs4all]

Nope. Woz rolled his own standard. MUCH faster and MUCH more reliable than the Kansas City standard.

In fact, of all the cassette interfaces of the day, including some REALLY elaborate S-100 ones developed by Don Tarbell, the Apple 1 (and Apple ][) cassette interface stands alone in reliability and "forgiveness" to things like playback level and tape quality. That is because Woz depended SOLELY on the timing of ZERO CROSSINGS of the signal, which, unless the tape speed is enormously out of whack, or enormously unstable millisecond-to-millisecond, have ZERO (pun intended) effect.

I have some original engineering notes by Woz regarding his development and testing of the cassette interface, where he describes his own amazement at how reliable his cassette interface is. He describes hunting down the cheeziest recorders, and the most horrible cassettes, to try it on, and it works like a charm. It just also happened to be either the fastest, or the second fastest (and easily the most reliable!) of anything available. Before or since.

Neither Steve nor Woz owns an Apple I?

[SteveWoz]

I still own a couple Apple I's.

Re:Alternative tools

[Detritus]

I've done work on recovering data from old analog tapes. You could clean up the signal by running it through a DC-blocking filter, which is a high-pass filter with a very low cut-off, and a software AGC with a short time-constant. Some of the noise can be eliminated with a low-pass filter. There isn't much that can done to eliminate speed variations. The best thing that you can do is to write a demodulator routine that can track the speed variations and has generous allowances for timing errors in the signal. Looking for zero crossings works OK on relatively clean signals. For more challenging material, you want to integrate over the time period of each bit.

Re:Those were the days...

[Detritus]

It's those traction motors. They are notorious for emitting a magnetic field that is strong enough to erase magnetic tape. If they can move a train, they can erase your tapes. The same thing used to happen to couriers transporting 9-track magnetic tapes in NYC via the subway.