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1200-Baud Archeology
Posted by
kdawson
on Tue Jul 15, 2008 04:32 AM
from the when-men-were-men-and-interpreters-ran-in-4k dept.
from the when-men-were-men-and-interpreters-ran-in-4k dept.
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."
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Alternative tools (Score:5, Interesting)
Re:Alternative tools (Score:5, Interesting)
Yes - I thought this too - the article's slashdotted at the moment but the summary makes me think he made a mountain from a molehill. In the Sinclair Spectrum world, loading Speccy tapes on to a PC, and encoding them in a useful format (TZX) has been a solved problem for years.
All these tape formats were physically pretty similar when it comes to how they were encoded, and the same techniques could have been used by looking at any home computer emulator that loaded stuff from tape even if the details were different.
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Re:Alternative tools (Score:4, Funny)
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Re:Alternative tools (Score:5, Insightful)
I think the emphasis is more on the historical significance, given the rarity of the tapes and the fact that the only digitised copy floating about has been patched.
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Re:Alternative tools (Score:5, Interesting)
Didn't the Apple I use PSK instead of FSK encoding for the tape audio?
I though instead of Shifting frequency they shifted phase which is quite a but harder to detect than frequency shift.
BTW: Computer Tapes worked great to load software across Ham radio. 2 meter radio, I would load a game from a friend across the city over 2 meters by simply patching audio from the rig to the computer.
Luckily the C64 had a very slow bitrate (even the floppy drive was slow as hell) for it's storage tapes so it worked great.
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Re:Alternative tools (Score:5, Interesting)
There used to be a Dutch radio program in the 80s called "NOS Hobbyscoop" that had their own basic interpreter for many computers of the day. (MSX, Acorn, Sharp MZ, etc.)
They actually broadcast computer programs every week on medium wave AM. They'd count down, you start the cassette recorder and you had some new programs.
Fun for the whole family, even if a bit painful on the ears!
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Re:Alternative tools (Score:5, Interesting)
here in brasil, UFRGS (federeal university of rio grande do sul) broadcasted test programs using stereo FM. one chanel would broadcast audio, the other encoded text to be decoded by an MSX computer and displayed on screen. the idea was to make talk radio accessible to def people.
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In Federated Brazil... (Score:4, Funny)
the idea was to make talk radio accessible to def people.
Like Mos Def and Def Leppard?
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Re:Alternative tools (Score:5, Interesting)
No. It's FSK, but it used a constant number of zero crossings per bit, rather than a constant bit period. So, to decode it, you count the average time between zero crossings rather than the number of zero crossings in a fixed time window.
On the plus side, it seems like the Woz scheme has some benefits. If you assign the shorter bit period to the more common bit value (likely, '0'), you shorten your average recording length a little. On the minus side, if you get an extra zero crossing in there (say, due to noise that wasn't filtered away by a Schmitt trigger or other hysteresis somewhere), recovery may be awkward.
BTW, the C64 floppy drives were slow as heck, but that had nothing to do with the bitrate for its media. There's some goofy history there, involving bugs in the shift register on the VIC-20's PIA, the decision to use CPU control loops instead to determine the bit period when communicating between the drive and the machine, and then the greater cycle-stealing period of the VIC-II as you get to the C64 throwing a monkey wrench in the works. The fast-load carts worked by restoring the native CIA hardware shift register to get rid of the CPU-controlled bit shifting to read bits from the floppy, restoring its speed to performance levels similar to the old IEEE-488 based bus they used back in the Commodore PET era. But that's a different story for another day.
--Joe
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Re:Alternative tools (Score:4, Interesting)
Probably the most elaborate scheme I've seen so far is the Mattel Electronics Keyboard Component [spatula-city.org]. It encodes everything into blocks of 32 10-bit words, each protected by a 5-bit detect-2-correct-1 Hamming code. The 32 words are then re-interleaved as 15 32-bit words. Each 32-bit word gets prefixed with a 5-bit framing header. That's prefixed by a block of zeros and a special 64-bit sync pattern per block. (I believe the whole sync structure was about 256 bits long.) The whole shebang is then Manchester encoded and put out to tape. Furthermore, the drive had a carrier-detect signal that it supplied in addition to the Manchester decoded bits.
The net result is a fairly robust protocol. Because the 32 data words were interleaved, dropout errors would get spread among multiple words. Thus, a burst error would show up only as 1 or 2 bit errors in each individual word. In addition, if a given word shows up as non-correctable, it was sometimes recoverable by flipping bits based on where the carrier was lost and trying again.
Other nifty aspects of its design: There was an additional pre-header on blocks that was recorded at (I think) 1/3rd the normal carrier frequency. This allowed the drive to detect interesting headers while fast-forwarding or rewinding. Since the drive was computer controlled, it allowed for fully automatic operation, including hardware seeks.
What was the bit rate? Well, I believe the raw bit rate for data bits coming off the tape was about 3000 bps. Factoring in encoding overhead, though, I'd say the final throughput was less than half that. A quick calculation suggests a nominal throughput around 1200 baud, give or take.
--Joe
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Re:Alternative tools (Score:5, Insightful)
When I was a teenager, I used to decode FAT tables and directory structures by hand, using pen and paper and printouts from a raw hex dump of a hard disk. I didn't do this because there was a problem needed to be solved; I knew what was on the disk and there sure were plenty of tools to read the data (like MS-DOS). But it was a fun challenge and I learned how FAT worked.
I can see how this is a similar challenge. It's nothing more than a geeky sudoku.
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Re: (Score:3, Insightful)
The code being simple does not necessarily mean the though process of coming to those 15 lines was simple and/or straightforward.
Re: (Score:3, Interesting)
Actually, If I remember right (digging back oh, nearly 30 years into the recesses of my mind), the C-64 had a fairly unusual tape format, unlike almost everyone else, just like their disk drive was unique (everyone else used a fixed number of sectors/track, where Commodore used a variable number, with more sectors/track as you moved out, to get more data on the disk)
this makes me want to take a dump of my own (Score:4, Funny)
*off to bathroom*
I'll report my findings later
Re:this makes me want to take a dump of my own (Score:4, Funny)
be sure to check the logs
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Sounds like a great idea for an iPhone app. (Score:4, Funny)
It would be way cool to have an Apple I emulator on my phone. Come to think of it, a DEC PDP-1 emulator with SpaceWar would be pretty sweet, too.
-jcr
Teach it! (Score:5, Insightful)
They don't feel confortable enough in less than 1 GB, what if they had just 4 KB?
Re:Teach it! (Score:5, Interesting)
I think that at least the basic interpreter should be taught to the new generations.
To rain on the parade, I wonder if there is a copyright violation in posting the code online un-edited. How long is copyright nowdays?
It's something we need to address in this age of IP property where the market has expired years ago but the copyright is in force for many more decades.
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Re:Teach it! (Score:5, Insightful)
Functionally "forever".
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Re:Teach it! (Score:5, Insightful)
If Apple tried to sue, Woz would likely pay for your defense.
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Re:Teach it! (Score:5, Interesting)
Of course I would! But this gets sticky. I gave away the Apple I schematics and monitor ROM (256 bytes to replace a front panel with a keyboard) but only a couple of copies of the BASIC, which I also intended to be in the open domain. I don't know if I ever gave away the completed Apple I BASIC because by then it was virtually the same for the Apple ][ (now completed) as well. We took the steps to retain the copyright for BASIC on the Apple ][. Any steps we took for the Apple I would be in the gray. I can't believe that it matters to Apple at all anyway.
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Re:Teach it! (Score:4, Funny)
There's a simple solution; post it as a hex (or binary or octal) dump. You can then claim that it's just an excerpt of the first trillion or so digits of pi. By the time Apple's lawyers determine it isn't, the guys who decoded & published it will be long dead.
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Re: (Score:3, Insightful)
How long is copyright nowdays?
How old is Mickey Mouse?
Re: (Score:3, Insightful)
Yeah, but runners sometime train with weights on to build strength. When they remove them their speed improves.
Electron ROM Ripping, ol' school (Score:5, Interesting)
Re: (Score:3, Informative)
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: (Score:3, Funny)
Re:Electron ROM Ripping, ol' school (Score:4, Funny)
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Very interesting (Score:3, Interesting)
I have a TI-99/4A that has been dead for nearly two decades, along with several hours worth of data stored on cassettes. I would love to recover the data off of those tapes. Most of it is the type of stuff a 10 year old would write in TI BASIC (and Extended Basic!), and it would really bring back some fond memories and certainly some good laughs.
Are there any generic utilities that can extract binary out of low-baud modem audio files? With the advantage of performing various audio processing and analysis in a non-linear, non-realtime manner, certainly data could be extracted by modern software that not even the actual legacy computer could decode.
Re:Very interesting (Score:4, Interesting)
http://www.cs1er.com/ [cs1er.com]
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Uhh... (Score:5, Funny)
That must be one weird family...
Nick Hodge Says (Score:4, Interesting)
Apple Inc does own an Apple I
It is actually owned by Apple Computer Australia, and on loan and display at the Powerhouse Museum in Sydney.
For many years, it was under a glass box in the foyer of the Apple Australia offices.
Memories (Score:3, Insightful)
I tell my kids about loading programs off cassette tapes but they just don't get it. I guess they'll never know the agony of having a program ruined by fragility of magnetic tape.
Re:Memories (Score:5, Interesting)
Tape, yes. But do you remember 45rpm floppy 7" "records" that came with magazines? Or programs broadcast over the radio late at night?
I've just written this article, coincidentally:
http://mark.tranchant.co.uk/2008/07/a-unique-generation [tranchant.co.uk]
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MP3 ? (Score:4, Informative)
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 (Score:4, Informative)
http://pom1.sourceforge.net/ [sourceforge.net]
Amazed it actually works (Score:5, Interesting)
The interesting thing about this article is:
This could have been done so much more easily :)
There is a MUCH Easier Way... (Score:5, Informative)
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...
Those were the days... (Score:4, Funny)
Many seasons ago, in a high school computer lab in the Bronx. I would save programs from computer labs Commodore PET to tape and wonder why they would always be blank the next day. Over time I realized that riding the NYC trains with my school bag on the car floor was not such a cool idea. NYC trains were somehow erasing the tapes when they were place closed to the floor. Until I figured this out there were many nights spent pondering what the gods of computing had against me. Curse you Number 6 Line! Curse you!!!
"Ahhh the suffering...."
Re:Those were the days... (Score:4, Informative)
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.
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Why they didn't use MESS? (Score:3, Interesting)
MESS emulates the Apple I, can read WAVE files, and the entire source code is available. :)
I miss my old CoCo3, but I hated cassette tapes. The saddest thing is that Audio Cassettes were designed to be lousy as a data storage media - they used two sides (interference), and were created to record just human voice. The only other option were floppy drives, and back then they were expensive and/or overpriced ($200 and up) which is equivalent to $400+ now in 2008. Most drives had to include the entire controller I/O inside the unit, and probably also a disk OS.
6502 assembly - now there's a blast from the past (Score:3)
I disassembled a few dozen bytes of the dump to see what it looked like. I have no idea what it's supposed to be doing, but seeing the code does take me back a few decades...
E000 4C B0 E2 JMP $E2B0
E003 AD 11 D0 LDA $D011
E006 10 FB BPL $E003
E008 AD 10 D0 LDA $D010
E00B 60 RTS
E00C 8A TXA
E00D 29 20 AND #$20
E00F F0 23 BEQ $E034
E011 A9 A0 LDA #$A0
E013 85 E4 STA $E4
E015 4C C9 E3 JMP $E3C9
E018 A9 20 LDA #$20
E01A C5 24 CMP $24
E01C B0 0C BCS $E02A
E01E A9 8D LDA #$8D
E020 A0 07 LDY #$07
E022 20 C9 E3 JSR $E3C9
E025 A9 A0 LDA #$A0
E027 88 DEY
E028 D0 F8 BNE $E022
E02A A0 00 LDY #$00
E02C B1 E2 LDA ($E2),Y
E02E E6 E2 INC $E2
E030 D0 02 BNE $E034
E032 E6 E3 INC $E3
E034 60 RTS
Back in my C64 days, I used to practically think in 65xx assembly code... ah, memories.
Neither Steve nor Woz owns an Apple I? (Score:5, Informative)
I still own a couple Apple I's.
Re:Doing it the hard way (Score:5, Interesting)
Yes. But a few people did some very magical things with tapes before the became obsolete. I saw a demo of a turbo tape system on an Atari 800XL which could load games "faster than a disk drive". Actually it about tied, but that was still impressive. The disk drive could probably managed 9600 baud sustained.
The modulator / demodulator was lump of potted electronics I could easily fit in my hand. Potting compound was a blank gunk you applied to electronics you didn't want people to tamper with, in this case to stop people seeing the components used. But whatever they were they could modulate and demodulate data at around 9600 baud. This was in the 80's back before DSPs too, so whatever circuit was used must have been made of Op Amps, transistors and passive components.
I never worked out how it worked. Though I can imagine exploiting the stereo nature of the tapes to send one carrier and phase shifted signal might work. Phase modulation is easy and demodulation is too if you have the carrier. Still phase modulation at 9kbaud+ would be a tight fit on an audio tape. I don't think things like QAM would be possible given the size of the package, the selling price (about twenty English pounds, or $40), and the primitive nature of 80's technology.
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Re: (Score:3, Interesting)
QAM (Score:3, Interesting)
Still phase modulation at 9kbaud+ would be a tight fit on an audio tape. I don't think things like QAM would be possible given the size of the package
Quadrature amplitude modulation was in use in the 1960s: it's just two AM carriers out of phase by 90 degrees. The color encoding in NTSC and PAL used QAM.
Re:Doing it the hard way (Score:5, Interesting)
The computer I designed and built around 1981 did 9600 bps with some TTL logic i designed myself. The format used was Manchester II, very simple to encode and decode if the clock can be recovered (the difficult part) for the decode phase. I think I used less PCB space than what was needed for the common 300 bps Kansas City format.
When the Atari sent data to the tape it had an internal modulator. But IIRC the demodulator was in the tape deck. And in any case you could output data to the disk drive, when it was a selectable baud 0-19200 rate and not modulated. So it seems like the turbo tape interface could use custom software to get 9600 baud TTL data to or from the tape and do its 9600 modem baud magic internally with a handful of components.
Tapes are stereo, so you could send the clock one one channel and the phase shifted clock (the signal) on the other.
You need an oscillator and a phase shifter made out of an XOR gate to modulate. Shifted single goes on one channel say left, unshifted one on the other, say right
To demodulate you use a phase detector made out of an XOR gate to compare the phase of the two channels.
This would be analogous to Manchester BPSK coding, except that you use one of the two audio channels to store the carrier so you don't need to spend expensive electronics regenerating it.
So something like this seems plausible. Unfortunately I didn't know enough about electronics back in the Atari days to try it.
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Re:Apple I BASIC? (Score:4, Insightful)
Who cares? The fun is in recovering it, not using it! I'm sure that the archeologists that find mummies don't want to mummify dead people, they just want to learn about how the ancient people lived.
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Re: (Score:3)
If you can read Portuguese, check my blog [blogspot.com]
If you can use Babelfish, check out his blog [yahoo.com] :-)
Re:Apple I BASIC? (Score:4, Funny)
The fun is in recovering it, not using it!
The real fun is in irritating Woz by reporting all the bugs to him again.
Hey Woz. I just found this small program you wrote and boy, is this code terrible!
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