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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."
Probably would have been useful for the person to look at how C64 emulators and people handle transfer C64 tapes to PC.
*off to bathroom*
I'll report my findings later
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
The only title of honor that a tyrant can grant is "Enemy of the State."
I think that at least the basic interpreter should be taught to the new generations.
They don't feel confortable enough in less than 1 GB, what if they had just 4 KB?
Maybe Computers will never be as intelligent as Humans.
For sure they won't ever become so stupid. [VR-1988]
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.
echo -e 'global _start\n _start:\n mov eax, 2\n int 80h\n jmp _start' > a.asm; nasm a.asm -f elf; ld a.o -o a;
Reminds me of my housemate and I at university ('92-'95) using the tape control relay on an Acorn Electron wired to a PC serial port to rip the ROM so we could start writing an emulator. A small BASIC program PWM encoded the whole ROM in about an hour IIRC. Was a great start to the project, we got as far as CPU emulator, multi-window debugger, VGA display driver, and had it running basic no problems. He got it reading WAV's of games recorded from tape too. Got as far as the in-game screen of Chuckie Egg before we ran out of knowledge and became stuck trying to fathom the hardware keyboard input. (for the BASIC interpreter we just injected characters into the key buffer). Ahh, happy days. :o)
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.
Better known as 318230.
That must be one weird family...
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My 6502 system accessed the tape at 300 baud. I used an old cassette recorder for the job. I had my eyes on my uncles reel to reel hi-fi system. I reckon I could have got 9600 baud out of that just by exploiting the frequency response.
http://michaelsmith.id.au
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.
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.
"Technology.....the knack of so arranging the world that we don't have to experience it." Max Firsch
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.
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.
echo -e 'global _start\n _start:\n mov eax, 2\n int 80h\n jmp _start' > a.asm; nasm a.asm -f elf; ld a.o -o a;
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.
Stop worrying about the risks of nuclear power and start worrying about the risks of not using nuclear power.
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/
"Without curiosity and knowledge, the mind is a vast void. Without the mind, curiosity and knowledge are nonexistent."
The interesting thing about this article is:
This could have been done so much more easily :)
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...
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...."
Isn't that the buggy version that Woz built (and hated?)
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.
So say we all
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.
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.
If you can read Portuguese, check my blog
If you can use Babelfish, check out his blog :-)
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It's not quite equivalent. The recorded wave file is heavily bandlimited so you can't reliably use a comparator to convert it to a period shift signal.
Which is why the decoder uses two comparators: one at 32768/DIVISOR and one at -32768/DIVISOR. In order to count a pair of transitions, the decoder has to see the waveform cross below the negative threshold and then above the positive threshold. This Schmitt trigger provides a bit of hysteresis that cleans up the signal. The value of this DIVISOR corresponds to the tape deck's volume control.
If there were any noise around the zero-crossing point, it wouldn't work at all.
Such noise is what the DIVISOR is supposed to filter out. I disassembled parts of the Apple II BIOS once, and it turned out that the original Apple I/II cassette interface also implemented period shift keying.
He's lucky that method works.
If there were more noise than the DIVISOR could reject, then wouldn't it fail to load on the original hardware too?
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.
the idea was to make talk radio accessible to def people.
Like Mos Def and Def Leppard?
Bow-ties are cool.
I still own a couple Apple I's.
OK a new size TV
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!