Half a croissant, on a plate, with a sign in front of it saying '50c'
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Home micro storage system

An idea whose time has come and gone and come back again
  [vote for,

As you are probably aware, I’m something of a retro fan and also easily distracted. Consequently I’m in the process of working out how to use a Commodore 64 to type documents and avoid distraction before transferring them to a memory stick and posting them on the web. Hence whereas this idea may appear to be decades out of date, it is in fact very much a current issue for me.

One of the odd things about late-’70s and early-’80s home micros was the huge mismatch between their video and cassette interface bandwidths. For instance, the ZX81 averaged about 250 baud on the cassette side but was able to fling six kilobytes of pixels (albeit character-mapped) at a TV screen twenty-five times a second. Although this might at first suggest the use of VHS cassettes to store data, this doesn’t work because VHS cassettes have rubbish bandwidth. Even so, this means that the cheapest computer ever made was able to achieve a bandwidth comparable to broadband back in 1981. Of course it’s not much use nowadays and unclear how this could’ve been exploited. I still steadfastly maintain, however, that this is an absurdly uneven asymmetry present in all first generation home computers.

I am currently confronted with the difficulty of getting a Commodore 64 to communicate with laptops, tablets and the like without further financial outlay. However, there does seem to be a straightforward way round this: use a QR-code style system.

Write a program which displays 2x2 pixel characters on a 40x25 text screen with appropriate simple compression algorithms. This is, due to caution, way below the maximum limit on bandwidth. Proceed to take screenshots with an ordinary digital camera, perhaps part of another device. This would enable a minimum of 750 bytes per screen to be transmitted, allowing an uncompressed 64K address space to be transmitted in eighty-seven frames. In fact rather less than this would need to be transmitted since the video RAM, firmware and whatever space is taken up by the software in RAM are all out of action. I’m primarily talking about text as well, which can fairly easily be compressed.

At this point you might imagine I’m going to suggest taking a video and allowing the whole 64K to come across in less than two seconds. I’m not, because this is about a quick and dirty solution to the problem. However, I am going to suggest borrowing an idea from video compression and only recording the pixels which change. The idea is that you do something like press the spacebar to change the screen and take another photo.

This could also, in fact, even have been done back in the day with an instamatic camera, but the problem is that without a scanner or some other optical device it would’ve been difficult to get the information back into the computer. There are various ways round this though, such as printing the photo using magnetic ink, and in fact even the camera could’ve been bypassed by just outputting the data on a thermal printer or something.

nineteenthly, Oct 14 2018

https://developers.chirp.io/ https://developers....orials/command-line
Chirp doesn't exist on a C64, but you might be able to borrow some ideas from their data-to-audio implementation. [zen_tom, Oct 20 2018]


       Thanks [Jutta] for sorting everything out by the way. I really appreciate it.
nineteenthly, Oct 14 2018

       So much fun giving a bun. Thanks Jutta and Inyuki!!!!
pashute, Oct 18 2018

       What about using a modem, plugged into the expansion port ?   

       C64 modems supported 600/600 duplex, or 1200/75 Prestel. Slow, but practical. Just use a cheap PABX and a PC modem as the intermediate.   

       Then there's the cassette port. The output is TTL levels; link it to a PC printer port via an opto-coupler, then decode the data stream.
8th of 7, Oct 18 2018

       IIRC, somebody made a tape backup program for the Commodore Amiga that recorded onto a standard VHS video tape by displaying the data on the "screen" (which was expected to be connected to the input of the VCR).   

       For the life of me I can't remember how you were supposed to restore the data from these tapes.
Wrongfellow, Oct 18 2018

       [Ian], there is a CBM-64 mouse, used on GEOS, and also a light pen.
nineteenthly, Oct 20 2018

       You could encode the text into morse, then play that as a stream - that way, depending on your recording medium, you'd effectively have an open-ended amount of data- transfer, limited only by the baud-rate, the capacity of your recording medium, and the length of time required to transmit your data. If you use ascii (petsci, but who's counting), that's a 4-byte space, covering 127 characters. The C64's SID chip's specs suggest it's able to cover 8 octaves, which gives you between 64 and 96 separate frequencies (depending on whether you include semi- tones) which *should* be plenty, if you slim down the charset to alphabetic (upper and lower), numeric, and a selection of punctuation characters. Let's say a set of 64. Then, if you experiment, you might be able to play a character note every 0.1 of a second, giving you transmission speed of 10 characters a second. At that rate, you'd be able to transmit War and Peace (which contains 587,287 words, at an estimated 6 characters per word) in about 98 hours. You might be able to trim that down with a bit of tuning, and perhaps some special bleeps for common dipthongs, common words etc. Even more with some compression. Anyway, the point is, audio may be the way to go.
zen_tom, Oct 20 2018

       I just looked up Chirp, they claim to attain a speed of 32 bytes in 4.52s (56.6 bps). So that's War and Peace in 17.5 hours. or 0.057 W&Pph. (apparently people read at about 0.03 W&Pph, so it's still twice as fast than reading it out loud into a dictaphone).
zen_tom, Oct 20 2018


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