Tape Data Recorder
Posted: February 6, 2010
Here's another old project that I decided to redo to see if I could do better.
Back in the days when the Commodore 64 and Atari 400's first came out, most home computers would store and retrieve data from standard audio cassette tapes. Lucky for me, my C64 had a floppy disk drive so I never had to suffer through waiting for games to load off tape, but years later I thought it would be fun to recreate this system myself. So after playing with some IR communication projects, it seemed like I could take this firmware and make some pretty minimal changes to make it record data on tape and read data off tape.
The original project is using an Atmel ATtiny2313 with an LM2917 frequency to voltage chip to filter out the data. Looking back at this, it seems like the chip I should have used is the LM567 frequency decoder. While the LM2917 will give an output voltage based on the frequeny it detects on the input, the LM567 will will create a voltage or no voltage based on if a specific frequency is there or not. Along with the redo of this project, I also later used the LM567 for the Tin Can Modem project.
While working on some W65C265SXB (6502 / 65C816) projects I started to realize the tone generator they had built into the chip that I was using to create music was probably there for an industrial purpose instead. This was the reason I decided to use the W65C265SXB board this time around.
For the original project I used a pretty nice tape deck that I used to use to mix down music with. It was very clean / noise free and was so perfect for this. For the new project I originally tried to use a crappy Craig microcasette recorder, but it proved to be too noisy. Luckily I had an old TRS-80 CCR-81 tape recorder laying around (I should really throw this crap away.. or sell it on Ebay) that was used to record data or alternatively plain audio. This was ended up being clean enough to do this project.
Another interesting project I did after completing this one is similar but instead of tape it reads data from a Swedish pancake: PANCAKE-ROM.
Related Projects @mikekohn.net
To play tones on the original project, instead of using the microcontroller to create the carrier frequency I used a 555 timer chip. This was kind of a bad decision, but I had 555's laying around that I wanted to use. For the W65C265SXB project I used the built in tone generator of the W65C265SXB. For the original project I used a carrier frequency of 10kHz, but for the new circuit I picked 4.5kHz. The reason for the lower carrier frequency is the microcassettes have a much lower frequency response (4kHz - 5kHz). Regular sized cassettes seem to be around 20kHz.
The techique to record data is to record bursts of tones where the length of the tone decides what kind of bit it is. The format for the original circuit was 7.2ms start bit, 3.6ms logical 1, 1.8ms a logical 0, and 1.8ms spaces of silence between. The data comes in Start Bit, LSb to MSb, and 1 stop bit. The new circuit is 9ms start bit, 4ms for 1, and 2ms for 0.
Because of this PWM format, the data rate and amount of data that can be recorded to tape depends on what the data is. I wrote two programs that can write and read files from tape. For the original circuit, recording a 2k GIF file it seemed to average 20 bytes per second. At this data rate one side of a 90 min tape (45 min per side) can hold about 52.7k. Not bad if this was 1970 :).
I used 2 NAND gates to buffer the output of the LM2917 and turn a 4v output to a logical 0v or 5v (so it's equivalent to the IR receiver in functionality). The NAND gate wouldn't react to the output of the LM2917 directly so I put a transistor in between. Looking back at this, this seemed like another weird decision.
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