Simple casette data encoder

227

01:49:10

FSK encoder for recording 500 bits per second to casette tape. I think the frequencies are good enough, but I'm not sure.  With the higher frequency below 5kHz, a hi-fi tape deck is probably not required.  (Not that I know where you'd find a lo-fi tape recorder any more! :)

~ Operation:
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The 555 oscillator varies its frequency depending on its control input.  This input goes to an internal resistor chain, so a single external resistor suffices to adjust the (CMOS) input to a suitable range.

For the tape, I want equal width high and low periods.  The flip-flop provides this by halving the frequency from the 555.

The voltage divider adjusts the level for a line input and the capacitor blocks DC.  

The 20kR resistor represents the tape recorder's line input.  It's pessimistically low; the nominal input impedance for a line input is 50kR.

published 7 years ago

jason9

7 years ago

Interesting. So how exactly do cassettes work?

hurz

7 years ago

dont be to afraid about the 1:1 thing. its anyway a sinewave and the 3rd or 5th harmonic not much enough to form sonething like a rectangular shape. so the recording will be more a sinewave shape anyway

eekee

7 years ago

Thanks hurz, I'll look into it.

eekee

7 years ago

@jason9: The tape carries magnetizable material, perhaps iron filings. A tape read/write head contains a coil. (A single coil for mono, 2 for stereo.) On record, the current flowing through the coil makes an EM field which affects the magnetic material, recording the signal as tiny magnets of varying strengths. On playback, these tiny magnets moving past the head make a signal in the coil. Disks work on the same principle, providing something nearer random access. Hard disks must additionally allow finer-grained storage on their rigid surfaces, but I have no knowledge of the physics behind that.

eekee

7 years ago

Incidentally, Atari's cassette deck was stereo, but they only used one channel at 600bps. I think it's because the decoding hardware was primitive; the cassette deck being as cheap as possible. (It was their only cheap peripheral, all others had microcontrollers.)

jason9

7 years ago

And it’s coded with FSK? Not regions of positive magnetization and negative magnetization to code ones and zeros?

eekee

7 years ago

Yup. I think magnets effectively lie along the tape, so the signal needs to be AC.

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