wiki/ LV0/ Payload/ LV0 Modem

LV-0 300bps Audio Frequency Shift Key Modem

AFSK Modulator
?AFSK Modulator Schematic

Quite a mouthfull, huh? Well don't let it overwhelm you. It's just a plain old modem that was high-tech back in 1975. "Audio Frequency Shift Key" (or AFSK) simply means that we use one tone for a 1 (2225Hz) and another tone for a 0 (2025KHz). We take these two tones, and we put them into the microphone input of the Amatuer TV (ATV) Transmitter's audio board. Down on the ground, we'll decode these two tones back into 1s and 0s.

"300bps" means that we can only send 300 bits every second which is only 30 characters per second (in asynchronous mode with 10bits including start and stop bits). Way slow. But we have to make sure that the two tones can fit in the bandwidth of the ATV transmitter, so again we're being conservative. Next flight we'll push the bandwidth as much as possible.

Note: when you watch the video of the flight, the audio channel is a high pitch screaming of the two tones see-sawing back and forth. You can actually here the data coming down! Very cool.

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