FSK Modem

Question

Hello 
 I work with a modem for transmitting data on a sound line. It's a FSK modem. Frequency, shift, modem. Two tones are transferred. 1000 Hz and 1160 Hz. 1000 Hz is a 0 and 1160 is a 1. I've made two butterworth filters on these frequencies. I need help creating an interface between the filters and a digital output on a GPIO pin. A comparator must be created which detects which filter is has the largest audio signal and converts it to 0 or 1 on the output. Can anyone help with that? The transmitter I have made with two sine oscillators and one mux. Mux is controlled with a GPIO input. Regards Helge Fykse

KJBob · Accepted Answer

Hello Helge, 
 Here's a basic setup which compares the two filter outputs: 
 
 The Peak Envelope blocks (1) act as full-wave rectifiers or AM detectors -- they measure the filter outputs. The ABCD Comparator blocks (3) compare their a and b inputs. If the a input is larger, the block outputs its c input. If the b input is larger, the d signal is output. DC levels feed the c and d inputs via aliases (2). 
 Looking at your modulation scheme -- switching between the mark and space oscillators causes jumps in the resulting waveform. Instead, use one VCO driven to produce the needed frequencies. It's not hard to set up this way and I can help if you would like. 
 Best regards, 
 Bob 
 RTTY-2.zip)

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FSK Modem

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