Our modem signal would drive that without the 1 KHz tone.
Albert Cahalan <[email protected]> wrote:
>> - any device with two audio input/out ports (like two USB audio
>ports)
>> and a GPIO port / serial port for switching the radio can be used. a
>FPU
>> is needed for codec2 encoding/decoding.
>>
>> - between the radio and the modem device, there needs to be a small
>> interface board. This consists out one single capacitor per
>> audio-channel (to block DC bias on the audio-signal) and a diode, a
>> resistor and a transistor to switch the PTT of the radio (add one
>more
>> capacitor is switching is done via the serial port instead of the
>GPIO
>> port).
>
>You need 3 output signals, but you're requiring 5 or more.
>Each USB audio device is 2, and the serial port is more.
>
>You could do left channels for encoded audio, and right
>channels for normal audio.
>
>Alternately, you could be compatible with fldigi's PTT circuit.
>This circuit keys the transmitter via a 1 kHz tone in the right
>audio channel. This type of keying has the advantage of
>matched latency; the PTT signal doesn't arrive at the radio
>long before or long after the modem output transitions.
>
>Circuit here:
>http://www.w1hkj.com/FldigiHelp-3.21/PseudoFSK.html
>
>Lots of modern PC hardware doesn't even come with a serial port.
>
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