> Well, that is true if you assume that the USB audio-dongle supports stereo. > > One of the things about GMSK is that the best result is got using cheap > USB devices, this because of the lack of low-pass filters on the input > and output. However, you would be surprised to see how many of these USB > audio-dongles only support capturing in 1 channel!
If it only supports mono, you're wasting power and space. The low-pass filter should be passing everything below about 22.05 or 24 kHz, right? Even an imperfect filter should be fine. > Another issue is that voice-capturing/playback is done by a different > application that does GMSK modulation/demodulation. This was a very > explicit choice to have a additional level of flexibility so that audio > / codec2 encoding (which needs a FPU) can be done by a different device > then the GMSK modulation (which can be done without FPU). I hope you can #if this away. >> 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. > > That would indeed be an interesting option. The comment about the modem output itself driving the circuit is interesting. That would seem to take away any ability to adjust relative timing, but maybe there could be a preamble. > That's why there is the option to drive the PTT via the TX-pin. When you > send a continues stream of 0x00 or 0xFF to the serial port (depending if > it TTL or RS232 voltage levels), it is "up" most of the time. The > periode the pin is low can be bridged by a simple capacitor. Send 0xAA or 0x55 and the fldigi circuit should work. > For PCs, that I think not such an issue. Most of them have more then > sufficient USB ports; so simply use a USB-to-serial adapter. USB adds latency. It's also not likely to match the audio latency. ------------------------------------------------------------------------------ Precog is a next-generation analytics platform capable of advanced analytics on semi-structured data. The platform includes APIs for building apps and a phenomenal toolset for data science. Developers can use our toolset for easy data analysis & visualization. Get a free account! http://www2.precog.com/precogplatform/slashdotnewsletter _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
