On Fri, Apr 12, 2013 at 6:08 PM, Kristoff Bonne <[email protected]> wrote: > On 12-04-13 17:14, Albert Cahalan wrote: >> [Kristoff Bonne probably]
>>> 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. > > I don't understand this. Both the GMSK signal and the voice signal are mono. Both signals are mono, so they can fit on a single stereo device. One gets the left, and the other gets the right. This means you don't need to power two devices and have room for two devices. >> The low-pass filter should be passing everything below about >> 22.05 or 24 kHz, right? Even an imperfect filter should be fine. > > From what I understand from the people of free*Star who have done tests > with different audio-dongles is that a number of them also filter on the > low-part of the audio-spectrum. That would be a high-pass filter then, not a low-pass filter. Uh, shift the signal upward? Make an approved hardware list? >> The comment about the modem output itself driving the circuit >> is interesting. > > True, as said, feel free to implement it and upload the code to github. > >> That would seem to take away any ability to >> adjust relative timing, but maybe there could be a preamble. > > I don't understand this. What exactly do you mean by this? The radio will not switch instantly. You may want to trigger it some amount of time (configurable) before sending any data. Triggering too early means garbage on the air, while triggering too late means that parts of the transmission go missing. Triggering via the same audio device used for transmit will at least get you close. This is because the transmitted data and the triggering signal are affected by the same sound card (USB device) latency. >>> 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. > > Euh ... I don't understand your remark about "match the audio-latency". > > The demodulator simply grabs a piece of audio (40 ms) and demodulate it > and then tries to decode it based on its state in the c2gmsk protocol. > > What exactly does latency come into this? Audio devices tend to have a large and unknown amount of latency. USB devices also tend to have a large and unknown amount of latency. If you send two signals (example: transmitted audio and a PTT signal) via two different devices, one may get to analog form long before the other. If you can send them via the same device, such as left and right channels, then they are highly likely to show up in analog form at the same time. For example: One person buys a fast USB-to-serial device and a slow USB-to-audio device. Another person buys the opposite. Can they now use the same software? With manual adjustment of a latency parameter, yes they can, but this is error-prone and thus annoying. ------------------------------------------------------------------------------ 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
