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So I have an IC-2730, which doesn't have external inputs for packet radio, but it does have optional board for Bluetooth connectivity and there's also an optional Bluetooth headset for it. I'm trying to evaluate feasible it would be to use Bluetooth to transfer say 1200 baud audio.

Now, it's been some time since I worked with Bluetooth and I haven't worked with Bluetooth audio at all, so I'm wondering what types of issues I could expect if I try to use Bluetooth to send audio data to the radio.
I expect that compression could be an issue here. From what I could dig up, the module IC-133 module which implements the Hands-Free Profile, Serial Port Profile and HeadSet Profile.

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    $\begingroup$ What kind of Bluetooth audio? There are a few, of varying degrees of fidelity. $\endgroup$ – Phil Frost - W8II Sep 3 '16 at 15:51
  • $\begingroup$ @Phil Frost That's the thing, specifications are not available, or at least I can't find them. Therefore answers giving general overview are welcome. $\endgroup$ – AndrejaKo Sep 3 '16 at 17:20
  • $\begingroup$ @Phil Frost Whoops, sorry, reading comprehension fail! $\endgroup$ – AndrejaKo Sep 3 '16 at 17:31
  • $\begingroup$ Headset Profile is the lower tier of Bluetooth audio protocols (as opposed to A2DP, which is designed for music and only one-way). Can't advise on whether it'll be adequate for packet. (Hands-Free Profile just signals button-pushes.) $\endgroup$ – Kevin Reid AG6YO Sep 3 '16 at 18:22
  • $\begingroup$ Based on just how subjectively crappy headset profile audio sounded the few times I used it, I wouldn't have high hopes for it working well for digital data. $\endgroup$ – Phil Frost - W8II Sep 3 '16 at 20:43
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Sounds a lot easier to reverse-engineer the Hand Controller Interface, especially since that comes with a handy RJ45 connector, which would make it cheap to build an adapter that allows you to poke around. Pretty certain that carries raw analog audio on any 2 of the 8 wires :), as they actually sell a mic adapter cable (OPC-589)! (turns out somebody did the work, and you can use that info) (oh furthermore turns out the ICOM manual actually has the pin assignments...)

What issues can I expect when using Bluetooth for packet radio?

Point is that the bluetooth headset profile uses a digital audio compression optimized heavily for speech – and not for preserving the digital audio symbols. It might work to transmit low-rate data through that, but it would be equivalent to adding a significant noise source to your system.

In other words: you'd be making your transmission worse without need.

To be honest, I'd expect a system that has up to 50W power output to be very cleanly modularized – you can probably easily identify the PA chain when you open it (it's what attaches to the antenna port and has big heat sinks!). Developing something that only generates / demodulates packet radio audio signal pre-distorted to counteract the Bluetooth compression as far as possible sounds a lot more complex than just building your own packet radio/FM modulator and directly piggy-backing that onto your radio :)

Hence, if I needed to build something that takes 1200bd of data for transmission wirelessly, I'd go for something like a Pi Zero, a RJ45 connector soldered onto a PCB that would plug directly on top of the Pi Zero and that contains both a 8V/5V voltage converter to power the Pi from the Mic port, and a I²S or USB sound card IC to generate/receive the audio, plus some kind of data connectivity (bluetooth, if you will), plugged in directly to the Pi. That way, I'd have something relatively small, which only transmits the raw data bits over your bluetooth link (no quality loss there), and does the modulation as close as possible to the radio. Of course, you'd need some kind of packet radio audio modem software running on the pi, but there should be something readily available :)

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