Sunday, April 5, 2009

D-Star data port?

I almost bought the IC-92AD at AES superfest. I had planned on it actually until I discovered a design flaw.

Apparently there a number of people reporting that the display goes blank after VHF transmit in DV mode. So something isn't shielded the best. Icom will fix this is you discover it and send it in. But that's just a bummer when you drop cash to buy a new radio only to have to send it in.

What did pique my interest for a while was the 12 pin connector on the handheld. There is an optional (overpriced) GPS speaker mike that interfaces to this port.

I knew it was wishful thinking that there might be access to the raw (signed 16-bit, Little-Endian, 8kHz, non-stereo audio packets.) AMBE decoded audio. Such is not the case. The 12 pin port is solely for the utterly useless slow speed data and some cloning functions.

Why anyone would bother with a 1200 baud digital data port is beyond me. We had that kind of speed with packet in the 80's. It makes sense to use that reserved 1200 baud for on-board text messaging. Or with that crazy priced GPS mic.

So it makes sense me to me that hams might want to interface to the digital audio part and callsign routing to try and create a SIP bridge.

Anyway you can't get at that DV stream which is a shame. So to experiment with interfacing a D-Star radio to Asterisk you really need to buy that overpriced DV-Dongle. Which of course lacks an over the air demodulator so you need to connect it to an analog radio.

What a bunch of bull.. D-Star isn't for me yet. I have other things to spend my money on, and there really isn't anyone around my neck of the woods with a D-Star radio. But it is interesting to read up on and play with some of the software being developed. Perhaps one day there will be such a standardized port, much like the standardized packet radio data port.

Above is a block diagram of how a D-Star radio works. As you can see analog microphone audio hits a single-channel, Analog Devices Front-End Speech Processor. This is actually what does the analog to digital and digital to analog conversion.

From there we have a RAW digital audio stream to the AMBE-2020 vocoder. This raw stream is signed 16-bit, Little-Endian, 8kHz, non-stereo audio packets. At this point this raw stream is a non-license encumbered codec, close in resemblance to ulaw (g711). The Linux SoX tool can convert most file formats, like WAV or MP3 to this RAW format.

The AMBE-2020 Vocoder Chip is configured to transmit and receive digitized speech to and from most linear, a-law, or u-law A/D-D/A codecs though its serial interface.

It kind of bugs me that there isn't a way to interface to this digital audio. I guess it's not a super big issue on a user end radio as you can always interface analog using the speaker microphone jacks.

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