[aprssig] PIC processor for APRS info for D-STAR mobiles?
Pete Loveall AE5PL Lists
hamlists at ametx.com
Tue Jul 8 06:28:50 CDT 2008
Something else to think about: which is a more reliable transmission (more likely to be received intact), a 1200bps 2 tone modem signal or a 1200 bps interleaved portion of a 4800 bps GMSK signal? In fact, it is the 1200 bps 2 tone modem signal because the demodulation process has inherent forward error correction (you don't need full quieting reception to properly demodulate the 2 tones). As previously stated, the 1200 bps control subchannel of the D-STAR transmission has no forward error detection and no error detection (it is part of a bit stream so there are no acks, etc.). Therefore, -any- noise on the signal can corrupt the control subchannel yet the voice gets through without a problem because the voice protocol has forward error correction built-in.
It was mentioned "That seems to be contrary to everything that Icom says about it". The Icom manuals were written by marketers. Like any manufacturer's document that is written as a user's guide, not a technical spec, there are some statements that are generalities but not necessarily comprehensive in their definition of the technical aspects of the device (yes, this is "intentionally obtuse") and not comprehensive in the "if you do this, these are the side-effects" department. One thing to also consider: the data port exposed on the radio has a maximum throughput of less than 750 bps due to other uses of the control portion of the bit stream.
I mention these points here as they were heavily studied and taken into account during the development of D-PRS. Understanding these and other issues are discussed in the D-PRS white paper/spec.
Pete Loveall AE5PL
pete at ae5pl dot net
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