[aprssig] D-PRS> D-STAR and APRS Explained

[Pete Loveall AE5PL Lists]

Since there appears to be a prevelant misunderstanding of the D-STAR DV (digital voice) protocol and how D-PRS works with it, I thought I would take a different approach and relate it to something that was originally designed by Bob back in the '90s.  That "something" is Mic-E.  Not the APRS packet format for Mic-E but how Mic-E was originally presented to work.

The basic concept of Mic-E was to have a single position packet appended to the end of every voice transmission by use of a standard 1200 bps TNC (with special circuitry, etc., of course).  If the person was using a voice repeater with a TNC tied to the repeater receiver, the TNC would see the position packet and retransmit it on a data frequency or, in later times, gate it to APRS-IS.  The repeater controller would block the tones from being retransmitted so other hams on the repeater frequency wouldn't hear the blap of the AX.25 packet.

This is a one-way use of APRS where the mobile station is tracker, nothing more.  This is exactly how the Icom D-STAR radios in GPS or GPS-A mode make use of the digital voice transmission with one big enhancement: instead of appending a single packet to the end of the voice transmission which is subject to being dropped due to bit errors, the Icom radios continuously send the position information while the person is transmitting helping to ensure at least one position will get through.  This GPS information is carried with the other control information sharing the voice transmission but separately encapsulated so the user never sees the other control information.  The other control information which is carried separately is information like callsigns, preprogrammed message, etc.

Why wasn't it ever the intent to make Mic-E operation bidirectional (again, talking about Mic-E functional operation, not the Mic-E format APRS packet)?  Because it would interfere with voice communications on that frequency.  That is the same reason that the "extra bits" accompanying the D-STAR digital voice transmission do not transform it into a "data" or "APRS information" channel.  Could Mic-E be bidirectional?  Yes, if the repeater doesn't block the modem tones.  Does it make sense to do?  No, because everyone would leave that frequency for one that they can talk on.  Same goes for D-STAR digital voice repeaters and D-PRS.

Hopefully this will help those who are failing to comprehend the difference between the serial data information exposed by the Icom D-STAR digital voice radios and data protocols like AX.25.  And hopefully it will help those who read the D-PRS information to understand some of the background information contained in those documents.

73,

Pete Loveall AE5PL
pete at ae5pl dot net