[aprssig] The APRS AX.25 Frame

Stephen H. Smith wa8lmf2 at aol.com
Wed Sep 21 12:26:46 CDT 2011

On 9/21/2011 9:32 AM, Joseph M. Durnal wrote:
> I need to think about the mic-e packet.  Obviously, if one takes their
> TNC and watches the 144.39 data go by on the terminal, they'll see a
> lot of mic-e.  I'm still working on how far into the weeds I should
> go.  I could keep it simple and just say that the destination address
> contains the encoded latitude, NS/EW,&  message and the info field
> contains the encoded longitude, course speed,&  symbol.  I don't think
> most of my audience will find much more than that very useful.

The things significant about Mic-E:

1)     The MIc-E format and hardware was originally created to create an 
extremely short position and ID burst (about 1/3rd to 1/2 second) at the end of 
VOICE transmissions when the radio's mic was unkeyed on a voice channel. Hence 
the name MIc-E (short for "Mic Encoder").

The intent is for a TNC to be attached to the receiver side of a voice 
repeater.  The TNC detects the tail-gate Mic-E bursts on voice transmissions, 
and retransmits them on the 144.39 APRS-only channel from an additional 
transmitter at the repeater site.   The TNC's "carrier detect" is used to mute 
the retransmission of the packet burst on the repeater's voice output channel.

2)     Mic-E is a highly-compressed format that is less than ONE-THIRD the 
length of a normal APRS packet, and ONE-TENTH the length of transmitting a raw 
NMEA position data string taken directly from a GPS.    The Mic-E format is 
valuable, even on a non-voice  data-only  channel. The shorter packet takes 
less air time to send, increasing the number of transmissions per minute 
possible on the RF channel.   Less obviously, it significantly increases the 
reliability of those transmissions because the much-shorter packet is less 
likely to collide with other stations, or be corrupted by mobile flutter or 
pops of noise.

3)     Mic-E format crams the latitude value into what is normally the 
"destination address" that usually begins "APxxxx".     The apparent 
"destination callsign" of a Mic-E station, as monitored on a TNC's output, will 
actually change as the north/south position of a mobile changes.

4)     Additionally, part of the Mic-E compression is achieved by placing TWO 
binary-coded-decimal digits of the latitude and longitude values in each 8-bit 
byte of data, instead of a single ASCII character in each byte.    As a result, 
Mic-E-formatted data is not human-readable like conventional APRS posits when 
viewed directly out of a TNC. (You see a seemingly random "gibberish" mix of 
letters, numbers and punctuation marks.)  However,  all APRS software,  
hardware (like Kenwood and Yaesu radios), and APRS websites (like findu.com and 
APRS.fi) know how to decode this "alphabet soup" correctly.

5)     The term "MIc-E" actually refers to several different things:

      o     A hardware device (the TAPR "Mic-Encoder") created decades ago, and 
long since discontinued. It was a box somewhat similar to a TinyTrack, that 
inserted inline between the radio and it's microphone, to generate these bursts 
each time you unkeyed on a voice transmission.      (Today's Tiny Tracks can 
still monitor the  PTT line, and burst-on-unkey on voice if desired, as can the 
Kenwood and Yaesu APRS radios.)

      o    The short-form data format involving  data-in-the-destination-field  
this device used, that is still used today by all Kenwood and Yaesu radios, and 
optionally by TinyTracks.

      o     The compression algorithm that these packets use; i.e. "That was a 
Mic-E encoded packet."

      o    A voice repeater equipped with this feature; i.e. "WR8XYZ is a Mic-E 



Stephen H. Smith    wa8lmf (at) aol.com
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