[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
=== Now relocated from Pasadena, CA back to 8-land (East Lansing, MI) ===
Home Page: http://wa8lmf.net
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