THis file is the original which layed out the design for the APRS
Mic-Encoder.  It is retained for reference purposes.

It has been upgraded and replaced by four other files:

Mic-E.txt
Mim.txt
Mic-Lite.txt
Telemtry.txt

VOX-APRS.txt        INTEGRATING APRS ON VOICE CHANNELS                APRS
                           and VOICE REPEATERS                 PROPRIETARY
                       Copyright 1993,4,5,6 WB4APR

     As mobile GPS/APRS continues to grow, we can eliminate the need for 
every mobile to have a TNC, digital radio, and second antenna by simply 
integrating the position report into a very brief tone burst at the 
end of a voice tramnsmission over any two-way radio.  With this scheme, 
no additional hardware is required in the vehicle, other than a GPS unit.  
The system not only reports position and vehicle type, but also one of 
7 canned messages and 4 analog telemetry values!

     By transmitting a position report at the end of a voice transmission,
not only is this a period of dead time due to the almost universal 
courtesy beeps found on amateur repeaters, but the tone burst can be 
easily muted out at the repeater receiver, so that the other mobile users 
DO NOT HEAR it!  If the tone burst is about 0.3 seconds, then it will be 
virtually transparent to voice repeater operation.  The APRS MIC-ENCODER
achieves a complete position report, course, speed, and digipeater 
information in about 30 bytes including header, vice the 90 or more 
bytes in a normal APRS position report.

     At the voice repeater receiver, a special APRS REPEATER NODE TNC 
picks off the position report and digipeats it out onto the dedicated APRS 
digital frequency for mobile position reporting using a path derived from 
the bits in the TO-SSID.  In addition, it appends the repeater frequency 
onto the end of the position report so that digital users can see where 
the packet originated.  If all voice repeaters digipeated onto the same 
digital position reporting channel (usually 145.79 if available) then 
anyone monitoring the APRS frequency will see ALL mobile position reports 
from ALL GPS mobiles on ALL frequencies!


HARDWARE:  To implement this APRS LOCATOR SYSTEM, there are two critical
elements needed.  First is an APRS MIC ENCODER to handle the integration
of the mobile GPS data into the users microphone connector.  Second 
is the specialized APRS REPEATER NODE that handles the digipeating of the
compressed position reports heard on the voice repeater inputs over to the
APRS digital channel.  (in the interim, any TNC with true Data carrier
Detect can be used at the repeater).  Details follow.


APRS MIC ENCODER:  In order to make the APRS LOCATOR SYSTEM practical, 
the MIC-ENCODER had to meet several design constraints:

   * Must interface to UNMODIFIED radios via the MIC connector
   * Must compress position report into about 0.3 seconds
   * Must Fail Safe so that MIC can always be used
   * Must be low enough in power to be powered from the MIC jack
   * Must have easily selectible user parameters
   * Must be very small for easy mounting and portability
   * Must accept the readily available NMEA output from GPS receivers
   * Must use standard AX.25 for compatibility
   * Optionally has 4 analog channels for telemetry

The result is a 1200 baud position report compressed to 32 bytes including
beginning and ending FLAGS.  This equates to about 320 ms, including
CALLSIGN, DIGIPEATER PATHS, and a minimum message capability.  Plus, the 
packets are still receivable on ANY AX.25 TNC.


PACKAGING:  Although the electronics could be miniaturized into about 
1 cubic inch, the requirement for user access to switches and the 
requirement for withstanding the pulling and tugging on the MIC cord
results in a robust dash board box design.  The cicruit is powered by the
mic connector and the only external input is via a standard 1/8 inch
phone jack to receive the NMEA data from the GPS unit.  This makes the 
entire mobile vehicle position reporting system as portable as the 
microphone!  Simply move the MIC from vehicle to vehicle, and as long as 
the radios are compatible at the MIC connector, then the vehicles are 
GPS ready!  The suggested front panel for the MIC ENCODER is shown below.
It connects to the radio with only a single 6 conductor pigtail back to 
either a back-to-back JACK/PLUG or soldered in parallel to the existing 
MIC jack.
                          
  ---------------------------------------------------------
  |  A U T O   D I R    P A T H  M S G    R A T E   O N   |                             
  |                    -----------------                  |
  |                    |       |       |    /^\           |
  |     O        O     |       |       |   /   \     O    |
  |    (O)      (O)    |   7   |   7   |  | (O) |   (O)   |                                   |
  |                    |       |       |   \___/          |
  |    OFF      OMNI   |       |       |           O F F  |
  |                    -----------------                  |
  ---------------------------------------------------------



     On the APRS MIC ENCODER, there are several configuration switches
that give the operator real-time control over other dynamic MIC choices
as follows:

     ON/OFF      Used to enable or disable APRS beeps (power switch)
     AUTO/OFF    Auto will transmit when repeater is silent more than N secs.
     DIR/OMNI    Used to set Directional or OMNI digipeating paths
     3 PATH BITS Used to set digi HOPS or North/South/East or West routes
     3 MSG BITS  Used to indicate up to 7 pre-defined messages


Normally the MIC encoder will only send a POSIT if the POSIT timer has  
elapsed AND the user has been talking and releases his PTT.  In the AUTO 
mode, however, after a specified AUTO time period, AND after the 
repeater has been silent for the QUIET period, then a position packet will 
be initiated and transmitted.  Most voice repeaters will never even key up
on such a brief burst due to built in ker-chunk filters.  The TNC on the
repeater input, however, will hear it and digi it over to the digital 
channel.



DIGIPEATER PATH BITS:  

    The OMNI/DIRECTIONAL switch and 3 routing bits are used to tell the
repeater how to route the packet.  There are actually two systems, one
that can use standard TNC's at the repeater and the other that takes 
advantage of a new specialized APRS NODE TNC.  We will call the standard 
mode the DIGI mode and the other, the SSID-NODE mode.  In the DIGI mode, 
the path switches just select how many hops along a given digi string
will be transmitted in the packet.   The asvantage of this method is that
it is compatible with ANY TNC and will work with existing systems.   The
disadvantage is that each digi hop takes 7 bytes, and just a 3 hop path
almost doubles the length of the packet.  The second mode uses only the
4 SSID bits for all routing information.  This keeps the packet short,
while also allowing for up to 7 hops in all directions!

     The following table shows how the routing is handled in both the
DIGI mode and the SSID mode.  The first path bit is the OMNI/DIR switch
and the lower 3 bits are connected to the thumbwheel switch.  In this
esample, assume the MIC-ENCODER has been loaded with the digi string
of RELAY,WIDE,WIDE,DIG4,DIG5,DIG6,DIG7
   
X PATH  SSID DIGI VIA TNC         APRS NODE DIGI's ONTO APRS CHANNEL
- ---- ---- -------------------  ----------------------------------
0 000  0   none                 direct 
0 001  1   RELAY                via WIDE-1
0 010  2   RELAY,WIDE           via WIDE-2
0 011  3   RELAY,WIDE,WIDE      via WIDE-3
0 100  4   DIG4                 via WIDE-4
0 101  5   DIG4,DIG5            via WIDE-5
0 110  6   DIG4,DIG5,DIG6       via WIDE-6
0 111  7   DIG4,DIG5,DIG6,DIG7  via WIDE-7
1 000  8   none                 via NORTH UNPROTO path
1 001  9   RELAY                via SOUTH UNPROTO path
1 010  10  RELAY,WIDE           via EAST  UNPROTO path
1 011  11  RELAY,WIDE,WIDE      via WEST  UNPROTO path
1 100  12  DIG4                 via NORTH UNPROTO path + WIDE
1 101  13  DIG4,DIG5            via SOUTH UNPROTO path + WIDE
1 110  14  DIG4,DIG5,DIG6       via EAST  UNPROTO path + WIDE
1 111  15  DIG4,DIG5,DIG6,DIG7  vai WEST  UNPROTO path + WIDE


First notice that in the DIGI mode, the paths 0 through 3 simply select
the number of digi hops in the original string to use.  The paths
4 to 7 start over again at the 4th position.  THis can be thought 
of as a completely independent second DIGI string.  Usually you would
load RELAY,WIDE,WIDE,WIDE,WIDE,WIDE,WIDE as the digi string.  If you
select 4 you get WIDE, if you select 5 you get WIDE,WIDE and so on.
THis separation into two distinct strings gives you the chance to
have a path beginning with RELAY or beginnning with WIDE.  This is
important for operating in areas which do not have the dual alias
WIDE-RELAY digipeaters yet.  Although the longest path is now limited
to four hops, anything beyond 2 WIDES is frowned on anyway...


Notice that the APRS NODE has two routing methods, the OMNI and the
DIRECTIONAL method.  In the OMNI mode, every APRS NODE repeats every
WIDE-N packet it hears and then subtracts ONE from the SSID.  They 
also keep copies of all such packets for 60 seconds and ignore all
DUPES of the same packet.  THis is a very effecitve OMNI reouting 
method that permits the packet to go out N hops in all directions.

If directional routing is selected, the APRS NODE uses one of four
unique directional paths stored at that site.  If DIRECTIONAL DX is
selected, then a WIDE is added to the end of that path.


IMPLEMENTATION:  The key to the success of the MIC-ENCODER is that it
is very versatile and can operate in all required modes.  This allows  
for growth and improvment in the APRS systems without obsolesence.
There are five possible operational situations as follows:

NO TNC  AT  REPEATER:  MIC-E path is set to 0 and anyone monitoring the
(OR SIMPLEX VOICE)     the repeater output with APRS can track users.

STD. TNC  @ REPEATER:  MIC-E path is set to 1, thru 7 in DIGI mode.  
                       The TNC with the alias of RELAY (or WIDE) 
                       repeats packets onto the APRS packet channel.

APRS NODE @ REPEATER:  NODE routes according to MIC-E SSID bits only.

OPS ON  APRS CHANNEL:  MIC-E path can be 1 thru 7 in DIGI mode.

Since the APRS NODE always routes on SSID bits anyway, the only difference
betweeen the MIC-E DIGI mode and the SSID mode is the length of the
packet due to the DIGI fields.  THis means for interim compatibility,
MIC-E users can operate generally with a path of 1,2, or 3 on all 
possible channels without specific configuring.  Of course, if they 
turn off the DIGI mode while on an APRS NODE repeater channel, their
packets will be much shorter, while still being routed by the SSID alone.



MIC-ENCODER ALGORITHM:  The MIC-E monitors the PTT line and will send a 
brief POSIT compressed into about 0.3 seconds when the PTT is released 
and as long as the POS-PERIOD has elapsed.  This prevents repetative 
reports when the PTT is used in rapid sequence.  The MIC-E also has an 
AUTO mode which will key-up the transmitter on its own at least every 
AUTO-PERIOD if the radio channel has been quiet for a given QUIET period.  
This permits mobiles not actively talking, to be tracked without 
interference to other users.

In addition, the MIC-E can append 4 channels of analog telemetry and/or
a BEACON text onto the end of the POSIT.  If the telemetry is added,
then it will always be 5 bytes.  

The MIC-E is based on the APRS Micro-Interface-Module (MIM) designed by 
Dr. Carl Wick (N3MIM).  The MIC-E is configured via its serial port
using a PC program called MIC100.exe.  This program provides a nominal 
TNC type command mode for setting the MIC-ENCODER configuration.  It 
provides the standard cmd: prompt.  Once the MIC-E is configured, you 
use the PERM command to cause the MIC-E to save the configuration in 
EEPROM.  The following items can be configured:

     MYCall           Sets the MIC callsign       
     MYSymbol         Sets the APRS symbol character
     VIA digi1,, etc  Sets the Unproto digipeater path
     TXDelay          Sets the key up delay
     PERiod           Sets the nominal MIC cycle period
     POSIT N          Sets POSIT period as N * cycle period
     TELEMETRY N      Sets TELEMETRY period as N * POS period
     BEACON N         Sets BEACON period as N * POS period
     AUTO N           Sets AUTO period as N * POS period
     QUIET N          Sets the QUIET period as N * cycle period
     BText            Sets the Beacon Text
     PTT (1:0)        Sets sense of the PTT signal.  For the MIC-E,  
                      this is 1 since an external PTT transistor is used.



MIC-E HARDWARE INTERFACE:

As noted, the initial MIC-E prototype is a standard MIM with a few external
components to integrate it to the user configuration switches and to the
microphone PTT circuit.  The 8 digital BIT inputs on the MIM are used as 
follows:

  D8, D7, D6, D5  Routing BITS.  These are placed in the TO SSID field 

  D4, D3, D2      Three message bits.  Active low.
                 
  D1              PTT INPUT.  Active LOW.
  
  A5              Multiples the PERIOD by N where N is between 1 and 15.

  HO              The Hold OFF is active LOW

  PTT             PTT output is ACTIVE HIGH since an external PTT 
                  transistor is required.

NOTE:  SInce all bits have internal pull up resistors, then the default
values are "1" so we use ACTIVE LOW negative logic.  A SWITCH to GROUND 
is considered to indicate the condition.

GPS INPUT STRING:  Currently, ONLY the $GPRMC is supported since
it has both position, course and speed.

   $GPRMC,123456.xx,A,3859.11xx,N,07629.12xx,W,123xxx,321.x,.....


HARDWARE ITEMS:  THere are two subtle hardware items in the circuit.
First, there is an LED on the PTT output line (diode isolated from 
the real PTT) that shows when the MIM is pullling the PTT low.  This
shows the user that a POSIT is pending and will be appended when 
he releases the PTT.

Secoond, the HOLDoff is connected to the receiver AND to a switch 
labeled AUTO.  If the switch is set to AUTO OFF, then the switch 
permanently GROUNDS the HOLDoff so that no matter what the receiver 
is doing, the MIC-E will never initiate an AUTOposit.  If thw switch 
is in the AUTO position, then the receiver audio establishes the value 
of the HOLDoff, and this permits a long period of silence to eventually 
(via the QUIET TIMER) to allow a POSIT to be transmitted regardles of 
the state of the PTT.

THere are three ways to wire the MIC-E to your radio system:

    A)  Put a MIC jack on the MIC-E and run a MIC cable from the MIC-E
        to the radio.
    B)  Wire the MIC-E in parallel to your mic at its connector
    C)  Wire the MIC-E internally to the radio or to an auxiliary input
        (But the MIC-E must be able to sense the MIC PTT independently)
    D)  Wire the MIC-E to a back-to-back plug jack combination

Option A will require you to hard mount the MIC-E so it doesnt get tugged
all over the dash-board.  Option B or C is suitable if you only have one 
radio and mic.  I prefer the universal option D where you simply solder 
back to back connectors and then run a 6 conductor cable of any length to 
the MIC-E.  Conveniently, a standard 8 pin plug and jack will fit very
nicely in a standard 1/2 inch to 3/8 inch copper reducing coupling.  Just
saw it in half so you can place it over the connectors after they are
soldered back-to-back.  Drill a 1/4 inch hole in the side for the cable
to the MIC-E.


   MIC JACK                       MIC PLUG
                     diode
      >--------*------|<---------*------>               * Note, this diode
      >--------|--*--------------|------>                is part of the MIC-E
      >--------|--|--*-----------|------>                schematic and can be
      >--------|--|--|--*--------|------>                placed here or in the
      >--------|--|--|--|--*-----|------>                MIC-E box
      >--------|--|--|--|--|--*--|------>
      >--------|--|--|--|--|--|--|------>
               |  |  |  |  |  |  |
               |  |  |  |  |  |  *---< PTT out to radio }
               |  |  |  |  |  *------< Audio to radio   }
               |  |  |  |  *---------< Audio, MIC gnd   }
               |  |  |  *------------* GROUND           }
               |  |  *---------------> 8 volts DC       } to MIC ENCODER
               |  *------------------> Receiver audio   }
               *---------------------> PTT input        }



MIC-ENCODER ASSEMBLY NOTES:

    The MIC-ENCODER is the original APRS TELEMETRY TNC transmitter on a
chip (MIM Module) with different code for the  MIC-ENCODER algorithms.  
To implement the MIC-ENCODER function use the following interface circuit.


 MICROPHONE                                                      RADIO
 ----------                                                     --------

 MIC >-------------------------------*--------------------------> AUDIO
                                     |     62k         1k
                                     *---/\/\/\--*----/\/\/-----> MIC gnd
                                                 |     
 +8v <----------*--------------------------------------*--------< +8 V
                |                                |     \ 1k
                |              D1                |     /
 PTT >---*---------------------|<----------------------\------*-> PTT
         |      |                                |     /      |
         |    |-|----------------------|   5.6k  |     *      |
         | D2 | *                    *-|--/\/\/--*     V  LED |
         *-|<-|-*    MIM MODULE        |              ---     |
   PTT in     |                        |               |  D3  |
              | *               *    * |  PTT OUT      *--|<--*
              |-|---------------|----|-|    5.6K     |/c  
                |               |    *----/\/\/\-----|   Q1 NPN
                |    *----*-----*                    |\e
                |    |    |     c\|  Q2 NPN            |
                | S1 *    |       |--------*-----||-------------< RCV AUDIO
                |  \      |     e/|        |   .2 uf   |
                |    *   ===    |          -           |
                |    |    | 6uF |          ^ D4        |
                |    |    |     |          |           |
 GND >----------*----*----*-----*----------*-----------*--------> GROUND


CIRCUIT DESCRIPTION:  D1 isolates the microphone PTT from the radio PTT
input so that the MIM can key the PTT line while also sensing the MIC
PTT condition.  D2 prevents the MIC-E from grounding the PTT lead when
the MIC-E is turned off.  Q1 is an open collector PTT transistor.  The
LED gives a visual indication that a PACKET is ready to go and that the
MIM is holding the PTT waiting for the user to release the MIC PTT.  D3
is necessary to isolate the LED from the PTT from the microphone.  The 
MIC-E audio is voltage divided by the 5.6 and 1k resistor to the
isolated MIC ground to minimize power supply noise and the 62k minimizes
loading on the existing MIC.  Q2, D3 and the lower two caps are an audio 
rectifier to drive the HOLDOFF input to the MIM when the radio is in use.  
S1 is the AUTO/OFF switch.  When closed, it asserts a permanent hold off, 
so that the MIC-ENCODER will never auto-initiate a posit on its own.  The 
following additional switches connect to the MIM to provide the user input  
for the MESSAGE bits and the PERIOD timing.


            D8    D7 D6 D5  D4 D3 D2         A5    +5v
            *     *  *  *   *  *  *          *      *
            |     |  |  |   |  |  |          |      |
            |   ---------------------        |      |
            |   |         |         |        |      /
            O   |    7    |    7    |        *----->\ 5k
           (O)  |         |         |               /
            |   ---------------------               \
            |        |         |                    |
          -----    -----     -----                -----
          ////     ////      ////                 ////
            

D8 is the OMNI/DIR switch.  D7 through D5 in the first Octal switch 
to select the DIGI path.  D4 through D2 in the second Octal switch select
the one-of-seven message bits.  A5 is an analog input which selects
the time period multipler from 1 to 16.  YOu can use a POT or a switch
with a 15K and 5K resistor.  This forms a 0.25 voltage divider so
that in the GND position the value is zero and in the up position the
value is 0.25 of 5 volts or about 4.  This will multiply your POSIT
period by 4.  (Zero is actually 1)

CONSTRUCTION:  Mount all components in a suitable enclosure that provides
easy user access.  Notice that all MIC-to-radio connections are straight
through except for the series diode in the PTT line.  You may either wire
the 7 leads in parallel with your existing mic plug or build a back-to-
back plug/jack as noted above:

MESSAGE BITS:  The one-of-seven message capability of the MIC ENCODER
will use the following default messages unless custom definitions are
provided.  Regardless of the message, the color of the symbol will always
correspond to the mesage number as follows:

   MSG    COLOR    DEFAULT DEFINITION
   ---   -------   -------------------
    0    normal    Off duty
    1    normal    Enroute 
    2    normal    In Service
    3    normal    Returning
    4    dim yel   Committed
    5    Brt yel   Special
    6    dim red   PRIORITY
    7    Brt red   EMERGENCY!

SERIAL PORT FOR GPS AND CONFIGURATION:  All users should consider using
the APRS standard 1/8 inch stereo phone plug/jack for their serial
data port on the MIC ENCODER and other small stand-alone-trackers.  
The phone plug is small, readily available, and is compatible with
the nominal mono 1/8 inch phone plug found on many GPS units:

    GPS UNIT or PC LAPTOP                 MIC-E or embedded TNC
    ---------------------                 ----------------------
    mono or stereo PLUG                   1/8th inch stereo JACK
    
    TXD (data out)   --------------------> TIP    
    RXD (data in )   <-------------------- RING   
    GND              *-------------------* SLEVE

To help remember, just think of the DATA comming out of the male plug tip.
This applies to the GPS by itself or to the laptop used to CONFIGURE the
stand-alone tracker.  By using a "closed-circuit" jack, an internal GPS
can be normally connected to the internal MIC-E or TNC, but plugging in
the LAPTOP opens that circuit and connects the laptop to the TNC...


DETAIL CONSTRUCTION NOTES:

1) First you must determine how to power the MIC-E.  Measure the voltage  
at your MIC jack with a 330 Ohm load.  If it is between 5 and 7 volts 
connect directly to the MIM 5 volt input WITH A 5 VOLT ZENER and a large
100 uF or so capacitor to ground.  This gives reasonably good power to
the MIC-E since the regulator is bypassed.  If 7.5 or more, connect to 
the Vin pin on the MIC-E.  Check all your radios with compatible connectors 
and plan to work with all of them... A good place to put the Zener and Cap
is on the end near Q1 between the 4.7k resistors and the corner screw pad.
Even if you have 8 volts, put the 47 uF cap across the 8 volt source.

2) Interfacing ANYTHING to your microphone circuit is not a trivial 
exercise.  Any ground loop will add noise to the MIC audio (remember
the alternator noise problems...)  Drawing 15 ma from the MIC circuit
adds to this problem too.  Separately powering the GPS from the 12 volt  
system and then connecting that data ground to the MIC encoder is also
a potential noise source.  Be sure to use the isolated MIC ground as 
shown in the circuit, and the large electrolytic to fiilter the power
or the circulating ground currents will degrade the packet audio.  If your
packets do not soundl clean, you may have to power your MIC-E with a
9v battery to get clean power and audio... 

3)  Do not solder to the MIM directly.  Plug it into some old IC sockets
and solder to them.  The pins on the MIM are larger than a DIP chip, and
will not fit all sockets.  Solder all of the front panel switches and 
the serial port jack to the appropriate DIP socket pins while still on 
the work bench.  All of the chassis parts can mount from the inside of the
chassis later.  TIN BOTH surfaces before tack-soldering to the pins.  The 
switches MELT easily, so pre-form the leads and solder QUICKLY with NO 
stress or the switch pins will deform.

4)  YES the OCTAL switches can be mounted from the back after soldering.
Just snap apart the two switches, remove the side plates, push them
separately through the square hole, re-assemble on the outside and
snap back into the panel.
  
      
USING ANY TNC AS A MIC ENCODER:  In order to encourage initial testing
with the MIC-ENCODER concept on your VOICE frequency, you can use any 
GPS capable TNC to simulate the MIC-E.  The big difference will be much
longer packets.  This is a risk, since your initial confrontation with
VOICE users will be WORSE than the final design...   Here is how:

1) Use the $GPGLL sentence (it is shortest)
2) Set your POS rate to about once every 2 minutes
3) Turn BText off and configure TNC for GPS operations.
4) set UNPROTO to APRS (via nothing)
5) set TXDelay to 3 instead of the default 33!
4) Connect a capacitor and diode to your MIC and TNC as follows:

                       *-----------------------*  Ext carrier detect
                       |      D1
   MIC PTT    *--------*------|<-----*---------*  Radio PTT
                                     |
                                    ===  C
                                     |
                                     |
                                    ---
                                   ////
NOTES:  
1)  Determine the value of C so YOUR radio stays keyed for the full 
    length of a posit.  My Alinco takes 30 uF, my TS-711 takes 220 uF.  
    Notice that the PTT from the TNC does NOT connect to the radio.

2)  These packets will be about 3 to 5 times longer than the MIC-E
    packets.  Setting no digipeaters will keep the packets shorter and still
    allow anyone monitoring the repeater to track you.  You can change this
    to ONE WIDE once you have a TNC installed at the repeater site...
    
3)  If you talk longer than your POS rate, TWO or more packets will be
    buffered up and go out at the end.  If your CAPACITOR is the right
    value, though, the additional packets should get cut off.


APRS REPEATER NODE:  This special TNC NODE is designed to be intgrated into
typical amateur voice repeaters.  The TNC performs a number of special
functions to fully implement the APRS LOCATOR SYSTEM:

   * It has true DCD to destinguish between voice and data
   * It uses this DCD signal to mute the repeater audio during packets
   * It digipeats all position reports from the repeater receiver to the
     dedicated APRS digital channel
   * It implements the APRS Directional Digipeating algorithm
   * It implements the APRS FLOOD-N digipeater algorithm for OMNI packets
   * It appends ADDText (usually the rptr freq) to the end of all packets
     ("Via 146.940") 
   * It uses the external carrier detect for the APRS packet channel for
     true CSMA effeciency, but this need only be an audio COR detector.
  

Notice that although the APRS REPEATER NODE function only listens on the
voice repeater input and only transmits on the digital APRS packet 
frequency, it must also have a secondary carrier detect on the APRS packet 
channel to avoid collisions.  This special APRS node function is NOT 
involved in any further routing on the APRS digital channel (I mean that 
it does NOT serve as a general purpose APRS digipeater on the digital 
channel).  All it does is to insert the appropriate directional or OMNI 
digipeater path and digipeat the packet.  This distinction, of course, 
is only a functional distinction, since APRS digipeater functions can be 
co-located, or even built into the same NODE box as long as dual digital 
receiver channels are maintained.


PERFECT DCD OR MUTE CONSIDERATIONS:

        Since the ultimate acceptance of the POSIT-PACKET on voice
repeaters will be determined by the minimization of the BRAAAAAAP sound
on the repeater output, the DCD and subsequent muting of the repeater
transmitter are very important.  The APRS NODE must therefore provide a
separate MUTE signal that is 99.99% percent accurate.  Since most voice
repeaters have simple analog delay lines of up to 50 ms to eliminate the
squelch tail, the actual MUTE decision can be made as late as 50 ms after 
the initial DCD, and still be able to mute the packet from the repeater 
output.


NOTES:   Notice that the APRS REPEATER NODE will also work on the digital
channel!  In other words, the APRS REPEATER NODE algorithms can also be
running simiultaneously in all of the APRS DIGI's so that the APRS compressed
format will be picked up directly on the digital channel.  These original
packets are distinguishable because they DO NOT have an original DIGI field.
Once a NODE processes them and adds the DIRECTIONAL or WIDE routing, they
will be forwarded as usual.  Notice that the NODE hardware can actually do
both functions as long as dual digital receive channels are provided.
       



APRS PROPRIETARY                                           APRS PROPRIETARY
