[aprssig] OT audio

[Andrew Rich (Home)]

very good thank you
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Andrew Rich 
Airways Technical Officer Grade 4
Surveillance - RADAR ADS-B
Amateur Radio Callsign VK4TEC
email: vk4tec at tech-software.net
web: www.tech-software.net
  ----- Original Message ----- 
  From: Stephen H. Smith 
  To: Andrew Rich (Home) ; TAPR APRS Mailing List 
  Sent: Wednesday, September 02, 2009 7:50 AM
  Subject: Re: [aprssig] OT audio


  Andrew Rich (Home) wrote: 
    I looked at the audio from my scanner coming from the car running an OT and FC-301 

    The tones are nicely formed, but of different amplitudes. 

    Does that matter ? 

    ie 1200 is one level 2200 is another amplitude 



  This is a standard FM pre-emphasis / de-emphasis issue and is quite normal.   


  Presumably, the scanner has normal receive de-emphasis on it's speaker audio.  If the tones are transmitted flat (i.e. directly into the TX modulator without the high-frequency boost of the mic amp), they go out over the air at equal levels (deviations).   Then the de-emphasis in the scanner RX will cause the high tone to come out the speaker at a lower level (about 2-3 db less) than the low tone.  [The de-emphasis is normally a simple RC network with a 3dB/octave rolloff over the audio band from 300-3000 Hz.]  The only way to avoid this is to directly tap the FM RX discriminator before the RX audio stages.  

  The long-standing packet convention is to feed 1200 baud packet into the radio mic jack (thus gaining PRE-emphasis on TX).  At the other end you take RX audio off the speaker or equivalent (thus getting ,matching  DE-emphasis at the RX end) yielding a net flat frequency response -- at least if the pre-emph and de-emph curves are correct which they are frequently not.   

  Normally the TX deviation is set about about 3.5 KHz on a mix of both tones provided by a TNC "test" or "cal" mode.  This typically means the low tone alone will deviate about 2.5Khz and the high tone about 4.2 KHz

  [This is done for the convenience of not having to modify or cut into the radio's innards to reach the direct RX discriminator and TX  modulator connections --  just "stuff it into the mic jack".]     Surplus commercial rigs often  have test and alignment connectors that let you access these points from outside the case without "hacking" the radio.  On newer ham gear, the 6-pin mini-DIN "data" or "packet" provides the same access.   

  On 9600 baud packet modes, you MUST make direct DC-coupled discr and modulator connections -- the simple "stuff it into the mic jack" mode that works on 1200 baud just won't work.  [The need to hack the radio is why 9600 baud packet never really caught on widely in the US - US hams tend to be plug-n-play appliance operators.]   Note that the direct-connect yields no pre-emphasis / de-emphasis.   

  To add to the confusion, the Kenwood APRS radios (with their built-in TNCs coupled directly to the RX discr and TX modulator) transmit "flat" (no pre-emphasis) on both 1200 and 9600 baud.    

  This occasionally causes problems for digipeaters using normal demphasized RX speaker audio hookups.   The receiver de-emphasizes the  already-equal tones so the high tone winds up reaching the TNC 3dB or so LOWER  than the low tone, resulting in unreliable decodes.   



------------------------------------------------------------------------------

  --

  Stephen H. Smith    wa8lmf (at) aol.com 
  EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
  Skype:        WA8LMF
  Home Page:          http://wa8lmf.net

  JavAPRS Filter Port 14580 Guide 
    http://wa8lmf.net/aprs/JAVaprsFilters.htm

  "APRS 101"  Explanation of APRS Path Selection & Digipeating 
    http://wa8lmf.net/DigiPaths 

  Updated "Rev H" APRS            http://wa8lmf.net/aprs
  Symbols Set for UI-View, 
  UIpoint and APRSplus:


    



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