[aprssig] OT audio

[Stephen H. Smith]

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
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