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Andrew Rich (Home) wrote:
<blockquote cite="mid:01B655B53C944E899C183FD3F09E0C10@home9e3111fa81" type="cite">I looked at the audio from my scanner coming from the car
running an OT and FC-301
<br>
<br>
The tones are nicely formed, but of different amplitudes.
<br>
<br>
Does that matter ?
<br>
<br>
ie 1200 is one level 2200 is another amplitude
<br>
<br>
</blockquote>
<br>
This is a standard FM pre-emphasis / de-emphasis issue and is quite
normal. <br>
<br>
<br>
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. <br>
<br>
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. <br>
<br>
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<br>
<br>
[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. <br>
<br>
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. <br>
<br>
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. <br>
<br>
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. <br>
<br>
<br>
<hr size="2" width="100%"><br>
--<br>
<br>
Stephen H. Smith wa8lmf (at) aol.com <br>
EchoLink Node: WA8LMF or 14400 [Think bottom of the 2M band]<br>
Skype: WA8LMF<br>
Home Page: <a class="moz-txt-link-freetext" href="http://wa8lmf.net">http://wa8lmf.net</a><br>
<br>
JavAPRS Filter Port 14580 Guide <br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/aprs/JAVaprsFilters.htm">http://wa8lmf.net/aprs/JAVaprsFilters.htm</a><br>
<br>
"APRS 101" Explanation of APRS Path Selection & Digipeating <br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/DigiPaths">http://wa8lmf.net/DigiPaths</a> <br>
<br>
Updated "Rev H" APRS <a class="moz-txt-link-freetext" href="http://wa8lmf.net/aprs">http://wa8lmf.net/aprs</a><br>
Symbols Set for UI-View, <br>
UIpoint and APRSplus:<br>
<br>
<br>
<br>
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