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<DIV><FONT size=2 face=Arial>very good thank you</FONT></DIV>
<DIV>----------------------------------------------------------<BR>Andrew Rich
<BR>Airways Technical Officer Grade 4<BR>Surveillance - RADAR ADS-B<BR>Amateur
Radio Callsign VK4TEC<BR>email: <A
href="mailto:vk4tec@tech-software.net">vk4tec@tech-software.net</A><BR>web: <A
href="http://www.tech-software.net">www.tech-software.net</A></DIV>
<BLOCKQUOTE
style="BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B>
<A title=wa8lmf2@aol.com href="mailto:wa8lmf2@aol.com">Stephen H. Smith</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=vk4tec@tech-software.net
href="mailto:vk4tec@tech-software.net">Andrew Rich (Home)</A> ; <A
title=aprssig@tapr.org href="mailto:aprssig@tapr.org">TAPR APRS Mailing
List</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Wednesday, September 02, 2009 7:50
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [aprssig] OT audio</DIV>
<DIV><BR></DIV>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>
<P>
<HR>
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Date: 09/01/09 20:03:00<BR></BLOCKQUOTE></BODY></HTML>