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Andrew Rich wrote:
<blockquote cite="mid:LGEKIKIAKJPNFGILGJBBOEDCDNAA.vk4tec@people.net.au" type="cite">
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<meta content="MSHTML 6.00.6000.16544" name="GENERATOR">
<div><span class="877001002-25112007"><font face="Arial" size="2">Gudday</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">I
thought I would share this experience - others may be able to help.</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">I
bought a KPC3 for 1200 baud to use outboard with the ICOM 706 Mk iiG</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">Seemed
to work well enough - using the "data" port on the ICOM 706. I seemed
to be copying most packets on ISS and PCSAT.</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">I
then migrated to AGWPE sound card and I noticed that it was not copying
packets 100%.</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">I
had a look on the Sound Card tuning aid and I saw something odd.</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">It
looked like 1200/2200 but was sort of riding on a low frequency sine
wave, in that the packet tones where not symetrical about zero volts.</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">OK,
time to drag out the CRO. Sure enough saw the same thing on the CRO.</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">Time
to try something different - the "phones" connection on the front panel
- perfect nicely balanced packet.</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">Fiddled
with the 9600 / 1200 baud lines and settings in the ICOM 706. Not much
joy.</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">Before
I dig deeper (service manual), anybody seen similar ?</font></span></div>
<div><span class="877001002-25112007"></span> </div>
<div><span class="877001002-25112007"><font face="Arial" size="2">I</font></span></div>
</blockquote>
<br>
<br>
1) What kind of computer and sound system were you using with AGW? <br>
<br>
2) What kind of interface/isolation were you using between the radio
and the computer.<br>
<br>
<br>
Some general observations and possibilities in no particular
order........<br>
<br>
<br>
o Motherboard-based sound systems are notorious for picking up
noise and hash from the computer's timing chains and clocks and DC-DC
power converters since they typically share the power supply and
ground/common busses with all the digital stuff on the motherboard.
The result is a lot of low-level trash superimposed on what ever you
feed into them. You don't hear this junk on the cheap speakers lacking
bass response typically used with computers, but critical DSP
applications such as ham sound card programs will be affected. <br>
<br>
o Most motherboard-based sound systems are brain-dead "gutless
wonders" that substitute massive software drivers for dedicated
hardware, and dump the sound processing tasks on the host CPU instead
of using dedicated hardware. In turn, the sound processing competes
for CPU "air time" and interrupts with all the other software running
on the CPU at the same time. The sampling rate and timing of the audio
A-to-D conversion actually varies depending on the number of other
tasks running at the same time. The timing errors and varying sample
rates of many motherboard-based systems is enough to severely degrade
the decode performance of AGW. <br>
<br>
Traditional add-on sound cards and external USB-connected sound
systems use dedicated hardware with accurate crystal-controlled
timebases and normally are much more accurate and consistent, and tend
to be much quieter. <br>
<br>
o If you don't totally isolate the computer sound system from the
radio with audio transformers, you have a common metallic chassis
ground/earth between the two devices. In turn, this allows 50/60 Hz AC
leakage currents from the power supplies of either the computer, the
radio or both, and hash/noise from the computer's switching power
supply, to circulate on the common shield wires of the audio
connections. <br>
<br>
In turn, these circulating currents (a.k.a. "ground loops") can cause
several 10s or 100s of millivolts of noise and hum to be superimposed
on the TX and RX audio. Note that completely isolating the PC and
radio requires both transformer-coupling the TX and RX audio lines
(typically with 600/600 ohm telephone-type audio transformers salvaged
from old modems or answering machines) -AND- isolating the PTT line
for keying the transmitter with either opto-isolators or relays. <br>
<br>
o The TNC has a very definite lower limit to it's audio frequency
response determined by the audio coupling capacitors on it's input.
(The TNC designer knows that nothing lower than several hundred Hz
needs to pass and sizes the caps accordingly.) The computer sound
system has an audio frequency response that is essentially flat clear
to 5 or 10 Hz (or even to DC). Any low frequency hum or hash picked up
by cable ground loops will pass much more readily into the computer
sound input, with less loss, than into the hardware TNC. <br>
<br>
o Another possible source for the low-frequency hum or hash is the
refresh clocks for LCD displays in the radio which are often scanned at
only a few tens or hundreds of Hz. The speaker or headphones output
typically has far less low-frequency response than the dedicated
data/packet jack which is often DC-coupled to preserve the DC levels of
9600-baud packet. Small value coupling capacitors in series with the
audio isolation transformers can reduce this kind of low-frequency "
garbage.<br>
<br>
o Finally, if you are using the MIC input of the computer sound
system rather than a LINE level one, you may need to attenuate the
receive audio fed into it by something like 50:1 to 100:1 to prevent
the mic amp from overloading. A simple resistive voltage divider
consisting of something like a 100K resistor and a 1K resistor will
usually do it. <br>
<br>
<br>
<br>
<br>
--<br>
<br>
Stephen H. Smith wa8lmf (at) aol.com <br>
EchoLink Node: 14400 [Think bottom of the 2M band]<br>
Home Page: <a class="moz-txt-link-freetext" href="http://wa8lmf.com">http://wa8lmf.com</a> --OR-- <a class="moz-txt-link-freetext" href="http://wa8lmf.net">http://wa8lmf.net</a><br>
<br>
NEW! World Digipeater Map<br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/APRSmaps">http://wa8lmf.net/APRSmaps</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>
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