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[aprssig] TNC vs AGWPE

Stephen H. Smith wa8lmf2 at aol.com
Tue Apr 18 15:42:33 UTC 2006


kc5zrq at gmail.com wrote:
> Don't use the "9600" out.  As far as I know, the ISS uses 1200 baud.
>

1)     The so-called "9600 out" is not a data output.  It's actually 
non-squelched non-de-emhasized AUDIO out,  directly from the receiver's 
FM discriminator.    (This is the kind of audio connection you MUST HAVE 
for connecting external TNCs running at 9600 baud, although it is also 
usable for TNCs running at 1200.)

The "9600" output is usually about 50millivolts which will overload the 
typical PC "mic input" and cause severe distortion unless you use about 
a 5:1 or 10:1 attenuator pad.  

Further, many PC mic input jacks have 3-5 volts DC on them to power 
active electret external mics.    The proper way to couple the radio's 
audio into the PC is to first use a 1:1 turns ratio audio transformer in 
order to avoid a common ground between the radio and the computer.  (The 
typical 600:600 ohm transformer used in telephone devices like modems, 
answering machines, etc. is ideal).   The secondary side of the 
transformer should be connected across a voltage divider network of two 
resistors in series.  The one closer to ground should be around 1K while 
the upper one typically will be somewhere between 4.7K and 10K.   
Finally connect the center point of the two resistors, to the PC audio 
input using something like a .1 to .5 uF series capacitor to block any 
DC that may be present on the mic input.   

Note that the PC mic input is SINGLE CHANNEL even though it uses a 
stereo TIP-RING-SLEEVE  (3-conductor) mini-plug.   Normally the TIP 
carries +5 VDC power WHILE the ring carries AUDIO.  The ring may or may 
not also have 3-5 VDC on it for use with mics that carry DC and audio on 
the same conductor.  




2)     Once you have your audio network in place, click the AGW icon in 
the system tray and choose "Soundcard Tuning Aid".  One the resulting 
screen, click the radio button for  " Oscilloscope Sine Wave".   Watch 
the display as packet bursts come in. You should see a clean sinewave 
display here.   Use the Windows RECORD mixer (not the default PLAYBACK 
mixer that you get initially when you click the speaker icon in the 
tray) to adjust the audio level for a clean sine wave.  Note that you 
will see white-noise random grass between bursts since this is 
non-squelched audio.  It's perfectly normal for this noise to clip on 
peaks.   The goal is that the sine wave DURING packet bursts be smooth 
and rounded with no flat topping. 

Note that the options in the Windows mixer for the mic input channel on 
many sound cards includes a checkbox for a "+20 dB Mic Booster". This is 
for the benefit of low-output non-amplified mics (i.e. passive dynamic 
mics) instead of electret ones.  You definitely want this box UNCHECKED.




3)     Most modern PCs, both laptop and desktop no longer have dedicated 
sound card hardware in them.  The built-in "AC97 Compatible" sound 
systems are basically an A-D converter and nothing else.  The "heavy 
lifting"  precisely-timed sampling and  processing of sound that was 
traditionally done by a dedicated processor,  RAM and accurate clock on 
the sound card is now done by the main CPU of the computer.   


The CPU clock usually isn't as accurate as the one on a sound card.  
Further, the CPU is sharing it's available pool of clock cycles between 
a varying number of other processes with varying interrupt response 
times (latencies).  The result is that the sampling rate is frequently 
far off  of the desired 11,025 or 8,000 samples/sec  that ham soundcard 
apps expect.   Further  IT VARIES depending on how many other programs 
are running at the same time!    There are several utilities that will 
measure the actual sampling rate of the sound card. 


Unfortunately, unlike some ham sound card programs such as mmSSTV and 
MixW, AGW has no provision for entering corrections for the sampling 
rate errors.   If the sample rate is severely off,  your only recourse 
is to try a different sound card --  in the case of a  laptop  this will 
mean either a PCMCIA -card based sound system,  or an external 
USB-connected one. 


Ironically, the older Pentium I and Pentium II laptops often had far 
superior sound systems based on dedicated Soundblaster, ESS, or Crystal 
Audio chip sets just like add-on  PCI-card sound systems in desk top 
PCs.    Today's hotrod P4 or Centrino laptops usually have the 
far-inferior "brain-dead" host-based AC97 sound systems. But AC97 
systems reduce parts count, power consumption and are CHEAP CHEAP.    I 
have a couple of 10-year-old Dell 3000 Pentium I  200MHz MMX - based 
laptops that I keep around exclusively for ham soundcard operating with 
mmSSTV, AGW, MixW and Echolink precisely because their sound systems are 
superior to my newer machines.   Further these "classic" machines have 
the audio LINE input in addition to the MIC input which is far superior 
for ham sound card applications.     With AGW, the true stereo line 
input actually allows you to create a DUAL-PORT tnc similar to a 
Kantronics KAM or 9624, with one radio connected to the left channel and 
a second radio on the right channel.







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


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