[aprssig] TNC vs AGWPE

Chris Howard w0ep at frii.com
Tue Apr 18 11:07:28 CDT 2006

I think this post would be a very good addition to the aprs wiki!
Steve, if you don't mind, can I put it up there?
Or if you would do it, even better.

We've got a page about agwpe that needs filled out
and another about soundcard as tnc.


On Tue, 2006-04-18 at 09:42, Stephen H. Smith wrote:
> 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
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