[aprssig] TNC vs AGWPE
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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