[aprssig] Fw: APRS questions

[Stephen H. Smith]

Andrew Rich wrote:
>  
> ----- Original Message -----
> *From:* Andrew Rich <mailto:vk4tec at tech-software.net>
> *To:* TAPR APRS Mailing List <mailto:aprssig at lists.tapr.org>
> *Sent:* Sunday, January 11, 2009 9:50 AM
> *Subject:* APRS questions
>
> What is the best ampliitude for AGWPE on the scope view ?
Anything that produces a clean unclipped sine wave at about 1/2 to 
2/3rds of the peak value where clipping begins.  

Note that this is a balancing act where you want to bring the audio out 
of the radio at a high enough level that residual hiss, hum and 
cross-over distortion of the radio's audio output stage is a small 
percentage of the total audio signal, but not so high that the audio 
stage clips, or the sound card input stage overloads.  I.e. ideally the 
radio's audio gain control is somewhere between 1/3rd and 2/3rds 
(assuming of course it affects the output you are using -- the 6-pin 
mini-DIN "data" or "packet" output is normally unaffected by the volume 
control).


>  
> What packets does the opentracker + decode ?

Any valid AX25 packet

>  
> Is a traditional TNC better at decoding and why ?

My experience is that on VHF 1200 baud AFSK on FM, there is not much 
difference. 

On 300 baud 200 Hz shift FSK on HF (where there is much more noise 
typically, and the relative levels and phase of the two tones is 
constantly shifting due to HF skywave propagation), I have found MixW in 
the packet/TNC emulation mode outperforms both my PK-232 and my KAM.    
With either AGWpe or MixW, it is critical to set the audio input level 
of the soundcard or equivalent (as the SignalLink USB) so it is not 
saturating the audio input (i.e clipping).  

Note further that the gain control stage of a computer sound is normally 
AFTER the mic input preamp stage.   If you use the Mic In rather than 
the Line Level Input (which unfortunately fewer and fewer sound systems 
seem to have), the audio input stage can be overloading and clipping, 
even if the gain control is at practically zero.  If you have to feed 
speaker-level audio into a mic input on the PC almost certainly you will 
need a resistive voltage divider of somewhere between 10:1 and 50:1 to 
keep the mic amp from clipping.  Ideally you want the input level padded 
so that you get a clean sine wave in the AGW tuning monitor, with the 
mic slider set to something like 50-75%.   [This wouldn't apply to the 
Signal Link USB which has appropriate level padding internally.]

I was also quite surprised, given the amount of hiss and background 
noise that comes out of the typical HF transceiver on receive (and the 
relatively narrow audio dynamic range due to AGC on SSB transceivers), 
that using a quality sound card with a -90 dB noise floor rather than a 
cheap one with a -50 or -60dB noise floor improved marginal weak signal 
copy on HF significantly.




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--

Stephen H. Smith    wa8lmf (at) aol.com
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