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[aprssig] *** TNC Test CD Update - Mirror Server Now Available ***

Stephen H. Smith wa8lmf2 at aol.com
Tue Oct 31 20:45:59 UTC 2006


ka8vit at ka8vit.com wrote:
> I have used MP3 recordings of packet, SSTV, PSK31 and Hellschrieber
> and they decode just fine...
>
> They were recorded at 8000 Hz,  8-bit, mono.
>
> 73
>
> Bill  KA8VIT
>
>


They may worked "just fine" on your particular setup.  

However, an 8 Khz sample rate on a waveform running as high as 2200 Hz 
means under 4 samples of the waveform per cycle.   This makes the 
zero-crossing time (i.e. phase)  of the waveform highly ambiguous.   
Further the MP3 coding process further "mangles" the original phase 
integrity.   (Human hearing is quite insensitive to phase errors in 
signals but many electronic devices are VERY sensitive to phase!)   For 
demodulators that use zero-crossing detectors or pulse counters (rather 
than analog PLL systems or tuned audio filter channels) the exact time 
of zero crossing of the waveform  is critical.  

I was striving to create a high-quality standard reference signal set 
for critically comparing the performance of various TNC designs.    A 
crude waveform re-constructed from less than 4 points  per cycle 
represents considerable phase jitter that might bias test results for 
some types of TNCs.  I recorded the original signals at the high quality 
sampling rate of 44.1KHz.  This had the added advantage that no 
re-sampling or sample rate conversion (with it's phase integrity issues) 
was required to make the  audio CD since commercial audio CDs are ALWAYS 
done at 44.1KHz.

I then choose to release the signal set as CD audio tracks rather than 
as .WAV or .AIFF files to discourage their playback through crappy 
unstable  PC motherboard sound systems (see my other post on this issue) 
that would make the findings by various parties running tests less than 
consistent. 





--

Stephen H. Smith    wa8lmf (at) aol.com
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