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[aprssig] Any audio recordings of 144.390 MHz ?

Matti Aarnio oh2mqk at sral.fi
Sat Oct 24 15:57:57 UTC 2009


I keep hearing rumors that in some big cities in US the 144.390 MHz
is under constant radio traffic.

Do you have any audio recordings of what those main digipeaters hear
on their radios?

I would like to hear the radio traffic in big US cities, at quiet night
time, "quiet" daytime, and at peak traffic time.

I would like to do some analysis on _uncompressed_  8 kHz 8 bit mono
recording, if any such exist.  That would be a bit under 29 MB per hour.
I can not use audio that has undergone any sort of lossy compression,
as those tend to skew signal phases.  Lossless compression is acceptable,
like ZIP, possibly also FLAC.

An 8-bit resolution sample is enough, IF you can make it so that maximum
signal amplitude is 80-95% of full scale, and never clips.
A 16-bit resolution lets you off with lower full range signal, but not
with clipping overamplitude.

I can receive them with ftp transfer, and of course retrieve from web
page, or whatever you can make yourselves.  Details to be negotiated
when there are files for me to receive.

By the way, do you KNOW when there are peak traffic times, and how busy
they are?  In Erlangs ?

One of very first things that my Aprx did was to present on telemetry data
an estimate of channel occupancy in erlangs based on number of bytes
received  in each packet plus some guesses overhead.   This method works
well enough at lowish traffic levels - at least under 0.3 Erlangs.

More precise measurement would take radio RSSI signal, and account
fraction of time when it is over some defined treshold.  That would
know not only of successfully decoded packets, but also of failures
due to two transmitters colliding at too even signal power levels.

73 de Matti, OH2MQK



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