| |
G0SYR > TECH 19.02.08 20:01l 67 Lines 3751 Bytes #999 (0) @ WW
BID : 42029G0SYR
Read: GUEST
Subj: Transmitter Audio Signatures
Path: IZ3LSV<IW2OHX<IQ0LT<IK5YZW<ON0AR<HS1LMV<CX2SA<GB7CIP<GB7CIP<GB7CIP
Sent: 080219/1341Z @:GB7CIP.#32.GBR.EU $:42029G0SYR
T:From: g0syr <g0syr@gb7cip.ampr.org>
T:Newsgroups: ampr.rec.tech
T:Message-Id: <1xfdcl5rh0zkx.bjdkphuyw87z$.dlg@40tude.net>
Hi All
I mentioned in a bulletin I sent a week or two back a problem with AKD
transmitters and although not directly connected with that, it reminded me
of using some DOS software called XMIT_ID which allowed the recording of
short audio samples from an FM receiver and displaying them as a scope trace
on the screen. As it is probably difficult to get modern sound cards running
under DOS I thought I would have a look for a more modern alternative.
The the theory behind this software was to identify a particular transmitter
from its audio fingerprint displaying a pretty well unique picture of the
way that the tranmitter both keys up/down and switches on/off its audio
modulation. Particularly with packet the combination of TNC and transmitter
give a complex fingerprint which is easy to identify. Originally I think it
was used to identify repeater abuse but is interesting to experiment with.
Although it would not identify the problem I mentioned with the AKD2001
radios it would probably give an interesting display to give a clue as to
what was happening.
The software I ended up trying is Audacity an opensource, free multitrack
recording and editing packege which allowed the production and display of
short audio clips very similar to those produced by XMIT_ID. Many people
will no doubt already be using Audacity as it's a useful tool for all sorts
of audio jobs and can be downloaded from many sources on the net if you
google for it but here is one such link that includes some simple
installation tips to get it going.
http://www.how-to-podcast-tutorial.com/15-audacity-download.htm
It's simply a matter of hooking up a suitable level feed of audio from an FM
radio to the soundcard and hitting record. When you've collected some
examples of various transmissions, could be packet or phone, you can view
portions of the waveform by zooming in, marking, cutting and pasting to get
an edited clip of start up and closing of the transmission. What you see
will depend on many things such as the way the synthesiser settles onto
frequency as its keyed or how the audio modulator stages come up. You can
clearly see how long the TNC txdelay is set by counting the clearly visible
flags preceding the data it's quite an eye opener. You should remember you
are simply viewing audio from a receiver with probably a 25 Khz bandwidth IF
filter with the audio AC coupled so this does limit exactly what you 'see'
In the example of the AKD radio that arrived on frequency from 500 Khz away,
you would only see the last 25 Khz worth as the transmitter appeared within
the receiver filter. You could obviously play with wider bandwidth receivers
and DC coupled A-D converters rather than just consumer audio electronincs
but it was still an interesting exercise with just the simple lash up.
For anyone interested I will zip up a couple of example files as an Audacity
Project file which will be called EXAMPLE.zip and put these out as a
seperate 7+ file. Audacity projects come as a .aup file with a data folder
with the same name. When you unzip the 7+ EXAMPLE.zip you should end up with
EXAMPLE.aup and a folder called EXAMPLE_data which both need to be copied to
the folder from which you want to open Audacity projects.
The example contains two signatures one from G8MNY and one from G8MM which
give an idea of the sort of results you should expect to see. To load them
into audacity select Open and open the file EXAMPLE.aup
Something for the technically minded to have a play with
--
73 de Bryan
AX25 mail: G0SYR@GB7CIP.#32.GBR.EU
amprnet mail: g0syr@gb7cip.ampr.org
internet mail: g0syr@beeb.net
Read previous mail | Read next mail
| |
