[arm-allstar] Noise on the output rpi2 and uri. (Jim Korenz)
Ken
ke2n at cs.com
Mon May 25 14:32:14 EST 2015
Another source of noise, on any system that uses a URI, is "ground-loop"
current in the audio connection.
The URI has a DC path from the USB-side through to the audio connections.
This means the zero-volt DC line of the RPI power supply is connected all
the way through to the zero volt DC line of your radio's power supply. Any
current that might flow between the supplies is flowing in the same ground
conductor that carries the audio (the audio shield connection). Any voltage
dropped due to this current appears in series with the audio. It will be
worse on transmit because signal levels are usually lower on the transmit
side and your radio draws more current from the power supply when it is
transmitting.
There are various ways to mitigate this. Probably the simplest is to
connect a heavy wire (like #10) from the negative side of the RPI DC supply
to the chassis of the radio. Here a flimsy cable on the audio connection is
your friend.
If the 5 volts for the RPI is derived from the 13.8 volts feeding the radio,
that might cure the problem too - provided the DC/DC converter has a solid
connection between the negatives of both voltage levels.
GL
Ken
1. Re: Noise on the output rpi2 and uri. (Jim Korenz)
----------------------------------------------------------------------
Message: 1
Date: Sun, 24 May 2015 11:09:41 -0700
From: Jim Korenz <n8pxw at yahoo.com>
To: ARM Allstar <arm-allstar at hamvoip.org>
Subject: Re: [arm-allstar] Noise on the output rpi2 and uri.
Message-ID:
<1432490981.7585.YahooMailBasic at web185305.mail.gq1.yahoo.com>
Content-Type: text/plain; charset=iso-8859-1
If it's RF getting into the audio, try a 1000pf cap from hot to ground. The
audio is AC coupled into the URI, if you use those inputs. If you still have
noise, try a 10uf cap in series with the shields, i.e. isolate the radio
ground from the URI ground, or use an audio transformer.
JimK N8PXW
--------------------------------------------
On Sun, 5/24/15, Doug Crompton <doug at crompton.com> wrote:
Subject: Re: [arm-allstar] Noise on the output rpi2 and uri.
To: "ARM Allstar" <arm-allstar at hamvoip.org>
Date: Sunday, May 24, 2015, 10:32 AM
#yiv7486914851
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#yiv7486914851 Andrew,
?? You didn't describe
what the noise sounds like but most noise comes from some switching
source. It could be a cooling fan, a switching power supply, etc.? Also
have you eliminated RF problems?
Since this is on TX only maybe put the transmitter on a dummy load just to
check. Are you using shielded cable for the radio connection? Are the
grounds all good?
You need to probe at different
points along the path to see where the noise is being introduced. Asterisk
Allstar itself would not introduce any noise, at least not objectional
noise as you describe.?
What are you using for a power
supply for the 5 volts?
73 Doug
WA3DSP
http://www.crompton.com/hamradio
Date: Sun, 24 May 2015
23:41:41 +1000
From:
andrew.gilbett at gmail.com
To:
arm-allstar at hamvoip.org
Subject:
[arm-allstar] Noise on the output rpi2 and uri.
Hi All,
I have just setup a RPi2 & URI combo and it work quite well. Except it has
quite a high noise level on the tx.
I noticed that this was also the case on the beagleboard xm and lox board
combo as well. I was hoping it was peculiar to that hardware but it is not
unfortunatly.
Basically it sounds like it is being encoded with gsm? and being
compressed with noise added to it.
If I connect via IAXRPT the signal is quite clear but not perfect so there
is a little bit of noise on the input but it is the output where it is
degraded again.
I am using Simoco SRM9005 radios using the rear io on the db15.
Could someone shed a little light on the audio path through asterisk and
where noise could be coming from there?
This is for an ultra portable repeater that is carried up hills for events
and I have found that apt_rpt and asterisk is awesome because it is a
complete voip switch. Keep up the good work.
Andrew
VK2XAG
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