Low End Audio Noise

k2yn
Posts: 1
Joined: Sat Aug 05, 2017 10:04 pm

Low End Audio Noise

Postby k2yn » Sun Aug 06, 2017 12:43 am

I am concerned about a condition I have with unwanted energy on the low end of my audio passband. There is low frequency energy present even when I disconnect my accessory plug (DB25) in the back. This disconnects my microphone and PTT connection from the rig. I have tried changing my power supply to no avail (still have the problem). I have tested the rig at full power and at 5 watts and the problem still exists.
CONDITION : There is energy at 30hz , 60hz and 120hz. There are "little carriers" 10 to 20db above the rest of the passband quiescent power. These blips of energy exist when everything is disconnected except the power supply and the computer. I have a picture of the condition and have submitted it. I can reduce these energy carriers when I drop the EQ bands in the 30hz to 120hz region. This kills the low end of my transmitted audio. What could this be??? The condition exists even if I use the front microphone connection, I transmit into a dummy load, no matter what!!! HELP!!! John K2YN
P.S. I am using HPSDR 3.4.2
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w-u-2-o
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Joined: Fri Mar 10, 2017 1:47 pm

Re: Low End Audio Noise

Postby w-u-2-o » Sun Aug 06, 2017 2:21 am

John,

Which radio are you using?

What are the conditions that obtain with each of the three pic's you posted?

What does it look like when DUP is turned on?

73,

Scott
w9mdb
Posts: 60
Joined: Sun Apr 09, 2017 5:53 pm

Re: Low End Audio Noise

Postby w9mdb » Sun Aug 06, 2017 3:31 am

It's quite possibly noise from your computer.
Do you have any extra cards in the computer you can remove? Or maybe a different computer you can test?

I went through something similar with another user and he found a 2nd network card that was giving broadband noise in his computer.

de Mike W9MDB
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W1AEX
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Re: Low End Audio Noise

Postby W1AEX » Sun Aug 06, 2017 2:42 pm

Hi John,

This morning I took a look at my own 200D (Orion 5.0 firmware) and noted that with my audio switched to MIC (which has nothing plugged into it) my screen looks similar to your third screenshot. I found that the bump near F-0 magically disappeared when I engaged the MON button. I can't hear anything at all when tuning across my signal with another receiver, so it may not be the same phenomenon that was reported by the other station who noticed a "tone" on your signal.

Image 1: DUP is OFF and MON is OFF
Image 2: DUP is OFF and MON is ON
Image 3: DUP is ON and MON is OFF
Image 4: DUP is ON and MON is ON

I'm not sure what that bump is, but my 200D also exhibits it although it does not seem to cause any issue on transmit.

Rob W1AEX


Image 1: DUP is OFF and MON is OFF
Image

Image 2: DUP is OFF and MON is ON
Image

Image 3: DUP is ON and MON is OFF
Image

Image 4: DUP is ON and MON is ON
Image
"One thing I am certain of is that there is too much certainty in the world."
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w-u-2-o
Posts: 1304
Joined: Fri Mar 10, 2017 1:47 pm

Re: Low End Audio Noise

Postby w-u-2-o » Mon Aug 07, 2017 4:04 pm

John,

I see that you are using quite a bit of COMP. 6dB is a LOT. In addition, inferring from the name you have assigned to your transmit profile, it would appear that you are using CFC processing as well. And some equalization, too, of course.

The amount of compression you get out of the CFC processor can be quite substantial, and the same is true of COMP. Together you are usually obtaining far too much of a good thing, and such a combination will dramatically enhance even the smallest noises in your audio chain.

I took a quick look at the input TX audio noise floor on the 8000 I have here, but I did it using the supplied "Default DX" profile, which has not processing enabled. The results, as shown below, are quite benign and devoid of any artifacts. I should add that activating MON or DUP in any combination makes no difference in this spectra. Nor does switching to line vs. mic inputs. Turning on the 20dB mic boost does raise the noise floor a bit, but not a great deal.

I suggest that what you are seeing is naturally occurring noise being picked up by the CODEC ADC and being greatly enhanced by the CFC and COMP processing. Check and see what it looks like without that processing turned on.

As for how to eliminate it I suspect it will take some modifications to the CODEC circuit itself. What happens if you ground the mic input to the CODEC? If the noise remains, then it is noise being coupled into the CODEC internally.

73,

Scott

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