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PC audio TX setting causes tempoarary distortion in Phones and Rear audio output

Keith Richards
edited February 2018 in SmartSDR for Windows
Flex 6300 v1.4 with PC setting selected for audio TX

As soon as the 6300 has booted and green LED is solid I notice if I talk into the mic I get distorted audio "breakthrough" output from the rear connection and Phones outputs. I have external powered speakers connected and the output is fairly loud. This happens without SmartSDR starting and I'm not keying the radio. 

The distorted mic sourced audio (harsh clipped "Darlek" like sound) gradually gets quieter and disappears after around 30 seconds !

I noticed this phenomenon not because I like shouting into the mike waiting for the boot-up but because the problem manifests itself in a more disruptive way when SmartSDR starts and clean resolved RX audio is fed through to the audio outputs and speakers - the mic picks up the RX audio and is output to the audio jack in a distorted form, mixing the two together.

I thought what was happening was that the AGC was set to "off" or "fast" and clipping was occurring - this is not the case. So the net result is badly distorted RX audio when the radio first starts. The problem gradually disappears after about 30 seconds from switch on.

As soon as I switch to ACC TX input and restart the radio this phenomenon disappears. 




  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited March 2017
    Is anyone else seeing the problem that Keith has described?
  • Bill -VA3WTB
    Bill -VA3WTB Member ✭✭✭
    edited February 2018
    I wonder if it is a strange connection or grounding problem?
  • Keith Richards
    edited March 2015
    Hi Tim,

    I'm trying many scenarios here to isolate this to confirm if its something local to my installation.

    Doesn't seem to be a wider issue as surely somebody would notice the same - other than many would be remote to their radio for obvious reasons and wouldn't typically be listening to the local AF outputs. I just use the digital PC option to help circumvent RFI in the TX. I guess you can't recreate at your end ?

    Do you think there is any rationale behind what I'm experiencing ? Bizarre how the distortion tails off and vanishes after 30 seconds - I'll try and capture on a video.

    One thought  - I use a RME Fireface UFX external DAC (Firewire interface) to convert Mic audio via class A mic preamp into the computer. The software is low latency ASIO driven and presents the computer with several audio ins and outs so I am able to choose between setting many devices (allbeit one at a time) as the default record or playback device which equates to a particular fader / channel. http://www.rme-audio.de/en/products/fireface_ufx.php

    This is a pro level piece of kit and drivers are rock solid - used in many pro recording studios.

    Anyway its no big deal for me as Ive switched back to the ACC input so I can also use the RCA PPT - digital PC option doesn't support that control option yet need to click MOX (hopefully will later though ?) 

    Best Regards


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