Remote Audio Distortion

Paul Christensen, W9AC

I've been trying to isolate an audio distortion problem when running SSDR in remote mode on a tablet PC.  It also shows up using Maestro and K6TU's iPad app. 

For Maestro users especially, perhaps you can help confirm the distortion by trying the test below.  Please listen with stereo headphones: 

1) In SSB mode, find a reasonably strong QSO on Slice A.  Let's say it's 14.200.000 as an example.

2) Now, activate Slice B and set B to the same freq. as Slice A (e.g., pressing A>B button). Both Slice A and B should read 14.200.000

3) Next, move Slice B up 100 Hz (in this example to 14.200.100). Do you hear severe distortion? 

4) Move Slice B back 100 Hz to 14.200.000, does the distortion disappear?

I've noticed the audio distortion does not appear at my Flex 6700 headphone jack.  It is only present when using remote audio mode.  Yes, hardware and software has been re-set.

*** This test is NOT for users listening at the Flex transceiver's headphone jack ***

Paul, W9AC

ka7gzr

I tried the test and experienced the distortion. However it only occurs at certain volume levels of slice A and B. The distortion zone moves as I lower or up the gain of slice A and then adjust slice B in the same manner.

Jim

ka7gzr

Bob K8RC

I bet if, during the "distortion", you mute either slice the distortion will magically disappear. Sounds like what you've set up is a demonstration of how to generate phase distortion using a Maestro.

The reason you don't usually hear it is that the 2 slices' audio channels are kept separate in the radio and moving the frequency(phase) slightly moves the stereo image around between the 2 speakers. If you take those two signals and mix them to a single speaker, however, the phase differences generated as you move the frequency of one slice will add or cancel as you move and that is the distortion you hear. Some audio frequencies adding while others are cancelling really chops up the bandpass.

I don't have a Maestro but I have done just as you described before I changed my setup at home to full 2-channel instead of a monaural mix.

Paul Christensen, W9AC

That's correct Bob.  And to Jim's point, the distortion is maximum when AF level and AGC-T are the same between Slices A and B.

It's the first time I've heard distortion in split mode and to repeat, the test produces clean audio at the transceiver's headphone jack.

Paul, W9AC

Tim - W4TME

I have added this report detail to the open HelpDesk ticket #3619 regarding distorted Maestro RX audio.

Paul Christensen, W9AC

Thanks, Tim!

Paul, W9AC

Steve-N5AC

Paul, I tried this with both the radio and the Maestro and just to my ears the distortion was similar.  I didn't run any measurements with test equipment.  You will always have something like this in an SDR that uses the same antenna/SCU because of the precision in the decimation and demodulation -- the receivers are perfectly aligned.  I'm guessing that this is just an observation and there's not a "real world" reason to do this (place two receivers right next to each other with some of the same frequency components).

There's one more thing at play -- in remote mode was pass the audio through the Opus codec which is designed to encode a single audio or music channel.  With the phase offset between the two channels we could easily be confusing the codec and causing it to produce audio that is not as good as audio from different places that are combined.

Paul Christensen, W9AC

Steve,

That's right.  The slice spacing was only used to exaggerate the main issue -- which is slightly noisy/dirty audio when running the remote CODEC on any remoted device like Maestro, SSDR-ios, K6TU, and SSDR on a PC in Remote Audio mode.  It's most noticeable on single-tone CW.  I don't hear this artifact when using other CODECs (e.g., RemoteRig, IP Sound and DF3CB's RemAud. I've even used some of the lossier CODECs available with RemoteRig.  It's subtle for sure. 

Here's another Flex Community discussion that may add more insight.  Perhaps it's due to Opus bandwidth scaling (or lack thereof at the moment?):

https://community.flexradio.com/flexradio/topics/remote-audio-codec

Paul, W9AC

Steve-N5AC

I think the ultimate solution here is to allow for uncompressed audio locally.  It's not terribly complicated to do this and it's actually easier on everything but the network.  Then the user would have an option for the tradeoff between ultimate audio quality and network bandwidth.  Do you agree that this would be a good solution?

KY6LA_Howard

The issue is how do you identify local remote audio vs remote remote audio.

A lot of remote Apps use remote audio. And bandwidth is an issue when Remoting remote. Nerd Knob?

Ken - NM9P

Yes, I think the "easiest" solution would be to make it user selectable, either with a nerd knob or with double buttons --- [LAN Remote]   [WAN Remote] 

Ken - NM9P

Paul Christensen, W9AC

">I think the ultimate solution here is to allow for uncompressed audio locally.  It's not terribly complicated to do this and it's actually easier on everything but the network.  Then the user would have an option for the tradeoff between ultimate audio quality and network bandwidth.  Do you agree that this would be a good solution?"

Steve,

I think you mean remotely, not locally?  Flex audio is superb when listening to the 6x00 hardware's headphone jack.  I call that local audio.  By contrast, "remote audio" as used in the upper right corner of SSDR, or using Maestro, or SSDR-ios, is what I refer to as remote audio and that's compromised at the moment with these subtle audio distractions. 

So, I think the ability to either let OPUS truly adapt to available network bandwidth -- or allow the user to select from a small menu of compressed and uncompressed CODECs just like RemoteRig, RemAud and IP-Sound is the way to go as Flex gets closer to SSDR version 2.0.  Of course, a choice of "Auto/Manual" would be even better.

Paul, W9AC

Jay Nation

All

Not whinging here, but.

I think of "Remote" as a connection via WAN ethernet.
I think of "Local" as  a connection via LAN ethernet.
I think of "Headphone, Speaker, Line Out" as a direct connection to the radio.

Maybe I'm wrong? Maybe I misunderstood the buzzwords,?

Maybe someone will tell me?
   
 SDRgadgets

73, Jay - NO5J

Steve-N5AC

Sorry for the confusion-- I mean use uncompressed audio in your LAN, compressed audio on the WAN

KY6LA_Howard

You will still need a nerd switch until V2 because WAN via VPN looks like LAN to SSDR Perhaps one could select audio as LAN Remote and WAN Remote as per Ken's suggestion.