Changing audio levels in SO2R

Tim K9WX

I’m trying to understand something that occurs with the receive audio on the 6600.  Not sure if this is operator error (something I can control and change with a different setting) or something more innate to the radio itself or even a bug.  I am using SSDR 2.4.9.

The issue occurs when I operate SO2R with Slice A audio in my left ear and Slice B audio in my right ear, so that I am listening in stereo, a different audio source in each ear.  The mode is typically CW with AGC set to medium and the threshold set to around 50 for both slices.

There are occasions when I will be trying to work a station that is on Slice A (left ear) and the station is relatively weak or has QRM/QRN/QSB.  To improve my ability to copy, I will switch the audio from stereo to mono, so that the Slice A audio is now in both ears. 

But I have always had the perception that switching from stereo to mono also boosts the audio level in the original ear which, in this scenario, was the left ear.  In other words, it is as if I increased the AF gain for the left ear when I switched from stereo (Slice A audio in just the left ear) to mono (Slice A audio in both ears).  I sometime think that I would have been able to work the Slice A station by listening to it in just the left ear if the left ear Slice A audio had originally been at the audio level I experienced in the left ear after switching to mono.

The phenomenon is the same if the weak station is originally heard on Slice B (right ear) and I switch from stereo to mono so that Slice B audio goes to both ears.

I have never been positive of what I was hearing, thinking that perhaps what I experienced as an increase in the left ear audio when switching from stereo to mono was a false impression attributed to the fact that I was now hearing the Slice A audio in both ears.  So, I did some testing to evaluate this more objectively. 

With my setup in SO2R configuration, mode set to CW, AGC at medium and threshold set to 50, I recorded just the left channel audio while switching back and forth between stereo and mono.  My recording was made from the radio’s left speaker audio (Slice A audio) to an iPhone, with the phone at a fixed distance from the speaker. The right speaker was unplugged so that the right channel was not contributing any audio to the recording regardless of whether I was listening in stereo or mono.  I tuned Slice A to a busy 3 kHz swatch of FT8 spectrum on the theory that, for a given 15-second FT8 interval, the overall audio level would be relatively constant.  The speakers involved are plugged directly into the Flex, so there is no Maestro in the mix.  I manually switched the receive audio from stereo to mono and back several times during the test using the tilde (~) key in N1MM+.  There are no changes being made to any SSDR setting when the stereo to mono switches are being made: AF gain, AGC, AGC-T, equalizer etc. are held constant.

The image below, which was created by opening the iPhone recording in audio editing software, shows the result.  It seems clear to me from this graphic that what I have been experiencing as an increase in the left channel audio level with my ears was not subjective but that there is in fact a clear increase in the left channel, Slice A audio when switching from stereo to mono.  The level changes visible on the graphic correspond exactly with the mono to stereo and stereo to mono switch.  Just eyeballing the graphic, it looks like a 3 or more dB increase, so at least a doubling.  And let me reiterate, this graphic is showing only the left channel audio, not both channels combined.

My question, then, is why is this the case?  Is there some setting I can alter to eliminate this doubling and have consistent audio on a given audio channel, left or right, regardless of whether that channel is being listened to in mono or stereo? 

And you might ask, why is this a problem?  In a contesting scenario, you want to keep your hands on the keyboard as much as possible and you don’t want to be reaching for radio controls more than necessary or type more keystrokes than necessary.  You want to set the audio to a comfortable and predictable level for the majority of signals and then type callsigns and exchange information as quickly as possible, with no extraneous keystrokes.  An uncontrollable doubling or halving of the audio is counter productive to high rate, an important metric in contesting situations.

Although not an apples to apples comparison, a similar observation about changing audio levels on an individual slice with multiple slices open was recently made here: https://community.flexradio.com/flexradio/topics/bands-switching-audio?

Tim K9WX

Andy - KU7T

Very good write up and explanation of your test method. I am having the same feeling when switching audio from 1 to 2 ears. I have always thought it's just easier to copy with 2 ears. Whether this is a bug somewhere or just a normal result of audio mixing, I hope this could be adjusted so the level is the same. 73, Andy KU7T

Kari Gustafsson SM0HRP

This effect is called binaural summation. Tests have showed that you win some 2-3 dB listening in stereo with both ears than just one ear. It is very evident in SO2R operating.

Andy - KU7T

Kari, thanks for the information. It makes sense that this is the case, we can just hear better with 2 ears.

However, this effect is cumulative to what Tim mentioned. Look at Tim's graph. If he measured 3 db, and binaural summation is somewhere around 3 dB, it would be proper to adjust with 6 dB when switching the audio from left to both, or right to both. 

Gotta try this out, as its only noticeable with weak signals and proper switching...

73
Andy
KU7T

Joel N9LQ

I have noticed the same thing as Tim.  An easy way to see the effect is to just put on one half of your headphones and put the other earpiece on the top of your head.  As you switch to stereo, the ear that you are listening with gets louder.  I think this is not very good for SO2R operations.  I agree it would be good to be able to adjust this effect out.  73, Joel N9LQ

Eric-KE5DTO

Tim,

Thanks for the excellent write up and description of the issue that you are seeing.  This helps a great deal when we are looking into problems.  Can you tell me how you are switching from stereo to mono?  Is this just changing the balance control on the Slice in SmartSDR?  Are you clicking on the L/R button or dragging the slider?

Andy - KU7T

Tim, 

I see the same issue. In my case, the situation arises when using SO2R audio switching via the OTRSP port. I have worked around the issue in some custom code in Slice.PropertyChanged event handler:

                        case "AudioPan":
                            if (slc.AudioPan == 50)
                            {
                                slc.AudioGain -= 6;
                            }
                            else if (slc.AudioPan == 100)
                            {
                                slc.AudioGain += 6;
                            }
                            else if (slc.AudioPan == 0)
                            {
                                slc.AudioGain += 6;
                            }
                            break;
Works like a charm.

The others mentioned SO2R, so I am guessing they may also be using OTRSP...

73
Andy
KU7T

Tim K9WX

I have only noticed the change in audio levels when using N1MM+, where I use the tilde key (~) to go from stereo to mono and back to stereo.  So the change is, in effect, instantaneous, without the delay that might be associated with adjusting the sliders.  As Andy KU7T suggests, there is an OTRSP port involved.

Tim K9WX

Eric-KE5DTO

When you notice the change, is there a Slice coming or going?  I ask because there is scaling that happens as a result of how many Slices are in use (and whether or not they are muted).

Andy - KU7T

When I use this, I have 2 slices and they are staying. OTRSP just switches the audio.

Andy
KU7T

Eric-KE5DTO

Is it muting one of the Slices?

Tim K9WX

Eric: Not sure if the question about a slice coming or going was directed at me.  But my SO2R setup would typically have two slices open and they remain open throughout.  When listening in stereo, both slices have the audio turned on.  When I use the ~ key in N1MM to switch to mono so that I have the same slice in both ears, the other slice becomes muted (shows the little red circle with the slash mark on top of the speaker icon for that slice in SSDR).

Eric-KE5DTO

The muted slice would explain this.  We will revisit the decision about whether to include a muted Slice in our contributing Slice calculation.  Thanks for the info.