Positive vs Negative Asymmetry

[ka1tdq]

I've been using the REA modulation monitor on both my 75 and 40 meter class E rigs.  Both show positive peaks into the 150% range with negative peaks never going over 100%.  And actually, I probably clip them too low on the 40 meter rig with negative peaks never going above 90%.

Anyway, my question is about asymmetry.  There's an asymmetry meter on the modulation monitor that goes positive and negative 25.  My 75 meter rig rides positive, but my 40 meter rig reads negative.  Both sound fine with the audio playback.  What exactly does asymmetry mean?

Feeding my 40 meter Heising circuit is a car audio amplifier.  If I swap the speaker leads going to the heising circuit, would I reverse the negative meter so that it would read positive?

Jon

[flintstone mop]

hi Jon
Asymmetry, in my old mind, seems like an audio processor function to force the audio to be more positive, than negative. Certain human voices can create a lot of asymmetry. The technical end of AM transmitters is that you should NEVER go over 100% negative. To be safe be around 90%. Past 100% the carrier is cut off and splatter and interference is created in the freq spectrum.
In a plate modulated transmitter, that is a disaster to the modulation transformer. The secondary is seeing an open circuit (?) and eventually will be damaged. 130% POS is about max that most receivers can handle. The POSITIVE peaks is your loudness. I miss my FlexRadio SDR 1000.......an oldie, for sure, but tremendous audio with little processing needed. The magic of computers.

My Kenwood TS850 can give me 125% positive peaks and broadcast audio. My chain goes directly to the balanced modulator and the radio is tricked into no filters in the transmit. 10hz to 10khz transmit and receive audio out of the ext spkr jack. Easy radio to convert to hi-fi audio. Still need an SDR. Using a SoftRock radio with I/Q output to a high quality USB audio device, in battle conditions.

[WD5JKO]

Jon,

    I recall that you typically use the QIX 3 diode circuit to minimize over modulation in the downward direction, and you let the positive peaks fly where they may. If it were me, I'd carefully look at your modulation at a level well before those diodes start doing their magic, and see what kind of asymmetry you have. If the audio polarity can be swapped somewhere (at a low level or at a high level), see which way your voice into your microphone favors for most upward modulation versus downward modulation. That way, you can crank the audio up, and the reliance on the QIX circuit will be minimized.

   Many people really get excited about asymmetrical modulation causing distortion in diode detectors. Realize that this is a detector load issue, and usually has an RC time constant to it. The upper frequencies at a high level when coming down will have a tail to them, but the lower frequencies will be less affected. If the transmitted audio is good, the received audio will still be pleasant to listen to, even if there is a slight distortion (I'd say way less than 10%) on the upper audio frequencies that might be, but probably are not at a high level.

   What really kills a diode detector in BA receivers is AM from a balanced modulator type transmitter when the negative peak exceeds 100%. When this happens, the phase reverses, and although not distorted in a sync. or product detector, the BA diode detector follows this phase reversal, and the results are just dreadful...often like copying SSB without reinserting the carrier.

Jim
Wd5JKO

[Mike/W8BAC]

If I swap the speaker leads going to the heising circuit, would I reverse the negative meter so that it would read positive?

Try swapping the speaker leads, if that doesn't work move back to the mic input leads. Continue down stream till you find the polarity you want Jon. Good luck. I would have never considered a car audio amp but it obviously works for you. 

Mike

[ka1tdq]

I think I will try reversing the speaker output leads going to my 40 meter transmitter and seeing what that does. I use the same microphone and mic preamp for both my 40 and 75 meter rigs, and the asymmetry is positive for the 75 meter one. The mic preamp feeds both audio modulators for both transmitters, but from different outputs.

Everyone tells me the audio sounds great from both rigs, and the audio playback from the REA modulation monitor sounds good too.

But, then again, when was the last time someone has said that someone else's audio sounded like crud? I bet I could put a stock Apache on the air with a carbon microphone and I'll get tons of great audio reports. We're all too nice.

Jon

[ka1tdq]

That works!  Asymmetry is now positive and the negative peaks are still 90% or below, and the highest positive peak was at 150%. 

Jon

[flintstone mop]

That works!  Asymmetry is now positive and the negative peaks are still 90% or below, and the highest positive peak was at 150%. 

Jon

Good deal Jon......Ya did it!

[KD6VXI]

But, then again, when was the last time someone has said that someone else's audio sounded like crud? I bet I could put a stock Apache on the air with a carbon microphone and I'll get tons of great audio reports. We're all too nice.

Jon

We have a couple people out west who give unsolicited reports.

Listen on the 75 ghetto for K6FMB.   He's a broadcash engineer at a couple stations in San Diego.   If he says you sound good,  you've done it.   There is also a couple in Washington whose calls escape me.

I've found its the unsolicited comments that tell ya when you've gotten it right.    In my limited time in playing on ham hi fi,  it's those guys who let me know.

Otherwise,  you're right.   It's always you are 30 over and sound great!   AM's equivalent of 'Your five nine,  number xxx'.

--Shane
KD6VXI

[N1BCG]

An important point about asymmetry that rarely gets the attention it deserves is that it is inconsistent across your vocal range. That is to say that certain sounds will cause completely opposite surges, such as "oooooooo" and "aaaahhhhh". Operators will determine the "polarity" based on whatever is most common, but lower frequencies can work against that, which is why a low roll-off filter (HPF) often helps avoid the oppositions tremendously.

All broadcast processors include a phase rotator to address this issue as well as to achieve a higher average level (RMS) by reducing the volume swings entering compression stages.  There are phase rotator boards pre-built on the net for under $20. An amazing value.

[flintstone mop]

But, then again, when was the last time someone has said that someone else's audio sounded like crud? I bet I could put a stock Apache on the air with a carbon microphone and I'll get tons of great audio reports. We're all too nice.

Jon

We have a couple people out west who give unsolicited reports.

Listen on the 75 ghetto for K6FMB.   He's a broadcash engineer at a couple stations in San Diego.   If he says you sound good,  you've done it.   There is also a couple in Washington whose calls escape me.

I've found its the unsolicited comments that tell ya when you've gotten it right.    In my limited time in playing on ham hi fi,  it's those guys who let me know.

Otherwise,  you're right.   It's always you are 30 over and sound great!   AM's equivalent of 'Your five nine,  number xxx'.

--Shane
KD6VXI

Reversed phase of the microphone will never show itself in an audio kinda way. You will not notice a difference in audio quality with wrong polarity of a microphone.
Only a 'scope, or the REA monitor will reveal if you are exceeding negative peaks.

[KD6VXI]

But, then again, when was the last time someone has said that someone else's audio sounded like crud? I bet I could put a stock Apache on the air with a carbon microphone and I'll get tons of great audio reports. We're all too nice.

Jon

We have a couple people out west who give unsolicited reports.

Listen on the 75 ghetto for K6FMB.   He's a broadcash engineer at a couple stations in San Diego.   If he says you sound good,  you've done it.   There is also a couple in Washington whose calls escape me.

I've found its the unsolicited comments that tell ya when you've gotten it right.    In my limited time in playing on ham hi fi,  it's those guys who let me know.

Otherwise,  you're right.   It's always you are 30 over and sound great!   AM's equivalent of 'Your five nine,  number xxx'.

--Shane
KD6VXI

Reversed phase of the microphone will never show itself in an audio kinda way. You will not notice a difference in audio quality with wrong polarity of a microphone.
Only a 'scope, or the REA monitor will reveal if you are exceeding negative peaks.

I beg to differ.

100 pct positive and 150 negative (the carrier cut off for an appreciable portion of the cycle) you most certainly can tell.   It sounds horrible!

Now,  I'll agree with you on the basis of the carrier not being cut off.   IE,  100 pct neg peaks and 75 pct positive.

Hence the need to assure your phasing is proper before trying to exceed 100 pct on pos peaks.   Your carrier will be cut off long before if your peaks are flipped improperly.

--Shane
KD6VXI

[flintstone mop]

But, then again, when was the last time someone has said that someone else's audio sounded like crud? I bet I could put a stock Apache on the air with a carbon microphone and I'll get tons of great audio reports. We're all too nice.

Jon

We have a couple people out west who give unsolicited reports.

Listen on the 75 ghetto for K6FMB.   He's a broadcash engineer at a couple stations in San Diego.   If he says you sound good,  you've done it.   There is also a couple in Washington whose calls escape me.

I've found its the unsolicited comments that tell ya when you've gotten it right.    In my limited time in playing on ham hi fi,  it's those guys who let me know.

Otherwise,  you're right.   It's always you are 30 over and sound great!   AM's equivalent of 'Your five nine,  number xxx'.

--Shane
KD6VXI

Reversed phase of the microphone will never show itself in an audio kinda way. You will not notice a difference in audio quality with wrong polarity of a microphone.
Only a 'scope, or the REA monitor will reveal if you are exceeding negative peaks.

I beg to differ.

100 pct positive and 150 negative (the carrier cut off for an appreciable portion of the cycle) you most certainly can tell.   It sounds horrible!

Now,  I'll agree with you on the basis of the carrier not being cut off.   IE,  100 pct neg peaks and 75 pct positive.

Hence the need to assure your phasing is proper before trying to exceed 100 pct on pos peaks.   Your carrier will be cut off long before if your peaks are flipped improperly.

--Shane
KD6VXI

OK FB Shane at least you do not totally disagree with me.