How Out Of Phase Looks

[flintstone mop]

Hello AM folks
I would like a link to a 'scope picture showing what an out of phase looks like to someone using a 'scope to monitor their transmitter?
OR I could get creative and take some pictures myself.
And a verbal explanation to go along with it. I have not had good success with Google.

There's a thread elsewhere on this site where someone is asking what is microphone phasing.
I'm dealing with a broadcaster that is kinda wondering what the hell I'm talking about.

thanks
Fred

[KU8L]

Hi Fred:

I might be the one that asked about mic "phasing".

If you search Google, you will see hundreds of mic phasing hits...but they all have to do with multiple mic phasing and phase cancellation...that was my normal exposure to phasing.

I suspect that for our AM modulation perspective, mic phasing is really about ensuring that any asymmetric waveform applied to the mic creates excess POSITIVE peaks in modulation and not the other way around.

BTW, depending on the waveform, it MIGHT look just like an overmodulated example with zero breaks in carrier but with normally formed peaks.  Reverse the polarity of the mic and it changes the mod pattern with proper valleys and 100% + peaks

You can find links to graphics I think if searching for asymmetric modulation or similar line.

Curt
KU8L

[kb3ouk]

If you want to see for yourself what it looks like, just reverse the polarity of your mic and see what happens. One way will have the positive peaks higher than negative, and the other will be the opposite. When the positive peaks are higher than negative, the mic is in phase.

[W3RSW]

Uh, I can't see negative peaks, just more crossings of the zero line, i,e., more beads for same positive peaks in 'bad' phase or higher positive peaks with same amount of beads when in the 'good' phase, properly polarized.

Hope that's not confusing but really shows up on a CRT or similar mod monitor.

A d104 really shows the difference more than any other mike I've tried in direct or non-processed audio to a typical AM plate modulated transmitter.. Very marked difference in my 32V2.  The 'good' phase easily shows 150 percent or better modulation from the non-symmetrical,  crystal audio generator of Astatic's mike.

Been mentioned many times before.

[W7TFO]

Fred,

What you are referring to is related to the natural asymmetry of some human voices, not how mics in multiple are set up for recording.

Some of us have this effect on a microphone, some don't. 

I've also seen it 'flip' for different voices, i.e. one has many + peaks where another will have - peaks on the same mic.  That being said, you should set the phasing of the mic according to your voice after testing it on a scope or BC modulation monitor.

The same phenomenon can drive BC FM limiters wonky, too.

73DG

[kb3ouk]

And depending on how your audio chain is set up, make sure everything is in the same phase and isn't switching pases at any point. If there's any phase switching in the audio chain, it will mess up any asymmetry.

[flintstone mop]

hmmm I'll have to create a little video clip and send it thru email to the station involved.
I'm not concerned about multiple mics, just one and the polarity effect on AM.
 
Correct, certain voices will give the nice Christmas tree look. (Christmas tree laying on its right side) When the polarity is wrong the tree will try to display on its left side, very little positive peaks, and a lot of base-lining.

But I'll look into Google for Asymmetry display on a oscilloscope.


Fred

[KU8L]

Here is one...with explainin'

http://www.nu9n.com/am.html

[Steve - K4HX]

Polarity is probably a better term, but everyone wants to use phase.

Improperly phased.

1. Mic audio

2. Modulation Envelope

[Steve - K4HX]

Proper polarity.

[W2VW]

Proper polarity.

Happy St. Patty's Day.

[W1AEX]

Hi Fred,

I have a simple home-made YouTube video of Dirk WA2CYT demonstrating in-phase and out-of-phase with his Viking II on 75 meters. It's located at the link below. It definitely shows what it looks like, but it probably won't mean much to someone who doesn't understand what it indicates.

http://www.youtube.com/watch?v=pvowzE_XYl0

There's a couple of waveform screenshots from the video below.

[flintstone mop]

Rob ans Steve
Thanks
Those are great illustrations and very informative. Pretty obvious how it affects the modulation.
Thank  you
Fred

[W3RSW]

Very nice screen shots Steve.
Really shows what's going on.

Polarity is probably a better term, but everyone wants to use phase.

I guess it's because 180 deg. diff. in phase is a total reversal, other side of the circle, happy and sad...   kind of like "off" and "on." 

[steve_qix]

There's even more to it..  If you are "in phase" - A.K.A., your positive modulation is greater than your negative modulation, as in Steve's pictures, you will actually be able to put more audio on the carrier - and therefore be "louder" without distortion - CAVEAT: *IF* the detector at the other end does not introduce distortion.

Here is a very interesting example.  It is Guy W1FRM running some phase tests.  One of the most dramatic examples of asymmetry, and how reversing the phase makes a huge difference, that I've ever seen.  These are actual pictures of what Guy's modulation looks like on a receiver-connected modulation monitor when he's in phase, and when he's out of phase.

First, in phase:


Now, out of phase:

[steve_qix]

Forgot to point out - in the out of phase condition, Guy is actually overmodulating in the negative direction.

In the out of phase condition, the Asymmetry meter is pinned at negative 30% asymmetry - meaning the average  modulation is more than 30% asymmetrical in the negative direction.

The reverse is true in the in-phase screen shot of the monitor.  The Asymmetry meter is pinned at positive 30% modulation.

Most signals I see are not nearly as asymmetrical.  Guy's signal is very much so.

[Steve - K4HX]

Good shots. It doesn't get more clear than that!

[flintstone mop]

Correct Steve.
I ordered this long-awaited mod monitor from QIX. Shaaaazaam I'm in heaven!

[ke7trp]

Fred, The setting that shows the higher positive peaks on the scope is the correct setting.  You can also use a PEP watt meter.  Just toggle the phase and the setting with the highest output power on the pep meter is the correct phase.

Keep in mind that you might need to back the audio down for in and out of phase testing. If the audio is flat topped, the ratio of the mod trans wont allow greater peaks or the modulator is just simply out of steam, Each setting might show the exact same peak output power.  By lowering the gain, the correct setting will jump out.

Same goes for testing using some form of audio processing that is limiting.  You could be on the limiter in either phase.. Then you would not see the difference.  Disable the limiter for testing.

Sometimes it can be very frusterating to find the correct setting if you have alot of processing going on.

C