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Matrix processing on FM?

K

kevtronics

Listening to my favorite Classic Rock station on a hi-fi monaural table radio, I can't help but notice that whenever a Beatles song, or something else with very wide separation comes on, the loudness falls off the map. And this is on a major Top 10 market station transmitting HD, so I'm sure they're using the latest-and-greatest processing, or something very near to it.

C-Quam AM Stereo avoids this problem by using matrix processing, but I've always been told this is impossible on FM. But with today's new techniques and DSP processing power, is there any effective way to accomplish it on FM?

I know many of today's processors let you run the AGC in matrix (L+R/L-R) mode, but I don't know how effective that is, because the audio still has to go through a whole bunch of limiters and clippers operating in the normal L/R mode before the rubber meets the road.

Is the best you can do just to restrict the L-R level to about 70-75%, to make up for some of the loss of loudness, at the expense of reduced separation?
 
Kevin,

Matrix processing has been used in FM. What must be considered with it, is the possible exaggeration to multipath, *if* the L-R level becomes overly enhanced. That has generally been a reason why it's not more commonly used in FM.

-Frank Foti
 
Adding to the previous answer, matrix processing was a sort of passing fad in the mid '80s. I know of several stations that used it for a while. When used correctly, it can enhance your stereo presence and it has a unique, hard-to-duplicate sound, but there is always a potential lurking tradeoff in multipath. If your station is in a flat area with good coverage of your primary population, it can make you sound subjectively better, or at least, unique. On the other hand, if you already have multipath problems, it can make those worse. I believe that there is still at least one processor out there that specifically has an available L+R and L-R processing function.
 
Vorsis processors have a "multipath controller" which they claim "can help mitigate the audible effects of multipath as well as reduce receiver-induced stereo blend by limiting the amount of L-R as a percentage of L+R for a more consistent and predictable sound."

Current Optimods can run the AGC in L+R/L-R matrix mode, also with the option to reduce the L-R level, although the final limiting/clipping is still done in the conventional left/right domain.

I'm not sure what matrix processing features (if any) current Omnia processors have... so those in the know, feel free to fill us in on the details. :)
 
Kevin,

Excessive L-R caused by 'unmanaged' matrix processing can not only lead to wildly varying mono loudness levels, it can increase perceived multipath by exaggerating stereo receiver blending. In fact by its very nature, mono loudness *will* vary as a function of the stereo content in program material because of how that content's amplitude and phase adds to create the L+R component of the stereo signal.

Over the last few decades I've done quite a bit of research on matrix processing, stereo enhancement and L-R management techniques to minimize multipath-induced stereo blending artifacts. You might find the following of interest, especially the parts about managing mono loudness:

http://wheatstone-processing.com/in...2-new-findings-on-fm-stereo-multipath-control

http://wheatstone-processing.com/in...th-presents-new-findings-in-multipath-control

Jeff Keith, CPBE, NCE
Senior Product Development Engineer
Wheatstone Corporation
 
Jeff_Keith said:
Excessive L-R caused by 'unmanaged' matrix processing can not only lead to wildly varying mono loudness levels, it can increase perceived multipath by exaggerating stereo receiver blending. In fact by its very nature, mono loudness *will* vary as a function of the stereo content in program material because of how that content's amplitude and phase adds to create the L+R component of the stereo signal.
Click to expand...

That is all true if the matrix processing is followed by conventional left/right limiting. The extra L-R energy that the matrix processing adds would just show up as increased L and R peak levels, which the limiter would have to smash back down.

I was really just wondering if the L+R / L-R processing scheme could somehow be maintained all the way through to the stereo encoder and transmitter on FM, as it is with C-Quam AM Stereo. From what I've always been told, this is impossible, because with FM Stereo, 100% modulation is determined by whatever is going on in the left or right channels individually, not by adding them together.
 
kevtronics said:


That is all true if the matrix processing is followed by conventional left/right limiting. The extra L-R energy that the matrix processing adds would just show up as increased L and R peak levels, which the limiter would have to smash back down.

I was really just wondering if the L+R / L-R processing scheme could somehow be maintained all the way through to the stereo encoder and transmitter on FM, as it is with C-Quam AM Stereo. From what I've always been told, this is impossible, because with FM Stereo, 100% modulation is determined by whatever is going on in the left or right channels individually, not by adding them together.
Click to expand...

Kevin,

I've often wished FM stereo's L-R signal was an independent modulation signal like it is in C-Quam AM stereo, but it isn't. In the FM stereo modulation scheme the L+R and L-R signals aren't separate, but interleaved, and bear a strict mathematical relationship to each other. Trying to separately control L+R and L-R peak levels and also maintain precise control of total peak modulation is a no-go.

So back to your original question:

Is the best you can do just to restrict the L-R level to about 70-75%, to make up for some of the loss of loudness, at the expense of reduced separation?

It's the best we can do given the modulation scheme chosen primarily for its backward compatibility with mono receivers.

Jeff Keith, CPBE, NCE
Senior Product Development Engineer
Wheatstone Corporation
 
kevtronics said:
I'm not sure what matrix processing features (if any) current Omnia processors have... so those in the know, feel free to fill us in on the details. :)[/B]
Click to expand...

Kevin,

Actually, we were one of the first to offer control over the L-R level in Omnia.fm. Later on, we developed a stereo enhancement feature that monitors L+R/L-R ratios and makes adjustments to the sound field in a natural manner. The algorithm does not allow for excess L-R to occur on content that already contains moderate to significant levels of L-R energy.

-Frank Foti
 
Even with C-QUAM, I had to increase the L+R audio level so as to avoid excessive sidebands. This reduced the stereo separation slightly, but the station was loud and legal. The jocks, listening on headphones, liked the sound of the AM C-QUAM better than the sound of the FM. In their words ... "It sounds bigger."
 
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