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How Much Modulation is Enough for Music on AM?

C

cc333

OK, I'm not sure this is the proper place to post this, but I'll try:

I can push modulation up to around 100% (and even overmodulate a bit, up to 120% or so) without any noticeable distortion on any of my mono tuners, but for some reason, on AM Stereo tuners, I can only go up to about 70-80% modulation before it starts sounding crunchy from distortion. Is this a normal characteristic of C-QUAM, or am I doing something wrong with regard to processing?

c
 
What kind of transmitter are you using? The homebrew ones made and sold as kits today are not made to the same standard as commercial broadcast C-Quam exciters and laboratory signal generators.

Plus, on many there is a tradeoff between modulation level and RF output level. For example the manual for my Panasonic signal generator says that at maximum RF output level, it is only capable of 80% modulation. If you want 100%+ modulation you have to reduce the output power.

There's one 50 kW NYC talk station that is transmitting with about 85% modulation and yet they don't sound noticeably quieter than other stations on the dial.

And the C-Quam system is officially capable of the same +125% / -100% modulation as mono AM, although if you go above +100% positive modulation there is some channel crosstalk. And negative modulation (even on a mono signal) beyond about -95% will cause distortion on any receiver with an envelope detector, although on stereo receivers it's more noticeable due to their wider bandwidth. Plus the negative modulation of single-channel audio must be limited to around -70% to -75% to prevent overload in the C-Quam decoder chip. But with modern music that contains little to no exclusive single-channel audio, this is unlikely to occur.
 
cc333 said:
OK, I'm not sure this is the proper place to post this, but I'll try:

I can push modulation up to around 100% (and even overmodulate a bit, up to 120% or so) without any noticeable distortion on any of my mono tuners, but for some reason, on AM Stereo tuners, I can only go up to about 70-80% modulation before it starts sounding crunchy from distortion. Is this a normal characteristic of C-QUAM, or am I doing something wrong with regard to processing?

c
Click to expand...
Anything over 100% on negative peaks on AM causes carrier suppression and horrendous distortion. Up to 125% on positive peaks is acceptable, although high positives can cause distortion on some kinds of radio receivers.

Those are characteristics of AM itself. C-Quam is an ancient technology, and was never updated beyond the very early 1990's. The final "improvement" was reducing the motion sensation that made some people actually get sick or queasy while listening in vehicles.

The real question is "why would you employ C-Quam today when there are practically no radios capable of hearing it left?"

Note: I discontinued C-Quam in 1995 on 50 kw KTNQ in LA, but until that time it was more than capable of using 100% negative peaks as the limit (with the limits Kevin mentions). Our transmitter did not perform well on positive peaks, so we did not push that... and since loudness wars were pretty much over by then, it did not matter much. So we generally tried to keep positive peaks in the 90% to 95% range, not 100% because what you saw on electromechanical meters did not necessarily show very short duration higher peaks that caused all the issues Kevin also mentioned; for that, you had to watch an oscilloscope.
 
kevtronics said:
What kind of transmitter are you using? The homebrew ones made and sold as kits today are not made to the same standard as commercial broadcast C-Quam exciters and laboratory signal generators.
Click to expand...
I'm using this one: ASMAX2

boiseengineer said:
In other words, your processor must be C-Quam compatible.
Click to expand...
I'm using StereoTool, with the C-QUAM stuff turned on in the AM Transmitter section. Is that good, or do I need something more?

kevtronics said:
A non-matrix processor will inherently limit single-channel modulation to 50%. Which is fine, but mono loudness on wide-stereo recordings will suffer, just like it does on FM.
Click to expand...
What is a matrix processor versus a non-matrix one? How can I tell? Is StereoTool matrix or non-matrix?

DavidEduardo said:
The real question is "why would you employ C-Quam today when there are practically no radios capable of hearing it left?"
Click to expand...
Because I can! And also because I have some AM Stereo radios, and I thought it would be fun to try to see what they're capable of, since there aren't any local stations still broadcasting in stereo that I can receive well enough to reliably decode the stereo signal.

This is all very helpful information, thank you!

I'm thinking that perhaps the response time on the modulation meter in this transmitter's display is a bit too slow to be useful. Perhaps I should get my meters calibrated to some sort of known value (something like, -3dB on the VU meter corresponds to 80% modulation) so I can not cross that threshold (or indeed, so I can actually know with some certainty what, exactly the threshold is).

c
 
cc333 said:
Because I can! And also because I have some AM Stereo radios, and I thought it would be fun to try to see what they're capable of, since there aren't any local stations still broadcasting in stereo that I can receive well enough to reliably decode the stereo signal.
Click to expand...
Sounds like a fun project that will also increase your tech skills. Go for it!
 
@Greg Strickland @DavidEduardo Thank you! It's especially fun because it's something I've wanted to do almost as long as I can remember.

I'd share a link to the stream I set up (basically, I have an AM stereo tuner pick up the over the air signal, feed it into another computer and stream from there), but, licensing.

I guess if anyone is interested in sampling it, PM me (@vchimpanzee in particular, as he seems to have musical interests similar to mine, and I'd like some feedback).

I saw the suggestions for running the AGC in matrix mode (since edited away, for some reason?). I'll look into it because I'd wondered what it was for (hopefully I don't need an extra StereoTool license for it), and it sounds like it might help.

c
 
Did you try flipping the phase of the audio going into the transmitter? The absolute phase of computer sound card outputs (and really any unbalanced consumer-grade audio equipment) is unpredictable, so if you have StereoTool set to produce asymmetrical modulation, and it ends up producing greater negative modulation instead of positive, that will definitely cause distortion.
 
Anyway, here's some updates:

Putting StereoTools' AGC into matrix mode isn't making much of a difference, although maybe it is a little bit cleaner.

Engaging asymmetrical mode and setting it to 125% "top" (positive, I'm guessing) seems to enable slightly higher modulation without any noticeable increase in distortion, so I'm pretty sure the polarity is correct (I'm using a Firewire-based Focusrite Saffire Pro 26, which is a decent audio interface).

c
 
cc333 said:
I'd share a link to the stream I set up (basically, I have an AM stereo tuner pick up the over the air signal, feed it into another computer and stream from there), but, licensing.

I guess if anyone is interested in sampling it, PM me (@vchimpanzee in particular, as he seems to have musical interests similar to mine, and I'd like some feedback).
Click to expand...
What is it you want me to do?
 
vchimpanzee said:
What is it you want me to do?
Click to expand...
Nothing that much knowledge of radio beyond that of a dedicated and discerning listener. Just listen and let me know how the programming is, the sound quality, the flow from song to song, and any thoughts regarding song selection. If you can PM me, I can send you the link and set you up with a username/password.

TL, DR Version: I want something that is a tasteful blend of KABL, KFRC and KYA with a hint of various other similar stations to provide some glue and make it more interesting.

I'm striving to have a fairly wide selection of what might be considered MOR, with some easy listening, soft oldies, etc. I can't please everyone, but I want to try to have a fairly neutral sound that is relatively unoffensive and uncontroversial. Imm not trying to push any boundaries!

For now, there's no imaging, only music. I want to get the playlist settled before I worry about jingles and voice-tracking.

Am I being too ambitious?

c
 
cc333 said:
Nothing that much knowledge of radio beyond that of a dedicated and discerning listener. Just listen and let me know how the programming is, the sound quality, the flow from song to song, and any thoughts regarding song selection. If you can PM me, I can send you the link and set you up with a username/password.

TL, DR Version: I want something that is a tasteful blend of KABL, KFRC and KYA with a hint of various other similar stations to provide some glue and make it more interesting.

I'm striving to have a fairly wide selection of what might be considered MOR, with some easy listening, soft oldies, etc. I can't please everyone, but I want to try to have a fairly neutral sound that is relatively unoffensive and uncontroversial. Imm not trying to push any boundaries!

For now, there's no imaging, only music. I want to get the playlist settled before I worry about jingles and voice-tracking.

Am I being too ambitious?

c
Click to expand...
LIsten to what?
 
I'm sorry! Listen to my station via the "private" internet stream I've set up for myself and maybe two or so friends and acquaintances to listen remotely (you would be number three).

I'm getting over a head cold, so I've been a bit scattered lately....

c
 
cc333 said:
I'm sorry! Listen to my station via the "private" internet stream I've set up for myself and maybe two or so friends and acquaintances to listen remotely (you would be number three).

I'm getting over a head cold, so I've been a bit scattered lately....

c
Click to expand...
Again, you didn't day how.
 
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