Asymmetrical AM (Amplitude Modulation).

[DrStrangelove]

https://www.thebdr.net/measuring-am-modu...with%20the

Gosh.

[Kalee20]

The asymmetric modulation I was referring to is where the positive part of the AM signal goes above 100% modulation but the negative part doesn't go below 0% as that would be a disaster for sound quality.

That would be asymmetric distorted audio.

That's what I thought, too - unless the audio waveform is complex and asymmetric, and contrived so that the positive excursions are higher amplitude than the negative (maybe they're narrower, so that the average is still zero).

[Mike Watterson]

That would be asymmetric distorted audio.

That's what I thought, too - unless the audio waveform is complex and asymmetric, and contrived so that the positive excursions are higher amplitude than the negative (maybe they're narrower, so that the average is still zero).

What I meant, is in the link. You can't even go quite to zero carrier and if you measure as per the linked article for say 1% or 2% carrier minimum, it's rare audio that is net asymmetric as that would be a DC offset. The AC coupling will remove it.

Indeed you only need to look at a minimum carrier level. Getting to zero is a total fail and bandwidth (splatter) and distortion will jump.

In no case shall the amplitude modulation of the carrier wave exceed 100% on negative peaks of frequent recurrence, or 125% on positive peaks at any time.”

The author of the essay doesn't consider adaptive carrier power, which makes it seem like you get more than 100% if it's done wrong. It's not due to "asymmetrical" peaks on speech. On a pure AM system that never loses carrier there is no asymmetry because there is AC coupling. It doesn't matter what sort of loudness compression you have (they also sell adaptors for CBs, Ham and mobile radio and they always reduce readability because they over compress).
See https://www.eham.net/reviews/view-product?id=2343 W4RT Electronics One BIG Punch (OBP).

[Kalee20]

..., it's rare audio that is net asymmetric as that would be a DC offset. The AC coupling will remove it.

I'd have to disagree there. Although it may be rare audio, it's quite easy to have a waveform that is asymmetric, one peak much higher than the other, with no DC offset - so AC coupling does nothing to remove it.

Waveforms with even-order harmonics will often be asymmetric. The waveform I sketched is an example - no DC offset, but one peak much higher than the other.
   
Going back to the OP's starting post, with conventional audio it would be jolly difficult to arrange the phasing such that upward modulation exceeds 100% while downward never exceeds 95% (the point at which simple diode envelope detectors often start to distort), but with digital signal processing and a bit of delay between signal 'in' and signal 'out' then I daresay it would be possible.

[colly0410]

https://www.thebdr.net/measuring-am-modu...with%20the

Gosh.

Thanks for that link. Read somewhere that most American AM stations TX at more than 100% on the positive part to sound louder. Read somewhere that Radio 1's 247 metres network used 110% positive on some TX's to improve reception in the mush zones, never worked here in the Nottingham area though as it was always mushy...

[Mike Watterson]

..., it's rare audio that is net asymmetric as that would be a DC offset. The AC coupling will remove it.

I'd have to disagree there. Although it may be rare audio, it's quite easy to have a waveform that is asymmetric, one peak much higher than the other, with no DC offset - so AC coupling does nothing to remove it.

Waveforms with even-order harmonics will often be asymmetric. The waveform I sketched is an example - no DC offset, but one peak much higher than the other.

It's trivial to create asymmetric waveforms and maybe irrelevant what the initial DC was once it goes through an audio chain.

You are agreeing actually with what I meant to write and essentially what I did write.

BUT:
1) How common is such asymmetric audio and what if it's inverted?
2) What does an AM modulator with gain adjusted so carrier is never less than minimum (1%? 5%?) do with such a signal?
3) Is adaptive carrier power (normal on all but low power AM) going to affect it.
4) Surely it's only instantaneous peak power that's affected and not average power? So would the more than 100% positive when the carrier is never off be only an instantaneous value?

Certainly I think can actually generate suitable audio (I have a DDS based arbitrary waveform generator and physically measure effects on modulation. I have a second function generator with AM in and two valve based signal generators. At least one uses a transformer Audio modulator to supply HT to RF anode, so unlike a multiplier with injected carrier, it can in theory do more positive than negative modulation. However I'm sceptical that is what is done deliberately. I'm sure the AM transmissions use a regular symmetrical compressor, if "loudness" is wanted and adaptive carrier power based on the average audio level after compression.

Maybe someone expert with LT Spice or Labview can simulate it?

[Kalee20]

1) How common is such asymmetric audio and what if it's inverted?
2) What does an AM modulator with gain adjusted so carrier is never less than minimum (1%? 5%?) do with such a signal?

That's what intrigues me too!

I postulated that an asymmetric waveform is possible - after going through AC coupling etc.

If it's inverted, then trouble - you get less positive modulation than negative, which doesn't suit simple diode detectors at all. So I wondered if there was any waveform monitoring to detect this and invert the phase of the audio - obviously that couldn't be done very often else there would be repeated minor clicks.

With a pure sinewave, it's not possible to have the carrier midulated by different amounts in the positive and negative directions.

As for adaptive carrier power, won't that give rise to audio compression in receivers with AGC? (Or is is just not very noticeable?)

[Mike Watterson]

As for adaptive carrier power, won't that give rise to audio compression in receivers with AGC? (Or is is just not very noticeable?)

I think The defunct since April 2023 RTE1 on 252 kHz used it and I think R4 on 198 kHz uses it, if so it's not noticeable.

I used to listen in to LW a bit in the Library, but the only serious LW listening I've done in last 20 years is in the car where some compression helps! I swapped the stock radio in my 2008 Yaris this year so as to have decent R4 LW. Some work with cutters, Dremel etc needed on removable facia. Adaptor to standard connections via cutting an old adaptor and cutting off end of existing cables and screw terminal blocks. At least aerial plug was the same kind and the old radio mount to car chassis plates amazingly fitted. Also now I get a 3.5mm audio in. Kenwood with VFD display, though display goes into "demo" mode and you have to hit a button. Some day I'll look for a manual.

Does it cause audio compression on sets with AGC? Which is nearly all. There was maybe a couple of Ever Ready 1950s battery valve sets with no obvious AGC at all, not even on the DK96 (apart from pre-1934 sets I have). I think it would depend on the AGC time constant and algorithm for adjusting TX carrier power. Any compressor in the TX / Studio audio path will have more effect?

Also how much is carrier power varied vs whatever local gain the AGC action has, which might have some compression action on long time constant even on conventional AM?

Too many known unknowns and some unknown unknowns and too few known knowns on too many different radio designs.