Asymmetrical AM (Amplitude Modulation).

[colly0410]

I know that Radio 1 used to transmit at more than 100% modulation using asymmetrical AM on some of the TX's on it's single frequency 247 metres network, (I started a thread on this subject on a now defunct site) I understand this was to try & improve reception in mush zones. Are there any TX's still using this method? Did Radio Luxembourg do this?

[colly0410]

Been thinking (I know, my brain could explode if I think too much) did TV systems A, C, E or F ever us asymmetrical modulation on their AM sound carriers?Just a random thought that came into my bonce...

[ppppenguin]

No. There would be no need. for SSB or VSB. The sound took up so little bandwidth compared to vision that there would be no need to use anything other than ordinary double sideband AM with carrier. There was also ample space for FM, as used on most systems.

The audio BW of System A was over 10kHz and hence capable of better sound quality than ordinary radio broadcasting.

[BusyBee]

The audio BW of System A was over 10kHz and hence capable of better sound quality than ordinary radio broadcasting.

Is that the reason why TV sound has always, at least up until digital TV, seemed far better than radio (at least to me)?


Tracy

[ppppenguin]

The TV sound channel (whether AM or FM) has always been capable of full bandwidth audio. FM radio is similar. AM radio on MW and LW is of course far worse.

Most TVs have small speakers and hence relatively poor sound quality.

Much broadcast sound, whether on TV or radio, is pretty mediocre. The best is excellent - think of concerts on Radio 3 or properly produced drama. Much radio is the victim of loudness wars.

[Kalee20]

What's asymmetric modulation, as in the title of this thread?

(I've heard that some stations have varying carrier power, so that during quiet passages the power is reduced but the % modulation is raised to more fully modulate the reduced carrier, thus giving the same output from the receiver - AGC system assumed not to influence this of course!)

The audio BW of System A was over 10kHz and hence capable of better sound quality than ordinary radio broadcasting.

Yes I can remember VHF TV sound, it was as good as FM radio.

[ppppenguin]

I assume that "asymmetric" is being used to mean vestigial sideband, as used for the vision signal.

Reducing AM carrier power during quiet passages is routine for LW/MW/SW transmitters. Saves on the electric bill. https://en.wikipedia.org/wiki/Dynamic_carrier_control

Reducing the electric bill is a big consideration for AM broadcasters. Doherty modulation was an early approach: https://en.wikipedia.org/wiki/Doherty_amplifier

[Mike Watterson]

The logical ultimate vestigial sideband AM was to use SSB with an additional reduced carrier instead of suppressed carrier. Maybe only much used 1.7MHz to 6MHz marine radios.

On that thought why did stereo on FM use double sideband suppressed carrier at 38 KHz (about +/- 16 kHz max) for the L-R and thus a 19 kHz pilot to aid detection. Why not say 24 KHz USB (for the L-R) with a "pilot tone" carrier. Easier to demodulate and less bandwidth?

[colly0410]

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. Read somewhere that Radio 1 used this system for some of their AM transmitters, presume it had a brick wall limiter on the negative part of the carrier to stop it going below 0%...

[Mike Watterson]

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. Read somewhere that Radio 1 used this system for some of their AM transmitters, presume it had a brick wall limiter on the negative part of the carrier to stop it going below 0%...

That would be asymmetric distorted audio.

No, what they do is vary the carrier power. The actual modulation stays symmetrical, as mentioned earlier.

In terms of coverage and a Public Service remit, nothing yet actually replaces AM. Closing AM is political more than a question of absolute costs.

Though "bean counters" wanting to close AM to save money, claiming that Streaming, or FM, or DAB or satellite provide the alternative are not looking at true cost vs area served and ease of access (Streaming & satellite may be good "coverage" but poor access), nor running cost for listener. Nor full picture. RTE closed MW long ago claiming no-one wanted it and closed LW RTE1 last spring citing high running costs. These were 0.1% of the top presenters' payments (many are paid via companies as a tax legal "fiddle" and to look better on salaries list). Low hanging fruit for "bean counters". RTE streaming falls over in Ireland if it's anything popular. The only cheap mass market delivery is satellite, but it's not very portable, rarely more than two rooms, rarely without the TV and doesn't actually have the coverage of LW. Tall buildings, valley, trees, apartment not allowed dish or wrong side etc. Still, Irish Bean Counters are technology agnostic, they closed DAB about April or May 2021 citing duplication (the few extra RTE channels, and it only had no local or regional radio) are on DTT (which is about 98% geographic vs 46% geographic for DAB) and Satellite. The main channels that people actually listened to are about 98% coverage on VHF-FM. DAB was costing more than RTE 1 LW, which was also covering most of England & Wales. Interference from Algeria in some areas.

Meanwhile on SW you can easily get Radio China in English.