Modulation Dependent Carrier Level Technology (MDCL) OK'd by the Media Bureau

[tomservo]

Saw this come across the digest the other day. The Media Bureau has given the OK to use technology like Adaptive Carrier Control to reduce power consumption by AM stations.

Will anyone take advantage of this? What does it mean for the end user (listener)?

 

[Lazy J]

Maybe I don't understand the technology... But it seems like if the carrier drops out when you have no modulation, you would get a lot of noise during quiet times. Say your running a talk format and the host pauses for a second or so, does the carrier drop to almost nothing? If so, the listener would get white noise, especially distant listeners.

Correct?

 

[frankberry]

It's nothing more than controlled positive carrier shift.
The upside is decreased power consumption.
The downside is increased impulse noise during periods of low modulation.
It won't be white noise, it will be static and adjacent channel hash.

 

[Kmagrill]

It's being used extensively at some test stations in Alaska. According to their group CE, they have reduced the AM transmitters' power bills by almost half with no observed reduction of service nor increased interference. I don't think it would be very compatible with AM IBOC, or AM stereo, though if that matters to anyone.

 

[Tom Wells]

It's being used extensively at some test stations in Alaska. According to their group CE, they have reduced the AM transmitters' power bills by almost half with no observed reduction of service nor increased interference. I don't think it would be very compatible with AM IBOC, or AM stereo, though if that matters to anyone.

In Alaska, this will work. Too much background noise in the "more populated areas".

This is old fashiioned carrier control. It always plays hell with receiver AGC response.
Amatuers tried to use this in the 30s and 40s. It can sound great under conditions of good signal to noise ratio
when you have a receiver that allows you set a manual RF gain with no AVC feedback.
If you don't have such a receiver, the normal AVC RF control will cause volume surging and ducking as
it attempts to counter-act the "savings" you have squeezed out of the power "wasted".

The same thing happens when radios with poorly designed AVC try to decode the signal of an AM with 150% pos modulation.
The audio volume gets strangely mangled when the RF level varies and the AVC time constant is too short.
Sounds like a really confused and poorly adjusted compressor/leveler at the station.

If interested I think I have some writngs about it by Frank Jones from 1936. I think I just paraphrased the summary.

 

[rfry]

From the online sources referenced in the FCC Notice, the carrier will not drop by more than 6 dB maximum for low/no modulation -- so a receiver will not see a huge noise increase then. Also the action of the AGC in the receiver will tend to increase the output from the AM detector as the carrier is reduced, which will help mask whatever the added r-f noise is at that time. Supposedly the attack/decay times for carrier level control are compatible with the r-f/i-f AGC characteristics of most AM receivers.

Reportedly tests done in Europe have shown this technique to produce negligible side effects for most listeners (see the slideshow included in the pdf document linked below).

http://transition.fcc.gov/Daily_Releases/Daily_Business/2011/db0912/DOC-309538A1.pdf

 

[w9wi]

It's being used extensively at some test stations in Alaska. According to their group CE, they have reduced the AM transmitters' power bills by almost half with no observed reduction of service nor increased interference. I don't think it would be very compatible with AM IBOC, or AM stereo, though if that matters to anyone.

FCC release says it's compatible with IBOC. Quoting Tuesday's release:

The National Radio Systems Committee has recently convened a subcommittee to investigate the effects of MDCL technologies on the hybrid IBOC AM signal, especially at the receiver. Initial tests by manufacturers have demonstrated that MDCL operation is compatible with hybrid IBOC transmission at the transmitter, including full compliance with the AM IBOC RF spectral mask requirements. Tests of the compatibility of MDCL with hybrid AM IBOC on various types of receivers are underway. We will permit AM stations broadcasting in hybrid AM IBOC mode to implement energy-saving MDCL technology provided the hybrid signal continues to comply with spectral emissions mask requirements in Section 73.44,7 and also provided that the relative level of the analog signal to the digital signal remains constant.

(http://transition.fcc.gov/Daily_Releases/Daily_Business/2011/db0913/DA-11-1535A1.txt)

It would seem to me self-interference would increase during periods of low modulation.

 

[BobOnTheJob]

Hams used this on a few AM rigs into the 1960's...not to save energy, but to save the expense of a high level plate modulated stage. IIRC, they called it Screen Modulation. Seems some of the lower end Heathit rigs used it. Good luck reading base currents using this one!

 

[Tom Wells]

Carrier control sounds weird on most radios, which is why it was left on the trash heap by amatuers 60 years ago.

If you have a tin ear and like 24 kbps bitstream audio, then, no, you won't hear anything "wrong"
with the audio from such a station.

And water and chalk in the milk is just fine.

It sounds like a great idea, but how it ACTUALLY sounds is another thing.
I've experimented with this. You can't avoid the interaction between RF level and audio level.

Saying it works well with "most radios" most likely means it will only sound bad on radios which
already have poor sound on AM. This is the same method used by ibiquity in promoting AM iboc.
Suggesting that the sidebands are inaudible means something along the signal path is deaf.
Likewise, suggesting the carrier-controlled decoded audio will not "pump" also would require a specific sort of deafness.

 

[W1DAN]

Hi:

Brown Boveri used this in the 1980s on their high power shortwave transmitters (no not screen modulation). WSHB shortwave used it. It worked well, no sonic side effects, except for the S-meter bouncing a bit.

With today's ability to delay audio cheaply, this could actually work better where the carrier would come back up from it's 6db drop right before the beginning of a word. Yes, and a sloppy AGC may cause some pumping.

Dan

 

[BobOnTheJob]

Regarding the pumping, find a station that stays on the same pattern but with different power levels. A station that drops from 5KW to 1KW (with no carrier drop) should come quite close to demonstrating how a given receiver AGC will act at a 6db carrier level drop. My guess is that it won't be heard. A 6db power cut results in a 3db field strength cut...signals vary far more than that just driving down the highway & most receivers handle that with grace. And 3db of field strength is going to be pretty hard to detect unless you're way out in the boonies IMHO. This technology is far less hair brained than HD Radio's equivalent of throwing endless empty fast food wrappers into the other driver's lane.

 

[boiseengineer]

http://www.rwonline.com/article/alaska-test-produces-lower-am-power-costs/23169

 

[Tom Wells]

Yes, I have heard stations drop/raise power level during music and heard how various radios handle the
variation. If you are willing to sound worse to a larger number of listers, fine.
Local listeners will probably not notice anything, agreed.
Those a bit further out, in the "medium signal/distance range" will hear the signal's noise level increase,
and it will sound to them as though they are now farther away from any such signal.
Those in the fringe or dxers will find 6db more of noise and the odd non-linear volume expander effect.

There's already enough trouble for the AM from non-compliant pt 15 devices, etc.
I suspect this is another idea hatched by the secret society for the destruction of AM.

The use of delay to avoid the surging is adding more cost/complication.

If you can't afford to produce full quieting, why take it out on the listeners?

It is easy to demonstrate such an idea where background noise is so low, and it's deceptive to
use such a location and say the results will extrapolate to everywhere else.

Heck, why not convert everything to single sideband supressed carrier if you want to save even more money.
Why waste money on a frequency reference for the listener? Let them inject their own demod "carrier".
No one will notice pitch shifts of 100 hertz, right?
And everyone agrees that a good sideband rig sounds much better than full AM :.

 

[[email protected]]

So this new technology saves energy thus reducing the electric bill. Once again AM radio is being driven by the bottom line rather than trying to improve the product for its consumers.

Clearly AM radios "innovators" haven't got a clue what the audience really wants. It's been downhill since the 70's.

 

[RadioFan2J3]

The use of delay to avoid the surging is adding more cost/complication.

Normally the transmitters, or rather modulations systems capable of CCM already have the delay circuits in the modulation control circuits, for that very purpose.

Continental manufactures solid state modulators for higher power tube type AM transmitters, capable of CCM.

http://www.contelec.com/swmodulatorfeatures.htm

Harris has this feature in all of their higher power DX series MW transmitters, especially any system using the 200 kW power blocks. I am not sure if this was an option or all of the Harris transmitters capable of the Harris version, Dynamic Carrier Control.

If you can't afford to produce full quieting, why take it out on the listeners?


Heck, why not convert everything to single sideband supressed carrier if you want to save even more money.
Why waste money on a frequency reference for the listener? Let them inject their own demod "carrier".
No one will notice pitch shifts of 100 hertz, right?
And everyone agrees that a good sideband rig sounds much better than full AM :.

If the modulation level is kept up, and there aren't pauses, the carrier power does stay up near the nominal carrier power of the transmitter.

As for SSB and audio quality, back in the days of AT&T and network audio distribution, those 5 kHz programs channels were SSB on the telco frequency division multiplex carrier systems. I worked with a 14 channel SSB multiplex program channel system that was good up to about 7 kHz. I will admit that I don't have any direct knowledge or experience with 15 kHz program circuits that were on the old carrier systems.

100 Hz shifts, Someone with a decent musical ear can hear shifts much lower in frequency than that.

 

[RadioFan2J3]

From the online sources referenced in the FCC Notice, the carrier will not drop by more than 6 dB maximum for low/no modulation -- so a receiver will not see a huge noise increase then. Also the action of the AGC in the receiver will tend to increase the output from the AM detector as the carrier is reduced, which will help mask whatever the added r-f noise is at that time. Supposedly the attack/decay times for carrier level control are compatible with the r-f/i-f AGC characteristics of most AM receivers.

Reportedly tests done in Europe have shown this technique to produce negligible side effects for most listeners (see the slideshow included in the pdf document linked below).

http://transition.fcc.gov/Daily_Releases/Daily_Business/2011/db0912/DOC-309538A1.pdf

One important thing to note is that the BBC's version, Amplitude Modulation Compounding (AMC), operates inverse of the Continental CCM and Harris DCC. With both the CCM and DCC, the carrier power drops back to the carrier cutback level during periods of no modulation. The BBC's AMC brings the carrier level of the transmitter back up during modulation pauses.

Continental's J. Fred Riley did a presentation on the subject of comparisons of CCM techniques at the IEEE Broadcast Symposium in 1994. I've also seen a white paper written by Continental's Bryan Weaver on the use of variable carrier level techniques in high power AM broadcasting.

 

[RadioFan2J3]

This is old fashiioned carrier control. It always plays hell with receiver AGC response.
Amatuers tried to use this in the 30s and 40s. It can sound great under conditions of good signal to noise ratio
when you have a receiver that allows you set a manual RF gain with no AVC feedback.
If you don't have such a receiver, the normal AVC RF control will cause volume surging and ducking as
it attempts to counter-act the "savings" you have squeezed out of the power "wasted".

The same thing happens when radios with poorly designed AVC try to decode the signal of an AM with 150% pos modulation.
The audio volume gets strangely mangled when the RF level varies and the AVC time constant is too short.
Sounds like a really confused and poorly adjusted compressor/leveler at the station.

If interested I think I have some writings about it by Frank Jones from 1936. I think I just paraphrased the summary.

I'd think you would have more variation in audio levels with a receiver on manual AGC because the varying carrier level with modulation would have far more effect than in a typical receiver with AGC turned on.

Most, if not all, of variable carrier power modulations systems have fast attack times to bring the carrier power back to the proper level for the audio, yet have a slower release time, slower on dropping the carrier power back, having a hang time for modulation pauses. Riley does note that there may be audible ticks when the carrier level is increased in anticipation of sudden audio increases, but he also comments the ticks are masked by the audio that caused the carrier to increase.

My casual monitor of a high power AM transmitter running a Continental Solid State Modulator in CCM mode using a few year old Jeep radio in the work vehicle doesn't show that AGC issue. Does it sound as good as if the transmitter were in straight AM? I don't know. Looks like I might have to do some experimentation and see if I can tell if the TX is running CCM or full carrier mode, simply by listening.

Numerous high power transmitters have been running various modes of variable modes of carrier level control, both MW and HF frequencies, since the mid-1990s, but granted, most of these transmitter operations are not intended for local listeners. The concerns with audio issues might be different for listeners within the primary daytime coverage area of an AM transmitter.

 

[Tom Wells]

Yes, pitch shifts of only a very few cycles are bothersome to me.

It certainly is possible to create a modulating scheme that is "transparent" or seemingly linear in behavior and sound.
Even for something such as controlled carrer.

The point I wished to make is that without standardized timing of AVC circuits, it's going to play differently with all radios.
And some will sound awful. Whether music is aired is important, too.
If there's never musical content, it should be "just fine" and sound like a very high quality CB signal
on a big "linear" amp.
If there is musical content, it's not a question of whether it's going to mangle the sound, it's a question of how much.

I have some radios here that sound fantastic with 200% positive AM asymmetric audio.
Then there are a few where the audio gets all muzzy and clumpy, and sounds like a there's a leaky old wax/paper audio coupling capacitor, which it can't be cause they're the newer radios, not the old tube sets.

Just like dolby systems, it would require a carefully crafted, coordinated encoding/decoding/timing response scheme to be
adopted by all. Engineering isn't done that way anymore.

I don't trust modern engineering to understand or respect analog properly.
Digital development for quite few years seems hell-bent on NOT understanding or respecting analog behavior.

 

[RadioFan2J3]

Yes, pitch shifts of only a very few cycles are bothersome to me.

Years back, in my receiver operating days, we monitoring an ERT transmission and the audio circuit between the studio and the transmitter site appeared to be on an FDM system and there was a frequency error of a few hertz along the channel. I could offset it with our receiving equipment, but the old RCA ISB receivers were not stable enough to leave it there. No one at the customer end of the circuit ever complained, so I did what all of the other guys did, just let the RCA lock to the carrier and then it didn't drift anymore.

It certainly is possible to create a modulating scheme that is "transparent" or seemingly linear in behavior and sound.
Even for something such as controlled carrer.

The point I wished to make is that without standardized timing of AVC circuits, it's going to play differently with all radios.
And some will sound awful. Whether music is aired is important, too.
If there's never musical content, it should be "just fine" and sound like a very high quality CB signal
on a big "linear" amp.
If there is musical content, it's not a question of whether it's going to mangle the sound, it's a question of how much.

I will agree with you, there are concerns with audio quality with the various types of variable carrier level techniques. I cannot comment on the comparisons to CB radio, as it has been far too many yars since I've had any involvement with CB, something on the order of 45 years.

I do not think the AGC timing has to be any more standardized to meet the variable carrier control systems timing, no more than radios have to contend with signal strength variations, especially for radios in vehicles. But the concern has not been avoided.

some radios here that sound fantastic with 200% positive AM asymmetric audio.
Then there are a few where the audio gets all muzzy and clumpy, and sounds like a there's a leaky old wax/paper audio coupling capacitor, which it can't be cause they're the newer radios, not the old tube sets.

Just like dolby systems, it would require a carefully crafted, coordinated encoding/decoding/timing response scheme to be
adopted by all. Engineering isn't done that way anymore.

I don't trust modern engineering to understand or respect analog properly.
Digital development for quite few years seems hell-bent on NOT understanding or respecting analog behavior.

Understand the concerns, but I am sure you do understand there is a difference between the variable carrier level modulation techniques and 200% asymmetrical modulation. Nothing in any of the variable carrier level modulation techniques requires any asymmetrical modulation at all, they only adjust the carrier level based on the modulation conditions.

As for modern engineering and digital vs analog, I would agree with you to a point. One of the major constraints with digial audio transport is the bandwidth required to handle uncompressed digital audio, as compared to the bandwidth to carry the audio in the analog domain. The bean counters had problems paying for the bandwidth and therefore we have the numerous compressed audio schemes.

Bottom line, the analog folks worked to perfect analog audio systems and the digital people take that analog audio (or video, for that matter) and work for the perfect digital transport mechanism, then they start taking that digital stream down to the point where only the better ears and eyes complain and then stop there. Kind of like Muntz enginnering, remove a part 'til it quits and then put that part back in, and then send the design off to production.

 

[RadioFan2J3]

The point I wished to make is that without standardized timing of AVC circuits, it's going to play differently with all radios.
And some will sound awful. Whether music is aired is important, too.
If there's never musical content, it should be "just fine" and sound like a very high quality CB signal
on a big "linear" amp.
If there is musical content, it's not a question of whether it's going to mangle the sound, it's a question of how much.

Either my ears are turning dead, or the Continental Solid State Modulator systems does a pretty fair job between CCM and straight AM. Coming into work this morning, I asked the guys to switch between CCM and AM and the audio sounded the same. I do not believe with an A/B blind test I could tell the difference and I suspect most listeners could not either.

Program content during my quick tests was all music, as there simply isn't much talk, other than a few news breaks.

Granted, this was just a quick and dirty test, while driving, vehicle speakers and typical road noise.

We use an Orban 9200 for processing and the modulation levels stay up quite high.

I would think if there were real problems with receiver AGC time constants, it would be far more an issue with voice programming and not music, considering the degree most audio is processed for high modulation levels. I have noticed on some of the announcers some artifacts, but I've not had the chance to listen to the feed audio to see if it is truly transmitter -induced artifacts due to the CCM or something more associated with the audio transport path or studio isses with certain announcers.

We never know where the studio source is located, because for us, at the transmitter, all the audio comes from a central point.

Sure would be nice, Tom, if you were within ground wave range and we could run some quick tests between regular AM and CCM.

73