• Get involved.
    We want your input!
    Apply for Membership and join the conversations about everything related to broadcasting.

    After we receive your registration, a moderator will review it. After your registration is approved, you will be permitted to post.
    If you use a disposable or false email address, your registration will be rejected.

    After your membership is approved, please take a minute to tell us a little bit about yourself.
    https://www.radiodiscussions.com/forums/introduce-yourself.1088/

    Thanks in advance and have fun!
    RadioDiscussions Administrators

Increasing the power of one sideband - how would it sound?

radiofan2023

radiofan2023

This RadioInsight article says the FCC just approved a petition allowing FM stations to increase the power of one sideband without having to increase the power of the other sideband, on an HD station. If they do that, will the station sound better/clearer, or will the signal just be able to be received in more buildings and the audio will sound the same to the average listener?

radioinsight.com

FCC Report 9/29: Potential St. George Move-In Shut Down For Circumventing Reallotment Procedured

FCC Actions The FCC has approved a 2019 petition filed by Xperi, NPR, and the NAB to allow FM stations
radioinsight.com radioinsight.com
 
It's not going to sound any different. Digital data is digital data. It either decodes or it doesn't. But because it's the same data in both the upper and lower carriers (they're not really "sidebands"), as long as you can decode one or the other, you'll get the HD data.

Asymmetrical power means you can use more power to push one set of carriers out when the other set would cause interference. Let's say you're WAWZ 99.1 in NJ - you can't put too much power into the lower carriers because you're short spaced to WUSL 98.9 in Philly, but you can increase power in the upper carriers with no issue.

Before, both sets of carriers would have been limited to a low power to protect WUSL. Now, the power on the upper carriers can be increased. And so now you might be able to lock those upper carriers closer to Philadelphia, increasing your HD coverage area.

It's important to note asymmetrical power isn't new. Stations have been allowed to use it for a few years. This FCC ruling just streamlines the process - they can be licensed with asymmetrical power instead of having to reapply every six months for experimental STAs.
 
Thanks for explaining! Would a station increase the bitrate of the HD station to take advantage of the increase in power, or not necessarily? My thought is that more power would also mean it would be easier to push more data, at a higher bitrate. Or would there be no advantage to increasing the bitrate?
 
Does it make sense now? Think about it this way: you could have a class A 6 kW station and a class C 100 kW station in the same market both using HD. Different power levels, but the same bitstream on both, right?

Same thing here - the only difference is that in this case the power levels are different on the upper and lower carriers of the same station.
 
fybush said:
Does it make sense now? Think about it this way: you could have a class A 6 kW station and a class C 100 kW station in the same market both using HD. Different power levels, but the same bitstream on both, right?

Same thing here - the only difference is that in this case the power levels are different on the upper and lower carriers of the same station.
Click to expand...
But the total power on each of the HD signals stays the same? That’s what I’m unclear about. This section makes it seem like changing the dBc would lead to a change in the total power:

“many more digital FM stations could increase power on at least one sideband above the current limit of -14 dBc with 6,120 of 10,875 digital FM stations studied able to increase power on both sidebands to -10 dBc under the current rules with an additional 3,496 stations could increase one sideband to -10 dBc if asymmetric sidebands were allowed.

As part of the grant, stations will need to notify the FCC of asymmetric operation indicating the digital ERP of of the upper and lower digital sidebands as well as the total digital ERP.”
 
Here's how to understand that:

dBc is just shorthand for "decibels below carrier power."

The original HD standard was -20 dBc, which equates to 1% of analog power. A station running 50 kW analog power would have 500 watts of power for its digital carriers, and yes, that's in addition to the analog power.

The FCC now allows -14 dBc routinely for most FM HD stations, which is about 4% of analog power, so a 50 kW FM station would have 2000 watts of digital power at that level.

With proper interference exhibits, the maximum power level now allowed for FM HD in the US is -10 dBc, or 10% of analog power. That's 5000 watts for your 50 kW station, and at that level most stations are finding that their HD coverage extends out to their primary analog coverage with solid signals, give or take adjacent-channel interference issues.

These numbers are all for symmetrical carriers - at -10 dBc, your 5000 watts of HD power is divided evenly between the upper and lower carriers.

With asymmetrical power, you could be -10 in the upper and -14 in the lower, for instance. Your 50 kW analog FM station in this example would have about 3500 watts of digital power, with roughly 2500 going to the upper and 1000 going to the lower.

So, yes, it's a change in the total power, but the idea is that if radios on the fringes can still hear those higher-powered upper carriers, even if they can't hear the lower carriers, the data is still there to decode the data and deliver a usable HD signal in some areas where no signal would be usable if it were a symmetrical signal at -14 on both sets of carriers.

Because of the extra power that goes into the HD carriers, we're all learning to spec transmitters with enough headroom to deliver more power than would have been required in the analog days.

I just did an HD install at WDKX here in Rochester, a class A station that's height-derated to 800 watts ERP. With the gain from a 3-bay antenna, our TPO was 863 watts analog, which was easy to make from a transmitter rated at 1000 watts.

But going to -14 dBc HD operation, we couldn't get by with a 1 kW transmitter anymore, because we need to produce about 35 watts of digital power and because transmitters are less efficient in HD mode. So we bought a 2 kW transmitter, which gives us plenty of headroom for both digital and analog operation.

That make sense?
 
Yes thanks for the examples. So it sounds like if stations want to take advantage of the change on HD, they may also end up needing or choosing to buy a new transmitter?

What’s the maximum amount of dBc an HD station could have in total on a super powerful station like WBCT? I mean mathwise based on their total analog power. As you probably can guess, I’m bad at math.
 
dBc is a ratio, not a power limit. The -10 dBc maximum means no station can have digital power that's more than 10% of its analog power.

For grandfathered superpower stations, however, the -10 dBc maximum doesn't apply. They are instead limited to -14 dBc (4% of analog power), though the FCC will consider exceptions under STA with a showing.

That would be 12.8 kW for WBCT *if* it's actually operating at -14. I can't tell if it is.

You are correct that increasing digital power from -20 to -14 or -10, whether symmetrical or asymmetrical, will often involve the need for a bigger transmitter (and sometimes combiner and/or filters at a shared site). That's why many stations that could increase HD power above -20 haven't automatically done so.
 
So, stations can now take advantage of the -10 dBc HD power increase? If so, it seems like there might be a few here in DFW that have done that. I can’t tell on the Cedar Hill tower farm stations, because I’m about 7 miles from there, but it seems like 89.7 KAWA is coming in quite a bit stronger at my location in Grand Prairie.
 
chriscervantez said:
So, stations can now take advantage of the -10 dBc HD power increase? If so, it seems like there might be a few here in DFW that have done that. I can’t tell on the Cedar Hill tower farm stations, because I’m about 7 miles from there, but it seems like 89.7 KAWA is coming in quite a bit stronger at my location in Grand Prairie.
Click to expand...
The FCC has allowed -10 dBc operation for a while now. It requires interference showings and an application, unlike -14, which just requires notification to the Commission.
 
fybush said:
The FCC has allowed -10 dBc operation for a while now. It requires interference showings and an application, unlike -14, which just requires notification to the Commission.
Click to expand...
Thanks for the info! I wasn’t aware that this was allowed yet. 🙂
 
Status
This thread has been closed due to inactivity. You can create a new thread to discuss this topic.


Back
Top Bottom