All digital Medium Wave transmission

With the approval from the FCC for all digital broadcasting on the Standard Broadcast (AKA AM, Medium Wave, Medium Frequency) band, it might be interesting to dissect Xperi’s HD Radio MA3 (HDMA3) standard a little bit. It might also be interesting to compare that to DRM30 which has been in use in many other places around the world for several years now.

First, I will dispense with the givens; HD Radio sounds better than its analog counterpart. I have also listened to DRM via HF, and that too sounds better than its analog counterpart. Of interest here is whether or not either digital modulation scheme improve reception reliability and coverage area. Medium Wave has a distinct difference from other frequency bands as it can cover vast areas. Something that has been dismissed in recent years as unneeded due to reduced maintenance schedules and the cost of keeping directional antenna systems in tolerance (thus increasing skywave interference).

Secondly; after reading several studies of HDMA3 and DRM30, I will concede that both systems perform betterAnnex E, Ref 2; Section III para C, Ref 6 than their analog counterparts in a mixed digital analog RF environment. Both systems have features which can be used to improve reception during night time operation. Skywave exists, whether or not people want it. If it is not desired as a reception mode, it still has to be dealt with from an interference perspective.

The two main complaints against Medium Wave broadcasting is perceived reduced audio quality (over FM) and interference. The interference comes in two flavors; electrical impulse noise and broadcast (co-channel and adjacent channel AM stations). Both are problematic. To some extent; both can be somewhat mitigated by an all digital transmission. However, if the interference noise becomes too high, the program will simply stop as the data loss becomes too great to reconstruct the audio program.

Of further interest here is the technical aspects of both systems and whether or not one would be superior to the other for Medium Wave broadcasting. I found this comment on a previous post to be particularly interesting:

DRM and HD both use OFDM, but the parameters are quite different, eg. the length of cyclic prefix which determines the performance in sky/ground wave interference are different by a factor of 9 (0.3ms vs 2.66ms). That is why DRM is much robust than HD.

http://www.engineeringradio.us/blog/2020/04/all-digital-am/

First of all, is this a true statement? Secondly, does the cyclic prefix make a difference in sky wave to ground wave interference? Which system might work better in a broadcast service where there are 4560 stations transmitting (as of 9/2020) and creating interference to each other? Finally, could the implementation of either system make a worth while difference in the quality and reliability of Medium Wave broadcasting in the US?

To answer these questions, I decided to begin with the technical descriptions found in the definitive documents; NRSC-5 D 1021s Rev GRef 1 for HDMA3 and ETSI ES 201 980 V4.1.1Ref 2 for DRM30.

There are many similarities between the two systems; both use COFDM modulation schemes, both have various bandwidth and data rates available, both use audio codecs that similar, both have some type of FEC (Forward Error Correction) system. I prepared a chart of these characteristics:

Feature/SpecHDMA3DRM30
OFDM subcarrier spacing181.7 HzVaries according to mode
Effective Data Rate, 20 KHz Channel40.4 Kbps30.6 – 72 Kbps
Effective Data Rate, 10 KHz Channel20.4 Kbps6.1 – 34.8 Kbps
Channel bandwidth10 or 20 KHz4.5, 5, 9, 10, 18, 20 KHz
CodecHDC-SBRHE-AAC, CLEP, HVXC
Operating Modes (QAM carriers and spacing)14
Protection Class (FEC)14
Features of HD Radio MA3 and DRM30

Both systems have 10 and 20 KHz channels available. This could be one feature used to mitigate adjacent channel interference, especially at night. In the US, physical spacing of transmitter sites helps prevent adjacent channel interference during the day. However, at night, half of the 20 KHz wide analog channel is in somebody else’s space and vice versa. Switching to 10 KHz mode at night would prevent that from happening and likely make the digital signal more robust.

DRM30 has additional advantages; multiple operating modes, protection classes and CODECs are available. Another advantage is the number of studies performed on it in varying environments; The Madrid Study,Ref 3 The All India Radio Study,Ref 5 Project Mayflower, Ref 4 and others.

Lets answer those questions:

  1. Are HDMA3 and DRM30 different? Yes, as the commenter stated, both use COFDM however, there are major differences in carrier spacing, symbol rate, and FEC. DRM30 has been designed at tested on HF, where phasing issues from multi-path reception are common. There are many configurable parameters built into the system to deal with those problems. My calculations of the Cyclic Prefix Length came out differently than those stated (I may have done it wrong), however, they are indeed different.
  2. Does the Cyclic Prefix Length make a difference in ground/sky wave interference? This is more difficult to answer. I would postulate that all of the configurable parameters built into DRM30 make it more robust. The various operating modes help mitigate phasing issues and the various protection modes help mitigate multipath reception issues. The only way to know that for certain is to do a side by side test.
  3. Which system would work better in high broadcast interference environments? Again, it is difficult to tell with out a side by side study. There have been numerous studies done on both systems; Madrid,ref 3 Project Mayflower, Ref 4 All India,Ref 5 WWFDRef 6 etc. In order to conclusively determine, one would have to operated HDMA3 on a station for a week, then DRM30 for a week on the same antenna system, with the same environmental conditions. Extensive measurements and listening tests would need to be performed during those tests.
  4. Is it worth it? Possibly. The big issue is the availability of receivers for both systems. Currently, only HD Radio receivers come as stock items in US automobiles. There are current and planned chipsets that have all of the digital radio formats built in (HD Radio, DRM+, DRM30, DAB/DAB+). If consumers want the service, manufactures will make the receivers. It would take a lot of effort to get this information in front of people and offer some type of programming that was highly desirable and available only on the radio. That is a big stretch.

Objectively comparing those two systems, I can see that both systems have advantages and disadvantages. There are some common items required for both systems; a reasonably well maintained transmitter plant, a newer solid state transmitter, and an antenna system with enough bandwidth so as not to distort the digital signal.

There are more receivers available for HD Radio, especially in cars. HD Radio MA3 is less configurable and therefore less likely to be misconfigured. There has been a lot of ink spilled in recent years about the declining number of radio engineers and the increased work load they are facing. Are there enough people with sufficient technical skills to implement and maintain even a basic all digital system? A topic for another post.

DRM30 is more flexible. Operating modes, protection modes and CODECs can be adjusted according to goals of station owners. There has been more testing done with all digital transmission of DRM30 using Medium Wave.

Are there enough reasons to allow a test of all digital Medium Wave DRM30 in the US?

Why not allow both systems and let the Software Defined Receiver decide?

References:

  1. HD Radio Air Interface Design Description Layer 1 AM Rev. G December 14, 2016
  2. Digital Radio Mondiale (DRM) System Specification, ETSI ES 201 980 V4.1.1 January 2014
  3. Digital Radio Mondiale DRM Multi-Channel simulcast, Urban and indoor Reception in the Medium Wave Band, Document 6A/73-E September 19, 2008
  4. Project Mayflower, The DRM Trial Final Report, BBC, April 2009
  5. Results Of DRM Trials In New Delhi: Simulcast Medium Wave, Tropical Band, Nvis And 26 Mhz Local Broadcasting, Document 6D/10-E March 28, 2008
  6. All-Digital AM Broadcasting; Revitalization of the AM Radio Service, FCC Fact Sheet, MB Docket Nos. 19-311 and 13-249, October 19, 2019

All Digital AM?

I have been reading, with interest, the saga of HD Radio on the AM (AKA Medium Wave) band. First question; if it goes all digital, will we still call it AM? Of course, there are other questions and concerns:

  • The proprietary nature of HD Radio, AKA MA3 or NRSC-5D as they are now calling it, is problematic. Xperi, the latest patent owner, currently (their word) has agreed to waive licensing fees for AM station owners who install their system. Is this a limited time deal for early adopters or in-perpetuity for all stations?
  • The NRSC-5D tests on WWFD, Fredrick, Maryland are hopeful, but as I pointed out before, it is one station with a well functioning antenna system. Many AM antenna systems are defective either in design or due to deterioration. Is the FCC going to start policing the AM band again to cure these self inflicted wounds?
  • Of course, the NAB wants zero oversight on the entire adventure. Under their proposal, small ownership AM stations would have a difficult time remediating interference issues from all digital co-channel stations be eliminating any required notification period, as proposed by the SBE.
  • The NAB also wants to nix a 1 Hz carrier frequency requirement, which would help with both the analog and digital interference issue, saying it would be too expensive. I disagree. In this day of universal GPS time keeping, it would be easy to implement this on all modern transmitters, especially if they were already installing an HD Radio exciter.
  • Denis Jackson’s Radio World Article states that reliable coverage can be had out to 0.1 mV/m. This seems very, very optimistic given that ambient electrical noise (non-broadcast related) on the AM band is at very high levels and still climbing. Further, once the all digital conversion starts, more and more co-channel digital interference will happen, likely cutting down that contour to a great extent. It works now, but may not work later. These types of statements seem naive or perhaps disingenuous. Again, WWFD is one digital signal in a vast ocean of analog carriers.

While I am skeptical of some of the statements made in various articles and comments before the FCC, I do believe that converting the Medium Frequency band to all digital will have benefits. The BBC DRM tests carried out in 2007 (The Plymouth DRM long term trial) show that digital on MF can work. DRM has been implemented in various countries with good results.

Getting rid of the hybrid IBOC/Analog is a step in the right direction.

My concerns are the small owners who are still making a go of it on AM. Those guys still doing community radio and serving the public interest. If they choose to wait, are they going to get buried under a digital dog pile and then have to pay the full license fee later? Something like that might be the end for them.

HD Radio in and of itself is not the panacea for the AM band. Other things have to happen to make it work right. The SBE speaks extensively about ambient noise on the MF band. They are entirely correct. In addition, there are many, many AM stations that do not have compliant antenna systems. There are stations operating a DA-2 system full time on the night pattern. There are stations operating a DA-2 full time on the daytime pattern and power. There are stations that are supposed to turn off at night, which stay on 24/7. There are stations not reducing power to night time levels. The list goes on. Simply putting digital carriers on everything will not reduce the station-to-station interference, especially at night.

I am cautiously hopeful that the FCC will look into the ambient noise problem, which simply cannot be over emphasized. They would also need to re-invigorating the Enforcement Bureau. Since they closed down most of their field offices, it has been kind of a free for all out here.

GatesAir FLX-40 one year in

I was at the WEBE transmitter site recently and took the time to look over transmitter we installed last year:

GatesAir FLX-40 transmitter, WEBE Bridgeport, CT
GatesAir FLX-40 transmitter, WEBE Bridgeport, CT

Overall, I would say that this transmitter has been very reliable.  We had to install a UPS for the exciter and HD Radio exporter, but that is not a big deal. During the first power outage, the exciter went dark first. It took longer for the transmitter controller board to lose power, in the interim the controller turned the transmitter power all the way up. When the generator came on line 10 seconds later, the transmitter returned to operation at 41.5 KW. This, in turn, caused one of the other field engineers to freak out and nearly lose his mind (stay away from the brown acid, FYI).

I installed the UPS a few days later.

WEBE TPO 35.3 KW with HD Radio carriers on
WEBE TPO 35.3 KW with HD Radio carriers on

Transmitter power output is 35.3 KW, which is getting into the semi-serious range. The reflected power goes up when it gets warm out and goes down in colder weather.  Over the winter, it was running about 50 watts.  Even at 138 watts, that represents 0.004% reflected power. The TPO forward goes to the 6 bay, 1/2 wave spaced antenna side mounted, 470 feet (143 meters) AGL. The station covers pretty well.

WEBE Pump station
WEBE Pump station, pump is running 2/3 speed and fans are running at about 1/2 speed

Overall, I would give the liquid cooling system an A grade. The transmitter still dumps a fair amount of heat into the room from the RF combiners and PA power supplies. Most of the heat, however, ends up outdoors. Previously, we had two Bard 5 ton AC units running almost full time. Now, only one AC unit cycles on and off except for the hottest days of the year. Outside temperature when this picture was taken was 81 degrees F (27.2 C).

Next year, we will have to send a sample of the coolant off to be analyzed.

Gates FLX-40, WEBE Bridgeport, CT
Gates FLX-40, WEBE Bridgeport, CT

I have had good experiences with the GatesAir FLX/FAX series transmitters. I would recommend this to a friend.

Status of AM revitalization

It has been about five years since the AM revitalization initiative was first proposed by the FCC and about five years since the first rules changes took place.  Those rules changes included:

  1.  FM translators for AM stations
  2. Allowing stations to use MDCL (Modulation Dependent Carrier Level)
  3. Changing some of the antenna radiation efficiencies requirements
  4. Changing some of the allowable interference towards other stations requirements
  5. Loosening some rules regarding proofs, MOM, night time coverage over city of license, etc

Things that were not addressed:

  1. Receiver quality and technical advances
  2. Ambient noise levels on Medium Frequency (among other) bands
  3. HD Radio or any other digital modulation scheme

Things that were discussed then changed subsequently as a separate initiative:

  1. The main studio rule, which was eliminated for all broadcasting stations

What has been the net effect of these changes?  Has any of this revitalized AM radio?  The net effect has been approximately more of the same.  There have been many stations that have applied for and received licenses for FM translators.  Those stations, in most cases that I am aware of, receive some benefit of extra revenue because of this.  Stations with carrier power levels of 10-50 KW have taken advantage of MDCL technology to save some money on their electric bill.  Nothing wrong with that.

For stations that use a directional antenna, proofs of performance and other DA matters with the FCC have become slightly easier.  Medium Frequency (MF) directional antennas are very large, require a lot of land, are expensive to build, license and maintain.  I know of several stations which have downgraded from a class B station with a directional antenna to a class D station with a single tower and greatly reduced night time power.   Those downgraded stations certainly benefit from an FM translator.

I have heard from more than one AM station owner who says after four years, they are going to “turn in their AM license and just keep the FM.”  I am sure that they are not informed regarding translator rules.  Perhaps, however, the FCC will allow this in the future; a sort of back door commercial low power FM station classification.

The AM band zenith occurred in November of 1991, when there where 4990 licensed AM stations in the United States.  As of June 30, 2018, the total stands at 4633.  That is a decline of 357 stations.  There are currently 90 AM stations listed as silent.  That represents a decline of approximately 9 percent or less than 1/2 of one percent per year.

The last number of AM stations actually transmitting HD Radio that I found was approximately 110, which differs from the iBiquity (and FCC) number of 240.  The FCC data base includes stations which are currently dark, or stations which where transmitting HD Radio at one time but have since turned it off.  Either way, it is a small percentage of licensed stations.  As of this time, AM HD Radio appears to be a non-starter.  In other parts of the world, Medium Frequency DRM seems to be doing well.  The difference seems to be that the DRM operation is all digital and the digital carriers have a much higher power level than that of the hybrid AM HD Radio being used here.

Of those 4633 standard broadcast stations, approximately 260 belong to iHeart radio, Cumulus owns approximately 120 and Townsquare owns approximately 80.   That accounts for 460 stations.  The remaining 4000 or so stations currently on the air are owned by medium sized corporations or individual owners.  The reason for the distinction; I have noticed that the large corporate owners tend to concentrate resources and effort on those licenses that will make the best return, e.g. FM stations.  Of course, there are a few exceptions to that trend, often in major markets.

Of those 4000 or so remaining AM stations, most seem to be treading water.  They are making enough money to stay on the air.  There are a few AM stations that are doing remarkably well.  Those are the ones with primarily  local content.  The vast majority of AM stations are running some type of syndicated talk.  News/talk and sports radio are the two most common formats.  Conservative news/talk seems to be the bread and butter.  Liberal news talk has been tried, but none have succeeded.

Last May, the Supreme Court overturned the Professional and Amateur Sports Protection Act of 1992.  That federal law prevented gambling on outcomes of professional and college sports games.  With the overturn of that rule, individual states can now legalize sports betting.  It will be interesting to see what states allow legalized sports gambling and whether that has any effect on the various sports radio formats.  I can see where individuals and odds makers may want to get good inside information regarding team dynamics and so on.  The sports network that can furnish such information may be in a good position to carve out a niche.

Music can and does sound good on AM when it is done correctly.  There is a great misconception that AM fidelity is poor.  That is not necessarily so.  There are a good many AM receivers these days which have much better bandwidth than the previous generation receivers.  I am noticing that car radios in particular sound much better.  Yes, there are still problems with electrical noise and night time interference.  There are still technological improvements that can be made for analog AM on the receiver side.

In summary; the revitalization efforts have benefited some AM stations in some areas.  The truth is, that many AM stations have been let go for so long that there is no saving them.  Other AM stations that are still viable are making a go of it.  In nautical terms; there is six feet of water in the hold, the pumps are working and the ship is not sinking… for now.