A bit of good news?

We were doing some overnight maintenance on one of the class A AM’s in New York the other night. The aged Automatic Transfer Switch on the electrical service entrance needed to be replaced, thus the power to the entire facility needed to be cut while the old switch was removed and the new switch installed.

During this period, we took the opportunity to do some maintenance on the main and aux towers. All went well. We also notified the National Radio Club that the station was going to be off the air so that their members could log some rare DX. My thought process here was that we might also find a few daytimers who were still on the air or a DA night who was operating with their daytime facilities. A quick look at MW list shows that there are several such stations on 770 KHz:

MW list, North American 770 KHz

Alas, the answer was no, nobody was on the air who should not have been. Reports from Cape Cod, Massachusetts; New Foundland, Canada; Manassas, Virginia; West Union, South Carolina; and south west, Ohio have Cuban and South American stations on the air (Radio Artemisa, Radio Rebelde, Radio Oriental) but all of the east coast daytimers are off.

The 180 degree main mast for WABC is in good shape. You can deride AM and say it is outdated. However, it still gets out and covers vast distances.

Resurrection of a different sort

I just finished a full alignment of my Kenwood R-2000 receiver and tonight I am treated with the pleasing tones of “Jazz from the Left,” on WRMI. Jazz from the left means the west coast sound, aka Smooth Jazz as I am given to understand. I spent some time on the west coast and beyond. I have fond memories of those years.

It is amazing to me still, that a simple AM receiver demodulating +/- 4.5 Khz audio bandwidth from 1,057 miles (1701 km) away can sound that good. That is being received directly; no Internet Service Provider, no satellite service, just a transmitter and a receiver.

There is an art to all this, which is being forgotten. A few minutes with a manual, a volt meter, a tone generator and a non-conductive screw driver can bring something that was neglected back to life sounding as good as the day it left the factory 35 years ago. Try that with with your very expensive iPhone 10,000,000x! Of course, you will need those tiny pentalobe tools to get the screws out. Apple would rather you return your expensive i device to their expensive i store so that their i geniuses can fix it for you.

I don’t know, maybe I am an old fart. Perhaps the right to repair the appliances that I purchased and therefore should own is an old fashioned point of view. After all, all of these corporations have my best interests at heart, right?

I recommend you support your not so local shortwave stations by listening and supporting their programmers. Even in 2021, there are still many shortwave broadcasts that are worth listening to!

Attention K-mart shoppers…

During the impending doom that is/was Hurricane/tropical storm Henri, I decided to bust out the old Kenwood R-2000. This venerable radio has been in my collection since 1989, when first purchased at the AFEES on Andersen AFB, Guam. Over many a year, it has given me lots of great service. However, last time I tried it a few years ago, the frequency tuning was all haywire and it seemed to be inoperative. I set it aside, as I always seem to have something important to work on. Not so much today.

As I discovered, there were two problems; first being a dead lithium battery and the second being the dirty pots on the VCO stepper, which are common failure modes for these units. I unsoldered the lithium battery and ordered a new one (CR 2032 with leads). The VCO stepper issue was corrected with a few slight turns of a small screw driver. There are a couple of Youtube videos on this procedure. Truth be told, the entire unit needs an alignment, which I will do once the replacement battery arrives. The service manual is available from several sources on line and it gives very good directions on how to perform an alignment.

Tuning around the Shortwave bands, I heard the normal things; some hams sending CW, some good some bad. A few messages from the Air Force Global HF network. On the broadcast side of things, Brother Stair seems to have multiplied… Then I came across an interesting signal on 9395 KHz. KMRT was broadcasting the K-Mart shopping sound track from the 70’s – 90’s, interspersed with spoof adds for “Plummet Mall.” As the story goes, these cassette tapes were saved from the rubbish bin by a store manager in 1992. Now, they are being broadcast for everyone’s enjoyment on the short waves. I can say, I felt like I was pushing that shopping cart down the very narrow isles looking for a pair of Adidas.

This is via WRMI, Okeechobee, Florida.

This was a lot of fun.

I will finish the alignment/repairs of the Kenwood R-2000 and put it back in service in the upstairs equipment rack. Enough of these software driven dongles, it is nice to just listen to the radio without having to boot up a computer.

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.

https://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