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

9 thoughts on “All digital Medium Wave transmission”

  1. FYI, for the Medium Wave broadcasting, DRM30 only recommends “Robust Mode A” which by design is exclusively used for MW band. And here are relevant parameters you can find in clause 8.2 (page117) of DRM doc “ETSI ES 201 980 V4.1.1 January 2014”

    Carrier spacing: (41+2/3)Hz
    Duration of guard interval: 2.66ms (this is the cyclic prefix)

    zfyoung

  2. Zfyoung, Thank you for the correction. I knew that the guard interval was the same as the cyclic prefix, I just didn’t see that chart. I also note that Robust mode A is the only mode used in MW DRM30.

    This brings to mind Ward Cunningham’s Law: “The best way to get the right answer on the internet is not to ask a question; it’s to post the wrong answer.”

  3. Congratulations on your thoughtful comparison! We in the DRM Consortium will tweet your article. We wish that more pressure could be placed on FCC so that it could be a bit more open rather than promote one proprietorial, fairly old standard only. We wish we could find a partner station in the US to demonstrate that DRM is the more modern, more flexible and cheaper option and also that it can operate in the country as well without necessarily replacing HD.
    And just a footnote: over 2.5 million cars with DRM receivers are already on the Indian roads and, as mentioned, the SDR receivers are the solution and the way forward for digital radio anyway. Nowadays it is not about “one platform fits all” but “the best platform fit for my needs, my listeners’ interest and my purse”. For more please go to: http://www.drm.org

  4. Omitted was the following;
    1. HDMA3 still uses a carrier which contains no information except the RF signal level to the receiver and it is over 90 % of the transmitted power. Extremely wasteful. DRM does not transmit a carrier, but a few pilot frequencies so that the receiver can set its gain and trim the fine tuning.
    2. HD radio uses identical sidebands either side of the carrier, but they are inverted with respect to each other. By comparison DRM uses all unique side bands.
    3. HD radio uses two different digital power levels so the coverage area of supplementary channels is poorer and likely to drop out. DRM on the other hand has all channels within a transmitted signal at the same power level giving equal coverage areas.
    4. No mention of DRM’s improved slideshow and text capabilities or tuning by broadcaster’s stream name instead of a frequency and HD channel number.

    It is a pity this tweet did not use the 2019 version of the DRM standard.
    For example DRM now specifies xHE AAC sound compression which gives better sound at even lower data rates which can include stereo sound.
    I suggest you read my submissions to the FCC.
    https://www.fcc.gov/ecfs/search/filings?q=%22Alan%20Hughes%22&sort=date_disseminated,DESC for “FCC Revitalization of all US radio” and “Options for conversion from analog to digital radio in North America”

  5. Alan, I appreciate your enthusiasm for this topic. Engineers and others in the broadcast field here in the US generally do not know too much about DRM because they rarely encounter it. This article was asking the question; will some type of digital modulation scheme be worth it and is HD Radio MA3 the only choice.

  6. Paul,
    Definitely. The questions to ask are;
    1. Since HD radio has been a standard for 20 years, why isn’t it universal in the USA? For example if you use xperi’s stated receiver sales over this time, and then compare it to the number of road worthy vehicles, it is only 25 % in the USA. This does not include the number of receivers in cars and on the shelf radios which have died during this time. Go into a TV retailer or online and see how many HD radio models are available!
    2. Since the digital signals in both AM and FM bands overlap into the adjacent transmission channels, and in the AM band case there is also a wasteful carrier, the digital transmission power is very low. This makes the reliable coverage area much smaller than the analog one even in pure digital. As a result in the hybrid mode the main channel blends back to analog or on the HD2 and above it just drops out. Now xperi wants receivers to not go analog but go to the wireless internet ie the telcos! How does the listener know what is being heard?
    3. The upper and lower digital sidebands are identical but phase inverted. This is so that if there is interference hopefully it is only in one sideband. This prevents the use of more intensive error correction.
    4. HD uses Orthogonal Frequency Division Multiplex, which does not contain gaps in transmission to allow for the dispersion of delayed signals. This is important in the FM band and in the AM band at dawn and dusk with ground and skywaves being present at the edge of the coverage area. The Coded Orthogonal Frequency Division Multiples receiver only decodes the beginning of each data burst which is used in DRM, and DVB TV. Not ATSC1.0 in the USA.
    5. Over the 20 years the level of impulse interference from switchmode power supplies which are in virtually all mains powered electrical equipment, arcing electricity lines and now with ageing electric cars, is greatly increased. This has made the very low powered digital signal particularly in the HD2 channel very unreliable.
    6. Ibiquity now taken over by xperi uses their own now very old and inefficient audio compression system. Only in the 96 kbit/s mode in the FM band HD1 could it be called “CD quality sound” All of the HD2 and above as well as in the AM band only a tiny data rate is available so the sound is of poor quality particularly for music.

    A far better option is to forget except in low population density areas, frequencies below 2 MHz due to the impulse noise and use now almost deserted analog TV channels 2 – 6 using DRM mode E. https://www.drm.org/wp-content/uploads/2020/05/DRM-Handbook-Version-5.pdf Page 48 and the standards in Annex A. The only power limitation is that from another transmitter on the same 100 kHz transmission channel.
    On a single site all digital transmissions can be on adjacent 100 kHz wide channels. There are enough channels for all current AM and FM broadcasters. There is much less noise, all programs have the same coverage area which is not limited with interference from analog, which can continue to broadcast as they are now including HD. There is a lot more data rate available better sound, for other functions such as slideshows, Journaline, electronic program guide, and more and faster downloads of traffic data for navigation systems and emergency warnings.
    In short HD radio was designed as a compromise by adding digital signals in the adjacent channels around a broadcasters’ analog signal. DRM is designed to fit within its digital only channel without the compromises giving more capacity and reliability

  7. Paul – What will the experience be for a dinosaur like myself (and the radio I use) when we tune to a MW digital transmission? A blast of noise? And when the transmitted signal is distorted a bit when bounced off the ionosphere, what’s the experience at the other end?

    I got to play with an FM HD radio about 10 years back, and didn’t find the experience to be so compelling that I had to run out and buy one…

  8. Titus SDR, Inc. has a multi-standard (DRM, DAB, and HD to be added anytime) ready-for-production digital receiver unit. In our view, FCC should allow DRM broadcasting on an experimental basis in parallel to the HD mode. The HD compatibility of car radios gives the incentive to broadcasters to transmit in HD. Drivers are a captive HD audience. Almost all radio listeners in a car are unaware that they are tuned to KLMN-HD1 and not to KLMN-FM station.
    Conversely in the US, DRM is left in the “chicken and egg” situation: No DRM broadcasters since there are no DRM receivers and no DRM receivers since there are no DRM broadcasters. Bottom line, as it is pointed out in section 4., the key is to offer highly desirable programming available only on DRM broadcasting. As Paul Thurst writes “That is a big stretch”. However, an effort should be made to initiate DRM transmissions in the US on an experimental basis. Could organizations and individuals sharing this objective start communicating/cooperating? Any ideas on how to accomplish this?
    pbenci@titus-sdr.com
    Note: The US Coast Guard has invested significant resources to evaluate the DRM protocol and FCC should take the report into careful consideration:
    HF Digital Radio Mondiale (DRM) Broadcast Summary Report For Long Range Dissemination of Maritime Information
    Distribution Statement A: Approved for public release; distribution is unlimited,
    April 2018
    http://www2.unb.ca/gge/test/SWL/2018-0568a%20DRM%20REPORT%20final%20PDF%20UDI%201732.pdf

  9. Geoff, Unfortunately if you tune in with an analog receiver, you will here white noise on channel. I feel that most stations in the US will be slow to adopt any all digital transmission. Regarding ionospheric propagation paths, DRM has several methods for dealing with phasing and multipath issues. One of the major problems facing US Broadcasters is the lack of compelling programming.
    Pier, That Coast Guard paper was interesting and is similar to various datacasting methods seen on ATSC UHF TV channels. Regarding receivers; there is a convergence of technology onto a single chip. STMicroelectronics makes the TDA7707 which has AM/FM/HD Radio/DRM/DAB/DAB+ on the same chip. That makes sense from a manufacturing standpoint. Make software defined radios that can do everything and enable the desired features. SDRs based on the RTL2832 can receive almost anything with the right plug in and codec.

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