September 2010
M T W T F S S
« Aug   Oct »
 12345
6789101112
13141516171819
20212223242526
27282930  

Archives

Categories

Digital Radio Mondiale, an alternative to HD radio?

DRM logoCould be.   Digital Radio Mondiale, or DRM, is a modulation scheme that a group of broadcasters and transmitter manufactures have been working on since about 1997 or so.  There are numerous shortwave broadcasters; the BBC, the CBC, Deutsche Welle, Radio France Internationale, the VOA, and others have been using DRM on shortwave for several years now.  I can state that the shortwave DRM system works well, I have a software decoder and use the sound card input on my computer to decode and listen to DRM shortwave broadcasts.

The goal of DRM is to establish a world wide open standard for digital broadcasting in the LF, MF, HF, and VHF bands.  In the early years of development,  DRM was designed for digital broadcasting on the bands below 30 MHz.  This system is now known as DRM30.  Since then, the DRM consortium has expanded that to the VHF band (up to 174 MHz)  as well (meaning where the current FM band is located) and have called that system DRM+.

DRM uses COFDM (Coded Orthogonal Frequency Division Multiplex) modulation, which is the same as IBOC HD radioTM.  This is a robust modulation system that employs multiple carriers at lower power (than an analog carrier) spread across the entire alloted bandwidth.

One of the claims is  DRM transmits less power and is more energy efficient.  In general, digital radio modulation does transmit less power, that is true.  However, transmitters have to be run more linear for digital due to the increased bandwidth.  This may not translate to greatly increased efficiency from the AC mains to RF standpoint.  Because of that, there is more waste heat, and thus more air conditioning is needed to cool the transmitter room.

Some of the advantages of DRM over Ibiquity’s HD radioTM are:

  • Open source system.  Royalties are paid by the transmitter manufactures only (and do date, most major US transmitter manufactures have already paid these).  There is no royalties paid by the broadcaster to install DRM or by the consumer when purchasing a DRM capable receiver. One company does not own the rights to the modulation system for all the broadcasters in the country.
  • Universally standard; accepted by the European Telecommunications Standards Institute (ETSI), International Electrotechnical Committee (IEC), and the International Telecommunications Union’s (ITU).
  • The CODEC is HE-AAC 4, which is widely used world wide.
  • DRM30 and DRM+ fits into existing band plans and will not interfere with other users on adjacent channels.  DRM30 is designed for 9 KHz channel spacing and DRM+ is designed for 100 KHZ channel spacing, all of which comply with existing FCC regulations.
  • Standardized receiver profiles, things that must be included in all DRM receivers. There are several advanced options as well, such as a media rich system that includes video.
  • DRM+ has several added features: DRM text, which is similiar to RBDS.  EPG or electronic programming guide, which shows what is coming up next and a searchable schedule of when programs may be heard out to seven days.  Some DRM+ receivers will have a TIVO like recording device that allows the user to record programs and play back later.
  • Traffic reporting and routing

In addition to that, DRM30 station have the ability to transmit low frame rate H. 264 video.  This is a distinct advantage for short wave stations that are seeking a way around fire wall blocking.  The video image is small, 176 x 144 pixels, and it is 8 frames per second, which is about as good as can be expected using a 9 KHz channel.

In some cases, DRM is capable of a hybrid mode (ed note: DRM calls this “Simulcast mode”), but what have we learned about hybrid mode digital radio:  It doesn’t work very well. In short, it would be better if DRM were employed in the digital only mode.  To many, this is a distinct disadvantage, but I don’t see it that way.  There have been many that have made the IBOC roll out/FM broadcasting roll out annology.  Frankly, those arguments don’t hold water.  When FM was introduced, no attempt was made to shoe horn it into the existing AM (Standard Broadcast) band, it was not designed to interfere with other stations or itself, power levels were sufficient for good reception using existing technology, quality over AM was markedly improved and programming was often separate (simulcasting with existing AMs did not start until later).  My point here is that any digital broadcasting should be introduced on a separate set of frequencies.  Some have proposed using TV channels 5 and 6, which makes some good sense.  Whatever the outcome is, we have learned, the hard and expensive way, that hybrid digital broadcasting does not work well.

A brief video about DRM30.

Currently, DRM30 is only allowed on shortwave broadcast frequencies in the US.  I asked a product development engineer from a major reputable broadcast transmitter manufacture about this, his response was:

  • Medium wave broadcasting in the US already has HD radio, so the FCC would be disinclined to allow a new standard
  • One might be able to apply for an experimental license to broadcast DRM, but it would likely have an expiration date
  • It is possible to operate DRM in a hybrid mode on the AM band and occupy the same bandwidth as HD radioTM (30 Khz), it might also be possible to squeeze that down to 20 KHz.
  • Most modern (read: solid state) AM broadcast transmitters should be able to transmit DRM without modification (antenna systems may be a different matter).

It might be fun to apply for an experimental license to broadcast somewhere in the 1600-1700 KHz range with DRM30 only and no analog modulation, except for an hourly station ID in morse.  A 1/4 wave tower in the middle of that band would be 141 feet tall.  With use of a skirt, a grounded tower can be employed.  That and a few above ground radials and the system would likely be pretty efficient.  Part of the experiment would include driving around and taking signal strength readings while recording the programming material.  This would give some real world testing on how the system would perform in wide spread use.

Of course, this would require a major about face by the FCC, which is not likely unless someone there grows or somehow acquires a back bone.

Some people question the need to do any type of digital broadcasting.  I am a realist, in one way, shape, or form, digital radio broadcasting will (or already is) take(ing) place.  It would make the most sense if the best system were used, which is not necessarily the first system proposed.  The big question is, will today’s terrestrial broadcasters be involved, or out of business.

Be Sociable, Share!

28 comments to Digital Radio Mondiale, an alternative to HD radio?

  • Gary

    Paul,

    Freakin’ awesome post! Let me help clarify a few things. OFDM, or COFDM, which is the same thing because no one that I’m aware of transmits in OFDM without first coding the livin’ s___ out of the data bits, is also used in almost all WiFi nowadays. If you’re using 802.11a or 802.11g, you’re using OFDM. The latest HomePlug standards for using your home powerlines for network connectivity are also using it. Why? Because its a scheme that works wonderfully in environments with lots of multipath. Guess what both powerlines and most suburban / urban environments have lots of? Yes, friends, its multiple paths for the signal to transit from the transmitter to your receiver. The problem with the name, OFDM, is that people think its some special form of modulation. It’s not. As they say in the Navy, “Now hear this… there are three AND ONLY three ways to modulate a carrier wave. It’s either amplitude, frequency, and/or phase.” That’s it. They ain’t no more. OFDM uses phase modulation, specifically phase shift keying (the “shift keying” part just means that the information is digital). What OFDM is doing is using a whole bunch of phase-shift keyed (PSK) carriers parked right next to each other. For example, in 802.11a/g (which are the exact same thing, just different frequency bands), the spacing is set at 312.5 kHz. The interesting part is that, even though many of the OFDM implementations, such as 802.11a/g, can change the overall bit rate, the symbol rate never changes. They change the overall bit rate by changing the modulation on the individual carriers, called “subcarriers”, as well as the amount of coding on each. (Its annoying that they picked a word that’s already used, since SCA and RDS signals are also passed as “subcarriers”.) So, on an 802.11g signal, the transmission typically starts using BPSK (binary phase shift keying) on each subcarrier. Then, as the system begins to measure the BER between the transmitter and receiver, it starts jacking up the modulation, from BPSK to QPSK (quaternary phase shift keying) to 16QAM (16-level quadrature amplitude modulation) to 64QAM.
    I imagine that, since DRM uses QAM on the subcarriers, then your efficiency should go down. Also, the antenna doesn’t make a difference so long as its flat in the passband of interest.
    Okay, I’ve hijacked this thread enough. Again, great post! I’m going to suggest it to Slashdot. If they accept it, expect to see a spike in traffic.

  • “Death of Digital Radio Mondiale in 2008 as well?”

    “From both formal and informal discussions among participants at the HFCC, it is now clear that the proposed DRM (Digital Radio Mondiale) system, that would have converted analogue Shortwave to digital, FM like quality reception would hardly be implemented if ever on a large scale, beyond the current experimental stage.”

    http://criticaldistance.blogspot.com/2008/02/death-of-digital-radio-mondiale-as-well.html

    “AM HD Radio versus DRM”

    “Unfortunately DRM shares many of the same flaws as DAB and HD Radio technology: Shorter broadcast distance as compared to analog AM signal when in hybrid mode; poor reception inside vehicles and buildings; and interference with adjacent channels.”

    http://en.wikipedia.org/wiki/HD_Radio#AM_HD_Radio_versus_DRM

    Only a relatively few SW stations have deployed DRM, and shortwave listeners have complained about the interference DRM causes. DRM is also no good on AM, as it causes interference as IBOC. With the world-wide flop of digital radio, haven’t they figured out that relatively few consumers are interested in digital radio.

  • Paul Thurst

    @Greg, that’s why I stated that the hybrid mode is the worst possible method to try and implement digital radio. Think of the FM band, separate set of frequencies for a new modulation scheme, that is what should happen. As far as DAB (Eureka 147), I don’t know enough to comment on it’s failure.

    Shortwave broadcasting is tough battle with any type of modulation because there is so much co-channel interference. Perhaps HF and MF would be best left to analog modes, but we won’t know until we at least attempt an experiment with an all digital broadcast and see how it does.

  • […] Follow this link: Digital Radio Mondiale, an alternative to HD radio? « Engineering … […]

  • Paul, an excellent summary of DRM. One comment regarding efficiency. Yes, we are able to cover the same ground with far less power in digital than with analog. Yes, a transmitter operating with DRM+ will operate less than Class C efficiency. But let’s say that we are at 40% efficiency rather than 80%. If we had 1/2 transmitter power, we’d be at break-even power consumption. But we’re not at 1/2, we’re at 1/10th. So we’re well ahead of the game.

    In AM, in the simulcast mode (as it’s called in DRM, hybrid in HD Radio) we can generally run the full analog power and sneak in the digital signal as well, at low power without wreaking havoc with the transmitter’s overall efficiency. So again, we’re ahead of the game here, too.

  • Paul Thurst

    Thanks, Hal. I enjoyed the webinar. On the efficiency, I stand corrected. I based that statement on what I have found with HD Radio installations and other digital radio/video services like media flow, etc. Generally, the transmitters produce gobs of heat, which has to be dissipated.

  • […] Radio, Paul suggests that Digital Radio Mondiale (or ‘Digital Radio Worldwide’) may be a better alternative than the US-adopted, proprietary IBOC system. But he’s concerned that the FCC is too far down […]

  • Cellar

    I’m not an engineer, but, hey, this is the intarwebz so I’m going to annoy you anyway.

    I don’t know which is better, but let me note that the biggest investment by far is in consumer radios, and therefore that to go digital at all means you’d have to have a truly compelling argument _for the consumer_ to switch over. And frankly, I don’t see it.

    More channels and more features are just more ways to throw advertising at me. And then I find that in my corner of the country the coverage of digital just happens to suck more than AM on LW because of I-don’t-care-what-quirk-in-the-system. And it degrades less gracefully. Or that it won’t work the next country over. Or that next week they ditched this system and switched over to another. And it doesn’t fall back to FM.

    The problem with engineering here is that this old analogue system *just works* and does what it needs to do well enough, the world over. I don’t see that with digital radio sets. They’re engineered for function, not against adversity. How many sets can be updated for a new modulation? Support all currently in fashion digital radio proposals? Or are there even world radios that can do all that and do LW, MW, SW, and FM? Because someone, somewhere, will not have updated and will still be doing analogue. Because the receivers are cheap and always work. Can “digital” top that?

    If not, it remains a fad for the advertising side.

    Because of that I’d say that open standards are good, and then we’ll sit on them for a decade and if we haven’t found anything better we might roll with it, after we’ve figured out how to make the protocol survive a chinese designed-and-manufactured ultra cheap set. And you’d still have to provide for analogue and updating for new digital methods twenty years down the road.

    Of course, none of this will happen exactly because where it isn’t a technology looking for a problem to solve the driving force is more broadcasting room for advertising, ensuring we’ll end up with a crap system and radio sets that’ll be useless in five years time, if they haven’t HCF in the meantime.

  • theaveng

    As an elevtrical engineer interested in free broadcast services I have to say:

    IBOC/HD Radio vs. DRM – there’s no real difference

    (1) Both broadcast analog-and-digital side by side
    (2) Both are designed to transition to 100% digital broadcast at ~60 kbps on AM and ~250 kbps on FM.
    (3) Both cause interference with long distance stations ~100 miles away.
    (4) Both will eliminate that problem when moved to 100% digital.
    .

    Also IBOC/ HD Radio has one advantage that Digital Radio Mondiale (DRM) does not have : Multiple programs. IBOC/HDR can broadcast 7 programs on a single station. For example my local station has Top40 hits on HD1, Dance music on HD2, 80s/90s on HD3, and a low quality traffic/weather channel on HD4. The DRM standard does not offer the same feature.

    As for the “open source” argument, HDR will also become open source in just ten more years when existing patents expire, and then it will be just like AM/FM Radio today – an open standard that anybody can copy and produce and sell.

    Bottom Line: I don’t see any reason that DRM is better than IBOC/HDR (and at least one reason it’s inferior). Nor any reason why we should throw out the millions of dollars already spent on HDR to switch.

  • theaveng

    P.S.

    Creating an all-digital service band for DRM or DAB would not work either. Why? No free space in the spectrum. In Europe they were able to eliminate the old analog TV channels 2-13 and repurpose them for DAB, but here in the US this option does not exist (2-13 are occupied by digital TV).

    The reason the FCC chose to shoehorn Digital IBOC/HD Radio on top of AM/FM bands was because (1) there was no other place to put it and (2) they knew it was only temporary; that analog would eventually be turned off as was done with analog TV.

  • Paul Thurst

    Story picked up by slashdot.org:

    http://tech.slashdot.org/story/10/09/25/1421220/Digital-Radio-Mondiale-a-Better-Standard-Than-US-Adopted-IBOC#commentlisting

    Very interesting comments list. Some are quite insightful.

  • R. Ethridge

    I would be very surprised should IBOC go completely digital. IBOC acceptance, particularly on AM/BCB, is extremely low and “flatlining”. Most receivers exhibit poor reception on FM and abysmal fidelity on AM/BCB. Also receiver prices don’t justify purchases. I’ve heard DRM on HF and found it very listenable with very little adjacent-channel interference. Of course the signal was considerably weaker than standard HF. I attribute some reception problems with the open-source DREAM software. I’m planning on paying for the DRM radio software from Fraunhoffer very soon. Stereo shortwave, WOW!

  • Mark Dulcey

    Channels 2-6 are free of digital TV in nearly all markets. Channels 7-13 are also free of digital TV in many, including Boston where I live – channel 7 originally returned to broadcasting on real channel 7 in June 2009 in place of their interim channel assignment on UHF, but lots of people had trouble receiving it so they asked for and received permission to switch back to the UHF allocation. (People south of the city who receive Providence signals do get those on channels 10 and 12.)

    I really don’t see a new spectrum allocation for digital radio as likely though. There are so many other competing demands for spectrum now; if channels 2-6 ever get completely deallocated from television they’ll be given to something other than radio broadcasting. Like it or not, digital/analog simulcasting on the existing bands is what we are going to get.

    I think HD Radio hybrid mode on AM is a disaster and should be abandoned; there is too much interference to adjacent channels, and these days there is very little AM radio programming where the improved sound quality matters. But it seems to work pretty well on FM, and unless DRM adds a multiple channel capability I don’t see it replacing HD Radio.

  • Rob

    An interesting read indeed. Over here in Aus we have no IBOC thankfully but digital radio has just started with DAB+ in its own VHF spectrum. Given commercial AMers are only allowed 5kw in Aus its taking off in places of high AM interference. I personally don’t think it matches FM for clarity but sure beats AM mixed with power line interference. The wide variety of extra channels appeals to some too.

    DRM has been kicked around as an alternative to AM on MW in country areas. That said the 24/7 50kw government owned AM stations have played a leading role in disaster information delivery so I suspect DRM will have a hard time replacing that role any time soon.

    Have read this blog from end to end and it on my must read list. Very informative and educational. Thanks Paul for the time you put into it.

  • J. Aegerter

    Terrestrial broadcasting was fine with analog. The digital craze is causing a lot of money to be spent with little return. When the USA finally follows the Europeans and the rest of the Socialist broadcasting world into bankruptcy, you will get digital broadcasts via your crummy cellular phone by paying for it.

  • DRM since the very beginning always allowed up to 4 radio programs to be simulcast in the same channel (in LW, MW, SW and VHF), and unlike HD Radio, you can even broadcast video in one of the channels.
    Unlike HD Radio, DRM works in SW, that is very important for international broadcasting.

    DRM is definitively much better then HD Radio, there is no doubt, and is also an open standard that uses a common codec (MPEG4 AAC) and not the industry secret not software implemented HDC that HD Radio uses.

  • The BBC ran a pretty successful trial of DRM on one of their medium wave frequencies back in 2007-2008; the report’s well worth reading.

    http://www.bbc.co.uk/devon/content/articles/2009/05/11/digital_medium_wave_report_feature.shtml

  • I’m not an engineer, but, hey, this is the intarwebz so I’m going to annoy you anyway.

    I don’t know which is better, but let me note that the biggest investment by far is in consumer radios, and therefore that to go digital at all means you’d have to have a truly compelling argument _for the consumer_ to switch over. And frankly, I don’t see it.

    More channels and more features are just more ways to throw advertising at me. And then I find that in my corner of the country the coverage of digital just happens to suck more than AM on LW because of I-don’t-care-what-quirk-in-the-system. And it degrades less gracefully. Or that it won’t work the next country over. Or that next week they ditched this system and switched over to another. And it doesn’t fall back to FM.

    The problem with engineering here is that this old analogue system *just works* and does what it needs to do well enough, the world over. I don’t see that with digital radio sets. They’re engineered for function, not against adversity. How many sets can be updated for a new modulation? Support all currently in fashion digital radio proposals? Or are there even world radios that can do all that and do LW, MW, SW, and FM? Because someone, somewhere, will not have updated and will still be doing analogue. Because the receivers are cheap and always work. Can “digital” top that?

    If not, it remains a fad for the advertising side.

    Because of that I’d say that open standards are good, and then we’ll sit on them for a decade and if we haven’t found anything better we might roll with it, after we’ve figured out how to make the protocol survive a chinese designed-and-manufactured ultra cheap set. And you’d still have to provide for analogue and updating for new digital methods twenty years down the road.

    Of course, none of this will happen exactly because where it isn’t a technology looking for a problem to solve the driving force is more broadcasting room for advertising, ensuring we’ll end up with a crap system and radio sets that’ll be useless in five years time, if they haven’t HCF in the meantime.

  • Sykie

    Dear Concern,

    We are a FM Analogue Broadcaster. Now we want to invest on FM HD Transmitter. But we’re afraid to do this. Becoz, their has few important disadvantage I heard. i.e.
    1. Are FM HD Tx can able to broadcast LIVE program with RJ Endorsement?
    2. Are FM HD Tx can able to broadcast LIVE Spots from Stadium?
    3. Are FM HD Tx only broadcast recorded program?
    4. What is the audio code format of FM HD TX: MPGE-1; MPGE-2; MPGE-4?
    5. What is the minimum & maximum bandwidth (KHz) require for FM HD broadcast?
    6. Which is the right work way of FM HD Tx:
    A. Stereo Audio Source (from studio)>>>Input to HD hardware (Importer &
    Exporter)output >>> Input to TX Exciter >>> Exciter Audio Signal go through TX PA
    (Analogue Power Amplifier)>>> than 10kW TX Combiner>>> to FM Antenna.
    B. Stereo Audio Source (from studio) >>> Input to HD hardware (Importer &
    Exporter)output >>> Input to TX Exciter >>> Exciter Audio Signal go through TX PA
    (Digital Power Amplifier)>>> than 10kW TX Combiner>>> to FM Antenna.

    Thanks

  • Neil

    So far HD FM radio has not drastically improved anything. HD radio works in the range where FM does not have much noise, but drops out and then analog keeps going. Your money would be best spend buying a high quality analog based audio processor and stereo generator. The only place where HD radio has improved audio is classical, as long as the signal is broadcast at near lossless compression. But for the average rock, pop, jazz, talk, or oldies station, HD radio is no improvement whatsoever, unless you plan on routing the audio without processing.

    One thing you should do is try to eliminate too many DA AD conversions in your station. Use an analog mixer and stay analog as long as you can after that. Some stations are using a digital mixer with analog inputs from digital sources. So it goes from digital to analog, then analog to digital then digtal to analog, then analog to digital at the audio processing. It sounds really crappy by the time it gets through the transmitter.

  • Neil

    As far as AM goes, it is easier to manufacture an AM radio that has active DSP to eliminate noise on AM radio, that it is to use digital radio. If you have noise and interference on AM radio, you can usually still hear the program. But where there is noise and interference, Ibiquity IBOC and DRM do not work at all.

    I am under development of a technology that can greatly reduce the noise and interference on AM radio…and my invention is Analog!

  • Neil

    Tried DRM using a sound card and a PC. It worked.

    My question is with DRM+ on FM, what would be the advantage? This format is lossy audio compression. Current analog radio is not. Ibiquity has very low loss compared to DRM’s compression.

    Again, neither of these digital systems resolve the problems of static and interference. When static and interference are a problem, both DRM and Ibiquity “Loose the digital signal”. None of these extend the listening range of AM or FM.

  • zhongfanyang

    Some facts have to be made clear;
    1.DRM or DRM30 maximum bit rate is about 20kbit over 10KHz bandwidth, DRM+ is about 180KHz so near CD quality in FM band should be no problem.
    2. Some may argue “neither of these digital systems resolve the problems of static and interference.” Do you really think analogue signal can get rid of this curse??Even worse than that, when analogue signal is effected by interference, no matter what kind of measures you take, the noise is still there. In digital mode, at least the broadcaster could use a more robust modulation parameter or(and) progress in modern technology do allow some fancy tricks to combat this problem: multi-antenna Tx/Rx, adaptive filtering….And why can’t we learn the success story in the evolution of 3G, 4G, 5G broadband and put to use in radio broadcasting? Do you really think the status quo (considering the heavily polluted air waves in today’s E.M. environment)would satisfy any media savvy younger generation?

  • Paul Thurst

    A couple of things to note; this post was written five years ago and some things have changed. The broadcast digital technologies in general are getting better. On the FM band (VHF Band), HD Radio has been allowed a power increase respective to their analog carriers This has somewhat improved performance, but the digital signal is still less than its analog counterpart in most installations. Since this was written mostly comparing DRM30 to HD Radio, most of the assertions made still hold true.

  • Ralph

    There have been improvements of late, notably the adoption of an improved codec for DRM
    and a method developed by Nautel for transmitting multiple HD digital only services in an FM band slot. If I read their webinar correctly it is compatible with the existing HD receivers so should allow similar advantages to the larger USA broadcasters as DAB+ does in other countries.

    The reality is the broadcaster chooses the implementation of these technologies: there is a conversation in the UK about new DAB+ mostly in mono using low bitrates, which the commercial broadcasters seem to prefer as quality is not really a factor in their business model.

    I expect any implementation of DRM will be the same compromise – 2 or 3 low bitrate services where only 1 is capable of full audio and sufficient robustness for a quality transmission.

    Last year I re ran some practical field tests to determine how much quality and dropouts would be acceptable. To my own ear, it was absolutely necessary in DRM to use minimum of 64QUAM, practically no error protection and 10k RF bandwidth to achieve good sounding audio. But AM transmission to the AMAX standard was superior in all cases. The droputs kill any digital system, I can tolerate interference better than silence.

    http://radioalchem.blogspot.co.nz/2012/05/holding-pattern.html

    I then re-read the BBC papers on the various trials. With a critical eye this time, not the rose – tinted glasses. Dropouts are an unresolved problem. Transmitter powers need to be higher to achieve satisfactory coverage. It was all there in those papers, I had just glossed over those conclusions in my desire to prove the system was worthy – its not.

    Now that All India Radio has completed a huge DRM MW band implementation, I look forward to getting a proper car radio to do some more tests. I do hope it works for them, but for less outlay the same could be done with AMAX and / or AM stereo. If its true that some HD receivers also do AMS / AMAX, that would a useful advancement to broadcasting today. The current radios marketed here (Oceania / Pacific) are DSP based but do a poor implementation with distorted FM on multipath and the usual low AM audio bandwidth hardly enough for a talk station. The only advancement they offer is shortwave, another medium closing down fast.

  • zhongfanyang

    About the field trial I have a few questions, Ralph:

    1. what is the DRM source coding parameter? hint: with or without SBR? which will have big difference in audio quality at very low bit rate.

    2. Are the bandwidth of two systems equal (this is an apple to apple test, right?)

    3.I can hear the constant hissing background noise in AM stereo which only gets somewhat attenuated at some point(possible where signal strength is getting stronger?)To be fair, DRM also suffers drop-out at some point, but what about AM stereo at that same spot(same time and location)?

    4.This is a relatively quiet test channel at a very short distance on a relatively flat landscape. How does it perform under some normal
    conditions (man-made noise.., hilly terrain with strong multi-path echo)?

    5. last but not least, is your DRM transmitter properly attuned to the some special requirement of digital system?(Although I’m not an expert working in transmission system like you, I’ve seen enough of some badly implemented digital Tx systems where self-generated noise resulted from non-linear modulation/amplification render the whole system useless.

  • Ralph

    hi Zhongfanyang
    I’ll answer briefly here and any further questions please go to http://drmrx.org/forum/
    1)The parameters were what was available with the DReaM transmit software, it doesnt do parametric stereo or the newer xHE AAC codec therefore cannot be as good audio like DW radio or Disc Palace samples on the blog. DRM implementation of audio less than 6kHz is “awful” to the ear. I spent a lot of time testing the available bitrates and protection schemes before doing the air tests.
    2)RF bandwidths not equal but with digital modulation you trade BW for power density etc. The one parameter I kept constant was average power.
    3)I apologise for the apparent poor AM stereo There was only ever one (Harris) modulator in NZ which I’ve not been able to locate so I had to make my own – very basic. It also developed a faulty power supply during the test leaving a buzzing noise on the signal. In the interests of science I published the results as the problems did not materially affect the trial.
    4)The 2 short tests in the update were closer, and started at distance moving closer to the transmitter. the other tests were done over 20km, over a hill, through a valley, past a substation and ending in a high electrical noise environment which even P25 digital radio could not maintain lock – the performance should scale with power and distance to a typical daytime MW broadcast setup.
    5)Yes the transmitter is performing perfectly well in this application easily verified looking at the 64QUAM constellation and I can watch the limits as it approaches the 1kW PEP in reality I can run it at 200W average power without S/N degradation on DRM but I wanted a clean comparison.

  • zhongfanyang

    From my experience as a regular SW Listener, I found AM listening demanding: the constant background noise is distracting and can easily causing hearing fatigue; due to the time-varying multi-path inherent in SW, the distorted audio is never clearing enough like it always covered in a thin veil.
    On the contrary, I much enjoy my DRM listening: clear and quiet. eg. I can listen to Radio New Zealand International DRM broadcasting for hours but for its AM counterpart, it is really off-putting.

    Of course digital drop-out is annoying esp. during the program.But I think that can be improved at least technically. Now I’m implementing dual channel diversity reception by merging info. from 2 antennas/receivers. As to the interfering problem, I can apply adaptive filter to cancel out the offending noise automatically. Again, I could apply the same technique to AM signal, but for DRM it is much more cost-effective, just consider the difference between digital drop-out(complete silence) to 100% recovered audio.

Leave a Reply

 

 

 

You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

Axiom


A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
~1st amendment to the United States Constitution

Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
~Benjamin Franklin

The individual has always had to struggle to keep from being overwhelmed by the tribe. To be your own man is hard business. If you try it, you will be lonely often, and sometimes frightened. But no price is too high to pay for the privilege of owning yourself.
~Rudyard Kipling

Everyone has the right to freedom of opinion and expression; this right includes the freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers
~Universal Declaration Of Human Rights, Article 19

...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.
~Alan Weiner

Free counters!