Speaking of Radio…

I was talking to a friend from Russia about history, my job, and various other things that are going on in my life. I received this reply, which I thought was interesting on a number of levels:

I’m glad we are on the same page about the era of the ‘cold war’. We were interested in your life even more than you in ours. We had almost no sources of information except for ‘The morning star’ which is a newspaper of the Communist party of Great Britain. The Voice of America and the Liberty (or Freedom, I have no clue because for us it was ‘RADIO SVOBODA’) were extremely hard to tune on. All foreign broadcasts were jammed. So to listen to the station you should maximize the volume up to the limit which was dangerous. Soviet houses are not at all soundproof and your neighbors could easily rat on you. Since that time I’d been dreaming of a small radio with could receive a clear signal from abroad. Of course we have the Internet broadcasting now but they often use old recording instead of live air and the signal depends on your data carrier. You should be online, you should have an app and unlimited data on your contract, your phone should be charged all the time. Too many conditions. Unfortunately a lot of foreign sites are banned here and the trend is to make this number bigger and bigger.

I find that perspective interesting.  We take for granted our ability to listen to information and listen to different points of view, even those we don’t agree with.  There are still trouble spots in the world and some people are not as fortunate.  It is very easy to block internet traffic and there are several countries that currently block access to some or all of the internet, for the safety of their citizens, no doubt.  Ideas are dangerous.

VOA/RFE transmitter site, Biblis Germany
VOA/RFE transmitter site, Biblis Germany. Photographer: Armin Kübelbeck, CC-BY-SA, Wikimedia Commons

In the last ten to fifteen years, many large government shortwave broadcasters have reduced or eliminated their programming favoring an internet distribution model.  This is a mistake.  It is very difficult to successfully jam terrestrial radio broadcasts.  Shortwave Facilities are expensive to develop and maintain, there is no doubt about that.  However, as the Chief Engineer from Radio Australia (ABC) once told me “HF will get through when nothing else will.”  Ironically, ABC has eliminated its HF service on January 31, 2017.

It seems to me that a sort of “Shortwave Lite” version of broadcasting might be the answer.  Use more efficient transmitters with lower power levels closer in to the target areas.  Such transmitters could be coupled to rotatable log periodic antennas to target several listening areas with one system, thus greatly reducing the number of towers and land required.  Solid-state transmitters with a power of 10-50 KW are much, much more efficient than their tube-type brethren.

DRM30 (Digital Radio Mondiale) has not gained widespread use in the MF and HF bands.  Like its HD Radio counterpart, the lack of receivers seems to be one of the adoption issues.  As of 2017, there are only four DRM30-capable receivers for sale not counting software plug-ins for various SDRs.  That is a shame because my experience with DRM30 reception has been pretty good.  I have used a WinRadio G303i with DRM plug-in, which set me back $40.00 for the license key (hint for those nice folks at the DRM consortium; licensing fees tend to quash widespread interest and adoption).

CFRX, Toronto coverage map, average HF propagation conditions
CFRX, Toronto coverage map, average HF propagation conditions

Finally, I have advocated before and still advocate for some type of domestic shortwave service.  Right now, I am listening to CFRX Toronto on 6070 KHz.  That station has a transmitter power output of 1 KW into a 117-degree tower (approximately 50 feet tall) using a modified Armstrong X1000B AM transmitter netting a 15-32 µV received signal strength some 300 miles away.  That is a listenable signal, especially if there is no other source of information available.  The average approximate coverage area for that station is 280,000 square miles (725,000 square kilometers). That is a fairly low overhead operation for a fairly large coverage area.  Perhaps existing licensed shortwave broadcasters should be allowed to operate such facilities in domestic service.

The point is before we pull the plug on the last shortwave transmitter, we should carefully consider what we are giving up.

Brother, can you spare a theorem?

A theorem is not, indeed, a fact.  It is rather, an idea that is deduced and supported by other proven facts.  Thus, a theorem is generally believed a truth.  It should be of interest to the “All Digital” AM (AKA Medium Wave) proponents that noise on the digital channel will reduce data throughput as a function of channel bandwidth and Signal to Noise Ratio.  This is known as the Shannon-Hartley theorem:

 C =  B \log_2 \left( 1+\frac{S}{N} \right)

Where:
C is the channel capacity in bits per second;
B is the bandwidth of the channel in hertz (passband bandwidth in case of a modulated signal);
S is the average received signal power over the bandwidth (in case of a modulated signal, often denoted C, i.e. modulated carrier), measured in watts (or volts squared);
N is the average noise or interference power over the bandwidth, measured in watts (or volts squared); and
S/N is the signal-to-noise ratio (SNR) or the carrier-to-noise ratio (CNR) of the communication signal to the Gaussian noise interference expressed as a linear power ratio (not as logarithmic decibels).

With this equation, one can discern a fundamental flaw in all digital logic.  One of the main issues with AM Medium Wave broadcasting is the ever-increasing noise floor.  Our society has changed drastically in the last one hundred years or so since AM was invented.  Electrical noise generators; computers, plasma screen monitors, mobile phones, appliances, energy-efficient lighting, data over power line, street lights, poor utility line maintenance, and even electric cars, it seems, generate a cacophony of noise in the Medium Wave frequency band. A digital modulation scheme, be it HD Radio or DRM, will mask the noise to a certain extent, that is true.  However, once the SNR exceeds the ability of the receiver to decode the necessary bits, the receiver will mute.  While it is true, the listener will not hear noise, they may not hear anything at all.

I will also note; none of the current “AM improvement” schemes under consideration by the FCC addresses the noise issue on the AM band.  Without addressing the noise issue, any digital modulation scheme will be a temporary fix at its very best.  The noise floor will continue to rise and after it gets high enough, the all-digital modulation will simply not work.

It will be interesting to see the data from the all-digital HD Radio testing that is being done in various locations.  That is, if the NAB, et al. does not decide to treat that data like some kind of state secret; they have become reticent of late.  When somebody acts like they have something to hide, it makes me think they have something to hide…

What bitrate is needed to sound like analog FM?

As it turns out, 300 kbp/s or greater.  At least in critical listening environments according to the paper titled Perceived Audio Quality of Realistic FM and DAB+ Radio Broadcasting Systems (.pdf) published by the Journal of the Audio Engineering Society. This work was done by a group in Sweden that made various observations with different program material and listening subjects. Each person was given a sample of analog FM audio to listen to, then they listened to various audio selections which were using bit reduction algorithms (AKA CODEC or Compression) and graded each one.  The methodology is very thorough and there is little left for subjective interpretation.

In less critical listening environments, bit rates of 160-192 kbp/s will work.

I made a chart and added HD Radio’s proprietary CODEC HDC, which is similar to, but not compatible with AAC:

SystemCodecBit Rate (kbp/s)
HD Radio FM; HD1 channel*HDC (similar to AAC)96 – 144
HD Radio FM; HD2 channel*HDC24-48
HD Radio FM; HD3 channel*HDC24-48
HD Radio AM*HDC20-60
DRM30 (MF-HF)AAC/HE-AAC34-72
DRM+ (VHF)AAC/HE-AAC700
DAB+AAC/HE-AAC32 – 128
DABMPEG II, Dolby Digital192 – 256
Blu-rayPCM**≥6 Mbp/s
DVDPCM, DTS, Dolby Digital>800
CD-APCM1,411
Web StreamingMPEG I,II,III, WMA, AAC, etc32-320, 128 typical
iTunesAAC128 – 256
SpotifyOgg Vorbis96 – 320
WimpAAC/HE-AAC64 – 256

*Hybrid mode
**PCM: uncompressed data

This is the composite Mean Basic Audio Quality and 95% confidence intervals for the system across all excerpts:

digital-analog-audio-compar

Over the years, we have simply become accustomed to and now accept low-quality audio from mp3 files being played over cheap computer speakers or through cheap ear buds.  Does this make it right?  In our drive to take something good and make it better, perhaps it should be, you know: Better.

Special thanks to Trevor from Surrey Electronics Limited.

WE2XRH and the NVIS antenna

WE2XRH looks like an Amateur radio call sign but it is actually the call sign of an experimental short wave station in Alaska.  Transmitting DRM on 4.85 MHz, 7.505 MHz and 9.295 MHz with a Near Vertical Incident Skywave antenna system, they hope to cover all of Alaska and almost nowhere else with shortwave broadcast.

WE2XRH DART coverage with NVIS antenna system
WE2XRH DART coverage with NVIS antenna system

This license was granted for two years in August of 2008 and renewed again this September until  July 2012.  According to the website Nextgov.com:

The company told FCC that its initial tests would be funded by and conducted for the Defense’s Joint Electromagnetic Technologies program, a classified operation whose mission is to develop technologies for use by special forces and intelligence units.

Defense also will supply surplus transmitters from the closed, Cold War-era Over the Horizon Radar, located in Delta Junction. The radar system bounced shortwave signals off the ionosphere to detect aerial targets, such as Soviet bombers, at ranges up to 1,800 miles.

The transmitters are 100 KW Continental HF units, which for this applications are running about 20 KW.  According to this Yahoo Groups posting, several Japanese shortwave DXers have received the station in late 2009, but nothing recently.  I shot an e-mail off to their information address, but did not receive a reply.

On High Frequency (HF) NVIS has been used for several years where line of sight VHF communications are not possible.  Soldiers during the Vietnam war noticed that if a vertical whip was bent over so that it was horizontal to the ground, the signal strength was slightly less but the signals were much less prone to fading.

Near Vertical Incident Skywave antenna angle vs. distance
Near Vertical Incident Skywave antenna angle vs. distance

In this case, WE2XRH is using a crossed dipole antenna which generates a circularly polarized field.  With traditional HF skywave, polarization is not a factor since the ionosphere usually causes some field rotation anyway.  It is interesting that the system had this design consideration.

The NVIS is a novel approach and it may work on Medium Frequency (MF) during the night time, but daytime coverage would still have to rely on ground wave signal.  The FCC has historically approached MF skywave as a secondary and unreliable transmission method.  The idea being to reduce the antenna take off angle to as low as possible, hence the popularity of taller than 90 degree towers.  There is good validity to that practice as mixing the ground wave and skywave components at a receive antenna will cause multipath fading.

Setting aside a new broadcasting frequency segment, say 1.6 – 1.8 Mhz, a system could be designed to transmit DRM by using groundwave during the day with a traditional 90 degree tower, and NVIS at night with a horizontal dipole antenna.    Then never the two should meet.  The night time NVIS system would have a small ground wave component, out to a couple of miles.  In addition to that, the night time NVIS system can run on an adaptive power system, when propagation conditions are poor, more power can be applied to the antenna input and in better conditions, power reduced in accordance with a remote receive monitor that reports the Bit Error Rate (BER) back to the transmitter controller.

The best NVIS antenna is the 1/2 wave dipole positioned between 0.1 and 0.2 wave lengths above ground. In the 1.6  to 1.8 MHz band, that equates a half wave dipole antenna 260 to 292 feet long mounted between 66 to 90 feet above ground level.

This would have many advantages over the current directional antenna based MF broadcasting system currently deployed.  The current system is based on pushing potential harmful signals away from a station that was licensed to the same frequency (or an adjacent frequency) earlier.  This puts the onus for proper operation on the broadcast license holder.  Most don’t have the know how or resources to insure that a n AM directional is operating properly.  I would estimate at least half of the directional AM antennas in this country are out of tolerance.  With a NVIS based night time antenna system, coverage areas would be assigned much like an FM allotment.

The BBC conducted medium wave DRM tests in 2007 with satisfactory results during the daytime, but poor reception at night time due to co channel interference.  That is why DRM will not work on the current AM broadcast band and if digital radio is to be broadcast on MF, a new frequency band would be needed.