Tag: DRM

Digital Audio Broadcasting Survey results

I have been looking over this data for a few weeks and there are some interesting data points. First, I would like to thank everyone who participated. This is not a scientific poll, but rather an informal survey of those who chose to participate.

The survey consisted of ten questions and was posted on Facebook, Reddit as well as Radio World. There were 114 responses, which is a relatively small sample size and is less than anticipated. There were 5 people who opened the survey and then did not take it. This may indicate a level of apathy towards the subject. Most responses were from the United States, but there were a few from China, Europe, and Brazil. The average time to complete the survey was 1 minute 40 seconds.

The first question was a warm-up question and it shows a lukewarm response at best with the top two responses being “It’s Okay,” or “I am indifferent.”

A vast majority of respondents feel that testing other Digital Audio Broadcasting systems such as DRM30, DRM+, or DAB+ would be a good idea. In MB docket 19-311, the FCC left the door open for such testing in the future, stating in paragraph 26 “Finally, we (the FCC) emphasize that by approving use of HD Radio technology, we do not foreclose the possibility of authorizing alternative technologies in the future, if they are properly before us.”

Question #2 was an attempt to find out where most people are listing to HD Radio and radio in general. Not a great surprise that it is mostly in-car listening. The in-home listening is a little bit surprising. What is even more surprising is that 30% of the respondents do not have an HD Radio. HD Radio has been the digital audio broadcasting standard in the US since October 2002, when the FCC first authorized its use. Receivers are still an issue some 20 years into the project. I know that when I purchased a new vehicle (Ford) in April of this year, HD Radio was not an option in any but the highest trim packages. My first HD Radio receiver was a tabletop Sony XDR-S3HD purchased in 2006 or so for $200.00 which was a lot of money. A quick look on Amazon shows that the least expensive HD Radio is the Sangean HDR-14 for $70.00.

Questions #3 and #4 deal with the “analog sunset,” as originally proposed by iBiquity, the developer of HD Radio technology. After a period of time, according to the original plan, stations would turn off their analog signals in favor of all digital transmissions. In October of 2020, MB Docket 19-311 the FCC has allowed AM stations the option to do just that. Thus far, four AM stations have transitioned to all-digital broadcasting, one of which is off the air since the owner passed away.

According to the survey respondents, by a slim margin of 53-47%, all-digital AM is supported. The FCC has yet to consider all digital FM and by an equally slim margin of 46-54%, all digital FM is not supported.

Question #7 asks about perceived audio quality. I received a few email comments about this question. Three respondents noted worse audio quality on HD-2, HD-3, or HD-4 channels due to reduced bit rate CODECS. Five people skipped this question.

This gets to the crux of the problem; for radio station owners, it is expensive to purchase and install HD Radio equipment. If there are no great perceived improvements, what is the point? I find AAC audio codecs to be okay, however, there is a noticeable difference between CD player PCM and streamed audio no matter what the source. Low-bit rate codecs sound like they are coming from underwater. Why do we listen to the radio? Information and entertainment. I posted something many years ago: Listening to the Radio is like doing Cocaine. For the maximum dopamine effect, I like my music to sound like music, not some watery approximation.

Question #8 asks about additional features, most people find Program Associated Data (PAD) useful. Even in non-digital FM stations, RDS is an important feature and stations will get phone calls if the RDS is missing or stuck on one song for a prolonged time. Listeners have become used to glancing at the radio to answer that age-old question; what’s the title of this song?

In response to question #9 (How many hours per week do you listen to the radio (including streaming terrestrial broadcast radio stations via a website or smart device)? The average was 19.6 hours with a minimum of half an hour and a maximum of 90 hours. Interestingly, there were four people who put in 0 weekly listening hours.

Question #10 is very interesting. In spite of the lukewarm feelings it seems that most respondents would favor the FCC mandating a transition to all digital audio broadcasting by a margin of 62% with most opting for “at some point in the future.”

HD Radio has been stalled for some time. The technology has not lived up to the hype and for most stations, it is a way to feed an analog translator with additional programming. There is an overall lack of interest, the majority of those who did take the survey stated HD Radio was “okay.” Receivers are expensive and still difficult to obtain. All digital AM (HDMA3) has not progressed very far since the FCC allowed its use. Yet, the people who did respond felt that additional testing of various Digital Audio Broadcasting systems should be allowed. I don’t know, that ship may have already sailed.

On the subject of All Digital Medium Wave

I want to explore all digital modulation methods for Standard Broadcast (AM, Medium Wave, or Medium Frequency). The most pressing technical problem for AM reception is electrical impulse noise. Can digital modulation solve this problem? Perhaps, but I am a natural-born skeptic.

To start out; I will say up front that the hybrid HD Radio (MA1) employed on AM was (or still is) a travesty. It never worked very well and it created massive interference +/- 20 KHz of the assigned frequency, especially when employed at night. Secondly; the all-digital version of HD Radio (HDMA3) remains a proprietary system with non-standard codecs. The current owner, Experi, has a license fee structure based on station type (AM, FM, LPFM, or Non-commercial) which ranges from $5,000 to $10,000 one-time fee for a five-year period. In all fairness; DRM pays a technology license fee to Fraunhofer for MPEG codecs used by receiver manufacturers and broadcast equipment. This is estimated to be between $0.13 to $1.13 US per receiver.

Those things being said, I thought a deep dive into the technical side of HDMA3 and DRM (Digital Radio Mondial) would be interesting. I did an article comparing MA3 and DRM a while ago: All Digital Medium Wave Transmission

What challenges are there to transmitting digital radio on MW? First, there is the very limited bandwidth of the channel itself. In North and South America, AM channels are spaced every 10 KHz (9 kHz in other places). On Medium Wave, the analog channel is +/- the carrier spacing, e.g. 20 KHz (or 18 KHz) with half of that channel potentially interfering with the adjacent channels. On a 20 kHz channel, this limits data transmission rates to 72 kbps or less with DRM and 40 kbps or less with HDMA3.

Secondly, skywave propagation is a potential difficulty for all digital broadcasts. Ionospheric changes can create multipath and fading, especially as the sun rises and sets causing the D layer to form or dissipate. Changes in the E and F layers can make or completely break skywave reception. Ground wave reception is reliable out to the limits of the noise floor, and varies based on transmitter frequency, power, and ground conductivity, and electrical noise in the area.

Everything that can potentially mitigate noise and skywave reception problems is a trade-off between robustness and data throughput.

Screenshot of an HF DRM exciter from RF Mondial showing a 10 KHz wide channel on HF.

Screen Shot of an HF DRM transmission showing mask, courtesy of DRM Consortium

This is a screenshot of an SDR showing an HF DRM transmission received from a distance:

Radio Romania International, 13,650 kHz 90 KW, 7,530 KM path

The receiver is not quite on bearing for this broadcast, however, it seems to be doing well. This is Radio Romania International’s Spanish broadcast targeting South America. The Pan Adaptor shows the signal is 10.2 kHz wide, but that doesn’t mean much from a $30.00 RTL SRD. The waterfall display below shows it is spectrally dense compared to the analog signals to the left and right. Note that with DRM there is no analog carrier being sent. Instead, a series of pilot tones are attached to various OFDM subcarriers for the receiver to lock onto.

A short Primer on COFDM

The modulation method for both systems is Coded Orthogonal Frequency Division Multiplexing (COFDM), which is the same system used by mobile phones, cable systems, WiFi (802.11), ATSC 3.0 TV, etc. COFDM consists of a group of subcarriers multiplexed onto one channel. The number of subcarriers and the subcarrier spacing relates directly to the data throughput and the robustness of the signal. OFDM is a very robust method that works well in the upper VHF, UHF, and SHF bands. It can work well in lower frequencies, however, there can be issues with multipath and Doppler effect. The coded part consists of forward error correction, which may include interleaving and subtracts from the data throughput.

The ability of an OFDM signal to reject electrical impulse noise, and deal with potential fading or multipath interference is based on a few things. The cyclic prefix sets the Guard Interval for the OFDM frame. The length of the Guard Interval should be the same as the multipath delay which helps mitigate inter-symbol interference and inter-subcarrier interference. Since the Medium Wave channels are fairly narrow, the number of OFDM carriers and spacing between carriers have a great effect on robustness. The fewer carriers the more robust the signal. This comes at the expense of data throughput; the fewer carriers the less data can be sent.

Raised Cosine impulses, similar to Orthogonal frequency-division multiplexing. (2023, May 10). In Wikipedia. https://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

A short Primer on QAM

Each individual OFDM subcarrier is modulated with a Quadrature amplitude modulation (QAM) signal. The advantage of this is that each individual carrier sends data at a relatively slow rate and the aggregate data rate is the sum of all the subcarriers. QAM uses two carriers 90 degrees out of phase. The amplitude of each carrier determines the resultant vector of the modulated wave to create a data bit. For example; the sum of the carriers equals +45 degrees at 25% amplitude a 1101 data bit is sent.

16-QAM Constellation diagram. (2022, December 17). In Wikipedia. https://en.wikipedia.org/wiki/Constellation_diagram

Both HDMA3 and DRM can use 16-QAM or 64-QAM. The larger the QAM constellation the more data can be sent. Smaller QAM constellations are more robust. HDMA3 can also transmit QPSK, which is Quadrature Phase Shift Keying. The resultant waveform from QPSK is identical to 4-QAM.

Bringing it all together

Radio Romania International, 11,975 KHz 90 KW 7,530 KM path, English service to Western Europe and North America,

A DRM-modulated HF and MF transmitter uses both sidebands to transmit unique information. There is no carrier present but rather a few pilot frequencies for the receiver to lock onto.

WFAS White Plains, NY All digital HDMA-3 signal

I like the waterfall display available with many SDR software programs. It gives a good indication of modulation density. With WFAS HDMA-3, the area +/- 5 KHz of the carrier signal has more power than the areas that are +/- 5 to 10 KHz from the carrier.

An HDMA3-modulated MW carrier sends the same data on upper and lower sidebands, effectively halving the data rate of DRM. There is a full carrier present, which represents approximately 25% of the transmitted power and does not contain any data. Currently, there are four three HDMA-3 stations transmitting in the US.

Both systems can make pre-corrections to the modulated signal in the exciter to compensate for amplifier non-linearities. This can greatly improve the MER and SNR.

The other perceived technical issue with AM radio is sound quality. This has to do mostly with poor-quality receivers, although there are some AM stations that are transmitting reduced-quality audio as well. There is a false notion that anything “digital” sounds better than analog. I would posit; it depends on several factors. Low-bit-rate audio codecs can sound abysmal. That being said, the newer high-efficiency audio codecs can sound quite good, but there are limits. With HD Radio, there is only one codec available; HDC+SBR. With DRM there are several; xHE-AAC, HE-AAC. xHE-AAC is designed to work with voice and can use bit rates as low as 12 kbps. It is possible for a robustly transmitted low-bit-rate codec to sound good with voice. It can sound okay with music, but not as good as analog FM.

Conclusion

Can an all-digital modulation format work well on the Standard Broadcast Band? The answer is; it’s complicated. One of the big positives of AM is that it is a very simple and well-tested system. Adding many layers of encoding and decoding is a violation of the KISS principle. That being said, using a digital modulation method that has been refined for mobile use over the years is a step in the right direction. There still is an issue with digital receivers; both HD and DRM. From what I have read, both formats are currently being included in several radio chip sets, yet I do not find those options in most car radios. There is a lack of public awareness, at least in the United States about digital radio in general. When someone says digital, most people think of streaming. When I am driving a rental car, I seldom find HD Radio, I do find Sirius/XM and all types of internet connectivity via smartphone apps.

What time is it?

This is an important question these days. We are running into more situations where timing is important, especially when audio and video codecs are concerned. If there is too much time differential, the codec will unlock. More often, digital transmission methods require precise timing to prevent jitter and dropouts. Some equipment has 10 MHz or 1PPS inputs. Some equipment does not and relies on NTP to keep things in sync.

While searching online for GPS time sever, I came across this post where Austin built a Stratum 1 level time sever with a Raspberry pi and an inexpensive GPS receiver. I thought to myself; damn that sounds interesting. While a Raspberry pi is a hobbyist toy, the same setup can be done with a more serious computer to create a solid NTP server for a facility or LAN.

A little about NTP time servers; Stratum 0 server is directly connected to an atomic clock. Since GPS satellites have atomic clocks, that makes them a Stratum 0 server. Stratum 1 servers are connected to Stratum 0 servers. Stratum 2 servers are connected to Stratum 1 servers and so on. The time accuracy for a Stratum 1 server is 10 microseconds.

First, I wiped my SD card and loaded a fresh install of Raspberry pi OS. Then followed along with the instructions. For this install, I opted for the cheaper GPS receiver, the GT-U7 (not an affiliate link) from Amazon for $10.99. It comes with a cheap little antenna, which actually worked sitting inside on my desktop while I was configuring the software.

This little module is designed for a drone but works well in this application. The 1PPS output looks clean on the scope. Here is the pinout between the GT-U7 and the Raspberry pi:

GT-U7 pinpi pinUseColor
vcc1+3.3 vdcGreen
gnd6groundBrown
txd8rxdOrange
rxd10txdRed
pps12GPIO 18Yellow

I found this really nice aluminum case in a pile of disused junk at a transmitter site. It used to be for a digital TELCO STL circuit. I figured it would be nice to put the Raspberry pi and GPS receiver in a suitable home.

Raspberry pi 3 is mounted on a piece of scrap sheet steel designed to slide into the aluminum case.

We have several of these nice Panasonic GPS antennas left over from various installs. I pressed one into service on the roof of my house.

Panasonic CCAH32ST01 GPS antenna

I think a high-quality antenna is pretty important to get consistent good performance from this setup. There are three slight problems, however. Unfortunately, this antenna has been discontinued by the manufacturer. Also unfortunate, the GT-7U boards have one of those little IPX RF connectors. Fortunately, I found a short jumper with an F SMA connector. Finally, it requires +5 VDC and the GT-7U runs on 3.3 VDC. The pi does have a 5-volt rail, so I used this 2-way power divider to feed 5 volts to the antenna from one port and the received RF from the antenna goes to the GT-U7 from the other port.

If you are interested, here are the commands to get this thing running:

sudo apt get update
sudo apt get upgrade -y
sudo apt install pps-tools gpsd gpsd-clients chrony

The next step is to make sure the serial port is turned on and enable the ssh login shell since this is going to live in the basement and I don’t want to run down there to fool around with it.

sudo raspi-config

Then go to interface options, serial interface, and enable. The login over the serial interface can be left off. If ssh access is needed, enable ssh, then exit.

Once those packages have been downloaded and installed, some config file editing is needed. You may use whichever method you like, I tend to use nano. First, the /etc/config.txt and add the following to the file:

‘dtoverlay=pps-gpio,gpiopin=18’
'enable_uart=1'
'init_uart_baud=9600'

The uart needs to be enabled if you want to receive NMEA data (NMEA stands for National Marine Electronics Association) It is helpful to see if or how the GPS is working.

Next, the /etc/modules and add:

'pps-gpio'

Reboot, then see if the pps module is working:

lsmod | grep pps

The output should look like this:

Next, there are a few more configuration files that need to be edited.

/ect/default/gpsd – there is a default file that comes with the package, it needs to be modified to start the daemon automatically and look for the pps signal on ttyS0.

START_DAEMON="true"
USBAUTO="true"
DEVICES="/dev/ttyS0 /dev/pps0"
GPSD_OPTIONS="-n"

Reboot

Now check and see if the GPS module is working by typing cgps or gpsmon. The output should look something like this:

It did not take the module too long to find and lock onto GPS. If you don’t see something like this in five minutes or so, go back and check your wiring, and make sure that the data connections are made right. The GT-U7 has a little red LED that is lit when the PPS pulse is not being sent. If this light is not on at all, check your power connection. If it is on steady, check your antenna. If it is flashing, but you are not seeing any output in cgps or gpsmon, check your data connections.

Next and last configuration file is the /etc/chrony/chrony.conf file. At the top of the file, I added the following lines:

#custom lines for PPS
server time-a-g.nist.gov iburst
server time-d-g.nist.gov
server 3.us.pool.ntp.org
server time.windows.com
server time.apple.com
# add refclock pps
refclock SMH 0 delay .1 refid NEMA
refclock PPS /dev/pps0 refid PPS
#my home network
allow 192.168.1.0/24

Leave the rest of the file alone. Basically, the time servers are added to compare the GPS time and act as a backup. The hosts on my home network are allowed to query this host and use it as an NTP server.

Restart Chrony:

sudo systemctl restart chrony

Wait a couple of minutes and check the chrony console to see what is happening: chronyc sources. Should look something like this:

This was after the server had been running for a day. Chrony is great because it measures the hardware performance and creates a delay file. This is used to anticipate any hardware-added delays that the system might have. The last sample column is of interest, the number indicates the offset between the local clock and the source at the last measurement. The far column is the margin of error or greatest variation +/- of the expected values. A value of 0.0000000042 seconds or 0.042 microseconds is pretty good for an $11.00 piece of hardware. Now every host in my house is syncd to satellite within 0.042 microseconds, in lockstep through the time-space continuum.

If I were to do this professionally, I would use better hardware. I think the pi 4 has better serial and ethernet interfaces, more RAM, and a quad-core processor. Last I looked they were $75.00 at Newark.

The GPS module was the cheapest I could find on Amazon. I am slightly concerned about the longevity of this device. Perhaps it will run for a long time, or perhaps not. A quick search brought up several “hats” (plug directly into the 20-pin header). These range in price from about $30.00 to $60.00. What is required of any GPS module is 1PPS output. The configuration would be about the same although some use GPIO 4 instead of 18.

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, nighttime coverage over the 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 nighttime 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 were 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 database includes stations that are currently dark or stations that were 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 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 that 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 nighttime 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.