Broadcasting, from AM to DAB



The first radio broadcast in the Netherlands dates from november 1919, i.e. over 100 years ago. The transmission, a live concert in AM, Amplitude Modulation, is said to be a european and even a world scoop.

AM transmission in the Medium waves

For a long time AM was the modulation type of choice, from the 20-ties to the 50-ies and 60-ies of the previous century. The type of modulation is very simple: just vary the amplitudes of the carrier in the rythm of the audio. The most simple transmitter would be an oscillator with something like a carbon mike in the antenna lead.

After the second worldwar, a slow transision to FM, Frequency Modulation took place. The drawback of AM, primarily transmitted on medium and shortwaves, is the sensitivity to interferences and noise. These interferences have effects on the amplitudes of the received signal, and since the amplitudes (or better the deviations in the amplitudes) are the information carriers, AM is - especially in the built environment - rather noisy. All kinds of electrical equipment, especially those small power adapters, generate sufficent noise to make listening to AM transmissions not always a pleasure.

Of course the advantage of using medium and shortwaves is that - depending on conditions - large distances can be bridged. In the 30-ties there were radio connections between the Netherlands and Indonesia wwith large shortwwave transmitters.

A second drawback of AM is that the transmission is not very energy efficient, the signal has a carrier and two sidebands (see picture). Since the information is stored in both sidebands, and since the carrier takes 50 percent of the transmitted energy, the efficiency is not more than 25 percent.

am signal

Of course, the main advantage of AM is that equipment needed for receiving AM transmissions can be extremely simple, crystal receivers were the most basic ones. Books and magazins in the 30-ies of the previous century showed schemes and construction descriptions for DIY receivers varying from one tube straight receiver to many tube superheteodyne sets. Even in the 50-ies and early 60-ies, radio magazins (we had Radio Bulletin) gave building descriptions, including layouts for the chassis (I build a number of them).

Amateurs use a variant, e.g. transmission of a single sideband of the signal, on the 80 meter band the Lower sideband (LSB) is commonly used, on the bands on higher frequencies the Upper side band (USB) is commonly used. Decoding USB or LSB signals with a common AM receiver sounded more like ducks croaking.

There were - already in the 30-ies - several attempts to transmit stereo signals using AM. One approach was to use the side bands to carry the left resp. right signal, another approach, eventually leading to a standard in the US standard is using C_QAM. In this approach the L+R signal is modulated as a regular AM signal, the L-R signal is modulated as a phase modulated signal with a limited amplitude and added to the AM signal.

usb signal

FM transmision

While already invented in the 20-ties of the previois century, the development of FM really started after the second word war. With Frequency Modulation, the information is encoded in variations of the frequency. Amplitudes do not play a role, and FM is therefore less sensitive to the interferences that effect amplitudes.

FM is positioned in the FM broadcast band, a VHF band from - in Europe and the US - 88 to 110 MHz. An important practical difference between AM and FM is the bandwidth required, which makes sense since the information is encoded in frequency variations. While an AM transmission in the MW and SW uses a bandwidth of 9 KHz (with 4.5 KHz as highest frequency in the payload), current FM transmissions take a bandwidth of over 100 KHz. Of course this comparison is not very fair, in the FM transmission a stereo signal with an audio frequency range from 1 .. 15 KHz is transmitted, together with additional RDS information..

The first FM transmissions were all mono, later a stereo facility and a data facility were added. So a logical question is how to encode a stereo signal, such that upward compatibility is ensured.

For FM that issue was solved by creating a single signal from the addition of the Left and Right component of a stereo signal, i.e. L+R and transmit that and adding an L-R component somewhere else to the signal. Mono receivers then receive just the L+R signal and ignore all other components, stereo receivers extract both the L+R and the L-R signals and reconstruct the left and right channel data.

The L-R signal is AM modulated on a carrier of 38 KHz which is added to the original signal. The 38 KHz carrier, used for modulating the L-R signal is the second harmonic from a 19 KHz pilot signal that -as the name suggests - is added to the original signal as 19 KHz signal.

A small data component is also added to the signal, the RDS signal. Encoded as an 1187 Hz digital signal, this RDS signal is put on the third harmonic of the pilot, i.e. on 57 KHz.

A picture of the signal is given. The picture on the right gives the spectrum as it is being received, the picture on the left shows the decoded signal, to the left the L+R signal, then the 19 KHz pilot signal, then the AM modulated signal, and at 57 KHz the RDS signal.

fm signal

In the (late) 50-ies and 60-ies of the previous century, radio magazines presented all kinds of DIY receivers. Since the transmissions were in the VHF band, an other type of antenna was needed. Propagation of signals in the FM band is limited, so requirements for antenna and equipment were more strict. The receivers were more complex. While an AM receiver usually has a single MF stage - usually something like 450 or 470 KHz, FM receivers usually needed two or more MF amplification stages, usually on 10.7 MHz, and - of course - decoding is more complex and takes more electronics than a simple diode. The introduction of transistors made a real difference, especially when transmissions became stereo transmissions with additional RDS data. While the early receivers were essentially "addons" to existing AM receivers, with a separate tuner, but a limited number of tubes, later on there was no limit to the number of transistors in FM receivers. o

Digital radio

Second half of the 90-ies of the previous century, attention was given to the use of digital data to encode radio and television signals. In Europe that lead - using Eureka projects - to the development of two standards for digital radio, DAB, Digital Audio Broadcasting, and DRM, Digital Radio Mondiale. The name digital is somewhat of a misnomer, in the end the signals transmitted and received are all analog, the payload is encoded digitally.

Characteristic for digital transmissions is the spectrum. Rather than a single carrier on which the payload is modulated, both DAB and DRM use many carriers - close to each other, each carrier being modulated with a small part of the payload. DAB uses 1536 carriers with a spacing of 1 KHz, DRM, depending on the mode 256, with a spacing of app 45 Hz. A DAB channel therefore takes app 1.5 MHz, the distance between DAB channels is app 1.7 Mhz. DAB is being transmitted in the "old" Band III, there is room for 4 DAB channels per TV channel, and there are 39 DAB channels.

A single DAB transmission carries data for what is called an emsemble, a group of between 10 and 20 services. Here in the Netherlands, we have the NPO (The National Broadcasters) with app a dozen services, and the Commercials with app 20 transmissions in a single ensemble.

Where I live I can receive DAB data on about 5 to 6 channels using a single whip as antenna, so in total I have a choice among over 60 services. But, as everywhere, quantity does not imply quality.

DRM, on the other hand, is meant to be transmitted on medium and shortwaves, in broadcast bands where also regular AM transmissions are. DRM defines different modes and spectrum requirements that can be used. Most transmission use a bandwidth of 9 KHz, similar to the bandwidth of AM transmissions. Since a DRM transmission carrier digital data, more than a single service can be encoded in a single transmission. Up to 4 services can be encoded, although I have never seen a transmission with more than 2 services.

A variant of DRM, DRM++ is meant to be transmitted in the FM broadcast band and has a bandwidth of 96 KHz, also with 1 to 4 services. The transmissions I have seen (well, actually only recordings of the raw data of these transmissions) carry 4 audio services.

DAB (and its later developed variant DAB+) is the "system of choice" in Europe, commercials, both in papers, on (FM) radio and TV urge us to abandon FM and switch over to DAB.

The picture shows the 1.5 MHz wide spectrum (top left) and the waterfall of the spectrum. Top right (hard to see) is the constellation of the signal, i.e. a cloud of dots.

dab signal

In Europe DRM was not very successful, while in the first decade of this century there were quite some countries with transmissions, I only see a DRM transmission from Kuwait of a couple of hours in the afternoon, and a couple of transmissions from Romenia, in different languages, each transmission for up to an hour. It seems that in other continents DRM is gaining in importance.

drm signal

Radio equipment and software

It is quite obvious that using a simple diode detector, or even a handful transistors, capacitors and resistors, is not enough to build a receiver for the various forms of digital transmissions. Next to something to receive the analog signal and convert it to samples, one does need computing power. Of course, once computing power is available this can be used for AM, single side band, and FM as well.

When applying computing power, e.g. a PC, radio reception requires a separate component for receiving signals, if possible some filtering, transforming the received signal to one on an Intermediate Frequency of 0 Hz, and converting the analog signal into a sequence of samples. Currently there is a tremendous amount of such devices on the market, they all have in common that the covered range of frequencies is up to about 2 GHz or more, and they all can be controlled programmatically using the USB interface.

Having such a device, decoding AM, single sideband and FM, using a computer is trivial and the most basic form, AM decoding, is essentially only taking the absolute value of a complex sample. In a program this is usually one or at most a few lines of program. Decoding the amateur modes usb and lsb requires some phase shifting, but in software that is no real problem, decoding is - again - expressed in one or two lines of program. The complexity of the software for processing these analog modes is more in filtering and controlling the radio device sending the samples, than in the actual decoding of the signal.

Processing current FM transmissions is slightly more complex, not because of the basic decoding of the signal, which amounts to computing the phase difference of successive samples, but because the raw decoded signal contains an L+R component, a pilot signal, an L-R component and a digital RDS signal. Since for extracting the L-R signal and the RDS signal the pilot is required one needs to implement some DSP algorithms.

Processing digital modes is - again - slightly more complex, usually an OFDM technique is used to - on the transmitter side - encode the sognal using an inverse FFT processor into a stream of bits, the through a DA converter into a nice analog signal. On the receiver side the process is reversed, the analog signal is converted into a stream of bits using an AD converter, using an FFT processor these bits are then converted into someething that is then eventually transformed into data for audio.

As a rule of thumb, decoding samples from an analog AM signal takes a couples to - at most - a couple of dozens lines of code, decoding samples from a analog FM stereo signal takes a couple of hundred lines of code, and decoding samples from an analog DAB signal takes a few thousand lines of code.

As mentioned, in the late 50-ies and 60-ies - my teener years - DIY "radio" was really "in", there were magazins with schemes and building instructions for radios (and of course amplifiers and even TV's) and it was really fun seeing the glowing of the tubes when switching on a newly built (variant of the) radio.

After being retired, rather than heating my soldering iron, I started with simple forms of digital signal processing. While starting with data transfer between equipment and computer using a sound card, the fun really started after obtaining (very) decent equipment that is capable of (a) being tuned to a frequency between a few KHz and up to a few GHz, and (b) send IQ samples with higher rates to the computer,

The result is

  1. a so-called SW receiver, a program that handles a number of broadcast and amateur modes and supports the SDRplay devices, the Hackrf kit, and - limited, due to frequency constraints of the device - dabsticks (i.e. RTLSDR devices). Typical broadcast modes supported are AM and DRM, amateur modes are rtty, psk, CW, utility modes are navtext (518 KHz) and weathercharts.

  2. a specialized software receiver for decoding DRM transmissions, supporting the same set of devices as the SW receiver does.

  3. a specialized software receiver for decoding DRM+ transmissions (although in this region there are no DRM+ transmissions), Since DRM+ transmissions are in the FM band, the software supports the devices that can handle the frequencies in that band, SDRplay devices, Hackrf devices, Lime devices, AIRspy devices, and the Adalm Pluto devices.

  4. a rather extensive FM receiver, supporting the same set of devices as the DRM+ software;

  5. A DAB decoder, i.e. a decoder for DAB signals with different forms of appearance.

    1. The GUI based versions Qt-DAB4 and Qt-DAB5, two versions using the same decoding and processing modules, differing in the GUI. During the evolution of that software, more and more functionality was added, leading to more and more displays and controls. While the GUI of earlier versions already consisted of more than one widget, the number of controls on the main widget was growing and growing. In the "5" version of the software the main widget was simplified to contain just a handful of controls, oter controls (and displays) can be made visible in secundary widgets.

    2. The command line version, a version where the functionality for decoding DAB is implemented as a library, and a handful of example programs is available.

    3. The ETI generator. ETI, Ensemble Transport Interface, is a formt for transporting - as the name suggests - the full data for an ensemble.

    4. Some more gadget type receivers, such as a terminal DAB version where the interaction is on a command window, and a very simple web DAB version, where - as the name suggests - the interface is using a browser.