Understanding DRM: digital radio mondiale
My friend, John Piliounis, a Planet Analog blogger with shortwave call-sign SV1OCS, wrote an intriguing article on the Spectrum Monitor in May 2017 entitled DRM—Digital Radio Mondiale: Spectrum within a Spectrum4.
I have never operated a shortwave radio, nor have I ever had a license (I always feared the Morse Code test in my youth), nor have I ever had a call-sign (except for “Rocket Man” because of my NASA exploits). I had never heard of DRM, so I did some research into it.
Defining digital radio mondiale (DRM)
DRM is actually a digital radio standard which broadcasters designed for broadcasting. Transmitter and receiver manufacturers, along with regulatory bodies, played a significant part in defining this standard. The ‘raison dêtre’ is that a high quality digital replacement for legacy analog radio broadcasting in the AM and FM/VHF bands was needed.
DRM operates with the same channel and spectrum allocations which AM and FM/VHF now occupy (Figure 1).
Figure 1: The frequency bands in which DRM operates among the other shared services are shown here. (Image courtesy of Digital Radio Mondiale)
The DRM standard identifies two operating modes: DRM30 and DRM+, as can be seen in Figure 1. DRM30 modes use the AM bands below 30 MHz and DRM+ modes use the 30 MHz to 300 MHz spectrum centered upon the existing FM band.
DRM is an open system in which all manufacturers have free access to the complete technical standards, and are also allowed to design and manufacture equipment equally and freely. This DRM standard is the only one recommended by the International Telecommunications Union (ITU) to be used in short wave bands.
Modulating the DRM signal4
Here is how the DRM broadcasting system transmits a signal to DRM receivers:
Figure 2 Modulating the DRM signal (Courtesy of DRM.org)
Piliounis discusses the prime mechanism by which the broadcast content comes together and goes into the spectrum’s air. The technique is based on a content multiplexer-server, called the DRM Content Server. A Configuration Interface Unit, which is part of this server, using programmable or predefined broadcasting templates and schemata, multiplexes audio, data, and other desired content like GPS info and streams it in the form of frames through an OFDM modulator into the air.
There is now a transition taking place, moving away from analog transmissions to a full DRM system. In the interim, broadcasters are able to simultaneously broadcast both analog and DRM content through the same transmitter and antenna; thus they are simulcasting.
DRM uses a modulation method known as Coded Orthogonal Frequency Division Multiplexing (COFDM) where each separate carrier (with a maximum of four) is QAM modulated.
A quality DRM receiver1,2
Traditionally, radio communication signals with high signal quality and propagation over long distances have been elusive in the shortwave bands of up to 30 MHz. The analog modulation schemes previously used coupled with shortwave radio’s difficulty during atmospheric disturbances have made this task a difficult one.
In 2001, when DRM was initially developed, new digital coding techniques, audio compression, and data protection features for shortwave were newly implemented in this digital standard. This enabled not-for-profit humanitarian broadcasters, operating in underdeveloped nations, to have high-quality, long-distance transmission capability that was at a lower cost than satellite radio.
Piliounis states that "the fundamental advantages of DRM are its ability to deliver sound of exceptional quality, never heard before on AM, HF, FM and VHF bands, along with data content that compliments the audio experience. It offers a mix of other information services, such as emergency weather notifications, through any portable or car radios with DRM enabled receivers."
A good example of a quality DRM receiver that was designed to be capable of demodulating a single DRM transmission (channel) and then compared to the commercial DRB-30 receiver can be seen in references 1 and 2. Figure 3 shows the block diagram of this receiver.
Figure 3 A block diagram of the DRM receiver design in references 1 and 2. (Image courtesy of Reference 1)
This is an RF bandpass filter after the antenna that will improve the receiver’s out-of-band overload quality.
Variable gain amplifiers (VGA)/analog-to-digital converter (ADC)
The filtered signal next passes through the low noise figure (NF) VGA which improves the sensitivity of the receiver. The VGA’s 45 dB gain range and high third order intercept (IP3) point increases the receiver’s dynamic range (see Texas Instrument’s paper on Calculating noise figure and third-order intercept in ADCs). This signal now enters the ADC input and is digitized.
That resulting digitized ADC digital output is sent to the FPGA which is configured as a digital downconverter (DDC) that in turn downconverts the signal to baseband frequency as well as providing adjacent channel selectivity. The QAM I and Q components are taken from that data stream and automatic gain control (AGC) is applied to the signal.
USB link (microcontroller)
A Microchip PIC microcontroller receives the I/Q packets from the FPGA and sends them to a computer over the USB 2.0 connection. The computer runs a modified open-source DReaM software program which coordinates the AGC with the FPGA and also demodulates the DRM data which now becomes audio. Sourceforge has a software radio for AM and DRM.
More work was done to improve the receiver’s performance in Reference 2.