Developing an advanced digital audio broadcasting transmitter using the Digital Radio Mondiale (DRM/DRM+) standard. This transmitter must be capable of sending multiple audio services, text streams, and pictures, and it must work over the bands currently used for AM broadcasting. This transmitter also needs to take encoded source code data as an input. This product needs to be developed by a group of final-year students in a highly accelerated timeframe of six man-months.
Using thousands of built-in signal processing functions in NI LabVIEW and the NI Modulation Toolkit to accelerate the development of the IP for DRM/DRM+ along with the NI PXIe-5673 radio frequency (RF) vector signal generator to generate the signal and test it using a commercial third-party receiver.
Sandeep K. Yadav - Indian Institute of Technology Rajasthan
Anupam Mehta - Indian Institute of Technology Rajasthan
Amit Gaurav - Indian Institute of Technology Rajasthan
Shivansh Chaudhary - Indian Institute of Technology Rajasthan
Prateek Agarwal - Indian Institute of Technology Rajasthan
Ashish Katiyar - Indian Institute of Technology Rajasthan
Narendra Chaudhary - Indian Institute of Technology Rajasthan
Digital Radio Mondiale (DRM) is a standard developed for digital audio broadcasting technologies. It is a standardized digital broadcasting system for all broadcasting frequencies, including long wave, medium wave, short wave, and bands I, II (FM band), and III (above 30 MHz). This technology is used to work over AM broadcasting. DRM is capable of fitting more channels of better quality at a lower power utilization into a given amount of bandwidth than AM. Development of DRM revolutionized broadcasting technology and opens a new door to utilize limited available bandwidth. Efficient use of bandwidth is possible due to the availability of modern compression techniques and low-cost processing through computers. The DRM system is designed to be used at any frequency below 174 MHz, with variable channelization constraints and propagation conditions throughout these bands. DRM delivers FM-comparable sound quality on 30 MHz. An advanced version of DRM (DRM+) is also a consideration. This works well for very high frequencies between 30 MHz and 174 MHz.
We developed the transmitter for DRM and DRM+ using LabVIEW. It is capable of transmitting audio, text streams, and pictures, and can transmit multiple services using different modes. To satisfy these operating constraints, different transmission modes are available. A transmission mode is classified in two types, one based on signal bandwidth-related parameters, and the other based on transmission efficiency-related parameters.
The DRM architecture can easily be understood with the help of the given conceptual block diagram. Figure 1 shows the general flow of different classes of information such as audio and data, and does not differentiate between different services that may be conveyed within one or more classes of information.
The source encoder and precoders ensure the adaptation of the input streams onto an appropriate digital transmission format. For the case of audio source encoding, this functionality includes audio compression techniques. The output of the source encoder(s) and the data stream precoder may be comprised of two parts requiring different levels of protection within the subsequent channel encoder. All services have to use the same two levels of protection. To offer optimum quality during transmission, various source coding techniques are used with different bit rates.
The multiplexer combines the protection levels of all data and audio services. The DRM transmission super frame consists of three channels: The main service channel (MSC), the fast access channel (FAC), and the service description channel (SDC). The MSC contains the data for the services. The FAC provides information on the channel width and other such parameters and also provides service selection information to allow for fast scanning. The SDC gives information on how to decode the MSC, how to find alternative sources of the same data, and the attributes of the services within the multiplex. It can include links to analogue simulcast services.
The MSC contains between one and four streams. Each stream is divided into logical frames. Each logical frame generally consists of two parts, each with its own protection level. The lengths of the two parts are independently assigned. For modes A, B, C, and D, the logical frames are each 400 ms long. If the stream carries audio, the logical frame carries the data for one audio super frame. The logical frames from all the streams are mapped together to form multiplex frames of the same duration (400 ms). The audio super frames formed after source encoding are mapped to the logical frames, which are then multiplexed to form one transmission frame. Three transmission frames are then combined to form a transmission super frame which is 1.2 s long.
The FAC is used to provide information on the channel parameters required for the demodulation of the multiplex as well as basic service selection information for fast scanning. Each transmission frame contains an FAC block. An FAC block contains parameters that describe the channel and service parameters to describe one service along with the 8-bit cyclic redundancy check.
Energy dispersal provides a deterministic selective complementing of bits to reduce the possibility that systematic patterns result in unwanted regularity in the transmitted signal. The channel encoder adds redundant information as a means for quasi-error-free transmission and defines the mapping of the digital encoded information onto a quadrature amplitude modulation (QAM).
Cell interleaving spreads consecutive QAM cells onto a sequence of cells quasi-randomly separated in time and frequency to provide transmission in time-frequency dispersive channels. The pilot generator provides a means to derive channel state information in the receiver, allowing for a coherent demodulation of the signal. The orthogonal frequency-division multiplexing (OFDM) cell mapper collects the different classes of cells and places them on the time-frequency grid.
The OFDM signal generator transforms each ensemble of cells with the same time index to a time-domain representation of the signal. Consecutively, the OFDM symbol is obtained from this time-domain representation by inserting a guard interval as a cyclic repetition of a portion of the signal. The modulator converts the digital representation of the OFDM signal into the analogue signal in the air. This operation involves digital-to-analogue conversion and filtering that has to comply with spectrum requirements.
After implementing the DRM/DRM+ standard in LabVIEW, we tested our program by generating RF signals using the NI PXIe-5673 RF vector signal generator and received the DRM signal on a WiNRADiO G313e DRM receiver. Because the RF signal-generating range for the NI PXIe-5673 is 85 MHz to 6.6 GHz, we had to make certain changes to the devices’ settings, to generate the signal at 25 MHz. After certain debugging steps, we were able to hear single-service and multiservice audio streams on the DRM receiver. LabVIEW was a powerful tool for us because it filled the gap of knowing the theory and implementing the theory on the real hardware with an easy-to-use graphical programming language. LabVIEW made it easy for us to debug the code, since the graphical programming nature of the code closely mimicked the block diagram we created. The DRM/DRM+ transmitter project:
This project will be continued in the future so we can implement the transmitter on FPGA boards to build a transmitter system.
Sandeep K. Yadav
Indian Institute of Technology Rajasthan
Administrative Block NI Lab, IIT Rajasthan Old Residency Road, Near Ratanada Circle