Digital radio may not have succeeded in Canada; but engineers at the CRC who have experience with it say they’ve produced a new module for receivers that may find application elsewhere.
A Communications Research Centre newsletter reports that researchers developed CRC-DABDetect, a module for DAB, DMB and DAB+ receivers to overcome distortion induced by the Doppler effect. (DAB+ and DMB are extensions of DAB that promise more efficient audio compression and multimedia broadcasting capabilities, respectively. DAB is used in the U.K.; Korea is employing a DMB system for mobile television; Australia and Germany are migrating to DAB+, according to a CRC summary.)
“When the receiver [in a vehicle] is on the move, the Doppler effect causes each sub-carrier of the OFDM multi-carrier DAB signal to move about its nominal position in a random manner,” stated Louis Thibault, manager of the group that developed the module, in the newsletter. “When you have several adjacent sub-carriers moving randomly, they create inter-carrier interference. This causes signal distortion, which produces performance degradation in the receiver.”
The module’s algorithm estimates inter-carrier interference and removes it from the signal before the signal is passed to the error correction part of the receiver.
Thibault noted that DAB in Canada was intended to be carried at L band, around 1.5 GHz. “This frequency is 15 times higher than FM, which is at about 100 MHz. The Doppler effect is directly proportional to two things: radio frequency value and vehicle speed. The faster your vehicle moves, the more severe the Doppler effect. At 1.5 GHz, the Doppler effect is quite severe. This is what we’re trying to estimate and cancel.”
According to the article, Doppler induces distortion only in certain DAB/DMB/DAB+ transmission scenarios. “This introduced a deployment dilemma in Canada and it was the impetus for the audio team’s research.”
Read the detailed CRC explanation here.
Thibault concluded: “It is ironic to note that the solution to cut down the coverage cost of DAB/DMB/DAB+ at L band lies in the receiver. The next step for us is to promote our DABDetect and DABSync technologies to foster their endorsement by broadcasters and their adoption by DAB/DMB/DAB+ receiver manufacturers. We hope this will encourage the use of L band for broadcasting digital radio and multimedia services.”