Louis Thibault

Subjective evaluation of state-of-the-art two-channel audio codecs

The results of double-blind subjective tests are reported, which were conducted to examine the audio quality of several state-of-the-art two-channel audio codecs against a CD-quality reference. Implementations of the MPEG Layer 2, MPEG Layer 3, MPEG AAC, Dolby AC-3, and Lucent PAC codecs were evaluated at the Communications Research Centre in Ottawa, Canada, in accordance with the subjective testing procedures outlined in ITU-R Recommendation BS. 1116. The bit rates varied between 64 and 192 kbit/s per stereo pair. The study is unique in that this is the first time that these codecs have been compared in a single test. Clear results were obtained for comparing the subjective performance of the codecs at each bit rate. All codecs were software based and constituted the most current implementations at the time of testing. An additional hardware-based MPEG Layer 2 codec was included in the tests as a benchmark.

Performance of cyclic delay diversity in DAB/DMB

Cyclic delay diversity (CDD) is a simple and elegant technique to exploit transmit diversity in a MIMO-OFDM system by artificially increasing the frequency-selectivity of the equivalent channel. In this paper, we study the application of CDD in digital audio broadcasting (DAB), which is well motivated by the mismatch analysis between the channel inherent diversity and the capability of the DAB channel coding and interleaving. Simulation of DAB with CDD in rural area and typical urban area is carried out; the results demonstrate the advantage of CDD in creating a more uniform coverage area compared to the system without CDD. We further devise a lower bound which allows justification of tradeoff between the design parameters and the performance improvement

A Low-Complexity Robust OFDM Receiver for Fast Fading Channels

Orthogonal frequency division multiplexing (OFDM) receivers suffer significant performance degradation in time-varying fast fading channels because of the inter-carrier interference (ICI) induced by the Doppler effect. We previously proposed a novel iterative decision-directed channel estimation and ICI cancellation technique which can provide robust performance for mobile OFDM receivers moving at fast vehicle speed. The nutshell of the proposed technique is a channel estimator which is capable of estimating the ICI gains on each off-diagonal vector of the channel frequency response matrix of a doubly-selective fading channel, which corresponds to the ICI induced by the neighbor subcarrier symbol to current subcarrier. In this paper, we generalize the channel estimator and provide theoretical analysis on the achievable performance. As an application to digital video broadcasting terrestrial system, we provide intensive simulation results and detailed comparison on implementation complexity and performance with existing techniques to showcase the significance of the proposed technique in practical applications.

EIA/NRSC DAR systems subjective tests. II. Transmission impairments

For pt.I see ibid., vol.43, no.3, p.261-7 (1997). This paper describes the methods, results and conclusions of a series of subjective tests which were performed at the Communications Research Centre, Ottawa, Canada, to assess the audio quality of digital audio radio (DAR) systems in the presence of transmission errors. Testing has been performed in a laboratory environment on the hardware implementation of nine DAR systems. One of the systems operated in the AM broadcast band, five in the FM broadcast band, two in the L-band and one in the S-band. Testing was done in the presence of transmission errors generated by additive white gaussian noise, co-channel interference and five different multipath mobile channels. In addition, one system was tested with an interfering signal in the lower first adjacent DAR channel. The test results are presented and discussed, for each type of transmission impairments, in terms of (a) the E/sub b//N/sub 0/ ratios (D/U ratios for co-channel and adjacent channel interference) at the threshold of audibility (TOA) of transmission errors and at the point where transmission errors are so important that the audio quality is unacceptable (point of failure or POF) and (b) the failure margin which is a measure of how quickly DAR systems fail when the received signal power is reduced. A new parameter, labeled C/sub SP//N/sub 0/, is proposed to quantify the power efficiency of the overall DAR systems (i.e. source coding plus channel coding and modulation subsystems). The spectral efficiency of DAR systems is also presented and discussed.

Fast communication: Design of multichannel frequency domain statistical-based enhancement systems preserving spatial cues via spectral distances minimization

It is often very important for multichannel speech enhancement systems, such as hearing aids, to preserve spatial impressions. Usually, this is achieved by first designing a particular speech enhancement algorithm and later or separately constraining the obtained solution to respect spatial cues. Instead, we propose in this paper to conduct the entire systems design via the minimization of statistical spectral distances seen as functions of a real-valued, common gain to be applied to all channels in the frequency-domain. For various spectral distances, we show that the gain derived is expressible in terms of optimal multichannel spectral amplitude estimators (such as the multichannel Minimum Mean Squared Error Spectral Amplitude Estimator, among others). In addition, we report experimental results in complex environments (i.e., including reverberation, interfering talkers, and low signal-to-noise ratio), showing the potential of the proposed methods against recent state-of-the-art multichannel enhancement setups which preserve spatial cues as well.

Statistical Multiplexing for Digital Audio Broadcasting Applications

In a shared telecommunications medium such as the Eureka 147 Digital Audio Broadcasting (DAB) system, we expect that available bandwidth will be optimally allocated to different users. This paper proposes optimal bandwidth allocation by use of statistical multiplexing that both service providers and multiplex operators can benefit from. Moreover, an auditory-based objective measure (i.e., ITU-R PEAQ) has been employed to make sure that the encoded audio materials maintain ¿broadcast quality¿. In our simulations of statistical multiplexing of five different ¿broadcast quality¿ radio programs, we show that a variable bit rate (VBR) stream with an average bit rate of 192 kbps could be saved as extra bandwidth in DAB to create new services. We also show that this VBR stream provides at least 112 kbps of extra capacity 98.5% of the time. The extra bandwidth can be used to transmit new audio programs, data or any side information to augment the radio listening experience.

EIA/NRSC DAR systems subjective tests. I. Audio codec quality

The method, results and conclusions of subjective tests of the basic audio quality of nine digital audio radio (DAR) systems are presented. These systems were proposed to the EIA/NRSC committees for standardization in the USA. The subjective audio tests were carried out at the Communications Research Centre in Ottawa, Canada for the Electronics Industries Association (EIA). Clear comparative results were obtained. The discussion focuses on the subjective merits of the various systems in relation to bit rate considerations.

A Novel Differential Detection for Differential OFDM Systems With High Mobility

In conventional differential detection (CDD), the previous received symbol is used as the phase reference for the current received symbol, therefore the channel variation due to fast fading causes an error floor. We propose a novel iterative decision-directed differential detection (iD4) algorithm for orthogonal frequency division multiplexing (OFDM) systems employing differential PSK modulation in each subcarrier. The core of iD4 is a low complexity time and frequency 2-D estimator which can effectively estimate the channel variation experienced by adjacent time domain symbols. This estimated channel variation is then compensated in the differential detection process. Both theoretical analysis and computer simulations show that significant performance improvement can be achieved using iD4 over the CDD for OFDM receivers in fast fading channels.