Matthias Rüngeler

Design and Evaluation of Hybrid Digital-Analog Transmission Outperforming Purely Digital Concepts

Efficient digital transmission of analog speech, audio or video requires source coding which introduces unavoidable quantization errors. The bit stream produced by the source encoder needs to be protected against transmission errors by channel coding. The split of a given gross bit rate between source and channel coding is a compromise taking into account the design target of the worst case channel. Thus, even in clear channel conditions the quality of the decoded source signal is limited because of the quantization errors. Hybrid digital-analog (HDA) codes address this limitation by additionally transmitting the quantization error with quasi-analog methods (discrete-time, quasi-continuous-amplitude) with neither increasing the transmit power, nor the occupied frequency bandwidth on the radio channel. However, the decoding complexity of existing HDA codes is infeasible for the required block lengths. In this paper, the decoding complexity problem is solved by a new design approach. These HDA codes benefit from well-known digital codes. Furthermore, theoretical bounds are derived and explicit guidelines for the design of superior HDA systems are given. By experimental verification it is shown that the HDA concept may outperform conventional purely digital transmission systems at all channel qualities while additionally eliminating the quality saturation effect.

Hybrid digital analog transform coding

One problem of conventional digital transmission systems is that the transmission fidelity always saturates at the design channel quality (worst-case channel condition); this saturation is due to the irreversible quantization error introduced by the source coder. Hybrid digital analog (HDA) codes address this problem by additionally transmitting the inherent quantization error by pseudo-analog methods. However, the design and the decoding of these HDA codes is complex or even impossible for long block lengths (look-up table decoding). In this study we propose an HDA transmission system for sources with correlation supporting long block lengths. This is achieved by combining transform coding with HDA transmission using well-known digital channel codes. For the evaluation Monte Carlo simulations are used with a normalized discrete cosine transform (DCTN) and Turbo codes as the digital part and LMMSE estimation in the pseudo-analog part of the HDA system. The proposed HDA system with transform coding outperforms the purely digital transmission system at all channel qualities and exploits the expected gain due to source correlation.

Combined reduction of time varying harmonic and stationary noise using frequency warping

Speech enhancement under non-stationary environments is still a challenging problem. This contribution presents a noise reduction system that is capable of tracking and suppressing both time varying harmonic noise and stationary noise. In a first stage, the harmonic noise power is estimated and attenuated using a modified Minimum Statistics approach that performs frequency warping according to the harmonics fundamental frequency. A conventional noise estimation technique is applied in a second stage in order to reduce the random components of the noise spectrum. The performance of the proposed noise suppression system is shown to be consistently better than conventional approaches.

Properties and performance bounds of linear analog block codes

Linear analog block codes have been considered for transmission of discrete-time and continuous-amplitude data. In this paper, the fidelity measure parameter SNR (pSNR) at the receiver is derived for an arbitrary generator matrix P using an additive white Gaussian noise (AWGN) channel. In contrast, it is shown that the performance of linear analog block codes is dependent on the eigenvalues of the matrix PTP and not only on the dimensions of the matrix P. Surprisingly, the quality of the received values is independent of the code rate r, and e.g. a simple identity matrix has the optimal eigenvalues. Furthermore, the theoretical fidelity bound OPTA (Optimum Performance Theoretically Attainable) is used to assess the performance of a transmission system of continuous-amplitude data.

Hybrid Digital-Analog transmission for wireless acoustic sensor networks

A crucial task of a Wireless Acoustic Sensor Network (WASN) is the transmission of the acoustic signal to a central fusion node. Due to different microphone locations, the microphones may experience different or even varying radio channel qualities. Usually, the design of the digital transmission scheme is governed by the worst-case radio channel. For given radio resources, the end-to-end audio quality is specified by the compromise between the fidelity of the audio encoder and the required error protection capabilities of the channel encoder. The quantization noise of the audio encoder limits the end-to-end SNR even for increasing radio channel qualities. In the fusion node, the decoded signals are combined (fusion algorithm) and the overall output audio quality is limited due to the quantization noise even for error free transmission. In this paper it is shown that this unfavorable behavior can be avoided by using a Hybrid Digital-Analog (HDA) transmission concept. The key feature of HDA is that the audio output quality improves with increasing channel qualities. For typical fusion algorithms the HDA system supersedes purely digital transmission for all radio channel qualities. HighlightsNew transmission mode of signal from microphones to central fusion node.Avoiding saturation of quality due to quantization noise of purely digital system.Increased quality of output signal of fusion node for any type of signal combining.Simulation results for different types of fusion audio signal processing algorithms.Hybrid digital analog transmission always supersedes purely digital transmission.

Wideband speech coding with hybrid digital-analog transmission

Efficient digital transmission of speech requires source coding which comes at the price of unavoidable quantization errors. Thus, even in clear channel conditions, the quality of the decoded speech signal is limited due to the quantization errors. Hybrid Digital-Analog (HDA) codes circumvent this limitation by additionally transmitting the quantization error with quasi-analog methods (discrete-time, quasi-continuous-amplitude) with neither increasing the total transmission power, nor the occupied frequency bandwidth on the radio channel. So far, the HDA concept has mainly been applied to random parameters. In this paper, the HDA concept is adapted to the transmission of wideband speech signals using PCM and ADPCM coding. By experimental verification it is shown that the HDA concept may outperform conventional purely digital transmission systems at all channel qualities while additionally eliminating the quality saturation effect.

Hybrid Digital-Analog transmission taking into account D/A and A/D conversion

Efficient digital transmission of continuous-amplitude signals requires source coding which comes at the price of unavoidable quantization errors. Thus, even in clear channel conditions, the quality of the decoded signal is limited due to these source coding errors (quantization). Hybrid Digital-Analog (HDA) codes circumvent this limitation by additionally transmitting the source coding error with quasi-analog methods (discrete-time, quasi-continuous-amplitude) with neither increasing the total transmission power, nor the occupied frequency bandwidth on the radio channel. So far, for HDA transmission, the potential distortion additionally introduced by D/A and A/D conversion of the analog signal has not been considered. In this paper, the effect of clipping and limited resolution of this conversion on the performance of HDA transmission is evaluated. For random variables and speech signals, simulations verify that even with poor A/D and D/A conversion (3 bit resolution) the HDA concept outperforms conventional purely digital transmission systems at all channel qualities while additionally eliminating the quality limitation effect.