Browsing by Author "Van der Westhuizen, Ewald"

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    An efficient beam steerable antenna array concept for airborne applications
    (Radioengineering Society, 2014) Aliakbarian, Hadi; Van der Westhuizen, Ewald; Wiid, Riaan; Volskiy, Vladimir; Wolhuter, Riaan; Vandenbosch, Guy A. E.
    Deployment of a satellite borne, steerable antenna array with higher directivity and gain in Low Earth Orbit makes sense to reduce ground station complexity and cost, while still maintaining a reasonable link budget. The implementation comprises a digitally beam steerable phased array antenna integrated with a complete system, comprising the antenna, hosting platform, ground station, and aircraft based satellite emulator to facilitate convenient aircraft based testing of the antenna array and ground-space communication link. This paper describes the design, development and initial successful interim testing of the various subsystems. A two element prototype used in this increases the signal-to-noise ratio (SNR) by 3 dB which is corresponding to more than 10 times better bit error rate (BER).
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    Language modelling for code-switched automatic speech recognition in five South African languages
    (Stellenbosch : Stellenbosch University, 2018-12) Van der Westhuizen, Ewald; Niesler, T. R.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: Code-switching refers to natural, spontaneous language alternation by multilingual speakers during a conversation or utterance, and is prevalent in everyday conversations by multilingual South Africans. Automatic speech recognition systems are generally highly optimised for monolingual input and performance deteriorates when presented with mixed-language speech. This thesis addresses the automatic recognition of speech containing code-switching between English and four South African Bantu languages, focussing specifically on the language modelling of English-isiZulu, English-isiXhosa, English- Setswana and English-Sesotho. Due to the severe scarcity of code-switched speech data in South African languages, it was necessary to first develop a representative corpus. This new and unique 35-hour corpus contains segmented and transcribed code-switched speech from conversations in South African soap operas, which exhibit spontaneous utterances with regular code-switching in the target languages. Insertional, alternational, and intraword intrasentential code-switching are all represented in the data, as are some other special characteristics of fast, spontaneous Bantu speech such as postlexical deletion. The distribution of language switches is extremely sparse, however. In this thesis, a number of data-driven modelling approaches were investigated and applied to address the sparsity by augmenting the training data with synthetically generated data. Postlexical deletion was successfully modelled statistically with joint-sequence models, and these models were used to generate synthetic pronunciations which were demonstrated to lead to improved automatic speech recognition performance. Two new code-switched language modelling approaches were proposed to address data sparsity. First, parallel language-dependent language modelling (PLDLM), which consists of two monolingual language models with explicit language transitions, was demonstrated to outperform a conventional language-independent language model in terms of recognition word error rate. Second, language models in which word embeddings were used to synthesise probable unseen code-switched bigrams were considered. It was possible to achieve a reduction of up to 31% in language model perplexity across a language switch boundary by including such synthesised code-switch bigrams. Although smaller, improvements in the recognition word error rate were also observed.
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    Nie-destruktiewe klankonttrekking, restourasie en spraakverheffing van Edison-fonograafsilinders
    (Stellenbosch : Stellenbosch University, 2003-12) Van der Westhuizen, Ewald; Blanckenberg, M. M.; Schwardt, L.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: Two non-destructive methods of extracting audio from Edison phonographic cylinders were investigated. A recording device with high accuracy positioning was designed and manufactured. A microscopic image method was investigated first. Surface images of the cylinder were obtained using a webcamera. An audio signal was then extracted from the width modulation. Results were not pleasing as echoes caused by intergroove modulation were perceptable. The audio also lacked resolution. The true modulation of the audio is not embedded in the width, but in the depth of the groove. The second audio extraction method involved using a laser pick-up from a compact disc player to measure the depth of the groove. Three laser recording methods were investigated. The first was forward recording, that measured the depth modulation in the recording direction of the groove. The second method, backward recording, was identical to forward recording with the mechanical system moving in reverse. Four recordings from different positions in the groove were combined to create an audio signal. This combination of recordings showed a substantial improvement in the signal-to-noise ratio. A third recording method, transverse recording, that measured the whole depth profile of the groove was also investigated. The groove profile was then processed to an audio signal. A manual audio restoration program was written to replace visible sections of distorted data with better interpolations. Two speech enhancement methods were investigated, the first being the most commonly used speech enhancement method for digital audio restoration, Short-Time Spectral Attenuation (STSA). The second method is based on linear predictive coefficient (LPC) estimation of short-time frames. The two methods were evaluated by means of listening tests. The LPC enhancement method was preferred because it enhanced the intelligibility of the speech.