Data compression, field of interest shaping and fast algorithms for direction-dependent deconvolution in radio interferometry
- Authors: Atemkeng, Marcellin T
- Date: 2017
- Subjects: Radio astronomy , Solar radio emission , Radio interferometers , Signal processing -- Digital techniques , Algorithms , Data compression (Computer science)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/6324 , vital:21089
- Description: In radio interferometry, observed visibilities are intrinsically sampled at some interval in time and frequency. Modern interferometers are capable of producing data at very high time and frequency resolution; practical limits on storage and computation costs require that some form of data compression be imposed. The traditional form of compression is simple averaging of the visibilities over coarser time and frequency bins. This has an undesired side effect: the resulting averaged visibilities “decorrelate”, and do so differently depending on the baseline length and averaging interval. This translates into a non-trivial signature in the image domain known as “smearing”, which manifests itself as an attenuation in amplitude towards off-centre sources. With the increasing fields of view and/or longer baselines employed in modern and future instruments, the trade-off between data rate and smearing becomes increasingly unfavourable. Averaging also results in baseline length and a position-dependent point spread function (PSF). In this work, we investigate alternative approaches to low-loss data compression. We show that averaging of the visibility data can be understood as a form of convolution by a boxcar-like window function, and that by employing alternative baseline-dependent window functions a more optimal interferometer smearing response may be induced. Specifically, we can improve amplitude response over a chosen field of interest and attenuate sources outside the field of interest. The main cost of this technique is a reduction in nominal sensitivity; we investigate the smearing vs. sensitivity trade-off and show that in certain regimes a favourable compromise can be achieved. We show the application of this technique to simulated data from the Jansky Very Large Array and the European Very Long Baseline Interferometry Network. Furthermore, we show that the position-dependent PSF shape induced by averaging can be approximated using linear algebraic properties to effectively reduce the computational complexity for evaluating the PSF at each sky position. We conclude by implementing a position-dependent PSF deconvolution in an imaging and deconvolution framework. Using the Low-Frequency Array radio interferometer, we show that deconvolution with position-dependent PSFs results in higher image fidelity compared to a simple CLEAN algorithm and its derivatives.
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- Date Issued: 2017
An evaluation of the efficacy of digital real-time noise control techniques in evoking the musical effect
- Authors: Warneke, Andrew Travis
- Date: 2012
- Subjects: Music -- Acoustics and physics , Signal processing -- Digital techniques
- Language: English
- Type: Thesis , Masters , MMus
- Identifier: vital:8524 , http://hdl.handle.net/10948/d1020158
- Description: This study sought to determine whether or not it may be possible to evoke ‘the musical effect' – the emotional response perceived by music listeners – using white noise as a sound-source and real-time digital signal processing techniques. This information was considered to be valuable as in a world driven by technological progress the potential use of new or different technologies in creating music could lead to the development of new methods of – and tools for – composition and performance. More specifically this research asked the question 'what is music?' and investigated how humans – both trained musicians and untrained people – perceive it. The elements of music were investigated for their affective strengths and new fields of research explored for insights into emotion identification in music. Thereafter the focus shifted into the realm of Digital Signal Processing. Common operations and techniques for signal manipulation were investigated and an understanding of the field as a whole was sought. The culmination of these two separate, yet related, investigations was the design and implementation of a listening experiment conducted on adult subjects. They were asked to listen to various manipulated noise-signals and answer a questionnaire with regard to their perceptions of the audio material. The data from the listening experiment suggest that certain DSP techniques can evoke ‘the musical effect’. Various musical elements were represented via digital techniques and in many cases respondents reported perceptions which suggest that some effect was felt. The techniques implemented and musical elements represented were discussed, and possible applications for these techniques, both musical and non-musical, were explored. Areas for further research were discussed and include the implementation of even more DSP techniques, and also into garnering a more specific idea of the emotion perceived by respondents in response to the experiment material.
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- Date Issued: 2012