In a communication system, audio quality is one of the parameters by which the end user defines the value of a product. This thesis examines the term audio quality, breaking it down into two subsidiary components, speech quality and speech intelligibility. One key goal in assessing audio quality is quantifying it in an accurate and repeatable way. As a part of this project a system was developed that achieved this goal. The system was then used to evaluate a number of existing products based on speech quality and intelligibility. Using these results the relationship between the two parameters was investigated. Investigations were also conducted in order to determine and quantify the effect communication systems have on perceptual speech parameters, and examine the relationship between them and speech quality and intelligibility. Using the testing systems developed a possible method of audio quality optimization was investigated and tested. The analysis methods that were incorporated into the test suite included the Perceptual Evaluation of Speech Quality, the Speech Transmission Index, vowel space analysis and segmental, psychoacoustic based methods. The testing incorporated a number of different handheld portable radios as speakers.

Tactical radio communications used in military applications hasstrict requirements regarding security and has to be operable inrough environments in which there may be disturbances and disruptionson a radio link. The performance of the Secure CommunicationInteroperability Protocol (SCIP) operating in an asynchronouscommunication network with various levels of packet loss isinvestigated and found inadequate mainly due to problems withcryptographic synchronization between the transmitting and receivingunits. The introduction of additional counter data to each datapacket remedies this problem and allows the receiving units to fillthe holes left by packet losses with filler packets, maintainingsynchronization. The audio quality can then be measured using thePerceptual Evaluation of Speech Quality (PESQ) algorithm.Measurements are performed in an emulated radio link with aconfigurable packet loss ratio developed by Saab. The results showthat parts of SCIP can be used alongside the counter solution withoutimpacting the audio quality. The insertion of filler packets is shownto have a positive effect on the audio quality, while aggregation ofpackets to conserve transmission data rate is shown to have anegative effect.

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The emergence of Bluetooth Low Energy in combination with optimized coders has made it possible to transfer digital audio at very low bitrates, paving the way for small devices with longlasting batteries. The aim of this study is to compare the audio codecs LC3 and aptX, as well as peoples’ attitude towards audio quality in different contexts. Two open source implementations of the codecs are evaluated in terms of time for execution. Furthermore, the perceived audio quality of low bitrates are subjectively compared in a listening test in combination with a questionnaire regarding peoples’ attitude towards audio quality. The results show that LC3 is capable of delivering satisfying audio quality at very low bitrates, whilst also outperforming aptX. It will be interesting to see how LC3 will affect transmission latency, battery life and overall QoS once it is established in everyday products

The dissertation thesis focuses on objective assessment and reduction of disturbing background noise in a musical signal. In this work, a new algorithm for the assessment of background noise audibility is proposed. The listening tests performed show that this new algorithm better predicts the background noise audibility than the existing algorithms do. An advantage of this new algorithm is the fact that it can be used even in the case of a general audio signal and not only musical signal, i.e. in the case when the audibility of one sound on the background of another sound is assessed. The existing algorithms often fail in this case. The next part of the dissertation thesis deals with an adaptive segmentation scheme for the segmentation of long-term musical signals into short segments of different lengths. A new adaptive segmentation scheme is then introduced here. It has been shown that this new adaptive segmentation scheme significantly improves the subjectively perceived quality of the musical signal from the output of noise reduction systems which use this new adaptive segmentation scheme. The quality improvement is better than that achieved by other segmentation schemes tested.

Wireless technology has increased in popularity since its invention, being embedded in many devices, such as smartphones, laptops, earphones etcetera. One of those technologies is Bluetooth, which is a short-range technology that is widely used worldwide. While being a popular and important technology, it is not as optimized as it could be, especially the standard Bluetooth 4.2. According to (Bluetooth, 2023)Bluetooth version 5 has four times the range of Bluetooth version 4.2 which is one area of improvement in Bluetooth. While limited Forward Error Correction (FEC)functionality in terms of (15,10) Hamming codes is present in the Bluetooth classic4.2 stack, however, it is not utilized in that version of Bluetooth. This thesis aims to implement a functionality in Bluetooth that uses FEC to increase the range. Before the experiments were conducted, multiple measurements were done in order to check if the hardware, such as the Bluetooth modules, antennas, air sniffer, could provide reliable and valid data. The experiments were conducted by streaming an audio sample of 48 kHz via Bluetooth and increasing the range until a certain percentage of fail rate in data transmission has occurred. FEC was then enabled, and the same experiment was repeated. The results from the experiments show that the range can be increased up to a few meters. Not only that, but the transmission without FEC allows a high level of errors without a connection cut-off. With FEC, the connection does not allow high levels of errors which means that the audio quality would not drop as low as it would have without FEC which means that a certain audio quality is preserved. This also makes the case for the implementation of higher modes of FEC into the Bluetooth stack to potentially increase the range of Bluetooth classic multiple times, but that implementation is beyond the scope of this thesis.