Síndromes de fallo de médula ósea congénitos

Tables 8 to 13 show the statistical significance of each pairwise comparison based on the repeated measures ANOVA. Highlighted boxes show significant data at p<0.05.

4.6.1. Hearing Impaired Participants

Significance: Speech Clarity

Table 8. P-values for each pairwise comparison for speech clarity ratings, highlighted values <0.05 indicate statistical significance

Significance: Overall Sound Quality

Table 9. P-values for each pairwise comparison for overall sound quality ratings, highlighted values <0.05 indicate statistical significance

Significance: Enjoyment

Table 10. P-values for each pairwise comparison for enjoyment ratings, highlighted values <0.05 indicate statistical significance

Pearson correlation coefficients were calculated to investigate the apparent correlation between ratings for the hearing impaired group. Results indicated a strong correlation between all three factors rated in this experiment as can be seen in table 11. The results show that for the hard of hearing group sound quality, enjoyment and clarity are all closely interrelated.

Table 11. Correlation coefficients between scales for hard of hearing listeners, shaded cells indicate significance at p<0.05.

4.6.2. Non Hard of Hearing Participants

Significance: Speech Clarity

There were no statistically significant outcomes for speech clarity rating for this subject group at a 95% confidence level.

Significance: Overall Sound Quality

Table 12. P-values for each pairwise comparison for overall sound quality ratings, highlighted values <0.05 indicate statistical significance

Significance: Enjoyment

Table 13. P-values for each pairwise comparison for enjoyment ratings, highlighted values <0.05 indicate statistical significance

Calculating Pearson correlation coefficients between the three ratings indicates a strong correlation between overall sound quality and enjoyment as can be seen in table 14.

Table 14. Correlation coefficients between scales for non hard of hearing listeners, shaded cells indicate statistical significance at p<0.05.

4.7.

Discussion

4.7.1. Hearing Impaired Group

This work using the default stereo Lt/Rt downmix indicates that the recommendations generated from chapter 3 that were effective for three channel reproduction (attenuating left and right channels by 6dB) may have also improved perceived speech clarity and overall sound quality for hearing impaired viewers when presented over a two channel reproduction system. There is also evidence to suggest that applying Dolby’s dynamic range control compression process may have improved speech clarity and overall sound quality for this group.

The removal of non-speech channels entirely had been expected to improve speech clarity ratings when compared to the default Lt/Rt stereo downmix however no evidence was found to substantiate this expected result. The rating of centre channel

only was rated more poorly than the attenuated left and right condition (LR6dB) for both perceived speech clarity and for overall sound quality when presented over two

loudspeakers. It could have been expected that there may be some reduction in ratings due to the centre channel being presented as a phantom centre rather than over a discrete loudspeaker however this was the case for all of the conditions assessed here. This possibility is investigated later in chapter 5 of this thesis.

The question remains however as to why the centre only condition was so poorly rated even when compared to other conditions with speech presented using a phantom centre. The results here give no firm conclusion to this question although do raise an interesting point about the differences between assessment of intelligibility versus speech clarity. It is possible that there could be some positive impact from the presence of side channel information that may hide some audible distortion or imperfection caused by acoustical crosstalk. Had the test been for intelligibility, logic would suggest that participants would have recognised more keywords with less background, side channel, sounds present. When perceived speech clarity is being tested it may be that participants were trying to assess the clarity of the speech signalin isolation from the background sound.

With no background sound present the speech may have sounded wrong, or unclear,

because of frequencies being cancelled out. Those same frequency peaks and troughs may have been disguised to some extent by the presence of side channel sounds that were sufficient to ‘fill in’ some of the frequency gaps but not sufficiently loud as to cause difficulty in understanding the speech content. Indeed Vickers (Vickers, 2009a) paper points out that such notches in the frequency spectrum caused by two channel reproduction are often filled in by the effect of room reflections. Furthermore he

suggests that, “When audio content includes correlated (center) and decorrelated (side)

information, only the center content is subject to the comb filtering, reducing the salience of the notches.”.

4.7.2. Non-Hard of Hearing Group

Much less significance was found for the non-hearing impaired group. Centre channel only, when reproduced over two loudspeakers as a phantom centre, rated lower than all

conditions for overall sound quality (p<0.05) and for enjoyment compared to all conditions apart from LR6dB (p<0.05) for which result no significance was found.

Correlations between ratings showed that for hearing impaired participants speech clarity, overall sound quality and enjoyment were all closely correlated and that for the non-hearing impaired group only overall sound quality and enjoyment showed

correlation.

4.8.

Further Work

While it would have seemed reasonable that centre channel only would have been rated highly for clarity even if this were not reflected in sound quality ratings this was not the case. There was clearly a substantial difference in perception of the conditions

reproduced over two channel compared to three channel reproduction. Further work was therefore required in order to ascertain the cause of the poor performance of the

phantom centre channel only when using Lt/Rt downmixed stereo in these tests and to assess if this could be shown to be the consequence of acoustical crosstalk.

4.9.

Implications for TV Viewers

The research presented here lent further weight to the premise at the root of discussions undertaken with Ofcom and Dolby Laboratories aimed at providing a hard of hearing setting for STBs. Previous research showed distinct benefits to using attenuated left and right channels in order to improve clarity, perceived sound quality and enjoyment in a 3 channel reproduction system utilising a centre loudspeaker such as that found in

surround sound systems. This research indicates that the same technique could be effectively implemented in a 2 channel stereo reproduction system utilising the Lt/Rt downmix that is standard on set top boxes, DVD players and other Dolby equipment. Furthermore potential benefits for hearing impaired people could be gained by utilising Dolby’s dynamic range control compression processing. This has the potential to provide solutions for hard of hearing viewers who do not yet have surround sound reproduction equipment and who rely on the downmixed-to-stereo sound produced by STBs as default for two channel reproduction.