Reports on Progress:

Low-frequency limitation effects

The effect of degradation of basic audio quality due to switching off the LFE channel is shown in Fig. 1 (results are shown only for items having the LFE content in the original recording). As a result of switching off the LFE channel basic audio quality was degraded from "Excellent" (80-100) to "Good" (60-80) for 3 items. For one item (Pop F-B) this effect was perceivable (small shift of scores from 100 to about 95) but the audio quality was not degraded very much. The obtained results show that excluding of the LFE channel from surround audio reproduction systems may have significant effect on degradation of audio quality, however resultant quality is still "Good". Therefore it may be concluded that LFE channel is not critical (necessary) for good reproduction of typical surround audio material.

Fig. 1 Effect of switching off the LFE channel

Figure 2 shows the effect of switching off the LFE channel in conjunction with high-pass filtration of all channels. Effects of switching off the LFE channel without any filtration of the main channels are also presented in this figure in order to compare the magnitude of the effect for these two types of degradation. Switching off the LFE channel combined with high-pass filtration of the main channels degraded the audio quality considerably. For all items shift of the lower boundary of the bandwidth up to 500 Hz and switching off the LFE channel degraded the audio quality to "Poor" (20-40). Obtained results show that simultaneous reduction of low frequencies in all channels considerably degrades audio quality. A similar result was obtained for the front left and right channels (Fig. 3). High-pass filtration of front left and right channels in conjunction with switching off the LFE channels caused also substantial degradation of audio quality, however a bit smaller than in the previous case. Moreover, for some items (Pop F-F and Applause F-F) band-limitation of left and right channels caused small deterioration of quality in relation to degradation of quality due to switching off the LFE channel. It can be explained by the fact that loss of LF content in left and right channels was not so noticeable because of the pronounced LF components in remaining channels.

Fig. 2 Degradation of the basic audio quality caused by band-limitation of all channels

Fig. 3 Degradation of the basic audio quality caused by band-limitation of front left and right channels

In contrast to previous cases, limitation of LF content in the centre channel had an insignificant effect on audio quality for most items. Fig. 4 shows that switching off the LFE channel had a greater effect on quality than switching off the LFE channel in conjunction with band-limitation of the centre channel. For example, for the "Applause F-F" switching off the LFE channel deteriorated the quality from "Excellent" (80-100) to "Good" (60-80). However, switching off the LFE channel and high-pass filtration of the centre channel did not cause any further degradation of quality (insignificant differences between means).

Fig. 4 Degradation of the basic audio quality caused by band-limitation of the centre channel

High-pass filtration of rear channels had only a small effect on degradation of audio quality for most items (except of classical music). The degree of deterioration of quality due to switching off the LFE channel with simultaneous high-pass filtration of rear channels was only slightly greater than the degree of deterioration of quality caused by only switching off the LFE channel (Pop F-B, Pop F-F, Movie F-B, Applause F-F). For organ music (Classical F-B) and sport item (Sport F-F) limitation of LF content of rear channels up to 80 Hz caused negligible degradation of quality. For organ music the loss of LF content was difficult to notice (probably due to masking effect) because of the pronounced bass in remaining channels. The lack of differences in quality for sport item was caused by the spectral characteristic of this item in which there was no perceivable LF content below 80 Hz.

Fig. 5 Degradation of the basic audio quality caused by band-limitation of rear channels