CONCLUSION

H = ((fSq + 20.6 . ^ 2) . * (fSq + 12194 . ^ 2) . * sqrt((fSq + 107.7 . ^ 2). *(fSq + 737.9 . ^ 2))) ./ ((12194 . ^ 2) . * f . ^ 4); H(1) = 0;

Figure 18. Final A-weighting Filter Function

Although this now serves the “correct” purpose visually, we can see from the amplitude axis that we now have the opposite problem and the gain is being boosted too much (almost 35dB) in the high end which is much too loud at this level [fig 9]. In the conclusion we will look at how this could be rectified.

From our results we can see that toneControl() is working as it should, (when loudness = 0), we are seeing magnitude response results that are in keeping with what would be expected for different knob positions corresponding to gain boost and attenuation. Some examples below.

Figure 19. Magnitude and Phase Response of toneControl() for [n] input

CONCLUSION

The first thing to note is that the loudness control (although it works properly) is unusable at its current state because of how extreme the gain boost is in the low and high end. One way of rectifying this would be with root-mean-squared (RMS) amplitude normalisation. RMS amplitude is the square root of the mean of a signal [15].

Looking back on the results of the A-weighting filter throughout the project and knowing what we know now, it does seem like the original, discarded result [fig 17] is closer to what a RMS normalised relationship would look like between an input and an output signal.

One shortcoming of this project is a lack of listening tests. That is, testing the tone control and loudness control with two-channel audio signals and listening to the ways in which they are being affected. Had more of these been performed throughout the development process perhaps

the loudness control gain issue could have been spotted and fixed earlier. The clipping and distortion that is prominent when listening is not immediately obvious in the plots.

REFERENCES

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