4.7  Signals and noise

This is an interactive that can be used to record sound and display a visual representation of it in both the time and frequency domains. It can also be used to add a noise tone to the sound and then apply filters to remove it again. Because this interactive changes visually according to the nature of the recorded sound, you may need a sighted helper to describe the results to you.

The top half of the interactive contains the recorded signal in the time and frequency domains. On the left is the original signal. Similar to the interactives in previous weeks, this is displayed on a graph of amplitude against time, and looks like a series of pink vertical lines. The Record, Play and volume control buttons and time details are below the graph, with the Zoom slider underneath. At the bottom are two tick boxes labelled ‘Add noise tone’ and ‘Override noise tone’.

On the right is the amplitude plot of the fast Fourier transform of the original signal. This is displayed on a graph of amplitude against frequency, with the frequency ranging from 0 to just under 7 kilohertz. Below the graph is a tick box labelled ‘Use logarithmic Y scale on FFT’. Underneath this are two sliders, the right cutoff and the left cutoff, and at the bottom is a series of radio buttons labelled ‘Low-pass’, ‘High-pass’, ‘Bandpass’ and ‘Notch’. Next to these radio buttons is a button labelled ‘Apply filters’.

The bottom half of the interactive contains the filtered signal, and only displays any content once the ‘Apply filters’ button has been clicked. On the left is a graph of amplitude against time, and on the right is a FFT plot. These graphs use the same scale and colouring as the top two graphs, and show the effect of the chosen filter on the original signal.

This is a screenshot from Interactive 5, showing a recording of the word ‘yes’. This appears on the left-hand graph as a set of fuzzy pink lines in an ellipse shape, followed by a dense, lower-amplitude tail corresponding to the hiss in ‘yes’.

On the right-hand graph (the frequency-domain representation) is a series of vertical blue lines. These are quite high for the lower frequencies, to about 1 kilohertz, but there are smaller vertical blue lines (about one-eighth the amplitude of the lower-frequency waves) for all the frequencies, with the largest bunched around 4 kilohertz.

This is the same as Figure 32 except that noise has been added at 1.221 kilohertz. This shows as a thick band of colour around the horizontal axis on the time-domain graph on the left, and as a blue spike at 1.221 kilohertz on the frequency-domain graph on the right. It is clear that the noise gets mixed up with the signal in a way that is hard to see in the time domain, but it emerges as a clear, isolated vertical line in the frequency domain.