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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.

 4.7  Signals and noise