A voice recorder stores a mono sound clip. The clip lasts for 12 seconds and is sampled 8000 times each second. Each sample is stored using an 8-bit resolut...
Question 1 Report
A voice recorder stores a mono sound clip. The clip lasts for 12 seconds and is sampled 8000 times each second. Each sample is stored using an 8-bit resolution. The simplified waveform below shows five sample values taken during a small part of the recording. The vertical axis represents amplitude: values higher than the centre line are positive and values lower than the centre line are negative. The recording is stored without compression and no header information is included in the file-size calculations.
(a) State what is meant by the sample rate of a digital sound file. [2]
(b) Complete these calculations: number of samples, number of bits required and number of bytes required for the clip. [3]
(c) Convert the number of bytes required into KiB, using 1 KiB = 1024 bytes. [2]
(d) Give two changes that could improve the quality of the recorded sound. [2]
(e) Explain why a sample rate that is too low can cause aliasing. [2]
(f) Identify the labelled sample with the greatest positive amplitude. [1]
(g) State the effect of increasing the bit depth from 8 bits to 16 bits. [2]
(h) Give one disadvantage of increasing the sample rate. [1]
Answer Details
(a) The sample rate is the number of sound samples taken each second [2].
(b) \[12\text{ s}\times8000\text{ samples/s}=96,000\text{ samples}\] \[96,000\times8=768,000\text{ bits}\] \[768,000\div8=\textbf{96,000 bytes}\] [3]
(c) \[96,000\div1024=\textbf{93.75 KiB}\] [2]
(d) Increase the sample rate and increase the bit depth [2]. Better recording equipment or less background noise are also valid.
(e) If samples are taken too infrequently, rapid waveform changes are not represented accurately. Frequencies may then be recorded as incorrect, lower frequencies, which is aliasing [2].
(f) Sample C has the greatest positive amplitude because it is furthest above the centre line [1].
(g) Increasing from 8 to 16 bits provides more possible amplitude values, reducing quantisation error and representing sound more accurately. It also increases file size [2].
(h) A higher sample rate stores more data, so it requires more storage and/or bandwidth [1].
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