Storing Sound

1 Explain what is meant by sampling when recording sound digitally. 3

One mark for each of the following, to a maximum of 3.

• The amplitude of the analogue sound wave is measured
• Measurements are taken at regular intervals
• Each measurement is stored as a binary value

Model answer. Sampling is the process by which a continuously varying analogue sound wave is converted into digital data. The amplitude of the wave is measured at regular fixed intervals, and each of those measurements is rounded to the nearest available level and stored as a binary number. The complete recording is therefore a long sequence of binary values that approximates the shape of the original wave.

2 Explain the difference between sample rate and bit depth. 4

One mark for each of the following, to a maximum of 4.

• Sample rate is the number of samples taken per second
• Bit depth is the number of bits used to store each sample
• Higher sample rate captures the shape of the wave over time more accurately
• Higher bit depth records each amplitude more precisely because there are more levels

Model answer. Sample rate is the number of samples taken every second, measured in hertz, so it describes how often the sound wave is measured along the time axis. Bit depth is the number of bits used to store each individual sample, so it describes how precisely each measurement is recorded along the amplitude axis. Increasing the sample rate means the measurements are closer together in time, so the stored data follows the shape of the original wave more closely and less detail is lost between samples. Increasing the bit depth means there are more possible amplitude levels available, so each measurement is rounded by a smaller amount and quieter detail is preserved.

3 A 90 second sound clip is recorded in mono at a sample rate of 22,050 Hz with a bit depth of 16. Calculate the file size in megabytes. 4

One mark for each of the following, to a maximum of 4.

• Multiplies sample rate by duration by bit depth
• Obtains 31,752,000 bits
• Divides by 8 to obtain 3,969,000 bytes
• States approximately 3.97 MB

Model answer. File size in bits is sample rate multiplied by duration multiplied by bit depth, which is 22,050 x 90 x 16 = 31,752,000 bits. Dividing by 8 gives 3,969,000 bytes, dividing by 1000 gives 3969 kilobytes, and dividing by 1000 again gives approximately 3.97 megabytes.

4 A student is recording a spoken podcast that will be downloaded by listeners on mobile data. Recommend suitable settings for sample rate and bit depth, and justify your recommendation. 6

One mark for each of the following, to a maximum of 6.

• Recommends a relatively low sample rate such as 22,050 Hz or lower
• Speech occupies a narrower frequency range than music so a high rate is unnecessary
• Recommends a bit depth of 16, or justifies 8, for adequate clarity
• Explains that this keeps the file size small for downloading over mobile data
• Recommends mono rather than stereo, halving the size
• Acknowledges the trade off that quality is reduced compared with music standard settings

Model answer. A sample rate of around 22,050 Hz with a bit depth of 16, recorded in mono, would be a sensible choice. Speech occupies a much narrower frequency range than music, so the very high frequencies that a 44,100 Hz rate is designed to capture are simply not present in a spoken voice, and halving the sample rate therefore halves the file size with almost no audible loss of clarity. A bit depth of 16 keeps the dynamic range wide enough that quiet speech does not become distorted, and dropping to 8 bits would introduce noticeable background hiss on quiet passages. Recording in mono rather than stereo halves the size again, and since a single speaker has no meaningful stereo image there is nothing lost. Together these settings might produce a file around a quarter the size of a CD quality stereo recording, which matters a great deal to listeners downloading episodes over mobile data where both speed and data allowances are limited. The trade off is that the recording would not be suitable if music were included, and a studio producing music would need the full 44,100 Hz stereo settings.

5 Describe the effect on both quality and file size of halving the sample rate of a recording. 3

One mark for each of the following, to a maximum of 3.

• Fewer samples are taken each second
• The reproduction of the original wave is less accurate, so quality falls
• The file size is halved

Model answer. Halving the sample rate means only half as many measurements of the sound wave are taken each second. Because the samples are further apart in time, more of the detail between them is lost and the stored data follows the shape of the original wave less closely, so the playback quality is noticeably lower and the recording tends to sound duller. Since file size is directly proportional to sample rate, the file will be half its original size, which is the benefit being traded for that loss of quality.