3-Representing Sound

Learning Outcomes

Students should be able to:

• Describe factors that affect sound quality, including:

– sample rate;

– bit depth;

– bit rate;

… when recording sound.

• Explain the need for analogue-to-digital conversion in sound recording.

THE NEED FOR ANALOGUE TO DIGITAL CONVERSION

Sound is heard when vibrations in the air reach the human ear, and a computer SPEAKER (found either on a loudspeaker or within a set of headphones) will vibrate a conical structure in order to push air vibrations towards an audience.

A MICROPHONE detects sound waves and converts them to voltage variations that can subsequently be captured digitally by a computer through the process of ANALOGUE-TO-DIGITAL (ADC) CONVERSION.

This means that between any two points in time there are infinitely many minor variations in the wave of sound energy. With space limitations in digital storage and with human brains being incapable of detecting all of the minor variations, human need can be adequately met by taking an appropriate quantity of regular measurements of the sound wave, via the microphone, and storing these measurements DIGITALLY so that the original sound wave can be produced later by a speaker. This is called SOUND SAMPLING.

The microphone records lots of samples of the sound, these are stored in binary form on the computer storage device and replayed in order to recreate a digital version of the sound.

Sample Rate

The SAMPLE RATE (or SAMPLING FREQUENCY) is the quantity of samples taken per second. It is measured in Hertz (Hz). We tend to use kilohertz as a suitable measure for sound: 1 kilohertz = 1 kHz = 1,000 Hz

The amount of samples per second determines the quality of the sound. More samples per second will give us a much higher quality sound and a larger file size. Less samples will give us a lower quality sound with a smaller file size.

Bit Depth

The BIT DEPTH is the number of bits that is allocated per sample taken. As long as the sample rate is known, there is no need to store the time values, only the sample (A) values, thanks to the fact that the samples are taken at regular intervals. Consider this 5-millisecond fragment of the original sound wave’s data:

With N bits, 2N different binary values are possible. In this case, the numbers could be whole numbers (integers) in the range 0 to 255, or perhaps -128 to +127. However, restricting the values to being only whole numbers means that we cannot store the ½ measurements, and if we allow ½ measurements then the range of numbers shrinks to e.g. 0 to 127.5, or -64 to +63.5.

The amplitude A of the wave represents the volume of the sound at a given point in time. By INCREASING THE BIT DEPTH, this can INCREASE THE QUALITY of the digital sound. This is because the samples taken can more precisely measure the exact value of A for every sample. The difference is not obvious at close range. Study the two diagrams below to see the subtle differences in how closely aligned to the gridlines the samples are:

Bit Rate

The BIT RATE is the number of bits required to store 1 second of sound. The unit of measurement is bits per second (bps) or kilobits per second (kbps). Bit rate is a straightforward calculation:

Bit depth × Sample rate = BIT RATE

In other words:

• Take the number of bits that are required per sample (bit depth);

• Take the number of samples per second (sample rate);

• Multiply these together to reveal how many bits are required to store 1 second of sound.

Since a high bit rate indicates that either the bit depth is high or the sample rate is high, a HIGH BIT DEPTH indicates a HIGH QUALITY SOUND as a lot of computer memory ends up being devoted to storing the original analogue wave as accurately as possible, either through very regular measurements of its amplitude (A) or through very accurate measurements of the precise value of A, or indeed both simultaneously.

POSSIBLE EXAM QUESTIONS

Describe how digital sounds are created. (2 Marks)
Explain the term sample rate. (2 Marks)
Evaluate the affect of increasing the sample rate of a digital audio recording. (4 Marks)
Explain the term bit depth. (2 Marks)
Evaluate the affect of decreasing the bit depth of a digital audio recording. (4 Marks)
A song has a bit depth of 8, and a sample rate of 50. Calculate the bit rate.(2 Marks)

The song lasts for 4 minutes 30 seconds. What is it's file size?(2 Marks)

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