Representing sound

Lesson

Sample - Measure of the analogue signal at a given point in time

Sample rate - number of samples taken per second and is measured in Hertz.

Sample resolution - number of bits used to represent each sample

The size of sound files can be calculated using:

size of file = length (seconds) x sample rate x sampling resolution

For sound to be stored digitally on a computer it needs to be converted from its continuous analogue form into a discrete binary values. The steps are:

1. Microphone detects the sound wave and converts it into an electrical (analogue) signal
2. The analogue signal is sampled at regular intervals
3. The samples are approximated to the nearest integer (quantised)
4. Each integer is encoded in binary with a fixed number of bits

Learning Videos

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Questions

1. The first sound pattern below has been recorded three times using different digital sampling settings.

   

   Calculate the file size of the each of the recordings A to C: (Show your workings)

   
   

   A: Resolution has 8 points - 0-7. 3 bits per sample required to give a maximum of eight unique sample values. Frequency is 6hz or 6 times per second over three seconds = 18 sampled points 18 x 3bits = 54bits

   
   

   Note: This is a fractional sample of a sound wave, with a heavily simplified resolution and sample frequency. In reality, sound would be sampled many thousands of times a second (CD quality is at 44.1kHz or 44,100 samples per second) at a much higher resolution of perhaps 32 bits, (16bits for each stereo channel - Left and Right) producing much larger files! This example, with a sample resolution of only 3 bits per sample, and only six samples over 1 second, amounts to 18bits/s or bits per second. Bits per second is a measurement of recording quality. Even a compressed MP3 file would be recorded at at least 128kbits/s or 128,000bits/s.

   
   

   B: 16 sampling levels requires 4 bits per sample x 9 samples

   9 x 4bits = 36bits

   
   

   C: 16 sampling levels requires 4 bits per sample x 18 samples

   18 x 4bits = 72bits / 8 = 9 Bytes

   
   

   Allow one mark for each correct answer and an additional mark for the correct method.

2. Which of the recordings above will be of the highest quality?

   
   

   Recording C as it has the highest resolution and samples are taken more frequently.

3. State a device that is used to capture an analogue sound signal.

   
   

   Microphone

4. The length of a recording will heavily influence the size of the sound file. State two factors other than recording length that determine the file size of a recording.

   
   

   Sampling rate / interval / frequency.

   Sample resolution / (audio) bit depth.