4.5 - Representing Audio

Sound on a computer must be converted from analog to digital waves.

 

To convert analog sounds into digital waves (i.e. using a microphone to speak into) the sound is sampled using an ADC (Analog to Digital

Convertor) and stored as a binary value (such as 01010011) called a sample.

 

Digital sampling is discrete and not continuous, like analog waves. Therefore many samples are taken to recreate the analog wave as closely as possible.

Digital wave created with few samples

The number of samples taken per second is known as the sample rate.

The sample rate is measured in kilohertz (kHz), and CD quality is 44.1kHz (44,100 samples per second).

The sample rate of a CD is high, so the audio quality will be better as it closely resembles an analog wave.

However, the higher the sample rate, the larger the file size becomes as data is stored for each individual sample.

 

Generally, the higher the sample rate, the more values are converted and the higher the overall quality of the sound.

Sample Rate

 
 

Bit Depth

The bit depth is the number of bits available to represent each sample.

E.g. a sample with a bit depth of 4 could be 0101 or 0111 or 1010.

A sample with a bit depth of 8 could be  01010110 or 1010110 or 11001111.

The higher the bit depth, the more bits are available to be used and thus the quality is often higher as the wave closer resembles an analog wave.

 

The file size will also be larger if the bit depth is higher, as each sample stores additional bits. A standard bit depth is 16 bits.

 

Bit Rate

The bit rate is the number of bits that are processed per second. The bit rate is measured in kilobits per second (kps). Youtube has a standard bit rate of 192kps and iTunes is 256kps.


Generally, the higher the sample rate, bit depth and bit rate, the better the quality of the audio as there are more samples and more bits to represent each sample but the file size will be larger too.

Metadata

 

Metadata is additional data about an audio file, such as:

The artist, album, genre, recording date or song title (think about different column titles on iTunes or Spotify). Features like sample rate, bit depth and bit rate also count as metadata.

Questo's Corner

Representing Sounds:

a.  Describe what is meant by the sample rate.

b. Explain the difference between bit depth and bit rate.

c. File A has a lower sample rate than File B. Suggest what the outcome of this might be on file size and audio quality and why. 

Metadata:

a. Give three examples of possible metadata for a sound file.

Original analog
sound wave

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