File Size Factors & Compression (SQA National 5 Computing Science): Revision Note

Sampling rate

How is sound sampled & stored?

• Measurements of the original sound wave are captured and stored as binary on secondary storage

• Sound waves begin as analogue and for a computer system to understand them they must be converted into a digital form

• This process is called Analogue to Digital conversion (A2D)

• The process begins by measuring the amplitude of the analogue sound wave at a point in time

• Each measurement (sample) generates a value which can be represented in binary and stored

• Using the samples, a computer is able to create a digital version of the original analogue wave

• The digital wave is stored on secondary storage and can be played back at any time by reversing the process

• In this example, the grey line represents the digital wave that has been created by taking samples of the original analogue wave

• In order for the digital wave to look more like the analogue wave the sampling rate can be changed

What is sampling rate?

• Sampling rate is the number of samples taken each second from an analogue sound wave

• Measured in Hertz (Hz), where 1 Hz = 1 sample per second

• A higher sampling rate produces better sound quality because the digital recording is closer to the original analogue wave

• However, higher sampling rates increase file size because more data is stored per second

• Common examples:

  • Telephone audio: 8 kHz

  • CD audio: 44.1 kHz

  • DVD audio: 48 kHz

• In this example you can see that the higher the sampling rate, the closer to the original sound wave the digital version looks

Need for compression

What is compression?

• Compression is reducing the size of a file so that it takes up less space on secondary storage

• The impact of compression is:

  • Less storage space required

  • Less bandwidth required

  • Shorter transmission time

• Compression can be achieved using two methods, lossy and lossless

What is lossy compression?

• Lossy compression is when data is lost in order to reduce the size on secondary storage

• Lossy compression is irreversible

• Lossy can greatly reduce the size of a file but at the expense of losing quality

• Lossy is only suitable for data where reducing quality is acceptable, for example images, video and sound

• In photographs, lossy compression will try to group similar colours together, reducing the amount of colours in the image without compromising the overall quality of the image

• In the images above, lossy compression is applied to a photograph and dramatically reduces the file size

• Data has been removed and the overall quality has been reduced, however it is acceptable as it is difficult to visually see a difference

• Lossy compressed photographs take up less storage space which means you can store more and they are quicker to share across a network

What is lossless compression?

• Lossless compression is when data is encoded in order to reduce the size on secondary storage

• Lossless compression is reversible, the file can be returned to its original state

• Lossless can reduce the size of a file but not as dramatically as lossy

• Lossless can be used on all data but is more suitable for data where a loss in quality is unacceptable, for example documents

Lossless file formats

• In the image above, lossless compression is automatically applied to document formats such as DOCX and PDF with a different rate of success

• When you open a lossless compressed document the decompression process reverses the algorithms and returns the data back to its original state

• Lossless compressed documents take up less storage space which means you can store more and they are quicker to share across a network