1. Representation of Data in Computer Systems
Sound

2. How is sound stored? Representation of an Analogue Sound
Learning objectives
(a) Explain how sound can be sampled and stored in digital form.
(b) Explain and discuss how sampling intervals and other considerations affect the size of a sound file and the quality of its playback.
Representation of an Analogue Sound
Representation of an Digital Sound
What do you see?
What do you think it means?
How do you imagine these sounds may differ?

3. How is sound stored? Analogue vs Digital
Learning objectives
(a) Explain how sound can be sampled and stored in digital form.
(b) Explain and discuss how sampling intervals and other considerations affect the size of a sound file and the quality of its playback.
Analogue vs Digital
Analogue sound (basically sound waves that continuously vary) are pure and of perfect quality.
E.g.: Listening to a singer (without a mic) at a concert is real – the quality is pure.
Any computer recorded sound is not pure, not real and not of perfect quality.
WHY?
It is because sound has been digitised – it has been sampled at set intervals.

4. How is sound stored? What is Sampling?
Learning objectives
(a) Explain how sound can be sampled and stored in digital form.
(b) Explain and discuss how sampling intervals and other considerations affect the size of a sound file and the quality of its playback.
What is Sampling?
Sampling is simply recording snippets of sound at set intervals.
So, whereas analogue sound is continuous over time, digitised sound is made up of lots of ‘sound bites’ over time.

5. Representation of a Digital Sound
Sampling
Representation of a Digital Sound
011010
010010
111010
111011
001011
111001
011110
001010
111010
110001
011011
011011
001010
110010
111010
101011
111011
011101
010010
110110
111011
110010
001111

6. How is sound stored? Metadata
Learning objectives
(a) Explain how sound can be sampled and stored in digital form.
(b) Explain and discuss how sampling intervals and other considerations affect the size of a sound file and the quality of its playback.
Metadata
Like any other data, sound is stored on a computer as binary code (1s and 0s).
Metadata tells the computer what type of file it is so that the computer can use the file correctly.

7. How is sound stored? Sample Rate
Learning objectives
(a) Explain how sound can be sampled and stored in digital form.
(b) Explain and discuss how sampling intervals and other considerations affect the size of a sound file and the quality of its playback.
Sample Rate
The sampling process happens very, very fast.
In a typical audio CD, for example, the music has been sampled 44,100 times per second (or 44.1 kHz)
This can be carried out across 1 or 2 “channels”
1 = mono, 2 = stereo.

8. How is sound stored? Bit Rate
Learning objectives
(a) Explain how sound can be sampled and stored in digital form.
(b) Explain and discuss how sampling intervals and other considerations affect the size of a sound file and the quality of its playback.
Bit Rate
As well as sampling rate and number of channels the number of bits stored per sample also varies.
This is known as the “bit rate”. And a higher ‘bit rate’ will record more detail of the sound and therefore improve the quality.
A typical audio CD has a bit rate of 16 bits per sample, per channel. Therefore, each sample will be of a good quality.
So, in terms of the storage space needed that works out at 44,100 x 16 x 2 (i.e. 1,411,200 bits per second). A lot.

9. How is sound stored? MP3 files
Learning objectives
(a) Explain how sound can be sampled and stored in digital form.
(b) Explain and discuss how sampling intervals and other considerations affect the size of a sound file and the quality of its playback.
MP3 files
Such huge file sizes make it impractical to send CD audio tracks over a network such as the internet or via .
The invention of MP3 sound compression changed all that.
MP3 is a form of lossy compression.
It removes parts of the sound that are least likely to be noticed by the human ear.
Frequencies which our ears don’t pick up for example.

10. How is sound stored? WAV files WAV files are a native Windows format.
Learning objectives
(a) Explain how sound can be sampled and stored in digital form.
(b) Explain and discuss how sampling intervals and other considerations affect the size of a sound file and the quality of its playback.
WAV files
WAV files are a native Windows format.
WAV files are bigger than MP3 files.
This is because the “lossy compression” applied to MP3 files is not applied to WAV files.
This means they are less good for sharing than MP3s, but are usually better quality than MP3s (at an equivalent sample rate & bit rate).

11. How is sound stored? MP3 and WAV bit rates
Learning objectives
(a) Explain how sound can be sampled and stored in digital form.
(b) Explain and discuss how sampling intervals and other considerations affect the size of a sound file and the quality of its playback.
MP3 and WAV bit rates
The bit rate of an MP3 file or a WAV file can be adjusted.
So can the sample rate and number of channels.
The higher the bit rate and sample rate, the better the music quality will be, but a larger storage space will be needed to store the file.