3.1.3 representing sound

Analogue


Analogue data varies continuously (all values are possible), think about how an analog clocks hands sweep every place on the clock face.


Digital


Digital data varies in discrete steps(only certain fixed values as possible)
Signal

Analogue to Digital (ADC)

•First, the continuous (analogue) sound signal is sampled at regular intervals. This is called Pulse

Amplitude Modulation (PAM)

• Each value is then rounded down to an approximate value dawn from a range of possible

discrete (digital) values.

•The value is then stored as a binary digit

•The technical term for approximating a sound sample to the closest digital value is

quantization

•Each quantized number represents a Pulse Code Modulation (PCM)

•Each individual PCM (binary number representing one sample from the sound) is stored in

sequence in a file to record the sound in a digital, binary file

Sampling


there are two factors that affect sound quality

• Sound rate - how many times per second a sample is taken.
• Sampling resolution the number of bits available to encode each sample

Sound files


- WAV : 1 min = 2.5mb
- MPEG (used in .mp2, .mp3, .mp4)
discards frequencies that the ear/brain cannot handle


which make the file about 10% of the original
1min = 0.25mb


Streaming audio


-The server sends the sound bit by bit
- The client buffers it and plays it when it has enough to keep going whilst buffering more


Advantages
- no need to download the file - just start playing
- saves hard drive space
- makes copying harder


Disadvantages
-cannot listen when not connected
- affected by bandwidth


Nyquist's theorem


frequency is measured in Hz (KHz, MHz)
Nyquist's theorem states that: "we must sample at a frequency at least twice the rate of the highest frequency in the signal"