Representing Sound

Sound
- Sound is an air pressure wave that is sensed by our ears.
- Can either be analogue or digital.
- Analogue sounds are when pressure waves are captured by a transducer and produces an electrical current which varies with the sound pressure.
- The electrical signal produced can be transmitted by telephone, over the radio and can be preserved on magnetic tape.
- At the other end when being broadcasted or transmitted by telephone the electrical signal is used to re-create sound via vibrations.
- The higher the pitch of the sound the more rapid the vibration.
- A pure tone is a regular sine wave, but when two waves get put together they superpose meaning you then get a pulsating tone.
- Today most sound systems are digital.
- The electrical signal from the transducer is converted in to a digital signal meaning it is represented by binary (1's and 0's)

Analogue to Digital Conversion (ADC) and Digital to Analogue (DAC)
- Converts an anologue signal into an equivalent digital signal.
- A computer may be used to record sound, but first the sound must be converted into digital form.
- The analogue signal is sampled.
- Sampling is when a wave is measured at regular time intervals and then rounded to the nearest binary value.
- To play back sound the signal must be converted back to analogue.
- The technique to do this is called pulse code modulation.

Pulse Code Modulation (PCM)
- Samples of the analogue signal are taken at regular intervals of time.
- The sampling frequency or rate must be at least twice the highest frequency in the anologue signal.
- These samples are represented as narrow pulses of height which are proportional to the value of the original signal.
- This process is known as Pulse Amplitude Modulation (PAM)
- To produce PCM data, the PAM samples are quantised, meaning they are rounded to the nearest whole number.

Nyquist's Theorem
- We Must Sample At A Frequency At Least Twice The Rate Of The Highest Frequency In The Sampled Signal.

Storing Sound In Files
- One of the most notable sound formats is WAV (supported by most Operating Systems)
- WAV requires 2.5 MB of memory for one minute of sound.
- WAV is commonly used when storing music and sound to CDs.
- Most common nowadays is the MPEG format, this can be mp2, mp3 or mp4.
- MPEG compresses the sound file by 10% of what WAV does.
- 1 min in WAV = 2.5 MB   -   1 min of mp3 = 0.25 MB
- MPEG also removes frequencies that the ear and brain cannot detect.

Editing Sound
- When sound is stored digitally on a computer for example, the sound can be modified allowing the user to mix multiple sources and add effects.
- When a sound is edited it can be saved as a single sound file.

Synthesising Sound
- Musical Information Digital Interface does not store sound waves but stores a digital representation of the sound.
- It takes note of the notes to be played, the instrument being played and how long a note and intrument is played for.
- The resulting form is that it is very compact.
- It can easily be transposed and played on different instruments.
- Musical Information Digital Interface is also known as MIDI.

Streaming Audio
- When you stream audio the server sends it bit by bit.
- The client buffers it and plays it when it has enough bits to keep on playing and buffering at the same time.
- Advantages - No need to download the file
                     - Saves harddrive space
                     - Makes copying harder
Disadvantages - Cannot listen when disconnected
                       - Can be affected by bandwidth