Representing Sound

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A sound that humans can listen to is an Analogue sound

A sound processed by a computer is in digital form

Sampling

The sampling rate (frequency) is the number of samples per seconds per, measured in Hz

The sampling resolution is the number of bits given to each sample, more bits - more accurary

The highest representation of sound happens with the highest samping rate and the largest samping resolution

This presents two issues:

-Storage space will need to be larger

-Bandwidth needed to transmit the signals will be larger

Sampling sound at 20kHz for 1 minute will produce 10,000 x 60 = 600,00 samples

With a sampling resolution of 4 bits, a minute will need 600,00 x 4 = 2,400,00 bits, in bytes = 300,00 bytes, which in kbytes = 300 kbytes

Nyquist's Theorem

Sample at a frequency at least twice the rate of the higest fregwncy in the sampled sound to avoid missing detail in the original signal

Human hearing = 20Hz to 20,00Hz

Cd recording frequency must be sampled at 40,000Hz

This will produce 20,000Hz signal, sound for a CD is sampled at 44.1kHz

Steaming Audio

Instead of downloading the whole file, data is sent part by part playing the sound when it has enough to play.

Streamed data is buffered on arrival and played while the buffer is not empty. If thbuffer empties then the playing program normally paused until there is enough in the buffer.

Streaming means that large files do not need to be transferred in one go and it reduced the ease with whih files can be copied.

Editing Sound

-Further sound can be added

-The 'shape' of the sound can be changed'

-the pitch and duration can also be altered

-Sound can be processed to add effects, like the addition of sounds to make the sound appear to come from a concert hall or a room with an echo

-Stereo tracks can be sepearted out

Sound Formats

WAV - large amount of space, uncompressed

MP3 - compact sound file, removed frequencies outside the human range and compressed the resulting signal

MIDI - Musical Infomation Digital Interface, Format that stores the pitch, duration and instrument, very compact, allows editing

Analogue + Digital