Transcript Multimedia: Making it Work

Multimedia Sound
What is Sound?
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Sound, sound wave, acoustics
 Sound is a continuous wave that travels through a medium
 Sound wave: energy causes disturbance in a medium, made of
pressure differences (measure pressure level at a location)
 Acoustics is the study of sound: generation, transmission, and
reception of sound waves
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Example is striking a drum
 Head of drum vibrates => disturbs air molecules close to head
 Regions of molecules with pressure above and below equilibrium
 Sound transmitted by molecules bumping into each other
Measurement of Sound
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A sound source is transferring energy into a medium in the
form of sound waves (acoustical energy)
Sound volume related to pressure amplitude:
- sound pressure level (SPL)
SPL is measured in decibels based on ratios and
logarithms because of the extremely wide range of sound
pressure that is audible to humans (from one trillionth=1012
of an acoustic watt to one acoustic watt).
 0 dB SPL - no sound heard (hearing threshold)
 35 dB SPL - quiet home
 70 dB SPL - noisy street
 110 dB SPL - thunder
 120 dB SPL - discomfort (threshold of pain)
Digital to Analog Conversion
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Sampling: x(n)=x(nT), where
T is the sampling period
(horizontal). The opposite
transformation, x(n) -> x(t), is
called interpolation.
Quantization:
Q() is a rounding function
which maps the values of x(n)
into N levels (vertical)
Coding:
Convert discrete values to
binary digits
Sampling
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Sample Rate: refers to the rate at which
sound samples are recorded from the
incoming analog audio source and it is
recorded along X axis. Measured in Hz
 Higher
the frequency better is the recorded signal.
 CD quality audio : 44.1 KHz
Sampling Size: refers to the number of
bits used to record the incoming signal it is
recorded along Y axis.
 Mono & Stereo Channelising
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Quantization & Clipping
Quantization: The value of each sound
sample during ADC process is round off to
the nearest integer value.
 Clipping: During ADC process if the
amplitude of the sample is greater than the
interval available the value is clipped at the
top and the bottom.
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Nyquist’s Sampling Theorem
Sampling Frequency is Very Important in
order to accurately reproduce a digital
version of an Analog Wave form
Nyquist’s Theorem:
“The Sampling frequency for a signal
must be at least twice the highest
frequency component in the signal.”
Digital Audio File Size
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Disk space required =
Sample Size (Hz) X Sampling Rate X Channel multiplication Factor
A 16 bit sound system recording signal at 44 KHz in stereo will take up:
16 * 44000 * 2 = 176000 bits or 172 KB per second
Audio Editing Terminology
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Trimming: the process of removing blank spaces.
Splicing: the process of removing unwanted sound.
Reassembling: Cutting, Copying, Pasting of sound clips.
Volume Control: Increase & Decrease of either portion or
whole of the recorded audio.
Fade in fade out: smoothing of beginning & end of audio
file.
Resampling: Process of reducing the sound quality to
reduce the size.