Page 228 - DCAP303_MULTIMEDIA_SYSTEMS

Multimedia Systems

notes record sounds from musical instruments. For ordinary sound there are varieties of software such
as Sound edit, MP3cutter, Wave studio.
13.6.1 Digitized audio
Audio Signal can be stored as digitized one in the form of numbers. An audio signal is represented
in digital memory with a binary code that stores a massive amount of numbers that are used
to represent a signal. An ADC (Analog to Digital Converter) is a computer chip that is used
to convert an analog signal into digital information. This process is called sampling. Digitized
sound is normally called wave files and end with the extension wav for PC and with .aif or .aiff
for Macintosh. Digitized sound is nothing but a sampled sound. Every nth sample of the signal
is taken and stored as digital data in the form of bytes. Three sampling frequencies used in
multimedia systems are 44.1 KHz, 22.05 KHz and 11.025 KHz. Sample sizes are 1byte or 2 bytes.
Audio resolution determines the accuracy with which the sound can be digitized. The digital
waveform to be reconstructed depends on the sampling frequency selected. It is very difficult to
reconstruct the waveform if the sampling frequency selected is very low. Figures 13.1 below show
the analog waveform converted into digital one using the sampling frequency.
figure 13.1(a): original Waveform

figure 13.1(b): sampling frequency

figure 13.1(c): reconstructed Waveform

Once the signal is represented as binary numbers, the signal is then converted back to an analog
signal through a DAC (Digital to Analog Converter). Each sample value is rounded off to the
nearest integer to create the waveform. This process is called quantization. But this process can
create unwanted noise. Digital audio technologies include digital broadcasting techniques namely
Digital Audio Broadcasting (DAB), HD Radio, Digital radio Mondi ale (DRM), in band on channel
(IBOC) and storage techniques like digital audio player, digital audio tape, CD, DVD, digital
audio workstation and various audio file formats.
13.6.2 MiDi (Musical instrument Digitization interface)

Musical Instrument Digitization Interface (MIDI) provides a protocol or a set of rules, using which
the musical note is directly recorded into the computer from musical instruments. A MIDI file is
basically a list of commands to produce the sound. For example, pressing of a guitar key can be

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