Transcript GCSEComputing_Session10

Session Objectives#10
SHOULD explain how sampling intervals and bit rates affect file size and playback
quality
COULD explain how sound can be sampled and stored in digital form
MUST define the term sample rate
Create yousrc apps to move images and play sounds.
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Binary Representation of Sound
Starter:
Units of storage starter
Sound is usually in analogue form (continuously changing data)
but the computer needs to have this in digital form as a set of
binary numbers.
To convert sound from analogue into a digital form, the sound
wave is sampled at set intervals and the height of the sound
wave (amplitude) is recorded.
These values are then converted to
binary and stored.
The frequency of these recordings is
called the sample rate.
The higher the number of recording,
over a given period of time, the higher
the sample rate = better qual;ity
recording, and larger size file.
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Make Some Noise!
TASK: Use Audacity to make recordings. Export the sound files
using different file types and make a note of the file size of
each?
Use a WAV, MP3 and one of your choice.
What comparisons/differences do you notice about the different
types? Prepare to feedback to the class.
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Managing Sound Files Size
There are three ways of reducing the size of sound file:
1) Sample Rate: By reducing the sample rate (e.g. to 22kHz), you
reduce the amount of data you need to store. This has quite
serious effects on the quality of the audio
2) Sample Depth: Much as with colour, you can reduce the
precision of each datum, using only 8 bits per sample instead
of 24, for example.
3)Compression:
Ideally you would sample at the highest practical rate you can
and these use an encoding algorithm to reduce the output file
size (this is why Audacity projects are much bigger in size than
the files output). This can be
Lossless compression (e.g. FLAC and potentially MP3/AAC/Ogg
Vorbis files]) or
Lossy compression (e.g. most MP3, AAC and Ogg Vorbis files).
WAV and AIFF files are used for storing uncompressed audio.
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Binary Representation of Sound
This is what a computer file for a music track looks like.
Once again it is just binary, like our images.
The metadata tells the computer what type of file it is so that
it doesn’t try to display it as a picture or any other file.
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Binary Representation of Sound
In order to digitally record sound, a microphone is used, a
device that converts the analogue sound into a digital form by
altering the output signal from the device.
In order to store this digitally, the voltage is sampled at
frequent intervals (typically 48 000 times per second, or 48kHz)
and stored as a binary code (typically 16 or 32 bits per
sample).
This equates to
slightly over 1.5
million bits per
second, or 88 MB per
minute. An 80
minute album stored in
such a manner would
require 6.9 GB of
storage.
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Make some noise!
TASK: Create some recordings in Audacity and experiment with the
different sample and compression settings.
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Managing Sound Files
REVISION OF SOUND & IMAGES:
http://web.eecs.utk.edu/research/cs100modules/module4/
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