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Sound
Computer Science Higher
Lesson 5: Sound
Learning intentions
 To know how a computer stores sound files
 To know how a computer can reduce the size of sound files
Success criteria
 I can describe how a computer stores sound files whether they be sampled or
synthesised.
 I can calculate the file storage requirements for a sound file given the length
of time the file is, sampling frequency, number of channels and the sample
depth.
 I can describe how audio files are compressed using perceptual noise
shaping.
Synthesised Sound
 Synthesised sound is generated by a sound card and does not support vocals.
MIDI (Musical Instrument Digital Interface) files are an example of synthesised
sound.
 MIDI is a file format and protocol for communicating between devices.
 MIDI is used to send digital data about music between devices, rather than
the resulting analogue sound.
 A MIDI sound file does not store samples. Like a vector graphic, a MIDI file is a
set of instructions that is used to play music. Each note has attributes (sound,
length, volume, time etc) that place it into the sequence of notes in the file.
 MIDI files can be played back by a computer using its own digital instruments.
Sampled sound
 Sampled sounds are stored by digitising the sound wave recorded by a
microphone. The sound wave is sampled thousands of times per second and
the values are recorded into a file.
 The quality of a sound file is determined by how many samples per second
are taken, and the sampling depth of each sample (the number of bits used
to represent the sample)
 Sounds you can hear are in analogue format, in order to record a sound to be
stored on a computer a device such as a microphone must be used. This
converts the audio waves (analogue signal) into electrical signals by an
analogue to digital converter.
Sampled sound files
The size and quality of an audio file can be calculated provided you have
the following information:
1. The length of time the audio file is (in seconds)
2. The sampling frequency – The number of times per second that the sound
is sampled. This is measured in KHz. 31.6KHz would be converted to 31600
for the purposes of our calculations.
3. The number of channels – This depends on how many channels were used
when recording the sound. One = Mono, Two = Stereo
4. The sample depth - How many bits are used to store each sample, much
like the concept of bit depth in relation to a bit mapped graphic.
Calculating sampled sound file sizes
Audio file size = [time in seconds]*[sampling frequency]*[number of
channels]*[sampling depth]
Example: Calculate the file size for a 1-minute audio file with a sampling size
of 43.5KHz. The audio was recorded in stereo and has a sampling rate of 8
bits.
[60]*[43500]*[2]*[8] = 41760000 bits
/8 = 5220000 bytes
/1024 = 5097.65 kilobytes
/1024 = 4.97 megabytes
Calculating sampled sound file sizes
Audio file size = [time in seconds]*[sampling frequency]*[number of
channels]*[sampling depth]
Example: Calculate the file size for a 1-minute audio file with a sampling size
of 43.5KHz. The audio was recorded in stereo and has a sampling rate of 8
bits.
[60]*[43500]*[2]*[8] = 41760000 bits
/8 = 5220000 bytes
/1024 = 5097.65 kilobytes
/1024 = 4.97 megabytes
Sound compression
In a sound, lossy compression can be used to filter out parts of the sound
that humans cannot hear. Less data is stored which results in a smaller file.
Lossless sound compression compresses files by spotting patterns in the
binary that represent the sound. This is unlikely to reduce the size of the file by
much.
Perceptual noise shaping
 MP3 audio files are compressed using a technique called perceptual noise
shaping.
 For example:
 There are certain sounds that the human ear cannot hear.
 There are certain sounds that the human ear hears much better than others.
 If there are two sounds playing simultaneously, we hear the louder one but
cannot hear the softer one.
 Using facts like these, certain parts of a song can be eliminated without
significantly hurting the quality of the song for the listener. This is an example of
lossy compression.
Sound summary
 Synthesised sound is generated by a sound card and does not support
vocals. MIDI files are an example of synthesised sound.
 MIDI is a file format and protocol for communicating between devices. A
MIDI file is a set of instructions that is used to play music. Each note has
attributes that place it into the sequence of notes in the file.
 Sounds recorded via a microphone are an example of sampled sound.
The sound wave is sampled thousands of times per second and the values
are recorded into a file. The quality of a sampled sound file is determined
by how many samples per second are taken, and the sampling depth of
each sample .
Sound summary
 A microphone will have an analogue – digital converter to change the
analogue signals in the air to a digital format.
 Audio file size can be calculated with the following formula:
 Audio file size = [time in seconds]*[sampling frequency]*[number of
channels]*[sampling depth]