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Lecture 11

Computer Science 1033A/B Lecture Notes - Lecture 11: Limewire, A.D. Vision, Noise Reduction

Computer Science
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High frequency/pitch waves (close together waves) causes high notes... [low frequency = low
Larger amplitude causes louder sound... [small amplitude = quieter sound]
Nyquist Limit:
-must take at least 2 samples for every cycle of the wave
-measured in kilohertz (1000 samples per second)
Sample rate = number of samples we take per second of audio
Only need 8000 kilohertz for voices, AM is 11 KHz, FM is 22 KHz, CD Quality 44 KHz
Quantizing = how many samples we will have
-CDs use 16-bit sound (65536 possible values)
-More bits are higher quality, but that's a huge file size
Computer Audio
Computer have sound card which samples the sound wave from a microphone
-Analog-to-Digital Converter (recording), Digital-to-Analog Converter (playing)
Rearranging the waveform: cut, copy, drag, trim, parts of waveform
-Can overlap multiple pieces of audio
Modify volume by amplifying, fade-in, fade-out, envelope, normalization (scan to find peak or
average level)
Noise reduction (hiss reduction, noise reduction/removal) are options when editing sound, as well
as special effects
Compressing Audio
CD can hold max of 80 minutes of audio
No usually compressed because audio files are not that large
-Can reduce number of samples (sample rate), from 44KHz to 22KHz
-Reduce the depth (sample size), from 16KHz to 8KHz
-Reduce channel, from stereo to mono will reduce to half the file size
-Compress using appropriate codec (can be lossy or lossless)
Lossy compress a song from a CD to 1/11 of size (ex. MP3)
Common file formats that are uncompressed:
-.wav (very common)
-CDDA ("Red Book") -> used for CDs, high quality
MP3 (.mp3) -> good sound quality even though compressed, steamed over web, requires
browser plugin
Real Audio (.ra, .rx) -> high compression, very small files, streamed over web, not great quality,
needs plugin
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