Comp Sci - Lecture 11.docx

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Computer Science
Computer Science 1033A/B
Vivi Tryphonopolous

Lecture11 What is sound? • Sounds are pressure waves of air • Visualize a sound as a series of recurring waves called a wave form • Voice muscles vibrate and cause the air to moveD thus cause sound (a series of waves) The NYQUISTLimit • This rule says you must take at least two samples for every cycle of the wave. If you take less than two samples,you will get a completely different sound wave. • Sample rate= number of samples we take per second of audio or number of times per second the wave form is measured Typical Sample Rates • Voice only (telephone quality) =8KHz • AM radio quality= 11.025 KHz • FM radio quality = 22 KHz • CDquality music= 44.1KHz How does the sound wave get converted to be stored on our computer? • Computers have a sound card which samples (sets the number of samples and quantizes) the sound wave from a microphone • Sound log has an analog-digitalconverter (ADC) for recording ,and a digital -to-analog converter (DAC)for playing audio Sound Editing • Rearrange the waveform Cut,copy,drag,trim parts of the waveform Overlap two or more pieces of audio find words you want to edit out and cut them from the waveform • Modify the volume Use amplify, fade-in,fade-out,envelop ,normalize • Noise Reduction Hiss reduction - noise within a given frequency range Noise reduction/removal -software examines the audio and finds unusual differences from waveform and removes them .Need a longer piece of audio than for HISS reduction []software had to analyze the audio to generate stats on what is unusual • Special effects Adding echo,changing the pitch of a portion • Down sample and reduce the bit depth (i.e. compress) Sound Compression • reduce the number of samples (sample rate) • reduce the bit depth (sample size) • reduce the channels • compress using the appropriate codec Audio Compression • file formats that use lossless codecs/compression there are a few but not very common • common file formats that are uncompressed .wav (very common ,44KHz,16 bit) .aiff .cdda (Red Book )-standard for COs,44 KHz,16 bit/sample ,2 channels Note: .wma and .mp4 are lossy and allow for built-in lockdowns Audio File Formats Audio Format File Extension Advantages Disadvantages Advanced Audio .aac • Good sound quality • Copy protected C
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