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Week 11

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Computer Science
Computer Science 1033A/B
Pamela Glatt

COMPSCI – WEEK 11 Warm up questions • which of these will NOT affect the size of the video file? ◦ Video size (height and width) ◦ colour resolution ◦ type of font used for credits – at this pint, it's just compressing pixels ◦ number of frames per second ◦ length of video clip ◦ all of the above will affect the size • what is codec? ◦ Atype of video capture card ◦ a piece of software used for compression/decompression of videos and audio ◦ a piece of software used to edit video ◦ none of the above Introduction to sound • sound can ◦ set a mood ◦ sell, sell, sell ◦ educate/present information ◦ allow communication over the web via Internet audio conferencing ◦ internet telephone ▪ allows two-way, full-duplex audio conversations over the web. With Internet telephoning, you can talk to friends or family without the cost of a long-distance phone call. Where can you get sound? • Pre-packaged ◦ purchase a CD – must watch for copyright infringements when using sound on your site ◦ download sound from companies ◦ download iTunes • Create your own sound ◦ recording program with a computer's operating system (such as Sound Recorder) and speak into a microphone attached to the computer – quality will not be the best ◦ recording studio with equipment such as DAT (DigitalAudio Tape) devices that record sounds digitally. Produces a high quality commercial product ◦ electronic instruments such as synthesizers can be used to create music sound files. Connecting the instrument to a computer allows the sounds to be captures in an MIDI (music Instrument Digital Interface) format. What is sound? • Sounds are pressure waves of air • as sound waves hit the ear they vibrate our ear drum • transmitted by longitudinal waves / compression waves • visualize the sounds as a series of recurring called waveforms • Q: which part of the wave indicates the volume of the sound? ◦ Height • Q: which part of the wave indicates the pitch or frequency? ◦ Amplitude • voice muscles vibrate and cause the air to move and thus cause sound (a series of waves) • two people (or one person talking to music) talking causes two sets of overlapping waves. The overlapping waves actually form a new wave How do computers represent sound? • Computer must somehow represent the wave • Q: what two things does a computer always do when it needs to represent something? ◦ Take samples – the more samples, the higher the quality ◦ always sample and quantify things The Nyquist limit • this rule says you MUST take at least 2 samples for every cycle of the wave. If you take less than two samples, you will get a completely different sound wave • Q: what does the Nyquist Limit rule imply about taking samples for higher pitched sounds than lower pitched sounds? ◦ Implies that for higher pitch sounds you must take more samples Sampling • we MUST take 2 or more samples per wave • Q: what is the advantage of taking lots of samples per wave? ◦ Better representation of sound • Q: what is the disadvantage of taking lots of samples per wave? ◦ You're going to get a large file size • number of samples per second is represented in Hertz (Hz) • number of 1000 samples per second is represented by KiloHertz (Khz) • humans can hear between 20 Hz to 20 Khz • for CD quality, we need 44, 100 samples per second or 44, 100 HZ or 44.1 KHz Sample rate • sample rate – number of samples we take per second of audio or number of times per second the waveform is measured • it is breaking sound waves into chunks • typical sounds rates ◦ voice only (telephone quality) – 8 Khz ◦ AM radio quality – 11.025 Khz ◦ FM radio quality – 22 Khz ◦ CD quality music – 44.1 Khz • example: ◦ one of these was sampled at 8000 Hz (8 Khz) and one at 16000 Hz (16 KHz) ◦ Q: which one sounds better? ▪ Second Quantizing – (aka Sample Size) • CDs use 16-bit rate (65, 536 possible values could be given to each sample) • Q: what is the advantage of having a higher bit rate for the sampling size? ◦ More bits = better quality • Q: what is the disadvantage of having a higher but rate for the sample size? ◦ Bigger file size How does the sound wave get converted to be stored on our computer? • Computers have a sound card which samples (sets the number of sample and quangtizes) the sound wave from a microphone • sound card has an Analog-to-Digital Converter (ADC) for recording, and a Digital-to-Analog Converter (DAC) for playing audio • operating system (Windows, Mac OS X, Linux, etc.) talks to the sound card to actually handle the recording and playback Sound editing • now we have the sound in the computer, let's edit the sound bit. What can we do to it? • Rearrange the Waveform ◦ cut, copy, drag, trim parts of the waveform ◦ overlap two or more piece f audio ◦ find words you want to edit out and cut them from the waveform • Modify the Volume ◦ use amplify, fade-in, fade-out, envelope, normalize ▪ sometimes songs from CDs play much louder than others , even at the same volume setting. Normalization corrects this by scanning audio files to find pea
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