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
Where can you get sound?
◦ 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?
• Q: which part of the wave indicates the pitch or frequency?
• 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
• 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 – 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
◦ 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
• 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