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Digital Media

Digital Media. Dr. Jim Rowan ITEC 2110 Audio. What is audio?. First, some demos. Can you hear this? http://s3.amazonaws.com/listverse/audioillusions/highfreq1.mp3 “mosquito ring tone” Audio illusion “Creep” http://www.youtube.com/watch?v=ugriWSmRxcM. The nature of sound.

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Digital Media

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  1. Digital Media Dr. Jim Rowan ITEC 2110 Audio

  2. What is audio?

  3. First, some demos • Can you hear this? • http://s3.amazonaws.com/listverse/audioillusions/highfreq1.mp3 • “mosquito ring tone” • Audio illusion “Creep” • http://www.youtube.com/watch?v=ugriWSmRxcM

  4. The nature of sound • There are two special types of audio • Functionally and uniquely different than other sounds • Music • Cultural status • Can be represented as non-sound: MIDI • Speech • Linquistic content • Lends itself to special compression

  5. And it’s complicated • Converting energy to vibrations and back • Transported through some medium • Either air or some other compressible medium • Consider speech • Starts as an electrical signal (brain & nerves) • Ends as an electrical signal (brain & nerves) • But…

  6. And it’s complicated (cont)http://en.wikipedia.org/wiki/Ear • Starts as an electrical signal (brain & nerves) ==> • Muscle movement (vocal chords) • Vibrates a column of air sending out a series of compression waves in the air • Compression waves cause ear membrane to vibrate ==> • Moves 3 tiny bones ==> • Causes waves in the liquid in the inner ear ==> • Bends tiny hair cells immersed in the liquid ==> • When bent they fire ==> • Sends electrical signals to the cerebral cortex • Processed by the temporal cortex

  7. Audio Illusions • Play a 200 Hz pure tone • Softly at first • Gradually increase the volume • Most listeners will report that the tone drops in pitch as the volume increases • Complex tones are reported to have lower pitch than pure tone of the same frequency • Frequencies above human hearing affect how the lower frequencies are perceived even though they can’t be “heard”

  8. Why do you think… • You can’t tell where some sounds come from (like some alarms for instance) • You only need one sub woofer when you need at least two for everything else • You can’t tell where sound is coming from underwater • Two things running at the same speed make a “beating” sound

  9. Why do you think… (cont) • With your eyes closed you can’t tell whether a sound is in front of you or behind you • You hear sound that isn’t there (tinnitis) • Phantom sounds • Heard… but not there • Masking sounds • Not simply drowning them out • Can mask a sound that occurs before the masking sound is played

  10. Why do you think… (cont) • You can hear your name in a noisy room • Cocktail party effect • http://en.wikipedia.org/wiki/Cocktail_party_effect • Still very much a subject of research

  11. Why?It’s complicated! • Sound is physical phenomenon • Wavelength affects stereo hearing • Speed of sound affects stereo hearing • You can tell where a sound comes from if • the wavelength is long enough and • the speed that sound travels is slow enough to allow the waves arrive at your ears at different times • Sound is a sensory and perceptual experience • http://en.wikipedia.org/wiki/Psychoacoustics

  12. Processing Audio

  13. Processing audio • How can we look at sound? • What do you want to see? • Waveform displays • Summed amplitude of all frequencies & time • Amplitude & frequency components at one point in time • Amplitude & frequency & time

  14. Summed amplitude across all frequencies & time

  15. Amplitude & frequency components at one point in time

  16. Amplitude & frequency & time

  17. Summed amplitude & time joe took father’s shoe bench out

  18. Amplitude & frequency & time

  19. Digitized audio • As we have seen earlier this semester • Sample rate & quantization level • Reduction in sample rate is less noticeable than reducing the quantization level • Jitter is a problem • Slight changes in timing causes problems • 20k+ frequencies? • Though they can’t be heard they manifest themselves as aliases when reconstructed

  20. Audio Dithering • Book description on page 284 • Figure 9.9 • Add random noise to the original signal • This noise causes rapid transitioning between the few quantized levels • Makes audio with few quantization levels seem more acceptable

  21. Audio processingterms to know • Clipping • …but you don’t know how high the amplitude will be before the performance is recorded • Noise gate • has an amplitude threshold • Notch filter • 60 cycle hum • Low pass filter • High pass filter • Time stretching • Pitch alteration • Envelope shaping

  22. One thing about humans… • We can actively “filter out” what we don’t want to hear • remember the cocktail party effect? • Over time we don’t hear the pops and snaps of a vinyl record • Have you ever recorded something that you thought would be good only to play it back and hear the air conditioner or traffic roaring in the background?? • A piece of software can’t do this… • …not yet anyway!

  23. Compressing sound • Take the opposite approach than the one you took with images • With images you can toss out the high frequencies • With audio you can’t… high frequency changes are highly significant

  24. Compressing sound (cont) • Voice? • Remove silence • Similar to RLE • (how many seconds of silence?) • Non-linear quantization • “companding” • Quiet sounds are represented in greater detail than loud ones • Mu-law • A-law • Allows a dynamic range that would require 12 bits into 8 bits • 4096 (2**12) ==> 256 (2**8)

  25. Compressing sound (cont) • Differential Pulse Code Modulation (DPCM) • Related to inter-frame compression • It predicts what the next sample will be • It sends that difference rather than the absolute value • Not as effective for sound as it is for images • Adaptive DCPM • Dynamically varies the step size • Large differences were encoded using large steps • Small differences were encoded using small steps

  26. Compressing sound (cont)that are perceptually based • The idea is to remove what doesn’t matter • Based on the psycho-acoustic model • Threshold of hearing • Remove sounds too low to be heard • High and low frequencies not as important (for voice) • They require much more power to be heard • Loud tones can mask quiet ones • It can mask sounds before and after they occur

  27. Record & Playback • There are two ways to “record” and then “playback” the audio • Play the instruments • Record the performance • Play the recording back • Write the music down • Send the written-down music • Perform the written-down music

  28. How do you write it down? • There is another way to “write down” the music for performance later. • Instead of writing it down on sheet music… • Write it down as machine instructions • Timing, voice, amplitude and pitch …one form of this is MIDI • The music flows as instructions that are used to recreate the analog music

  29. MIDI • You can use software to create or capture MIDI music • You can use software to play back the MIDI stream • P 307, Tables 9.1 and 9.2 • Voice numbers • Drum kit numbers • Why are there 128 voice numbers? • Why not 129 or 127? • How many bits? • Why are there 32 drum kit numbers? • Why not 33 or 31? • How many bits?

  30. Questions?

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