Digital Media Dr. Jim Rowan ITEC 2110 Audio

What is audio?

First, some demos Can you hear this? – usions/highfreq1.mp3 –“mosquito ring tone” Audio illusion “Creep” – mRxcM

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

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…

And it’s complicated (cont) –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

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”

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

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

Why do you think… (cont) You can hear your name in a noisy room –Cocktail party effect – effect –Still very much a subject of research

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

Processing Audio

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

Summed amplitude across all frequencies & time

Amplitude & frequency components at one point in time

Amplitude & frequency & time

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

Amplitude & frequency & time

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

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

Audio processing terms 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

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!

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

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)

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

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

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

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

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?

Questions?