PH 105 Dr. Cecilia Vogel Lecture 24

OUTLINE  Electronic music  Theramin  Analog vs digital  components in electronic music

Announcements  final exam  Tuesday, 1-3 pm  new material and review  Same length as 1 st 3 exams  Counts the same as other exams  Office hours  Friday, 12-12:30 & 1:30-2  Monday, 9:30-10, 11:30-12  Tuesday, 10:30-11

Electronic Music  May refer to instruments that are amplified or mixed electronically.  May refer to instruments that are purely electronic  Theremin  synthesizer.  May refer to music composed, edited, and/or stored on computer.

Theremin  A Theremin is a totally electronic instrument,  source of sound is electronically driven speaker.  player controls pitch and loudness  Controllers are capacitors  where the player’s hand is one of the capacitor plates.  capacitance and thus current is controlled by moving hand closer or further from antenna.

Theremin 

Analog vs Digital  Electronic music must span the boundary of analog and digital.  digital generally means binary:  true or false, on or off, 0 or 1, nothing between  analog means a continuum of possibilities  example clocks 8:00:00 8:00:01

Advantages  Analog signal contains more info  for example, can show 8:00:00.3  Digital signal can be  more efficient because it doesn’t need to store extra info.  Less susceptible to noise, since 0.95 is wrong  Trade-off depends on  ability to measure or respond  Can tell how far between ticks second hand is?  perception or needs  Who cares if your late by 0.3 seconds?

Components in Electronic Music  Source of sound  purely electronic — digital  sampled from mike — analog converted to digital  Digital sampling rate determines the highest frequency can detect.  maximum frequency is half the sampling rate.  can’t detect changes that happen faster than you collect data.

Controllers in Electronic Music  amplitude (loudness)  variable resistor  frequency (pitch)  waveform  Equalizer can vary the amplitude of frequencies within a certain range  using filters  Variety of instruments with stored info about their timbre and envelope

Electronic Music Output  The final signal may be stored electronically as digital format,  but in the end, signal must be changed from digital back to analog  speakers are analog  vibrate due to continuously variable current  ear is analog

Electronic Music Output  Output may be monophonic or polyphonic  can you play more than one note at once?  how many?  Output may be multitimbral  can you play more than one instrument at once?  each a different timbre  If so, which can you control?

MIDI  Musical Instrument Digital Interface  Does not directly contain information about the waveform.  MIDI is more like a computer program  “words” tells the device how to play.  Digital information includes  pitch  loudness  timbre (which instrument)  MIDI keyboard

CD-quality  In digital audio recording,  analog electrical signal is digitized  A-D conversion.  The input is sampled 44,100 times/s;  means sample rate, R = 44.1 kHz.  Max freq able to be saved is about 22 kHz:  about the max freq human ear can hear.

CD-storage  Each sample is stored as a number  which takes up 16 bits of memory.  So each second of sound takes up  44,100 X 16 X 2 (for stereo)  or 176 kB.  That’s 10MB per minute!  That’s the amount of storage space it takes to store the sound wave as it is.

MP3  Can store audio info in smaller file.  shrink size by factor of  factor of 12 is typical  How does it do that?  “lossy” compression:  some information about the sound is lost.  If lose parts of the sound that our mind/ears would throw out anyway,  doesn’t affect the sound quality much.  Takes less space to store sound “as it sounds” rather than “as it is”

MP3 Compression  Step 1,  Divide the wave into short frames.  Step 2,  take an FFT of the wave in each frame  to get spectrum.  Step 3,  For each peak in spectrum,  compare to a psychoacoustic model.  Throw out any peaks that are unimportant according to the model.

MP3 Compression  Step 4,  Lossless compression,  throws out redundant information.  “Huffman coding”  decreases size about 20%  Step 5,  Make a header for each frame.  So can identify MP3 version, copyright, etc.  Each frame has a different header,  because some frames are more compressed than others.

Psychoacoustic Model  Any tones that would be masked,  unheard due to other sounds,  are considered unnecessary and not saved in the MP3 file.  Info that goes into model includes:  For two tones of similar pitch, if one is much quieter, it will be masked.  Lower tone will more easily mask higher tone, than vice versa.  Noise masks differently than pure tones.  How close in time they must be to mask.

Stereo  Since our hearing is binaural  directionality comes largely from differences in what 2 ears hear.  Stereo sound sends different signals to left and right speakers  so left and right ears hear different things  giving spatial effect.

Surround Sound  The 5.1 standard for surround sound  gives a greater spatial feel  because it uses 5 speakers instead of two  for spatial localization  plus one sub-woofer  very low pitch sounds aren’t directional.  Required six channels of sound  and 6 speakers

Directionality Cues  Binaural cues that help us determine directionality:  relative phase of low-frequency sound,  relative intensity of mid-range sounds,  relative time of arrival of high-freq sounds.  Non-binaural cue:  Which frequencies are strong in the spectrum?  Effects like diffraction around the pinna & the head,  mean that the ear has a different response to sounds from different directions.

Transfer Function  Transfer function tells what fraction of the free sound is actually transferred to the eardrum  It is different for sounds from different directions  for example, sound from the side has  more response from 500Hz-1500Hz and near 8 kHz  than sounds from front

SRS 3-D  If you increase the amplitude of components  from 500Hz-1500Hz and near 8 kHz  you can make it sound like the sound came from the side  even if it came from speakers in front of you  SRS technology  uses this idea  to create fake surround sound  Can also make the sound appear to come from higher than the speakers  if speakers are below TV, for ex

SRS TruBass  Another problem with speakers  is reproducing good bass  without using large, heavy speakers  TruBass  uses the idea of the missing fundamental  to create fake bass.  For ex, if the sound contains 50 Hz tone  but speakers aren’t responsive below 100Hz  then have speakers emit 100 Hz, 150 Hz, 200 Hz  the mind will fill in the missing fundamental at 50 Hz

Summary  Electronic music  purely electronic or with acoustic sources  is often analog in source, digital in processing, analog in output  may be polyphonic and/or multitimbral  Components:  analog or digital source  controllers  amplifiers, filters, mixers  analog sound output