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Music Hath Charms to Sooth the Savage Beast Introduction to Sound Processing Some Sources Used Richard E Berg: Physics 102: PHYSICS OF MUSIC , University of Maryland Robert Jourdain (1997). Music, the Brain and Ecstasy . Quill. Various bits of Wikipedia Dolby Sound Sound Is Analog

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music hath charms to sooth the savage beast

Music Hath Charms to Sooth the Savage Beast

Introduction to Sound Processing

CSE5060 -- Multimedia on the Web -- Lecture11

some sources used
Some Sources Used
  • Richard E Berg: Physics 102: PHYSICS OF MUSIC, University of Maryland
  • Robert Jourdain (1997). Music, the Brain and Ecstasy. Quill.
  • Various bits of Wikipedia
  • Dolby Sound

CSE5060 -- Multimedia on the Web -- Lecture11

sound is analog
Sound Is Analog
  • So there’s infinite variation
  • Like a rock thrown into a pond, there are waves:
    • Amplitude: how high the waves are -- Loudness
    • Frequency: how many waves per second -- Pitch
  • Loudness is measured in decibels
    • This is a log scale, so 20 is ten times as loud as 10, 30 is ten times as loud as 20, and so forth.
    • You can distinguish from just over 0 dB to 120dB
      • 37 quiet office (no air-conditioning)
      • 59 conversation
      • 76 loud factory
      • 110 really loud night club or rave
      • 140 threshold of pain (well, for some)

CSE5060 -- Multimedia on the Web -- Lecture11

sound is analog 2
Sound Is Analog, 2
  • Pitch is measured in Hz (Hertz, cycles per second) and kHz.
  • You can distinguish between a few Hz and 15 - 20 khz (this is age dependent)
    • Lowest note on piano 27 Hz
    • Highest note on piano 4,186 Hz (4.186 kHz)
    • Lowest vocal sound 80 Hz
    • Highest vocal sound 800 Hz
    • The A above middle C 440 Hz (used to be lower!)
  • But just a sine wave at these frequencies sounds sterile: it lacks the overtones, the harmonics, produced by all natural sources of sound.

CSE5060 -- Multimedia on the Web -- Lecture11

this is a sine wave which may represent a pure and so artificial sound
This is a sine wave, which may represent a “pure” (and so artificial) sound

Its frequency (tone) is the distance between crests -- hertz

Its amplitude (loudness) is the height of the crests -- decibels

With frequent sampling we can capture both frequency and amplitude in a single series of numbers

Any sound can be reproduced using a sequence of overlaid sine waves (Fourier transformations)

Two sine waves interacting

CSE5060 -- Multimedia on the Web -- Lecture11

notice how complicated the vibrations
Notice how Complicated the Vibrations

CSE5060 -- Multimedia on the Web -- Lecture11

sound is analog 3
Sound Is Analog, 3
  • An instrument vibrating produces lots of sounds above the fundamental tone.
  • Many of these are various octaves above the fundamental
    • Octave = double the frequency
    • To get realistic sound we have to pick up at least the 4th harmonic, 4x the frequency of the fundamental.
    • So we have to pick up to 12kHz for, say a realistic flute sound (where the highest fundamental is just under 4kHz
    • More is better until, say 20kHz where a 5 year old’s hearing cuts out.
  • So how frequently do we need to sample to get “realistic” sounds?

CSE5060 -- Multimedia on the Web -- Lecture11

poor sampling rate graphic
Poor Sampling Rate: Graphic

CSE5060 -- Multimedia on the Web -- Lecture11

nyquist shannon sampling theorem
Nyquist-Shannon Sampling Theorem
  • You have to sample at 2x the size of the smallest difference you want to catch.
  • Remember, we are sampling the volume (loudness) of a sound consisting of lots of superimposed fundamental and harmonic frequencies.
  • So there are 44,100 samples per second, each a 2 byte --16 bit between 0 and 64k
  • Sampling Demo Program

CSE5060 -- Multimedia on the Web -- Lecture11

slide10
MP3
  • MPEG-1, Layer 3 sound compression -- intended for movies on CD and DVD
    • 90+% compression possible
    • A typical song (50mb) goes to 5mb
    • Is a lossy compression, so the quality goes down
    • No encryption in any way
    • No “watermark” (watermark = a secret pattern of bits somewhere which indicates the source of the copy)
    • Much music publisher panic with the sudden popularity of the format.
    • Much more music publisher panic with the iPod and friends

CSE5060 -- Multimedia on the Web -- Lecture11

mp3 sound isn t very good
MP3 Sound Isn’t Very Good…
  • Having failed at my attempt to demonstrate how bad MP3 is in a tute
  • I will now (fail again?) demonstrate the loss of quality in MP3 yet again…
  • Roll it, monks…..

CSE5060 -- Multimedia on the Web -- Lecture11

sound into bits adc
Sound Into Bits (ADC)
  • Something that always confuses me:
    • The 16 bits used (0 - 64k) record the amplitude (loudness)
    • The differences between successive 16 bit samples contain the frequency (pitch)
    • Remember, a high wave also has a trough between peaks.
  • So how often do we have to sample to get enough samples
    • 2x the maximum difference we want to catch.
    • And we want to catch differences up to 20mHz, so we have to sample at at least 40,000 times a second.
    • So your CDs contain music sampled at just over 44,000 times a second.
    • So the digital signal bandwidth must be 16 x 44,000 = 704,000 bits per second or 88 kbps. With stereo sound, we have to have two such samples, so a 1x CD-ROM bus goes at 176 kbps, which we already knew!

CSE5060 -- Multimedia on the Web -- Lecture11

bits into sound dac
Bits Into Sound (DAC)
  • Amplifiers and speakers are analog devices, so
  • The CD player does DAC and passes the results as an analog signal to your stereo system.
  • It does the same if you listen to music off your CD-ROM drive.
  • But where does your sound card/chip do the conversion?
  • Hummmm…. Later is (far, far) better, because there’s lots of electrical interference inside your PC. Digital isn’t affected by this, but analog is.
  • So the perfect system would be all digital inside the computer and have its DAC inside the speakers

CSE5060 -- Multimedia on the Web -- Lecture11

so we have dac for both sound and video
So We Have DAC for both Sound and Video
  • At the same time
  • By two independent sets of hardware and software
  • Working on two independent files
  • How can we guarantee synchronisation???
  • This is a problem with Flash!

CSE5060 -- Multimedia on the Web -- Lecture11

midi general midi
MIDI & General MIDI
  • MIDI = Music Instrument Digital Interface
  • MIDI is to sampling exactly what vector is to raster graphics
    • A language for describing sounds
      • The notes
      • The instruments, each of which has a number
        • 128 instruments
        • Plus drum kit
      • The note characteristics
        • attack
        • sustain
        • decay
        • release
      • 2+ ways of making those notes
        • FM synthesis
        • Wavetable

CSE5060 -- Multimedia on the Web -- Lecture11

an mdi studio
An MDI Studio

CSE5060 -- Multimedia on the Web -- Lecture11

an audioacoustic editing lab
An Audioacoustic Editing Lab

CSE5060 -- Multimedia on the Web -- Lecture11

the parts of a midi note
The Parts of a MIDI Note
  • From the MIDI Manufacturers Homepage

CSE5060 -- Multimedia on the Web -- Lecture11

making midi
Making MIDI
  • FM Synthesis
    • Sterile sine waves
    • What gave computer music a bad name
  • Wavetable Sound Generation
    • The music gives the number of the instrument
    • Samples of the sound of that instrument are stored in ROM/RAM on the sound card
    • The samples are processed to give a far better illusion of the sound of the instrument
    • The more samples, the better, so 64mb of samples on ROM are better than 512k.
    • Wavetables may also be downloaded from CD-ROMS

CSE5060 -- Multimedia on the Web -- Lecture11

midi quality
MIDI Quality
  • Well, as always there’s the trade off:
    • Much smaller file size
    • Always somewhat less quality
    • Infinitely cheaper to create -- only one muso necessary
    • May require significant CPU processing

CSE5060 -- Multimedia on the Web -- Lecture11

channels voices and streams
Channels, Voices and Streams
  • A channel drives a speaker:
    • 2 channels for standard stereo
    • 4-5 channels for 3D sound (two may be faked)
    • 8+ channels for super sound in theatre movies
  • A voice is an instrument, etc. on a channel
    • MIDI supports a large number of voices: 32, 64. This is polyphony
    • The voices are superimposed, in digital or analog form, and then sent to the speakers
    • Again, multiple voices may load down the CPU
  • A stream is half voice and half channel
    • Lets you record a sound effect, a stream
    • When we need it, we superimpose it on top of the sound going to a channel
    • The sound card and/or CPU do the work

CSE5060 -- Multimedia on the Web -- Lecture11

channels voices and streams 2
Channels, Voices and Streams, 2
  • The higher the bandwidth into the sound card/chip
    • The more channels, voices and streams we can get at once
    • And the more processing work has to be done
    • So we either do more on-sound-card/chip processing or bog down the CPU
    • (Sound like the issues related to 3D accelerator cards?!)
  • Evolution in sound cards/chips rather slow
  • Most systems use sound chip on motherboard
  • But if you want to play games….

CSE5060 -- Multimedia on the Web -- Lecture11

games and computer sound
Games and Computer Sound
  • Games are one of several factors driving the evolution of graphics boards
  • Games are almost the only factor driving the evolution of sound cards
    • Who is sneaking up behind me? We need 3D.
    • What kind of sound does that alien make when exploded? We need lots of streams superimposed.
  • 3D illusion
    • Uses 3 speakers (woofer, + 2 satellite) and an algorithm to fox the ear by marginally delaying one stereo channel
    • Developed by NASA for space flight simulators
    • Can work well if don’t move your head
    • With 5 speakers, esp. with 4 channels, can work very well indeed
      • Note: deep tones non-directional, so we can use just one woofer
  • As the musicians are never behind you, not necessary for music. Whoops, sorry Berlioz, Allegri, Tchkovsky, etc.

CSE5060 -- Multimedia on the Web -- Lecture11

games and computer sound 2
Games and Computer Sound, 2
  • Competing 3D positional audio standards
    • A3D
      • From Aureal Semiconductor
      • On their widely used Vortex audio chips
    • Audio Extensions .EAX
      • From Creative Labs (who brought us Sound Blaster)
    • DirectSound3D
      • From Microsoft
      • Part of the DirectX set of Windows APIs/extensions, including Direct3D
  • Currently the first of these is the standard, but watch out for the rest.

CSE5060 -- Multimedia on the Web -- Lecture11