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Week 9 – Digital Sound Sound – Digital Audio Waveform Digital sampling of electrical signal Analogue to digital conversion Digital data stores the amplitude of the note Pitch is frequency of the sound - not specifically digitised Sound recreated (playback) through sound card and speakers

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Week 9 – Digital Sound

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sound digital audio
Sound – Digital Audio
  • Waveform
    • Digital sampling of electrical signal
    • Analogue to digital conversion
    • Digital data stores the amplitude of the note
    • Pitch is frequency of the sound - not specifically digitised
    • Sound recreated (playback) through sound card and speakers
    • Digital to analogue conversion
digital audio sampling
Digital Audio – Sampling
  • Sample rate is how often the incoming sound wave is measured
  • Most common sampling rates are multiples:
    • 11.025 kHz (voice only - telephone quality)
    • 22.05 kHz (Most frequently used rate)
    • 44.1 kHz (CD quality — Potentially)
  • Sampling rates are very important to the quality of the sound
  • Sound can be sampled as Monaural or Stereo
sampling considerations
Sampling Considerations
  • Human hearing range not more than 20Hz to 20kHz
    • often only 40Hz to 15kHz in later life
  • Highest frequencies cannot be recorded at 11kHz sampling rate
  • Speech needs 4kHz to 8kHz sampling rate
  • Music needs 22kHz to 44kHz sampling
  • Too high a volume may incur clipping
  • Too low a rate my cause quantisation to affect to reproduced sound
    • Small variations in the sound may be too small to record as different sample values
digital audio sampling size
Digital Audio – Sampling Size
  • The sample size is how much information is recorded at each sampling - also known as Bit Depth
  • The bit depth also influences sound quality
  • An 8 bit sample = 256 values
  • A 16 bit sample can store 65,536 values — A huge difference!
  • 16 bit sampling gives a cleaner waveform with fewer steps
quantisation clipping
Quantisation & Clipping
  • Quantisation is an integral part of the digitising process
    • It is only a problem when the variations between the discreet values recordable at a particular bit depth are too large to register the changes in the sound
  • Clipping occurs when the largest values recordable are less than the highest volumes recorded
digital audio the trade off
Digital Audio – The Trade-off
  • Mono, 8 bit, 11 kHz audio
    • 1 byte ´ 11,000 ´ 1 second = 11 KB per second
    • 11 KB/s ´ 60 second = 660 KB per minute
  • How much for Stereo and/or 16 bit and/or 44 kHz audio?
stereo and mono
Stereo and Mono
  • Mono, 16 bit, 22 kHz audio
    • 2 bytes ´ 22,000 ´ 1 second = 44 KB per second
    • 44 KB/s ´ 60 second = 2.64 MB per minute
  • Stereo, 16 bit, 44 kHz audio
    • 2 bytes ´ 44,100 ´ 1 second ´ 2 = 176 KB per second
    • 176 KB/s ´ 60 second = 10.56 MB per minute
digital audio compression
Digital Audio – Compression
  • Many flavours
    • ADPCM – 4:1
      • MicroSoft, MicroSoft IMA, Creative
    • CCITT – 2:1
      • A-law and -law
    • Audio MPEG – 20:1
advantages and disadvantages of compression
Advantages and Disadvantages Of Compression
  • Advantages
    • Smaller disk storage requirements
  • Disadvantages
    • Must be decompressed before use
      • Can take up to twice sound duration
    • Supported by good sound cards and specialist sound editing packages
digital audio file formats
Digital Audio – File Formats
  • Apple
    • Audio Interchange File Format – AIFF
      • .AIF or .AIFF or .AIFC
        • 8-bit, mono
        • 8-bit, stereo
        • 16-bit, mono
        • 16-bit, stereo
        • 32-bit, mono
        • 32-bit, stereo
apples sound formats
Apples sound formats
    • .AIF files support a range of sampling rates 8kHz, 11kHz, 22kHz, 44kHz and 48kHz
    • compression of between 2 to 1 and 4 to 1 is available using suitable codecs but causes reduction in sound quality
    • .AIFC is AIFF with IMA compression
  • Sound
    • .SND
other platforms
Other platforms
  • MS-DOS
    • Voice – a Sound Blaster format
      • .VOC
  • SUN
    • Sun Audio - (NeXT Audio)
      • .AU
      • .AU files support only 8kHz, 11kHz and 44kHz sampling rates
  • Wave and PCM
  • Adaptive Delta Pulse Code Modulation – ADPCM
    • All WAV (sometimes .PCM for PCM files)
windows sound formats continued
Windows sound formats continued
  • .WAV files support a range of sampling rates 8kHz, 11kHz, 22kHz, 44kHz and 48kHz
  • also a version with Microsoft’s own compression algorithm
  • Can exceed CD quality
  • Higher quality – Greater storage penalty
windows media audio
Windows Media Audio
  • A streaming audio format
  • Designed for network transfer and play-before-download replay
  • Available for UNIX, Mac and Windows
    • Formats:
      • .asf, .wma, .wmv
    • wide range of quality options
real audio
Real Audio
  • A streaming audio format
  • Designed for network transfer and play-before-download replay
  • Available for UNIX, Mac and Windows
    • Formats:
      • .RA
      • also as part of .RM, .RAM
    • wide range of quality options
other streaming formats
Other Streaming Formats
  • In addition there are other streaming formats including:
    • LiveAudio - .LA
    • LiquidAudio - .LQT
    • Streamworks - .MPA
    • Shockwave Audio - .SWA
    • The players for many of these can also play non-streaming audio
  • Most streaming formats deliver mono sound at 8kHz or less
  • MPEG Audio Layer-3
  • In 1987, the IIS started to work on perceptual audio coding in the framework of the EUREKA project EU147, Digital Audio Broadcasting (DAB).
  • In a joint co-operation with the University of Erlangen (Prof. Dieter Seitzer), the IIS finally devised a very powerful algorithm that is standardised as ISO-MPEG Audio Layer-3 (IS 11172-3 and IS 13818-3)
    • MPEG2
mp3 cont 2
MP3 cont 2
  • Without data reduction, digital audio signals typically consist of 16 bit samples recorded at a sampling rate more than twice the actual audio bandwidth (e.g. 44.1 kHz for Compact Disks).
  • So you end up with more than 1.400 Mb to represent just one second of stereo music in CD quality.
  • By using MPEG audio coding, you may shrink down the original sound data from a CD by a factor of 12, without losing sound quality.
mp3 cont 3
MP3 Cont 3
  • Factors of 24 and even more still maintain a sound quality that is significantly better than what you get by just reducing the sampling rate and the resolution of your samples.
  • Basically, this is realised by perceptual coding techniques addressing the perception of sound waves by the human ear.
typical data reduction using mpeg audio
Typical Data Reduction Using MPEG Audio
  • still maintaining the original CD sound quality.
mp3 cont 4
MP3 cont 4
  • By exploiting stereo effects and by limiting the audio bandwidth, the coding schemes may achieve an acceptable sound quality at even lower bit rates.
  • MPEG Layer-3 is the most powerful member of the MPEG audio coding family.
  • For a given sound quality level, it requires the lowest bit rate - or for a given bit rate, it achieves the highest sound quality.
typical performance data of mpeg layer 3
Typical Performance Data Of MPEG Layer-3
  • * Fraunhofer uses a non-ISO extension of MPEG Layer-3 for enhanced performance (MPEG 2.5)
mp3 sound quality
MP3 Sound Quality
  • In all international listening tests, MPEG Layer-3 impressively proved its superior performance, maintaining the original sound quality at a data reduction of 1:12 (around 64 kbit/s per audio channel).
  • If applications may tolerate a limited bandwidth of around 10 kHz, a reasonable sound quality for stereo signals can be achieved even at a reduction of 1:24.
mp3 sound quality cont 2
MP3 sound quality cont 2
  • For the use of low bit-rate audio coding schemes in broadcast applications at bit rates of 60 kbit/s per audio channel, the ITU-R recommends MPEG Layer-3. (ITU-R doc. BS.1115)
  • For more information take a look at our Layer-3 FAQ at http://www.fhg.de/layer3faq/index.html.
  • However, NN and IE do not offer support for MP3 yet
digital audio
Digital Audio
  • Creation & Modification
    • Apple – Passport’s Alchemy
    • Windows’ Sound Recorder
    • Sound Card or MM package supplied utilities – Creative’s Wave Studio
    • MicroSoft’s Wave Edit
  • Playback
    • Any editor
    • Windows’ Media Player
    • Most MM authoring packages
    • Many browser plug-ins
digital audio considerations
Digital Audio – Considerations
  • Can record speech
  • Can record complex noises
  • Can exceed CD quality
  • Higher quality – Greater storage penalty
  • Easily manipulated
  • Difficult to change inherent sound
sound with animation and video
Sound with Animation and Video
  • Wave recording may be linked to animations
  • Wave recordings may be incorporated into video clips
  • Wave recordings may be extracted from video clips
rules of thumb digital audio
Rules of Thumb: Digital Audio
  • Record at the highest practical bit depth and sampling frequency
    • Reducing quality after recording gives better results than recording at lower quality
  • Use the lowest resolution that gives the required results
    • “CD quality” stereo is 16 bit, 44 kHz
    • i.e., 16 ´ 44.1 ´ 2 = 176 KB per second !
    • Not all sound cards can handle the fidelity properly
  • Test your content at various sampling rates
audio for mm and web
Audio for MM and Web
  • Director can import:
    • SWA (via an xtra till v 7), AIFF, AIFC, WAVE (but not with Microsoft’s compression), AU (via an xtra)
  • Optimising audio for the web
    • keep it short
    • mono rather than stereo
    • sample at 8-bit rather than 16-bit
    • sample at 8kHz/11kHz for noises or speech and 22kHz for music
adding non streaming audio to a web page
Adding Non-Streaming Audio to a Web Page
  • There are four ways to do this:
    • Use a normal link: <A HREF=”audio/music.wav”>Play the music.</A>
      • the result may be the sound just plays when the page is opened
      • a plug-in player may open as a Web page
      • a helper application may open in a separate window
basic embedding
Basic embedding
  • Use a BGSOUND in IE: <BGSOUND SRC=”midi/music.wav” LOOP=infinite>
    • the result will be the sound plays when the page is opened
  • Use the OBJECT in IE tag to use an Active-X control to play the sound, e.g.:<OBJECT ID=pop CLASSID=”clsid:0FC6BF2B-E16A-11CF- AB2E-0080AD08A326” HEIGHT=60 WIDTH=145<PARAM NAME=”song” VALUE=”midi/music.wav”></OBJECT>
    • the result will be an Active-X control opens at the specified size in-line in the Web page
basic embedding 2
Basic embedding 2
  • Use the EMBED tag to use a plug-in to play the sound, e.g.:<EMBED SRC=”audio/music.wav” CONTROLS=”console” HEIGHT=60 WIDTH=145 AUTOSTART=”false” LOOP =”false”></EMBED>
    • the result will be a plug-in player (LiveAudio in NN 3 or later) opens at the specified size in-line in the Web page
adding streaming audio to a web page
Adding Streaming Audio to a Web Page
  • Use a normal link: <A HREF=”audio/music.ram”>Play the music.</A>
    • the plug-in player opens
    • however, the file linked to may be a reference file rather than the actual sound file
      • the reference file contains details of the actual audio file
  • Refer to the specific streaming audio documentation for details
  • The important features of the Script depend on the type of sound
  • For a recorded sound:
    • The activities that produce the required noise
    • Background noises (if desired)
    • The stereo effects required
    • The volume required
    • The duration
for music
For music:
  • The Form
  • The Melody
  • The Harmony
  • The Tempo
  • N.B. These are all terms used in music criticism – if you are unfamiliar with them you may need a third party who does to undertake your liaison with the musician – a music producer for example
  • Major to Minor key changes and their exact timing to synchronise with other on-screen events
  • The moods to be matched and their timings
  • The sound card standard to be used for playback