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Week 6 – 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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Sound digital audio l.jpg
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


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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


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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


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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


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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 small changes in the sound

    • Small variations in the sound may be too small to record as different sample values

  • Clipping occurs when the largest values recordable are less than the highest volumes recorded


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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?


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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


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Digital Audio – Compression

  • Many flavours

    • ADPCM – 4:1

      • MicroSoft, MicroSoft IMA, Creative

    • CCITT – 2:1

      • A-law and -law

    • Audio MPEG – 20:1


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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


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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


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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


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    Other platforms

    • SUN

      • Sun Audio - (NeXT Audio)

        • .AU

        • .AU files support only 8kHz, 11kHz and 44kHz sampling rates


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    Windows

    • Wave and PCM

    • Adaptive Delta Pulse Code Modulation – ADPCM

    • CCITT

      • All WAV (sometimes .PCM for PCM files)


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    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


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    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


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    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


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    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


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    MP3

    • 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


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    MP3 cont 2

    • 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.

    • 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.


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    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.


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    Typical Data Reduction Using MPEG Audio

    • still maintaining the original CD sound quality.


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    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.


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    Typical Performance Data Of MPEG Layer-3

    • * Fraunhofer uses a non-ISO extension of MPEG Layer-3 for enhanced performance (MPEG 2.5)


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    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.


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    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


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    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


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    Digital Audio playback

    • Playback

      • Any editor

      • Windows’ Media Player

      • Most MM authoring packages

      • Many browser plug-ins


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    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


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    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


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    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 l.jpg
    Audio for MM and Web

    • Flash can import:

      • SWA, MP3, AIFF, WAVE, AU

    • 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


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    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


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    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


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    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 opens at the specified size in-line in the Web page


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    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


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    Scripts

    • 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


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    For music:

    • The Form, Melody, Harmony and 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


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