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 –Digital to analogue conversion

Digital Audio – Sampling Sample rate is how often the incoming sound wave is measured Most common sampling rates are multiples: – 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 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

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 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

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 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 = MB per minute

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 –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 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.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 SUN –Sun Audio - (NeXT Audio).AU.AU files support only 8kHz, 11kHz and 44kHz sampling rates 8-bit, mono 8-bit, stereo u-law, mono u-law, stereo A-law, mono A-law, stereo Java/Web, mono

Windows Wave and PCM Adaptive Delta Pulse Code Modulation – ADPCM CCITT –All WAV (sometimes.PCM for PCM files) µ-law, mono µ-law, stereo A-law, mono A-law, stereo IMA ADPCM 8kHz, 4-bit, mono IMA ADPCM 8kHz, 4-bit, stereo IMA ADPCM 8kHz, 8-bit, mono IMA ADPCM 8kHz, 8-bit, stereo IMA ADPCM 8kHz, 16-bit, mono IMA ADPCM 8kHz, 16-bit, stereo

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 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 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 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

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 and IS ) –MPEG2

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 kHz for Compact Disks). So you end up with more than Mb to represent just one second of stereo music in CD quality.

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 –still maintaining the original CD sound quality. 1:4 by Layer 1 (corresponds with 384 kbps for a stereo signal) 1:6...1: 8 by Layer 2 (corresponds with kbps for a stereo signal) 1: :12 by Layer 3 (corresponds with kbps for a stereo signal)

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 Sound quality BandwidthMode Bit rate Reduction ratio telephone sound 2.5 kHz mono 8 kbps * 96:1 better than shortwave 4.5 kHz mono 16 kbps 48:1 better than AM radio 7.5 kHz mono 32 kbps 24:1 similar to FM radio 11 kHz stereo kbps :1 near-CD 15 kHz stereo 96 kbps 16:1 CD >15 kHz stereo kbps :1 –* Fraunhofer uses a non-ISO extension of MPEG Layer-3 for enhanced performance (MPEG 2.5)

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 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 However, NN and IE do not offer support for MP3 yet

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

Digital Audio playback Playback –Any editor –Windows’ Media Player –Most MM authoring packages –Many browser plug-ins

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 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 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 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

Adding Non-Streaming Audio to a Web Page There are four ways to do this: –Use a normal link: Play the music. –Use a normal link: Play the music. 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 –Use a BGSOUND in IE: –Use a BGSOUND in IE: 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.: –Use the OBJECT in IE tag to use an Active-X control to play the sound, e.g.: the result will be an Active-X control opens at the specified size in-line in the Web page

Basic embedding 2 –Use the EMBED tag to use a plug-in to play the sound, e.g.: –Use the EMBED tag to use a plug-in to play the sound, e.g.: the result will be a plug-in player opens at the specified size in-line in the Web page

Adding Streaming Audio to a Web Page Use a normal link: Play the music. Use a normal link: Play the music. –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

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

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