1. 1 CP586 © Peter Lo 2003 Multimedia Communication Introduction to Sound

2. CP586 © Peter Lo 20032 Sound and digital audio Sound is comprise of Spoken word Voice Music Noise

3. CP586 © Peter Lo 20033 Sound and digital audio Sound is based on Energy Transport medium Transmitter EnergyPressure Transport media

4. CP586 © Peter Lo 20034 Sound Produced by the vibration of matter. During the vibration, pressure variations are created in the air surrounding it. The pattern of the oscillation is called a waveform. The waveform repeats the same shape at regular intervals and each portion is called a period. Sound relies on energy and a transport medium (which is the air we breathe). Sound is created when changes in air pressure are detected by the ear drum. Two important characteristics are frequency and amplitude.

5. CP586 © Peter Lo 20035 Frequency Rate of sound The reciprocal value of the period. Measured in cycles per second Represents the number of periods in a second and is measured in hertz (Hz) or cycles per second (cps). A convenient abbreviation, kHz is used to indicate thousands of oscillations per second. (1 kHz = 1000 Hz) Higher the frequency, clearer and shaper the sound Human hearing capability: 50Hz to 20,000Hz

6. CP586 © Peter Lo 20036 Amplitude The measure of displacement of the air pressure wave from its mean or quiescent state. The amplitude could also be described as the height of a given waveform. Power or intensity of the sound Louder the sound, larger in amplitude Stronger in energy

7. CP586 © Peter Lo 20037 Pitch - Loudness Frequency and pitch are related. Pitch is the subjective quality our brains perceive from frequency. Greater amplitude results when objects are set in motion with greater force. We derive the relative perception of loudness from the absolute amplitude of a given sound wave.

8. CP586 © Peter Lo 20038 Computer Representation of Sound The continuous curve of a sound waveform is not directly represented in a computer. A computer measures the amplitude of a waveform at regular time intervals to produce a series of discrete numbers. Each of these measurements is a sample.

9. CP586 © Peter Lo 20039 Recording Sound To effectively record sound Twice the frequency that sound can be heard is needed = 40,000Hz The higher sampling rate, the better quality At lesser rate, subtle change in pitch and tone will be lost Effective sampling rate Human voice: 5,000 to 15,000Hz Music: 40,000 Hz

10. CP586 © Peter Lo 200310 Voice and music Consider following three classes of sounds Voice Talking, not singing Music Human singing or musical instruments Sound effect Voice or music More often created by natural events

11. CP586 © Peter Lo 200311 Sample Rate The rate at which a continuous waveform is sampled is called the sampling rate. Like frequencies, sampling rates are also measured in Hz. CD quality : 44,100 Hz Radio quality : 22,050 Hz Telephone quality : 11,025 Hz

12. CP586 © Peter Lo 200312 Audio Production - Conveying Realism Proximity: As you get closer to the source of a sound the volume should increase. Environment: Sound should be realistic using ambience and expected acoustics. Enhanced sound effects: Often sound effects are required to create a realistic feel in a virtual world.

13. CP586 © Peter Lo 200313 Audio Production - Conveying meaning Emotion : dramatic music. Time : Sovereign Hill. Geographic location : didgeridoo, drums. Association : Alarm, Harvey Norman. Signature and continuity : Jaws, 007. Memorability : Star Wars, Dr Who.

14. CP586 © Peter Lo 200314 Editing Digital Audio Trimming : Wave Edit deletes silence. Splicing and Assembly : Cut and paste. Fade In/Out : Volume slowly fades down. Noise Reduction : Dolby Digital Noise Reduction. Signal Processing : Signal processing can allow you to play sounds backwards and insert sound effects.

15. CP586 © Peter Lo 200315 Real Time A real time process is a process which delivers the results of the process in a given time span. Examples of real time applications may include missile guidance systems, nuclear reactor protocols and auto pilot technology. Multimedia often has temporal constraints that require real time scheduling.

16. CP586 © Peter Lo 200316 Real Time Audio (.ram) Using a fast down load process, users with standard modems can access near CD quality audio and high-quality video normally only available over faster connections. The buffer decreases sound and image loss on poor Internet connections, improving audio and video quality.

17. CP586 © Peter Lo 200317 Windows WAV Created by Microsoft and IBM. It specifies an arbitrary sampling rate, number of channels and sample size. It has a plethora of different compression formats. It is a standard used on most Windows machines. Uses application specific blocks.

18. CP586 © Peter Lo 200318 Sound Blaster VOC This is the format used by the Creative Voice SoundBlaster hardware used in IBM-compatible computers It specifies sampling rate as a multiple of an internal clock and is not as flexible as the other general formats. Portions of silence can be added to a sound file.

19. CP586 © Peter Lo 200319 Au Sun Audio (.AU) and NeXT. This format became very popular because it is closely associated with the Unix operating system. Unix is used in most universities as a teaching and research tool.

20. CP586 © Peter Lo 200320 AIFF AIFF stands for Audio Interchange File Format and was developed by Apple for storage of sounds in the data fork. The Macintosh OS includes support for playing and creating AIFF files. It allows the specification of sampling rates, sample size, number of channels, and application- specific format chunks.

21. CP586 © Peter Lo 200321 MPEG Audio MPEG stands for the "Moving Picture Experts Group”. MPEG compression is a lossy algorithm which can achieve high rates of compression without noticeable decreases in quality. Generally considered a Macintosh based sound format.

22. CP586 © Peter Lo 200322 MP3 MP3 is the file extension for MPEG, audio layer 3. Layer 3 uses perceptual audio coding and psycho acoustic compression to remove all superfluous information. The result in real terms is layer 3 shrinks the original sound data from a CD by a factor of 12 without sacrificing sound quality. Because MP3 files are small, they can easily be transferred across the Internet.

23. CP586 © Peter Lo 200323 QuickTime This is the Apple standard for time-based multimedia files. Versions 1.x support moving pictures, sound and later versions support text. This standard is often associated with Macintosh computers.

24. CP586 © Peter Lo 200324 MIDI Musical Instrument Digital Interface is primarily a standard for communication between musical instruments. It stores information about which notes were played in a time-line format. Often used in conjunction with synthesisers to reproduce musical arrangements.

25. 25 CP586 © Peter Lo 2003 Audio Digitizing sound

26. CP586 © Peter Lo 200326 Digitizing sound Characteristics for recording sound Amplitude Sound channels (monaural and stereo) Frequency rate Standard frequency rates for multimedia computer 11.025 KHz (kilohertz = 11.025 x 10 3 Hz) 22.5 kHz 44.1 kHz

27. CP586 © Peter Lo 200327 Digitizing sound Other frequency sampling rates Especially lower rates 5 KHz and 8 KHz For network applications Sacrifice some quality Significantly reduce amount of data Amplitude measurement 8 bits (one byte) for 256 levels 16bits for 65,536 levels

28. CP586 © Peter Lo 200328 Digitizing sound Calculating audio data size Characteristics Amplitude Sound channels Frequency rates Time

29. CP586 © Peter Lo 200329 Digitizing sound File size calculation b = bytes (1 for 8 bits, 2 for 16 bits) Amplitude c = number of channels (mono – 1 channel, stereo = 2 channels) Sound channels s = sampling rate in Hertz (cycle per second) Frequency rates t = time (seconds) File size = b * c * s * t

30. CP586 © Peter Lo 200330 Digitizing sound Example: file size for 1 minutes, 44.1 KHz, 16 bit, stereo sound c = 2 (2 channels for stereo) s = 44,100 (frequency) t = 60 (time) b = 2 (2 bytes for 16 bits) File size = b * c * s * t =2 * 44,100 * 60 * 2 =10,584,000 bytes

31. CP586 © Peter Lo 200331 Digitizing sound Digital sound characteristics FrequencyData sizeChannelSizeComment 44.1 KHz16 bitStereo10.5 MBCD-quality 44.1 KHz16 bitMono5.25 MBVoice or monaural, high quality 44.1 KHz8 bitStereo5.25 MBBest quality for playback on low-end PC 44.1 KHz8 bitMono2.6 MBGood trade-off for voice quality 22.05 KHz16 bitStereo5.25 MBDecent quality 22.05 KHz16 bitMono2.5 MBGood for voice 22.05 KHz8 bitStereo2.6 MBAcceptable for good quality playback 22.05 KHz8 bitMono1.3 MBThinner sound, TV quality 11 KHz8 bitStereo1.3 MBLowest quality for music audio 11 KHz8 bitMono1.3 MBLowest reasonable quality for voice

32. 32 CP586 © Peter Lo 2003 Audio Digital audio systems

33. CP586 © Peter Lo 200333 Digital audio systems The circuitry for digital audio system is responsible for Convert the audio signal from microphone or other source into a digital signal Transferring digital data to computer’s storage media Converting digital data from storage into analog audio signal, if necessary, amplify for playback

34. CP586 © Peter Lo 200334 Digital audio systems Digital audio hardware configuration Audio player or recorder Microphone Audio Speaker Digital Audio Circuitry Data storage Digital audio file Analog to digital converter Digital to analog converter Amplifier Line in Microphone Line out Speaker

35. CP586 © Peter Lo 200335 Digital audio systems Digital audio software support Playback Translate the audio into sound Record Capture audio data from external analog source Stop Halt playing

36. CP586 © Peter Lo 200336 Digital audio systems Fast forward Move to a later point in the data stream Reverse Move to a prior point in the data stream Rewind Move to the beginning of the data stream Note many of these functions are similar to controls found on standard commercial audio tape player

37. CP586 © Peter Lo 200337 Digital audio systems Sequential access and random access Sequential access Type must be physically advanced or rewound E.g. tape player Random access Able to jump to any point in the storage medium and access data E.g. CD-ROM