Digital Audio

1. Digital Audio

2. Sound gives multimedia application a better dimension; • Sound acts as a notice or a warning that something is happening. • Some Multimedia application contains soothing background music or a voice-over explaining the operation of a program.

3. What is Sound?? • The spoken words, voices,music and even noise. • Sound is based on two things: energy and a transport medium. • Energy from the source is converted into pressure that is transmitted by the air • The change in air pressure is detected by the ear ,and a vibration occur,that is sensed by the nerves and send to the brain.

4. Characteristics of Sound • Amplitude: it is the power intensity of the sound,the louder the sound the larger the amplitude. • Frequency:-it is the rate at which sound is measured.it is measured in Hz.The higher the frequency ,the clearer and sharper the sound • Human hearing is capable of detecting sound vibration ranging from 50 htz to 20,000 htz

5. Voice is defined as Talking Human voice does not have great fluctuations in the pitch and tone Voice can be captured at a much lower frequency rate Music can be created from human singing or musical instrument Music can have rapid changes in tone and pitch within very brief period of time Music requires higher frequency rate Voice vs. Music

6. Music is usually recorded and played in stereo or multiple playback tracks to capture the richness and fullness of many singing voices or the instruments of an orchestra • The capture of sound into digital format requires that a measurement of sound be made at specific interval of time • Higher capture frequency means more data sampling which requires more data storage space

7. Digitizing Sound • It means capturing the sound amplitude at a specified frequency .Three char. For recording sound: • Frequency rate • 11.025 Khz • 2.5 Khz • 44.1 Khz • Amplitude measurement • 8 bits for 256 level • 16 bits for 65,536 levels • Sound Channels • Monaural (1 channel) • stereo (2 channel)

8. The number of bits used for the amplitude measurement is based on how precise the level of sound is to be measured. • Eight bits allows for 256 levels, which is adequate for many types of sound. • 16 bits allows for 65536 levels of sound, which provides a much finer level of precision.

9. Calculating Audio Data size • c=No of channels • s=Sampling rate in Htz • t=time(seconds) • b=bytes File size=c*s*t*b • E.g.: file size for 1 min 44.1 khz,16 bit stereo sound File size=2*44100*60*2=1,05,84,000

10. Digital Audio System Digital audio system for multimedia computers are based on special circuitry that interfaces the computer to external audio components such as microphones, audio player, recorder and speaker

11. Digital Audio Circuitry Audio player or Recorder Line in Analog to Digital Converter Microphone Data Storage Digital Audio File Microphone Line out Digital to Analog Converter Speakers Audio Speaker Amplifier Digital audio system

12. It consist of: • Converting the audio signal from a microphone or other audio sources into a digital signal via A to D converter. • Transferring the digital data to the computer’s storage media • Converting a digital data from storage into an analog audio signal via a D to A converter and amplify it for play back using speaker

13. Digital Audio Software Support The digital audio system is managed by the S/W….it support functions: • Playback:translate the audio data into sound • Record:capture audio data from an external analog source • Stop:halt playback • Fast Forward: move to a prior point in the data stream

14. Editing Digital Audio • Trimming-removing dead spots from the beginning and end of a digital recording • Splicing and assembly-linking separate audio segments together,usually from different files • Volume adjustments-increasing the loudness or softening the sound level

15. Format Conversion-Changing the file format to move audio b/w computer system • Resampling and downsampling:-changing by reducing the frequency or number of bits used for recording • Fade-ins and fade-outs:-increasing and decreasing the volume within a segment of the audio data • Equalization:-used to clean up noise by reducing spikes or levels within the audio data stream

16. Time –stretching:-slowing down or speeding up the sound by removing or adding filler data stream • Digital signal processing:-digitally adding sound effects such as reverberation,echo ,inserting silence or sound level and other effects. • Reversing Sound:-reversing the data stream or play the sound backwards

17. Sound Controls Multimedia computers usually have software that acts as a control for the computer’s sound playback and recording.This s/w includes: • Selection of components • Volume level ,both mono and stereo • Recording input level • Bass and treble controls

18. Musical Instrument Digital Interface(MIDI) • It is an encoding scheme for recording and playing musical instruments on a computer • MIDI was originally developed as a standard for interchanging information b/w music synthesizers. • MIDI synthesizers are provided on most computer cards, MIDI interface are available for connecting music synthesizers to a wide range of computer system

19. MIDI • The MIDI specification organizes sounds into 16 families of 8 variations each. • This organizing or grouping of sounds makes it easy to reorchestrate a piece using similar sounds. • An Instrument patch map,part of the MIDI specification,lists 128 sounds,with corresponding MIDI program numbers.

20. Audio File Formats There are two general categories of digital audio files: • Sound audio files • MIDI files

21. Audio Files Audio files can be compressed to reduce storage space and can also be exchanged between computer systems.some of the file formats are: • Apple • the AIFF (Audio interchange File format) is actually used across a number of platforms and environments • AIFF files contain a header with information about the audio sampling frequency,data size,no of channels,file size and other information followed by the formatted audio data

22. Microsoft • Waveform or wave files are commonly used in the intel-based computer world • Wave files typically use a .wav extension and are based on the Microsoft Resource Interchange File Format(RIFF) • RIFF files are comprised of chunks of information including file type ,audio sampling frequency ,data size and number of channels and the audio data

23. CD-Audio • Red Book Audio standard:used for CD audio;format is called Linear pulse code Modulation • It typically provides 50 to 60 min of high fidelity stereo music captured to 16 bit ,44.1 Khtz data.

24. MIDI Files • It offer the advantage of being quite small compared to captured audio files. • They can be used to deliver relatively high quality synthesized music for a variety of applications • MIDI files are composed of chunks for a header and one or more tracks • The header chunks provides identification of the data ,arrangement of the data tracks and trimming • The track chunks provide the timing and type of event or instrument to be played

25. Using Audio in Multimedia application • For managing software function • To enhance other content • For content delivery