Uncompressed Audio Formats

Transcript

 Time varying media  Form of energy that propagates through a certain medium in the form of mechanical waves.  When an object oscillates through a medium, the vibrations are converted to sound.  Acoustics is a branch of science that deals with generation, propagation, transmission of vibrations, sound, ultra sound and infra sound.  Ex. Aeroacoustics, Audio signal processing, psychoacoustics etc.  Sound is one kind of longitudinal wave, in which the particles oscillate to and fro in the same direction of wave propagation. Sound waves cannot be transmitted through vacuum. The transmission of sound requires at least a medium, which can be solid, liquid, or gas.   condensation rarefaction  The wavelength, l is the distance between two successive rarefactions or condensations.  Positive peak in sine wave denotes compression, and the negative one denotes rarefaction.  Amplitude  Frequency  Wavelength  Speed  Waveform  Sound pressure  The amplitude of a periodic variable is a measure of its change over a single period.  Peak-to-peak amplitude is the change between peak (highest amplitude value) and trough (lowest amplitude value, which can be negative).  Peak amplitude  This is the maximum absolute value of the signal. 1: peak 2: peak to peak 3: RMS  For cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles per unit time.  Unit: Hertz(Hz).  Range of audible frequency for humans: 20Hz to 20 kHz: sonic frequency.  Ultrasonic > sonic >infraSonic  The period, usually denoted by T, is the duration of one cycle, and is the reciprocal of the frequency f: 1/T.  The distance over which the wave's shape repeats.  Distance between two successive troughs/ crests.  Higher frequency : shorter wavelength.  Indicated by Lambda  The speed of sound is the distance travelled per unit of time by a sound wave propagating through an elastic medium.  In dry air, at 20 °C (68 °F), the speed of sound is 343.2 metres per second (1,126 ft/s).  This is 1,236 kilometres per hour (667 kn; 768 mph), or a kilometre in 2.914 seconds or a mile in 4.689 seconds.  Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average, or equilibrium) atmospheric pressure, caused by a sound wave.  In air, sound pressure can be measured using a microphone, and in water with a hydrophone.  Unit: Pascal (Pa).  Deals with human listening capabilities, psychological responses, and physiological responses, and physiological impact of music and sound upon the human nervous system.   Threshold of hearing Masking  Range of audible frequency for humans: 20Hz to 20 kHz: sonic frequency. As age of the person increases, upper limit decreases.  Adults (on an avg.) cannot hear sound of frequency above 16kHz.  Human hearing is most sensitive for frequency range 1kHz to 5kHz.  Lossy audio data compression methods, such as MPEG/Audio encoding, remove some sounds which are masked anyway  1. The general situation in regard to masking is as follows: A lower tone can eectively mask (make us unable to hear) a higher tone 2. The reverse is not true { a higher tone does not mask a lower tone well 3. The greater the power in the masking tone, the wider is its influence { the broader the range of frequencies it can mask. 4. As a consequence, if two tones are widely separated in frequency then little masking occurs MIDI is a technical standard that describes a protocol, digital interface and connectors and allows a wide variety of electronic musical instruments, computers and other related devices to connect and communicate with one another.  Each MIDI-capable instrument is equipped with a transmitter and receiver, though certain peripheral devices, such as signal processors, may have only a receiver or a transmitter.  The interface operates at a 31.25 Kbaud asynchronous (serial) transmission rate. Each byte consists of a start bit, eight data bits, and a stop bit, for a total duration of 320 microseconds per byte.  It is a standard adopted by the electronic music industry for controlling devices, such as synthesizers and sound cards, that emit music.  At minimum, a MIDI representation of a sound includes values for the note's pitch, length, and volume.  When you play a MIDI instrument, it produces data that can be captured by a MIDI “sequencer.”  When the sequencer plays back the data, your original performance is recreated.  The General MIDI sound set allows you to instantly play back pre-recorded music on many MIDI instruments, or on a computer.  One MIDI device can control another, letting you use the most convenient tool for any job 5 pin DIN connectors: IN, OUT, THRU.  IN: I/p signals, OUT: o/p signals,  THRU: daisy chaining multiple instruments.  THRU 2nd of 1st instrument is connected to IN of and so on.  Suppose you’ve got a MIDI keyboard connected to a MIDI synthesizer box, or “module,” and you want to play the module from the keyboard  When you hold down the Middle C on the keyboard, the keyboard sends out a Middle C “key down” MIDI message to the module. The module says to itself, “Ah, right, Middle C,” and plays Middle C  When you let go of Middle C, the keyboard sends a Middle C “note off” message to the  Note-on and note-off messages are just the beginning, though. Most MIDI controllers can also sense how hard you play a note, transmitting a MIDI “velocity” value to the receiving device so the volume or brightness of its note responds to your touch.  Instructional based music Channel Messages System Messages specific Channel Voice System common change, Channel mode System real time description language.  Channel message applied to : channel.  Voice: control program change, note on, note off, change etc. pitch bend System exclusive  Commands that are not channel specific, such as timing signals for synchronization, positioning information in pre-recorded MIDI sequences, and detailed setup information for the destination device.  Common: meant for receivers of the system. Ex.: song select message is used to store songs and recall them.  Real time: synchronize all the clock equipments of MIDI system.  Exclusive: send data between MIDI devices. based  Digital audio is technology that can be used to record, store, generate, manipulate, and reproduce sound using audio signals encoded in digital form. A microphone converts sound to an analog electrical signal, then ADC—typically using pulse-code modulation—converts the analog signal into a digital signal. A digital-to-analog converter performs the reverse process, converting a digital signal back into an analog signal, which analog circuits amplify and send to a loudspeaker.  Digital audio compression, systems storage, may include processing and transmission components.  Conversion convenient to a digital format manipulation, allows storage, transmission and retrieval of an audio signal.  Sampling is the method of converting an analogue signal to digital data.  Audio sampling is based on the Nyquist-Shannon sampling theorem, which states that for accurate signal reconstruction the sampling frequency should be at least twice the bandwidth of the source signal. Therefore, if a source signal has a bandwidth of 0 1KHz, then the sampling rate needs to be at least 2KHz.  The vast majority of digital audio systems encode the numbers which represent the original audio waveform as binary data, by means of a process known as pulse code modulation ( PCM ).  The logical format of an audio CD (officially Compact Disc Digital Audio or CD-DA) is described in a document produced in 1980 by the format's joint creators, Sony and Philips.  The selection of the sample rate was primarily based on the need to reproduce the audible frequency range of 20Hz - 20kHz.  The Nyquist–Shannon sampling theorem states that a sampling rate of double the maximum frequency to be recorded is needed, resulting in a 40 kHz rate. CD Formats CD-digital audio CD-ROM CDInteractive CDRecordable CDRewritable Data stored in form of sectors /blocks.  1 sec sound= 75 blocks & 1 block= 2352 bytes.  Contents can be ripped and copied on PC.  Super Audio CD gives better playback than CD-DA.  Follows red book standards.  MODE 1: 2352 bytes=2048(original data)+304(error detection & correction).  MODE 2: error detection &correction codes are absent. Can play 75 blocks in 1 sec.  Follows yellow book standards.  Includes text, image, audio, graphics, video, comp. programs.  H/w to store and process the data by connecting to TV(direct plug in + remote).  H/w used: decoder, CD player, joystick, controller and mouse interface.   2nd part of orange book.  Write once and read many times.  Multi session recordings.  3rd part of orange book.  Data stored can be erased and rewritten.  Incompatible with drives which do not support multi-session. White book standard.  Capable to hold 75 mins of compressed video with resolution of 352*288 or 352*240.  Audio is stored at 224 Kbits/sec & video at 1150 Kbits/sec.  Xing MPEG player and WMP can play them.  Super VCD: better quality A/V.  Uncompressed audio formats — formats that use no compression. This means all the data is available, at the risk of large file sizes. A WAV audio file is an example of an uncompressed audio file.  Lossless compression applies compression to an uncompressed audio file, but it doesn't lose information or degrade the quality of the digital audio file. The WMA audio file format uses lossless compression.  Lossy compression will result in some loss of data as the compression algorithm eliminates redundant or unnecessary information — basically it losses what it sees as irrelevant information, small file size, it is easier to transmit over the Internet.  MP3 and Real Audio files uses a lossy compression.   The most popular sound file format these days is .MP3 for both Mac and PCs.  Other file formats include .AIFF (for Mac); .AU for Mac and UNIX; .WAV for the PC; and .RA for Real Audio, a proprietary system for delivering streaming audio on the Web. and playing  The goal of the MP3 format is to compress a CD-quality song by a factor of 10 to 14 without losing the CD quality of the sound.  With MP3, a 32 megabyte song on a CD compresses down to 3 megabytes or so.  This lets you download a song in minutes rather than hours, and it lets you store hundreds of songs on your computer's hard disk without taking up that much space.  MP3 offers exceptional quality at 1:12 compression. This format is best played with Winamp, although RealAudio Quicktime, and the new Windows Media Player, and many others also support the format.  The size of the MP3 file depends on which compression routine was used, near- CD or CD quality.  It can be broken up into pieces, and each piece is still playable. We can now easily transmit a song over the internet.  Main disadvantage of the MP3 format is that it needs to be decompressed when played.  Format used for storing sound in files developed jointly by Microsoft and IBM.  Support for WAV files was built into windows 95 making it the de facto standard for sound on PCs.  WAV sound files end with a .Wav extension and can be played by nearly all windows applications that support sound.  PCM is widely used CODEC since it uses uncompressed lossless storage method that keeps audio track intact & gives maximum audio quality.  When a PC plays back a WAV file, it converts numbers in the file into audio signals for the PC's speakers.  Strengths:  WAV files are simple and widely used, especially on PCs.  Later versions of Netscape Navigator (3+) and Microsoft Internet Explorer (2+) support the WAV format.  Weaknesses:  WAV is seen as a proprietary Windows format, although conversion tools are available to play WAV files on other platforms.  The AIFF format is associated with Apple QuickTime Player on Mac and Windows PC.  Even if you run under Windows, you will have no problem playing these files so long as you have Apple QuickTime installed.  It uses uncompressed PCM CODEC.  Windows Media Audio, WMA is a Microsoft file format for encoding digital audio files similar to MP3 though can compress files at a higher rate than MP3.  WMA files, which use the ".wma" file extension, can be of any size compressed to match many different connection speeds, or bandwidths. MPEG is a working group in a subcommittee of ISO/IEC in charge of developing international standards for compression, decompression, processing, and coded representation of moving pictures, audio, and their combination.  Applies a filter bank to the input to break it into its frequency components  In parallel, a psychoacoustic model is applied to the data for bit allocation block  The number of bits allocated are used to quantize the info from the lter bank { providing the compression  MPEG audio offers three compatible layers :  Each succeeding layer able to understand the lower layers  Each succeeding layer oering more complexity in the psychoacoustic model and better compression for a given level of audio quality  Each succeeding layer, with increased compression effectiveness, accompanied by extra delay.  The objective of MPEG layers: a good tradeoff between quality and bit-rate C:\MPEG PDF  AAC LC/LTP  HE-ACCv1  HE-ACCv2  AMR-NB  AMR-WB  FLAC  MP3  MIDI  Vorbis  PCM/WAVE