1 Digital Audio Storage Formats

2 Formats  There are many different formats for storing and communicating digital audio:  CD audio  Wav  Aiff  Au  MP3

3 WAVE  Waveform Audio (wave or.wav) is a file format in which Windows stores sounds as waveforms  The WAVE format is a subset of RIFF (Resource Interchange File Format) used for storing digital audio  Basic.wav files store information in PCM format with no compression

4.wav  Wave files contain 3 parts:  Descriptor  Format information  Data

5 Chunk Descriptor  ChunkID - Contains the letters “RIFF” in ASCII form  ChunkSize - This is the size of the entire file in bytes minus 8 bytes for the two fields not included in this count: ChunkID and ChunkSize  Format - Contains the letters “WAVE” in ASCII from

6 Format Sub-chunk  Subchunk1ID - Contains the letters “fmt”  Subchunk1Size - 16 for PCM. This is the size of the rest of the sub-chunk which follows this number  AudioFormat - PCM = 1 (i.e. Linear quantization). Values other than 1 indicate some form of compression  ByteRate - SampleRate * NumChannels * BitsPerSample / 8  BlockAlign - NumChannels * BitsPerSample / 8 The number of bytes for one sample

7 Data Sub-chunk  Subchunk2ID - Contains the letters “data”  Subchunk2Size –  NumSamples * NumChannels * BitsPerSample / 8  This is the number of bytes in the data  Data - The actual PCM sound data  8-bit samples are stored as unsigned bytes, ranging from 0 to bit samples are stored as 2’s-complement signed integers, ranging from to 32767

8.au  Sun’s audio file format  File is composed of three parts:  24-byte header  variable-length annotation block  contiguous segment of audio data

9 Header Information  Magic Number - 4 bytes - “.snd”  hdr_size - 4 bytes - offset to start of audio data  data_size - 4 bytes - data length, in bytes (optional)  encoding - data encoding type code  sample_rate - samples per second  channels - number of interleaved channels

10 AIFF  Apple’s Audio Interchange File Format  RIFF (and thus.wav) is derived from AIFF  File is split into:  Header, Common and Data Chunk ChunkID - Contains the letters “FORM" in ASCII. ChunkSize - This is the size of the entire file in bytes minus 8 bytes for the two fields not included in this count: ChunkID and ChunkSize. Format - Contains the letters “AIFF”  Common chunk – information on data format  Data chunk – PCM audio data

11 MIDI  Musical Instrument Digital Interface  Serial interface standard for interfacing between electronic instruments (31.25Kbits uni-directional)  Involves communicating sound events rather than sampled audio

12 MIDI  The information transmitted between MIDI devices is in a form called a MIDI message, which encodes aspects of sound such as: note played, note number, velocity sustain pedal, breath controllers, aftertouch, pitch bend, modulation

13 MIDI Message Format  Control messages are sent as groups of bytes, preceded by one start bit and followed by a stop bit

14 MIDI Message Format  There are two basic types of MIDI message byte  Status byte (starts with a ‘1’)  Data byte (starts with a ‘0’) 1sssnnnn 0xxxxxxx 0yyyyyyy 8 bits StatusData 1Data 2

15 MIDI Message Format

16 MIDI  There are sixteen basic MIDI channels and instruments can usually be set to receive on one (omni off) or all (omni on)  General MIDI sets the standard for the ordering and naming of sounds Group1-8 Piano65-72 Reed 9-16 Chromatic Percussion Pipe Organ Synth Lead Guitar Synth Pad Bass Synth FX Strings Ethnic Instruments Ensemble Percussive Instrms Brass Sound effects Plus 47 percussive sounds

17 MIDI  Using MIDI for transferring music can be problematic  Even with general MIDI the output depends on the quality and type of synthesiser used  Not so good for Hi-Fi  Very good for multimedia or games sound effects and music

18 MIDI Sequencers  Capable of storing a number of “tracks” of MIDI information and manipulating them

19 MIDI Sequencers

20 MIDI Sequencers

21 Fin