1. 4.1Different Audio Attributes 4.2Common Audio File Formats 4.3Balancing between File Size and Audio Quality 4.4Making Audio Elements Fit Our Needs

2. 4.1 Different Audio Attributes Sampling Rate and Sample Size Sampling is the process of converting sounds from analogue to digital The quality of digitized sounds is determined by: Sample size Sampling rate The number of capture points used to record one-second analogue audio Measured in Hertz (Hz) Common sampling rates: 8 kHz, 11.025 kHz, 22.05 kHz, 44.1 kHz The amount of information used to record an audio sample Measured in bits Common sample sizes: 8-bit, 16-bit

3. 4.1 Different Audio Attributes Sampling Rate and Sample Size Higher sampling rate → greater similarity between the sample waveform and the original one → higher quality Sampling rate/Sample sizeSound quality 44.1 kHz / 16-bitCD 22.05 kHz / 8-bitFM radio 11.025 kHz / 8-bitAM radio Common sampling rate and sample size combination Effect of Sampling Rate on Sample Waveform

4. A stereo audio file 4.1 Different Audio Attributes Audio Channels Number of audio channels determines number of waveforms in a recording Two relevant types of recording Stereo recording Contains two audio channels Offers richer listening experience Mono recording Contains one audio channel Requires half the amount of storage space for stereo recording A mono audio file

5. 4.1 Different Audio Attributes Audio File Size Uncompressed file size can be estimated by, Number of audio channels Time Audio file size (uncompressed) Sample size Sampling rate ×××= For example, the uncompressed file size of a ten-second stereo recording at a sampling rate of 44.1 kHz and 16-bit sample size should be: 44,100 Hz × 16 bits × 2 × 10s =14,112,000 bits =1,764,000 bytes =1.68 MB

6. 4.2 Audio File Formats Common Audio File Formats Audio file formatFile extension Wave Format.wav MIDI Format.mid MPEG Audio Format.mp3 Windows Media Audio.wma RealAudio/RealMedia.ra /.rm Advanced Audio Coding.mp4 /.m4a

7. 4.2 Audio File Formats Wave Format (WAV) Native audio format in Microsoft Windows Supports various sampling rates, sample sizes and audio channels Gives the best audio quality but has the largest audio file size For example, the file size of four-minute CD-quality stereo recording 44,100 Hz × 16 bits × 2 × (4 × 60s) =338,688,000 bits =42,336,000 bytes =40.37 MB Huge file size, not suitable to use on the Web Activity 1

8. 4.2 Audio File Formats MIDI Format (MID) Stands for Musical Instrument Digital Interface Stores music in the form of digital musical instructions instead of a set of capture points of analogue music Each instruction represents the volume, frequency and duration of a sound a specific musical instrument emits No analogue audio can be recorded in a MIDI file

9. 4.2 Audio File Formats MIDI Format (MID) File size Hundreds of times smaller than that of a WAV file Typically less than 100 KB Comparison of a WAV file and a MIDI file WAV fileMIDI file NatureDescribe sound through sampling Describe sound through MIDI code and instructions File sizeGenerally very largeMuch smaller Ease of editing More difficult to editAllow more precise editing Loading time Long because of the large file size Short

10. 4.2 Audio File Formats MIDI Format (MID) Most sound cards provide a MIDI interface for connecting external MIDI devices (e.g. MIDI keyboards) to a computer MIDI keyboard records keystrokes MIDI software converts keystroke sequence to MIDI instructions

11. 4.2 Audio File Formats MP3 and Lossy Compression MPEG-1 Audio Layer-3 (MP3) Popular standard for lossy audio compression Supports small file size and high sound quality Compresses storage space by removing inaudible signals in sound.

12. 4.2 Audio File Formats MP3 and Lossy Compression Bit rate / encoding bit rate Determines the audio quality and file size Specifies the amount of data (in kilobit per second, kbps) used for representing the compressed audio higher bit rate → better audio quality QualityEncoding bit rateFile size CD128 kbps2.95 MB Quasi-CD96 kbps2.21 MB FM radio64 kbps1.47 MB AM radio32 kbps757 KB

13. 4.2 Audio File Formats MP3 and Lossy Compression MP3 supports streaming media is sent continuously to the user instead of downloading the whole file before hearing the sound. Playing MP3 with Windows Media Player MP3 playing software Mobile MP3 players Extracting MP3 with Extraction software (or CD ripper)

14. 4.2 Audio File Formats MP3 and Lossy Compression Windows Media Player can extract music stored on an audio CD Winamp – popular MP3 playing software

15. 4.2 Audio File Formats Windows Media Audio (WMA, ASX) A compressed streaming audio format developed by Microsoft Supports both lossless and lossy compression The next most popular format after MP3 Music extraction – WMA supports both lossy and lossless compression WMA – a popular format for online radio live broadcast

16. 4.2 Audio File Formats RealAudio/RealMedia (RA/RM) A streaming audio format developed by RealNetworks Supports lossy compression only RealPlayer Playing RA/RM requires proprietary audio decompression software Example RealPlayer Can be downloaded from http://www.real.com http://www.real.com

17. 4.2 Audio File Formats RealAudio/RealMedia (RA/RM) Specialised RA encoding software usually provides different bit-rate options to cater for different Internet connection speeds Lower bit rate → smaller file size → lower quality of output audio Example RealProducer RealPlayer

18. 4.2 Audio File Formats Advanced Audio Coding (AAC) a lossy compressed streaming audio format developed by MPEG The most advanced audio format on the Internet The Apple’s iPod music player supports AAC format Do Short Q 2 + LQ 2 on p. 101