Frequency ( 頻率 ) e.g. 1kHz = 1000 Hz = 1000 peaks per second Frequency = # of peaks occur / second
Frequency Audible sound20 Hz ~ 20 kHz Human speech< 4 kHz Bat20 ~ 120 kHz
2. MIDI Musical Instrument Digital Interface – Stores musical notes played by piano, violin, etc. – Sound card synthesizes ( 合成 ) these notes MIDI stores codes only – File size ~ several kB
3. Recorded Audio Digital Audio – Sound recorded digitised, stored in PC – All kinds of sound recorded from mic. digital audio by sound card – Digitisation Analogue waves pattern Discrete values (Digital value)
3. Recorded Audio Digitisation Sampling Sampling Rate Sampling Size
Sampling Rate # of times measured height of sound per sec. measured in Hz Sampling Rate > 2 x Max. Sound Freq.
Sampling Rate Higher the sampling rate, better the quality, BUT larger the file size.
Sampling Rate Sampling Rate of Audio CD is 44.1 kHz – Max. Audible freq. is 20 kHz Sampling Rate of Human speech is 11.1 kHz – Max. freq. of human is 4 kHz
Sampling Size # of bits used to stored If sampling size small – very soft and loud sound can ’ t recorded quality of recorded sound depends on it
Sampling Size Sampling RateSampling Size Spoken voice11.1 kHz8 – bit CD-Audio44.1 kHz16 – bit Professional recording 44.1 kHz24 – bit
Mono and Stereo Mono – Single Channel Stereo – 2 Channels – Twice the file size of mono
Calculating Sound file size File size = Channel # x Sampling rate x Sampling size x Duration / 8 bytes
Calculating Sound file size e.g. Given that a sampling rate of 44.1 kHz and a sampling size of 16 bits is used to record a sound for one hour. What is the uncompressed file size if (a) the sound is mono, (b) the sound is stereo?
Calculating Sound file size Solution (a) For mono sound, the # of channel is one. Thus, File size= 1 x 44.1 x 10 3 x 16 x (1 x 60 x 60) bits = 2.54 x 10 9 bits = 2.54 x 10 9 / 8 bytes = 3.17 x 10 8 bytes ~ 300 MB (b) For stereo sound, the file size is twice, i.e. ~600MB
Calculating Sound file size Ex. Given that a sampling rate of 44.1 kHz and a sampling size of 24 bits is used to record a sound for half an hour. What is the uncompressed file size if (a) the sound is mono, (b) the sound is stereo?
Calculating Sound file size Solution (a) For mono sound, the # of channel is one. Thus, File size= 1 x 44.1 x 10 3 x 24 x (30 x 60) bits = 1.91 x 10 9 bits = 1.91 x 10 9 / 8 bytes = 2.38 x 10 8 bytes ~ 238 MB (b) For stereo sound, the file size is twice, i.e. ~476MB
Compressing Sound file Raw sound is WAV, uncompressed. WAV MP3, compressed. MP3 – Lossy compression – Can compress CD audio by 12
Bit Rate The speed of data transmission Bit Rate = Channel # x Sampling Rate x Sampling Size
Bit Rate e.g. The amount of bits for a 16-bit stereo music at 44.1kHz transferred in each second is 2 x 16 x 44.1k =1411.2kbps bps = bits per second