Part A Multimedia Production

Rico Yu

Part A Multimedia Production

Ch.1 Text Ch.2 Graphics Ch.3 Sound Ch.4 Animations Ch.5 Video

Multimedia

Text Graphics Audio information Animation Videos

Audio Information (Sound)

Nature of sound MIDI Recorded Audio

1. Nature of Sound

Wave pattern Loudness, pitch

Loudness (聲量 ) vs pitch (音高 )

Curve , sound louder Curve narrower, pitch

Frequency (頻率 )

e.g. 1kHz = 1000 Hz

= 1000 peaks per second

Frequency = # of peaks occur / second

Frequency

Frequency

Audible sound 20 Hz ~ 20 kHz

Human speech < 4 kHz

Bat 20 ~ 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 patternDiscrete values(Digital value)

3. Recorded Audio

Digitisation

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 Rate Sampling Size

Spoken voice 11.1 kHz 8 – bit

CD-Audio 44.1 kHz 16 – bit

Professional recording

44.1 kHz 24 – 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 103 x 16 x (1 x 60 x 60) bits

= 2.54 x 109 bits

= 2.54 x 109 / 8 bytes

= 3.17 x 108 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 103 x 24 x (30 x 60) bits

= 1.91 x 109 bits

= 1.91 x 109 / 8 bytes

= 2.38 x 108 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