SOUND

Sound?

Continuous sequence of vibrations of air (Why no sound in space? Contrary to Star

Wars etc.) Abstraction of an audio wave:

Ear translates vibrations into sound The slightest air-pressure fluctuations will

move ear drum back and forth

Speakers

If you create a sound that vibrates 200 times/second

Send to a speaker, head phone… Speaker vibrates 200 times in/out in a second

Frequency

The number of vibrations (waves), usually per second

Measure: Hertz Example:

Assume 2 second interval Vibration of 1,000 times Cycles/second in Hz? i.e. how many vibrations in one second?

Cycles: Example of Two Hz

Assume 1 second

Hz?

What is the frequency in Hz?

Wave to Digital

Pulse Code Modulation… (DVDs outputs it)

Sample the wave at a specified rate (samples per second) Sample Rate

Determine the associated amplitude at each sample Sample value

Store the binary value of the amplitude (sample value)

Example

Assume one second, one Hz

Made up Sample Data: Assume

Sample Rate = 72 ( measure amplitude 72 times/second)

Sample Size: 4 bits (2^4 = 16 possible values)

So: Every 1/72 of a second, determine amplitude using 4 bits and save as binary

PCM Example…

Sample Values: 7,9,11,12,13,14,14,15 etc.Binary nibbles: 0111, 1001, 1011,1100 etc.

Common Ranges

Humans Hearing: 20Hz to 20k Hz Dogs hearing: 40 Hz to 40k Hz Voice range 80 Hz to about 4 kHz ( 4,000

Hz)

Frequency and sampling

Nyquist’s theorem (revisited): For best pcm result

Sample at 2 x highest frequency What did we say was the highest

frequency of human hearing? So, what should be the sample rate? Actually we add 4,100 cycles…thanks to

Sony So final sample rate?

CDs

Sample rate: 44,100/second Sample size: 16 bits

What is the highest possible sample value? Sample size for DVDs is usually 24 bits

And we have 5 channels, not two (L,C,R,S1, S2) Sample rate 96,000 Hz, not the 44,100 as for CD

Storage of 1 minute stereo music on a CD

Sample rate: 44,100 Hz Bit depth ( sample size) 16 bits/sample

Allows for 2^16 sample values Stereo : two channels

Total number of Samples (for 60 seconds)

60 seconds x 44,100 samples/second = 2,646,000 samples

Number of bits to store 2,646,000 samples x 16 bits/sample = 42,336,000 bits

That’s for one channel For two channels:

42,336,000 x 2 = 84,672,000 bits perminute

Size…continued

How many bytes? 84,672,000 bits/(8 bits/byte)= 10,584,000 bytes

Kilobytes (KB)? 10,584,000 bytes (1024 bytes/KB)= 10,336 KB

MB? 10,336/(1024 KB/MB) = 10 MB for 1 minute

Compression

Save space Are Lossless and lossy compression Some uncompressed audio formats:

WAV, FLAC ( Use wav as master, use mp3 for deployment )

Phone (iphone and Android) format is compressed, lossless (.m4a)

Lossy compressed: mp3 Eliminates sounds we supposedly can’t hear

or don’t want to hear

More audio formats

aiff : non-compressed, pcm au: compressed, lossy, used by sun, unix,

java wma Windows Media Audio: lossy,

compressed

PowerPoint support

Audio and the Web

Html 4 supported formats Used embed, object, plug-ins, etc. Very messy

Html 5: Wav Mp3 Ogg (vorbis … professional, compressed

format)

Web Streaming versus downloading (Video and Audio) Three ways to listen to sounds:

Download and play Stream Progressive download

Methods 2 and 3 above yield same results, different technologies

YouTube streams File usually saved in a Temp folder

iTunes uses progressive The video file is not stored on the viewer’s

computer – the data is played and then discarded by the media player. This lets you maintain more control over your content. Better choice

Streaming

If you listen to a radio station online, you will stream the music, since you're listening live and couldn't download all the songs that they play.

Instead, your computer connects to the radio station and receives an ongoing audio feed, or stream, with the radio station's programming.

It begins to play when a sufficient amount is playable. While you listen, more audio is being streamed.

When you stream a file, you will get to listen to or watch it, but won't have the file on your computer to do things with later