Signal DigitizationAnalog vs Digital Signals

An Analog Signal

1 0 0 1 0 1 1 10 0 1 0 0 1 0 1 1 10 0

A Digital Signal

What type of signal do we encounter in nature?What type of signal is required by computers?

What are the issues involved in converting from analog to digital?

Signal DigitizationAnalog Holds a Lot of Information

The Analog Signal: A Closer Look

Can a computer represent an infinite number of values?

Amplitude

The amplitude can take on an infinite number

of possible values

TimeTo get an exact copy of the signal, an infinite

number of samples must be taken.

Signal DigitizationA way to overcome the “time” problem

Amplitude

Time

Sampling – breaking a signal up into discrete pieces by measuring the signal at regular time intervals. The higher the sampling rate the better the quality of recording!

Signal DigitizationA way to overcome the “value” problem

Amplitude

TimeQuantizing – grouping the infinite set of possible values into a

finite number of bins and then assigning a binary number to each bin.

1101

1111

1011

1001

1100

0111

1010

0011

1000

0010

0110

0001

0101

0000

0100

Signal DigitizationVoila! The analog signal is now digital!

0011

01000100

0100

01100110

01110111

0111

1000100010001000

1001 1001 1001 1001 1001

1010 1010 10101010

1010

10101010 1010

1011 10111011

1011

11001100

11001100 1100 1100 1100

1101 1101

1111 11111111

0011 0110 1010 1011 1001 1100 1001 1101 1100 0111 …

Storing Audio SignalsWave Files

a digital audio representation uses a sampling and quantization

to store sound

0 4 bytes 'RIFF' 4 4 bytes <file length - 8> 8 4 bytes 'WAVE' 12 4 bytes 'fmt ' 16 4 bytes 0x00000010 // Length of the fmt data (16 bytes) 20 2 bytes 0x0001 // Format tag: 1 = PCM 22 2 bytes <channels> // Channels: 1 = mono, 2 = stereo 24 4 bytes <sample rate> // Samples per second: e.g., 44100 28 4 bytes <bytes/second> // sample rate * block align 32 2 bytes <block align> // channels * bits/sample / 8 34 2 bytes <bits/sample> // 8 or 16 36 4 bytes 'data' 40 4 bytes <length of the data block> 44 bytes <sample data>

Storing Audio SignalsMusical Instrument Digital Interface (MIDI) Files

a description of musical events, NOT a digitization of an audio signal

representation of a sound includes values for the note's pitch, length, and volume

very compact relative to digital audioDelta Time(decimal) Event Code (hex) Other Bytes (decimal) Comment 0 FF 58 04 04 02 24 08 4 bytes: 4/4 time, 24 MIDI clocks/click, 8 32nd notes/24 MIDI clocks 0 FF 51 03 500000 3 bytes: 500,000 usec per quarter-note

0 C0 5 Ch. 1, Program Change 5 96 90 76 32 Ch. 1 Note On E4, piano 0 80 76 64 Ch. 1 Note Off E4, standard 0 FF 2F 00 Track End

Overcoming Limited BandwidthCompression and Streaming

Compression – reducing the amount of space needed to represent video or audio.

Streaming – instead of downloading the whole video or audio, get enough to start playback and then continue download in the background.

Which download technique will/won’t work when listening to web radio?

CD Trivia

How many samples per second are taken for a CD?

44,100 (sampling rate of 44.1 khz) How many bits per sample?

16

(Other factors: error correction, 2 sample streams for stereo)