cmpt 165 lecture one, Exercises of Computer Science

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Lecture 1

Digital Media

Prof. Shervin Shirmohammadi

The Internet

Application Type

Text

• Traditionally presented in ASCII (American Standard Code for Information Interchange) - 7 bits per character
• What about non-English characters?

Extended ASCII

• 8 - bit or larger characters

Audio

• Sound representation. Physically speaking, audio

is a waveform.

• Digital audio:
  • Logical representation (Midi, MOD)
  • Sampled (PCM)
• Compressed audio
  • Most common type for music : MPEG-1 Layer 3 (mp3) - At 128 kbps it is around 1 MB per minute
  • Windows Media Audio (wma)
  • Real Audio
  • etc. Prof. Shervin Shirmohammadi CMPT 165 1 - 7

Digitizing Audio

• Psycho-acoustic analysis: human ear picks up

signals from 20 Hz to 20Khz.

• Nyquist’s theorem states we need to sample

twice the highest frequency.

• Sample rate of 40Khz should be enough (44.

Khz in practice).

• 16 bits per sample is usually adequate
• Leads to 1.4 Mbps for standard audio CD

(stereo, PCM).

LPCM

• Linear Pulse Code Modulation ( LPCM ), most-widely used digital uncompressed audio format.
• Encodes one channel, at 8, 16, 20 or 24 bits per sample.
• Can support 1 to 8 channels (7.1 surround)
  • For 7.1 audio: 8 channels × 48 kHz × 16 - bit per sample = 6.144 Mbit/s (supported in video DVDs).
  • Multi-channel requires synchronization of multiple LPCM streams.
• Sampling frequencies typically 44.1 kHz (CDs), and 48 kHz (video DVDs).
• Also supports 96 kHz and 192 kHz for digital archival material. (^) How to compress audio?

Frequency Compression

• Two ways in the frequency domain: (a) The threshold of audibility for a human as a function of frequency. (b) The frequency masking effect.

MPEG-1 Audio

• Specified in MPEG-1 Part 3, supports:
  • Mono
  • Joint Stereo
  • Dual (two mono channels)
• Sampling rates: 32000, 44100, and 48000 Hz
• Bitrates:
  • Layer I: 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416 and 448 kbit/s
  • Layer II: 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320 and 384 kbit/s
  • Layer III: 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256 and 320 kbit/s

MPEG-1 Audio Layers

• Layer I (practically obsolete):
  • Same as Layer II, except it uses a smaller frame size and finer resolution for low-delay apps like teleconferencing.
  • It also has lower complexity suitable for early 1990s software/hardware.
• Layer II :
  • Uses Fast Fourier Transform (FFT) to generate 32 sub-bands.
  • Uses audibility threshold, frequency, and temporal compression.
  • Reduces quantization accuracy for less important sub-bands.
  • Best used for orchestras, human voices, audience applause, and percussive sounds such as triangle and glockenspiel.
• Layer III :
  • known as mp3 ; popular at 128-192 kbps for near CD-quality audio

Image

• Each natural image can be thought of as a set of pixels, each pixel with a different colour.
• The dimensions of this pixel (and its quantized value) will determine the quality of the digital picture compared to its natural version.

Pixel Colour

• Each pixel is represented by three values: Red, Green, and Blue, together known as the RGB value.
• The reason is that:
  • Any colour can be constructed based on various values for R, G, and B.
  • Traditional colour TVs actually use 3 dots or strips (phosphors) on their screen. If you look closely at a traditional color TV’s screen while it’s on you will be able to see the three R, G, and B dots close to each other.

YUV

• YUV and its digital successor YC b C r have also been proposed as image representation formats.
• YC b C r breaks the picture itself into three components:
• Y: Luminance , is an indication of the brightness of each pixel. This essentially leads to the greyscale version of the picture.
• Cb and Cr: The Chrominance components corresponding to blue-difference and red-difference chroma components. CC^ Yb r

Bitmap Image

• Essentially Bitmap image is the uncompressed digital image in RGB format.
• Using the Bitmap file format, one can store and transmit an image.