9. Multimedia Networking
2 9.1 Introduction to Multimedia People in all corners of the world are currently using the Internet to watch movies and television shows on demand There are many Internet movie and television distribution companies (Netflix, YouTube) People can upload and distribute their own user-generated content (YouTube), becoming Internet video producers as well as consumers Multimedia Is defined as two or more media (is a book multimedia?) The combination of text, graphics, images, video and audio used together; at least one must be continuous (time- dependent like audio, video, animation) Sometimes pure audio (or video) is referred to as multimedia
3 Multimedia System The integrated production, processing, storage, representation, and transmission of several time-dependent and time- independent media streams Audio (sound) wave An audio (sound) wave is a one-dimensional acoustic (pressure) wave Causes vibration in the eardrum or in a microphone Frequency range of human ear: 20 –20,000 Hz (20 KHz) Audio waves can be converted to digital form by an ADC - Analog Digital Converter
4 Analog Digital Conversion (ADC) Sampling : of the audio wave in every ∆ T secs (only 9 possible values in Fig. b) Quantization Precision of the digital sample depends on the number of bits (4 bits required in Fig. c since there are only 9 values) Quantization noise: error due to finite number of bits/sample Audio encoding - example a) A sine wave b) Sampling the sine wave c) Quantizing the samples to 4 bits
5 Audio Encoding - Standards Telephone 8,000 samples/sec (up to 4 KHz) Needs 64 Kbps (Pulse Code Modulation - PCM, 8-bit samples in Europe), or 56 Kbps (USA, Japan – 7 bits) Audio CDs 44,100 samples/sec (up to 20 KHz) 16-bit samples: quantization error is small but audible Needs 705.6 Kbps (44100x16) for monophonic audio (or commonly called mono ), 1.411 Mbps for stereophonic audio (or more commonly called stereo ) MP3 (MPEG-1 audio layer 3) compression Based on psycho acoustic models (128 Kbps)
6 Analog Video - Basics Sequence of images flashing faster than 50/sec Makes the impression of a continuous movie TV (black-and-white) An electron beam scans the image rapidly From left to right and from top to bottom At the end of the scan (a frame) the scan retraces NTSC scans 525 lines (483 effective), 30 frames/sec PAL and SECAM: 625 lines (576 effective), 25 frames/sec In both, the top few and the bottom few lines are not displayed - to approximate a rectangular image on the original round CRTs
8 Smoothness vs. Flickering These are problems in both analog and digital video Smoothness : affected by the number of different images per second Flicker : affected by the number of refresh of the display per second Good computer monitors rescan with > 70Hz The image is repainted from RAM 25 frames/s produces smooth motion, but flicker Interlacing solves flicker; 50 half frames (fields) / sec Non interlaced: progressive scanning
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- Winter '19
- DR. Tesfa
- Networking, Streaming media, Transmission Control Protocol, People