Operating systems

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COURSE: Operating System II
SEMESTER: 3rd (2010/2011)
PROFESSOR: Konstantinos Lambrinoudakis
STUDENT: Luis Cano Pérez



In this figure we see two possible video-on-demandinfrastructures. Each one contains three essential components: one or more video servers, a distribution network, and a set-top box in each house for decoding the signal.

The distribution network between the user and the video server must be capable of transmitting data at high speed and in real time.
The last piece of the system is the set-top box, where the ADSL or TV cable terminates.This device is, in fact, a normal computer, with certain special chips for video decoding and decompression. As a minimum, it contains a CPU, RAM, ROM, and interface to ADSL or the cable.

Getting back to multimedia itself, it has two key characteristics that must be well understood to deal with it successfully:
1. Multimedia uses extremely high data rates.
2. Multimedia requires real-timeplayback.

The real-time properties required to play back multimedia acceptably are often described by quality of service parameters.
The most common way to provide quality of service guarantees is to reserve capacity in advance for each new customer. The resources reserved include a portion of the CPU, memory buffers, disk transfer capacity, and network bandwidth. If a new customer comes along andwants to watch a movie, but the video server or network calculates that it does not have sufficient capacity for another customer, it has to reject the new customer to avoid degrading the service to current customers. As a consequence, multimedia servers need resource reservation schemes and an admission control algorithm to decide when they can handle more work.


In mostsystems, an ordinary text file consists of a linear sequence of bytes without any structure that the operating system knows about or cares about. With multimedia, the situation is more complicated. To start with, video and audio are completely different. They are captured by distinct devices, have a different internal structure and they are played back by different devices.

The next result isthat a digital movie may actually consist of many files: one video file, multiple audio files, and multiple text files with subtitles in various languages.

We need some way to keep the sub files synchronized is also needed so that when the selected audio track is played back it remains in sync with the video.

A simple set of multimedia files is shown in this figure:


2.1 AudioEncoding

An audio (sound) wave is a one-dimensional acoustic (pressure) wave.
In a similar way, when an acoustic wave strikes a microphone, the microphone generates an electrical signal, representing the sound amplitude as a function of time.

The frequency range of the human ear runs from 20 Hz to 20,000 Hz.

Sound is conventionally expressed in dB (decibels) according to the expression
dB= 20 log10(A/B)

Audio waves can be converted to digital form by an ADC (Analog Digital Converter). An ADC takes an electrical voltage as input and generates a binary number as output.

In this figure we can see:
a) A sine wave.
b) Sampling the sine wave.
c) Quantizing the samples to 4 bits.


Digital samples are never exact. The error introduced by the finite number ofbits per sample is called the quantization noise. If it is too large, the ear detects it.

Digitized sound can easily be processed by computers in software. Virtually all professional sound recording and editing is digital nowadays.

2.2 Video Encoding

The human eye has the property that when an image is flashed on the retina, it is retained for some number of milliseconds before decaying....