Chapter 5-Multimedia Playback Systems

Multimedia Playback Systems

Playback System Model

Playback System Model● A multimedia playback consists of a disk-read which

feeds samples into a (D/A) converter at the same rate at which they were sampled and a display which consumes data the rate at which it has recorded.

● The D/A converter creates a signal that closely approximates the original to the display (or playout) unit.

● If the samples are not delivered to the D/A converter at precisely the rate at which the sampling occurred, the reproduced sound (or video) will not match the sampled sound (or video).

General Playback Framework● The data is not compressed.

● Digitization of video done by using analog frames of video data at fixed intervals and storing them in binary form.

● Digitization of audio data is most commonly achieved by sampling an analog signal at fixed intervals and by storing the amplitude of each sample in binary form.

● Reading the digitized multimedia data (either video or audio or text or images, etc.) is used to refer to retrieving data from the storage device(s), while the term “consume” is used refer to data being consumed by the D/A converter for playout.

● Delays exist in storage devices and playback units.

Read Function

Consumption Function

Buffer Function

Delays● Artificial Delays

● Rate of data receiving may be much higher than the rate of consumption

● May be created in the disk system for optimal utilization of the resources

● Let a disk-read reads at the rate, rt = 4Mbs and the display of the system consumes the data at the rate, rc =

2Mbs. In this playback system, there could be an artificial delay at most 1 second. The artificial delay can be induced in slabs. tA = l*d + rc, where l = number of

artificial slabs and d = artificial slab duration, i.e., 0.1 second.

Delays● Unavoidable delays

● These are the delays may crop up due to mainly seeking the information on the disk or reading the data on the old disk systems which need to be read several times for fetching the correct data.

● Let a disk-read reads at the rate, rt = 4Mbs. During the

reading of data, let seek-time (time taken to positioning the first data-item) is 10msec and also, disk-read has tried around 50 times to get the correct 4MB data from the disk since the disk is old, say, 100msec spent for that. So, totally, it has taken 1:110 sec to get the data from the disk instead of 1 sec. The extra 110msec is the unavoidable delay.

Playout Buffers

No Buffer basedplayback system

A Buffer basedplayback system

Playout Time● Real-time scheduling approach● Optimistic playout approach● Ideal playout approach● Interval-based process playout● Playout Time Calculation

Playout Calculation Algorithm

Playout Calculation

Playout Calculation

Playout Calculation

Minimal Start Time

Continous Stream Playback System

Continuous Stream Playback System

Continuous Stream Playback System

Buffer Function forProposition 5

Buffer Requirements● Buffer Requirement for a playback with Artificial

Delays

● Buffer requirement for a playback with unavoidable delays

Buffer Requirement for multichannel

environment● The storage device is dedicated to the multimedia data playback.

● The number of playback applications is nc.

● Each channel has the same consumption rate, rc. The sum of the

consumption rates is at most equals to the transfer rate of the storage device, ncrc ≤ rt.

● Playback can be divided into reading periods during which Sb data are

read for each channel.

● During each reading period, there are some unavoidable delays of nonzero length that are bounded above by dmax.

● k is defined as the ratio of the data transfer rate of the storage device over the total data consumption rate, that is, k = [rt/ncrc], since we

assume that ncrc ≤ rt, it must be the case that k ≥ 1.

● lp is defined as the reading period length. This is the delay time plus

the reading time.

Minimum Block Size

Reading Period Length

Sharing Storage Device – Free Time Model● Free time should be long enough to do reading or writing

for other tasks● Consider some free time (tf)

to each reading period:

● This can be substituted into theequation for consumption rate,and solved for free time to obtain:

● Consider the fraction of timethat the storage device is free

● Increasing the block size will increase the fraction of free time and also, it happens by decreasing the delay.

Sharing Storage Device – Channel Model● Reserve some channels to multimedia data playback use, while

requests for non-multimedia tasks could utilize any available non-reserved channel

● Buffer estimation:

● The multimedia block size:

● MM block must be long enough to supplydata for the length of a reading period

● Equals

● The closer we come to using the maximum number of multimedia channels, the more costly it is to add other non-multimedia channels.

Multimedia Playback Server Requirements

● Scalability: both in storage capacity and in the number of video channels.

● High reliability: A failure of storage or a video channel component should cause at most only a momentary disruption in service

● High bandwidth: The media server should be capable of supporting multiple video streams at very high speed rates.

● Real-time performance: The media server must guarantee continuous streaming of video at the high speed rate, with low jitter.

● Short-term admission control: The server should control the number of active video streams

● Long-term admission control and Program packaging

End User Service Requirements● Service availability

– availability of each of the offered multimedia

presentations at any point in time.

– presentation instances should start with minimal delay.● Maximization of presentation quality

– Multimedia presentations must be performed with

minimal deviations from their specifications

Presentation Provider’s Requirements● Minimization of rejected customers

– Each provider wants to reach as many users as

possible such that the number of rejected users● Maximization of total presentation quality

– This parallels the discussion of maximizing the

presentation quality with respect to the users.● Individualize-able presentation-specific adaptation

profiles

– Service providers should be allowed to influence how

the system adapts instances of their presentations.

Playout Management Task● Device Management

● Data Stream Management

● Synchronization Enforcement

● Support of Interactivity

Performance-Based ReactivePlayout Management Model

PB-RPM● Best-effort prefetch process loads the multimedia data

● Current throughput of this process: Buffer safety level: S, Current playout presentation quality: Qp.

● PB-RPM enforcement process keeps track of current buffer

level B and identifies situations that require d- or i-adaptations

depending on B

● It determines a proper d- or i-adaptation schedule and realizes

the schedule.

● After all data already in the buffer prior to the adaptation is

presented, the playout process switches to Q1p implied by the

adaptation schedule.

● Without any delay, the prefetch process changes its

presentation quality Qp to Qb implied by the adaptation

schedule.