Home > Articles

iTV Food Chain

This chapter is from the book

3.2 Broadcast Model

3.2.1 MPEG Transport

MPEG transports are unbounded sequences of bits (i.e., bit-streams) grouped into 188-byte packets (see Figure 3.11). These bits are extracted from other layers including the physical layer modulation, such as the QAM and the QPSK.

03fig11.gifFigure 3.11. Example packet sequences for two 8 Mbps channels carried in a 19.2 Mbps transport.

Each 188-byte packet is associated with a Packet ID (PID). This is not a unique identifier of the packet, but rather an identifier of the data stream the packet is part of. Generic MPEG equipment enables management at the PID level, namely is able to identify components of the transport by the PID values within the 188-byte packet headers. More specialized equipment may be able to manipulate the data within the PID, namely, manipulate the data carried within the collection of packets having a common PID.

In addition to transmission of content, packets containing MPEG Program Specific Information (PSI) must be transmitted. PSI packets include those carrying the Program Association Table (PAT), Program Map Table (PMT), and other tables. The PAT contains a data structure specifying which PIDs will carry the PMT, and the PMT contains a data structure specifying which PIDs will carry the video, audio and data stream that collectively constitute the broadcast program. Standards built on top of MPEG may specify various constraints on the bandwidth and frequency of appearance associated with transmission of MPEG PSI.

Multiplexers decide which transport packets to insert and when. Multiplexers are responsible for ensuring that the average bandwidth allocated to the PSI and each program element as specified by the applicable standards and content characteristics.

3.2.2 Virtual Channels

Whereas with analog TV there were channels, each associated with a frequency range, with iTV (being based on MPEG-2 transports) that channel can be divided into sub-channels called virtual channels. A virtual channel is labeled by two numbers, its major and minor channel numbers. For example, instead of Channel 7, with iTV we have Channel 7.1, 7.2, etc., where the major channel number is the analog number, 7, and the minor numbers follow the '.'.

An MPEG-2 transport bitstream (carried over a single continuous frequency band) is divided by assigning each component to a distinct PID. Each virtual channel is associated with a set of PIDs using dedicated tables encapsulated in separate dedicated PIDs (see following subsection and Figure 3.12). This means that virtual channels may be interleaved. For example, it is possible to follow packets of Channel 7.1 with packets of Channel 7.2, which can then be followed by packets of Channel 7.1 (see Figure 3.11).

03fig12.gifFigure 3.12. An abstract depiction of the logical structure of an iTV MPEG-2 transport.

The virtual channel binding also associates the major and minor numbers with additional meta-data, such as network or channel name and Transport Stream ID (TSID), usable by electronic program guides or other receiver applications. Transmission of a new version of the virtual channel binding table changes the programs associated with each channel, and effectively changes the content of that channel.

3.2.3 Program Specific Information

In addition to transmission of content, also transmitted are packets containing MPEG Program Specific Information (PSI). PSI packets include those carrying the Program Association Table (PAT), Program Map Table (PMT), and other tables. The PAT contains a data structure specifying which PIDs will carry the PMT, and the PMT contains a data structure specifying which PIDs will carry the video, audio, and data stream that collectively constitute the broadcast program. Standards built on top of MPEG may specify various constraints on the bandwidth and frequency of appearance associated with transmission of MPEG PSI.

All MPEG-2 PSI tables should be broadcast repeatedly. A typical transport structure would contain in addition to the PSI, such as the PAT and the PMT required by MPEG-2, a table defining the mapping of virtual channels carried in that transport, such as the ATSC Master Guide Table (MGT) in conjunction with the Virtual Channel Table (VCT) or its DVB variants (see Figure 3.12). The PAT lists the MPEG-2 programs carried on the transport, and associates them with number identities unique within the transport, pointing to a PMT for each program.

Multiplexers decide which transport packets to insert and when. Multiplexers are responsible for ensuring that the average bandwidth allocated to the PSI and each program element are as specified by the applicable standards and content characteristics.

Whereas transport packets are the basic transport encapsulation, Packetized Elementary Stream (PES) is the basic streaming structure depicted in Figure 3.12; transport packets have segments of PES packets in their payload. PES is used for carrying both Video and audio frames in the payload of the packet. However, PES is rarely used for data carriage; Digital Storage Media Command and Control (DSM-CC) ISO/IEC 13818-6 is used to carry data (see Section 3.4 and Chapter 11 Transport File System). With PES, synchronization information, as well as other meta-data critical for the encoding and rendering of content, is carried in the header of a PES packet. A PES packet can be up to 65,542 bytes long, and thus its carriage may require up to 349 transport packets (each 188 bytes long).

3.2.4 Audio Content

Sound is pressure differences in air. When picked up by a microphone and fed through an amplifier, this becomes voltage levels. The voltage is sampled a number of times per second and converted into digital signals using analog-to-digital conversion. For CD quality audio there is a need to sample 44,100 times per second where each sample is a conversion of the analog pressure into a digital word with a resolution of 16 bits. When recording in stereo mode, this requires 1.4 Mbit per second, raising the need for compression. Various audio compression techniques (including AC-3 and MPEG AAF) are described in Chapter 9. When delivered using MPEG transports, audio data is broken into frames. The data bits representing the compressed audio frames are transmitted in the payload section of PES packets. It is synchronized utilizing a portion of the frame header called frame sync. See Chapter 9 for more details.

3.2.5 Video Content

Video consists of dynamically changing light patterns hitting a sensor. When picked up by a video camera, these patterns become voltage levels associated with location and time. The camera samples voltage levels to convert them into digital signals by dividing the camera viewport into a matrix of regions, called pixels, and recording light color and intensity for each pixel. Table 3.1 lists some video formats, their resolutions, and number of frames per second. For HDTV quality, a matrix of 1080 (DVD is 480) interlaced rows of 1920 pixels needs to be sampled 60 times per second (two fields of 540 lines, each sampled 30 times per second), where each sampled pixel contains 24 bits. Without appropriate compression, processing such data is not practical as it amounts to 540 × 1920 × 24 × 60 bits per second! State of the art sophisticated compression techniques are able to deliver HDTV video for less than 19.2 Mbps, a common DTV broadcast bandwidth.

Table 3.1. Summary of Video Format Standard

Standard

Resolution

Frames Per Second

Film

 

24/23.98

Analog NTSC

720/704/640 × 480/486/512

29.97/30/59.94/60

Analog PAL

720/704/640 × 576/612

50/25

Analog ITU-R BT.601-4

864/858/720 × 625i/525i/483i

50/30/25

HDTV SMPTE 260M

1920 × 1035i (Interleave)

30/29.97

HDTV SMPTE 296M

1280 × 720P (Progressive)

60/59.94

HDTV SMPTE 274M

1920 × 1080i (Interleave)

60/59.94/30/29.97/25/24/23.98

HDTV SMPTE 274M

1920 × 1080P (Progressive)

30/29.97/25/24

The bit stream representing the compressed video data is carried in PES packets which are converted into 188 byte transport packets and multiplexed into an MPEG-2 transport. Video data essentially encodes a sequence of frames, using an explicit or implicit representation of frames. The explicit representation of frames, called I-frames, is used to initialize a motion vector predictor. The two types of implicit frames, P-Frames and B-Frames, are processed by a motion predictor. Typically, an I-Frame is followed by one or two B-Frames, followed by a P-Frame, followed by one or two B-Frames, followed again by a P-Frame.

Synchronization between audio and video, however, depends on the performance of the video and audio decoders. Smooth performance and consistent synchronization should be maintained as long as the transmitting encoder and receiving decoder are both compliant with the MPEG-2 delivery contract called the Transport System Target Decoder (T-STD) buffer model. For details, see the MPEG-2 specification for video decoding.

3.2.6 Data Content

Data differs from audio and video in that it is often not associated with sampling rates, decoding, and delivery contracts. For example, the broadcasting of an iTV application composed of a single HTML file may be achieved by transport packets that are distant, long time periods apart, constrained only by overall bandwidth allocation and without a requirement to meet any MPEG-defined time constraints. Further, the format by which the data is placed in the transport, called encapsulation, may vary significantly (see Chapter 11).

In contrast to data, a video component of an MPEG-2 program is placed in a single elementary stream, using a single PID. Similarly, each audio track associated with that video component is usually placed in a single elementary stream using a single PID. Data is different as it may be more complex than both audio and video. A single application may be carried in several PIDs, and several applications may share PIDs (see Figure 3.12). Data may be carried using a broadcast file system (see Chapter 11). The unique characterization of data broadcasting, also known as datacasting, has numerous implications, including challenges related to multiplexing and re-multiplexing, transport, and receiver design issues.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020