MPEG-2 is an international standard for video coding and transmission. It is the standard used in the current European digital television standard, DVB-T (and also in other TV standards). This digital television standard uses the MPEG-2 standard for the encoding of the video and also uses the MPEG-2 standard for the transmission and signaling of the video. Previous post detailed the DVB-T television standard, the video encoding standard, and the signaling standard.
To transmit the compressed video, and also other extra media content and information, all the streams must be combined. The process of combining these multiple streams is known as multiplex. A multimedia program consist of compressed video, compressed audio, control data and user data. In the multiplex process, these multiple streams are combined into one signal over a shared channel. Currently there are several shared channels where the multiplexed stream can be transmitted such as radio frequency (UHF/VHF), satellite, cable TV, standard terrestrial communication, digital networks (ADSL), etc.
The multiplexed stream is transmitted over a communication channel, also know as shared channel or transmission link. Today they coexist several communication channels (UHF, cabe TV, terrestrial communication…) and the physical transmission mediums are very different. To provide communication services to multiple users, multiplexing techniques are required. There are currently several multiplexing techniques, also known as access methods, and the use of each one depends on the characteristics of the medium such as bandwidth or if its wired or wireless.
Several multiplexing techniques are used depending on the transmission system, bandwidth and characteristics of the medium. There are the following basic multiplexing methods for the transmission of TV signals to multiple users:
- Frequency Division Multiple Access (FDMA): The TV programs are carried by different independent carriers (in different frequencies), sharing the transponder bandwidth.
- Time Division Multiple Access (TDMA): The digitalized TV programs are time multiplexed onto a signal carrier. This technique is used when the transmission bitrate is higher than the bitrate needed.
- Code Division Multiple Access (CDMA): It employs spread-spectrum technology and a special coding scheme to allow multiple users to be multiplexed over the same physical channel. The modulated coded signal has a much higher data bandwidth than the data being communicated.
- Wavelength Division Multiple Access (WDMA): This system multiplexes all signals onto a signal optical fiber by using different wavelengths.
The multiplexing process
Once the A/V signals have been digitalized and compressed the different bitstreams are combined and multiplexed to be transmitted. The streams are combined onto the following streams:
Packet Elementary Stream (PES)
The elementary stream (ES) is packetized by encapsulating sequential data bytes inside PES packets. PES packets consist on a header and the Playload (0-64kB). Once PES packets are created from the elementary stream data, these PES packets are encapsulated inside Transport Stream (TS) or Program Stream (PS).
Program Stream (PS)
Program Streams are created by combining one or more PES packets (audio, video or data), which have a common time base, into a single stream. Audio and video PES packets includes the Time Stamp for a correct synchronization. PS are designed for reasonably reliable media such as DVDs, CDs, HDDs…
Transport Stream (TS)
Transport Stream specifies a container format encapsulating packetized elementary streams, with error correction and stream synchronization features for maintaining transmission integrity when the signal is degraded. TS are designed for broadcast applications.
Null Packet Optimization
Standards like ATSC or DVB impose strict constant bitrate requirements on the transport stream. In order to maintain a constant bitrate, the multiplexer insert some additional null packets (not contain any data). The receiver ignore its contents.
Program Clock Reference (PCR)
The PCR is a 42 bits number transmitted in the Transport Stream packets to enable a decoder to present synchronized content.
Conditional Access is the protection of content by limiting the access or requiring certain criteria. In the DVB standard is achieved by a combination of scrambling and encryption.
The data stream is scrambled with an 48-bit secret key, the control word (CW). The CW have been encrypted with the service key and the service key have been encrypted with the user key. The control word is encrypted as an Entitlement Control Message (ECM). The CA subsystem in the receiver will decrypt the control word only when is authorized by and Entitlement Management Message (EMM). The contents of ECMs and EMMs are not standardized and depends on each CA system provider.
CAT tables are used to indicates to the receiver the position of EMMs and important information and conditional access descriptors.
Technologies of conditional access systems
- Symulcript: The CW is transmitted through different ECMs at once. This allows the use of several conditional access systems at the same time. It uses multiple set-top boxes, each using a different CA system, to authorize the programs for display.
- Multicrypt: The CW is sent sequentially through different conditional access modules and its based on a DVB standardization. It allows two or more different encryption systems to co-exist in the same receiver.