MPEG Transport Stream, often known as MPEG-TS or MTS, is a standard digital format. It’s widely used for transmitting and storing audio, video, and Program and System Information Protocol (PSIP) data. MPEG developed this format and is a key component of broadcast systems. These systems include DVB, ATSC, and IPTV.
The MPEG Transport Stream format encapsulates what are known as packetized elementary streams. These streams offer features like error correction and synchronization patterns. These features are crucial for maintaining the integrity of transmissions, even when communication channels are less than ideal.
Unlike MPEG program streams, which are designed for reliable media like DVDs, transport streams differ. They cater to less reliable transmission methods, making them perfect for terrestrial or satellite broadcasts.
In this blog post, we will explore the complexities of the MPEG Transport Stream.
What is MPEG TS?
MPEG Transport Stream (MPEG-TS) is a standard digital container format for transmitting and storing audio, video, and Program and System Information Protocol (PSIP) data in broadcast systems.
MPEG-TS encapsulates packetized elementary streams. It includes features for error correction and synchronization. These features maintain the integrity of transmission when the communication channel carrying the stream is degraded. The transport stream may carry multiple programs, making it different from the MPEG program stream. The transport stream format was specified in MPEG-2 Part 1, Systems. It’s also known as ISO/IEC standard 13818-1 or ITU-T Rec. H.222.0. The format is designed for less reliable transmission, such as terrestrial or satellite broadcast, unlike program streams designed for reliable media like DVDs.
How Does an MPEG Transport Stream Work?
MPEG Transport Stream (MPEG-TS) works by encapsulating packetized elementary streams. These streams include audio, video, and PSIP data, which are packetized into small segments. Each stream is chopped into 188-byte sections and interleaved together. This process ensures less latency and greater error resilience, making it ideal for videoconferencing where large frames may introduce audio delay.
The transport stream format uses a network packet as the basic data unit. Each packet begins with a sync byte and a header, followed by optional additional headers and the payload. The packet identifier (PID) is a crucial component, as it helps identify each table or elementary stream in the transport stream. A demultiplexer uses the PID to extract elementary streams from the transport stream.
MPEG-TS also uses a concept of programs, with every program described by a program map table (PMT). The PMT lists the PIDs associated with that program. If a receiver wishes to decode a particular channel, it only needs to decode the payloads of each PID associated with its program, while it can discard the contents of all other PIDs.
Key Components of MPEG Transport Streams
Packetized Elementary Streams (PESs): These are the core data streams, which include audio, video, and other data. They are wrapped using either the MPEG codec or non-MPEG codecs.
Packet Identifier (PID): This is a 13-bit identifier that helps distinguish each table or elementary stream in the transport stream. It allows a receiver to decode specific streams by checking packets with the same PID.
Program-Specific Information (PSI) Tables: These include program association (PAT), program map (PMT), conditional access (CAT), and network information (NIT) tables. They provide essential information about the programs in the stream.
Program Clock Reference (PCR): This is transmitted in the adaptation field of an MPEG-2 transport stream packet. It generates a highly accurate time base in the decoder, synchronizing audio and video streams.
Null Packets: A transport stream may include null packets to maintain a constant bitrate. These packets have a PID of 0x1FFF, and their payload, which is expected to be ignored by the receiver, consists of all zeroes.
Why Is the MPEG TS Preferred for Broadcasting?
MPEG Transport Stream (MPEG-TS) is a preferred broadcasting format for several reasons.
Firstly, it’s designed to handle packet loss frequently occurring in broadcasting environments. The small packet size of 188 bytes allows streams to be interleaved with less latency and greater error resilience, making it a robust choice for unreliable transmission mediums like terrestrial or satellite broadcasts.
Secondly, MPEG-TS can carry multiple programs in a single stream. This multiplexing capability allows broadcasters to transmit several channels over the same frequency, efficiently using the available bandwidth.
Thirdly, using Program-Specific Information (PSI) tables provides crucial data about the programs in the stream, facilitating the decoding process at the receiver’s end.
Moreover, including a Program Clock Reference (PCR) helps synchronize audio and video streams, ensuring smooth playback at the viewer’s end.
Lastly, using null packets allows the stream to maintain a constant bitrate, a requirement in some transmission schemes like ATSC and DVB. This helps in preserving the quality of the broadcast signal.
Overall, the features of MPEG-TS make it well-suited for broadcasting demands.
Where Are MPEG TS Commonly Used?
MPEG Transport Stream (MPEG-TS) is commonly used in various digital broadcasting systems, such as:
- DVB (Digital Video Broadcasting),
- ATSC (Advanced Television Systems Committee),
- IPTV (Internet Protocol Television).
It’s also used in digital video cameras and recorders, where a timecode field is added to the standard packets for quick access and synchronization. Furthermore, MPEG-TS is utilized in Blu-ray Disc video titles with menu support and in BDAV (Blu-ray Disc Audio/Visual) audio/video recording format. Its ability to handle packet loss and carry multiple programs makes it ideal for these applications.
Conclusion
The MPEG Transport Stream (MPEG-TS) is a digital container format integral to many broadcasting systems. It’s designed to handle packet loss, a common occurrence in broadcasting environments, making it an ideal choice for terrestrial or satellite broadcasts. Its unique ability to carry multiple programs in a single stream allows efficient bandwidth use. Also, it uses Program-Specific Information (PSI) tables and Program Clock Reference (PCR) to simplify the decoding process and synchronize audio and video streams.
Furthermore, it uses null packets to maintain a constant bitrate, ensuring high-quality broadcast signals. These features make MPEG-TS a preferred format for broadcasting. It’s widely used in systems like DVB, ATSC, and IPTV, as well as digital video cameras, recorders, and Blu-ray Discs. With its robustness and versatility, the MPEG-TS plays a vital role in digital broadcasting and storage.
