The MPEG Standards
Many people that work with digital video use the terms “MPEG-2” and “MPEG-4” to refer to a specific digital format of video. Others use those terms to refer to digital audio, or sometimes those terms are used to describe audio and video that have been interleaved, or “multiplexed”, into a single stream of digital data.
What can be confusing about this short-hand use of those terms is that each of the two standards, MPEG-2 and MPEG-4, actually refer to an entire system of video, audio, multiplexing and more. So, using only those terms could refer to any part of the larger system, and not just video itself.
Why Terms Matter
This can lead to confusion when discussing what type of video your TelVue Broadcast server will play, what type of video your transcode software can generate, or even what type of video you already have in your library. You might say “we have MPEG-4 video”, but that is not enough information to actually understand what format of audio, what format of multiplexing, or even what format of video you have. This becomes particularly important when trying to determine if 3rd-party equipment will work with your TelVue broadcast server.
The Most Common Mistake
The area that is most often confused are the terms used for multiplex formats, which are particularly confusing because they do not always have commonly used distinct names to help separate them from the parts of the standard that cover coding of video or audio.
MPEG-1 and MPEG-2 introduced two types of multiplexing, called Program Stream and Transport Stream. Because Transport Stream first appeared in the MPEG-2 suite of standards, its often referred to as MPEG-2 Transport Stream.
However, Transport Stream can actually contain many types of video or audio. It itself is a multiplex format, that just specifies how to take different streams of data and combine them together into a single digital stream of data. So a common confusion might be to think that “MPEG-2 Transport Stream” always contains MPEG-2 Video, which it does not. In fact, it can contain MPEG-4 video, and many other types of video as well.
Because Transport Stream was designed for network or digital transmission, and has been available for a long time, it is widely used as a means to transport video, audio and other data together. In fact, ATSC digital transmission, digital cable, ASI, Blue-ray and many other forms of transporting digital video all use MPEG-2 Transport Stream, often times without using MPEG-2 Video compression.
TelVue Princeton HyperCasters receive and transmit compressed and multiplexed streams of video and audio in the Transport Stream multiplex format, but are not limited to MPEG-2 video or MPEG-2 audio compression.
How It Gets More Confusing
When MPEG-4 was created, several new multiplex formats were added, including MP4. If referring to Transport Stream as MPEG-2 Transport Stream was not a source of enough confusion, calling the multiplex format “MP4”, which many people use as a shorthand for all of the MPEG-4 standard, certainly created enough misunderstanding. In fact, the term “MP4” refers to a specific multiplex format, also called MPEG-4 Part 14 (but no one calls it that in common discussion), that differs from Transport Stream in many ways, though it was also designed to send multiplex audio and video over a network.
MP4 is today commonly used for Internet streaming, such as HTTP Progressive Download from sites like YouTube, or streamed over RTMP to Adobe Flash player. It is not used as part of any ATSC transmission, on any common digital-cable technologies, or in any CableLabs standards. At least part of the reason it is not used for these purposes is that Transport Stream already does those jobs extremely well.
Multiplex formats are not the only ways that the MPEG-2 and 4 standards differ or that products that implement parts of those standards differ. Each standard also builds in multiple types of compressed audio, in some cases multiple types of compressed video, and even within a specific type of compressed video, differing encoding parameters called Profiles and Levels. Outside of the MPEG standards, transmission and network technologies can send the same digital video over different network types, packet types, or with different Control Protocols.
An important point to keep in mind is that each of the standards, MPEG-2 and MPEG-4, are quite large and complex and cover the use of digital video and audio in many different circumstances. It was never intended that any one product would implement the entirety of the MPEG-2 or 4 standard, and that is why they are broken up into parts.
Its very easy to read a statement such as “Supports MPEG-4” and think that means the product sends or receives MP4 (which may or may not be true), or to read “Transmits MPEG-2 Transport Stream” and think that means you can only send MPEG-2 compressed video (which is not true).
While it can seem a little overwhelming at first, the good news is that common use cases are well understood throughout the industry and many, many products do interoperate quite well together due to the work done by the Motion Pictures Expert Group.
In addition, TelVue Corporation is always here to help, so please contact us with any questions!