Picture This: Square MPEGs and Data Holes
Apr 1, 2005 12:00 PM,
By Jeff Sauer
What standard will fit digital signage?
Most video content for digital signage systems today is MPEG-2, but MPEG-4 compression holds much promise.
Digital signage can mean different things to different users filling different needs. For displaying flight information in airports or showing schedules of events posted around large business or educational campuses, digital signage might involve basic text-based information with minimal fanfare. In those cases, the increased facility of a digital infrastructure may be enough to recommend the new technology. However, new digital signage technology owes its broad appeal to the fact that it can leverage all media types, including text, graphical images, photographs, and, particularly, motion video.
Today that video is most likely to be MPEG-2 compression. Moving forward, however, it will increasingly be MPEG-4. Displaying video is hardly foreign to most systems contractors, but the “MPEGs” of digital signage infrastructure might be less familiar. Is MPEG-4 better? Should you avoid old MPEG-2? The respective answers to those questions are probably “yes” and “no,” but it’s important to understand the differences between the two compression schemes.
STANDARDS IN PRACTICE
MPEG-2 is the video compression format of digital television, including digital cable to set-top boxes, and DVD-Video discs. Therefore, it’s very widely used and very well understood — and very robust. It’s also the most common format today for sending full-frame, high-quality video over IP for distance learning, point-to-point two-way communications, and e-meetings, as well as the burgeoning digital sign market.
MPEG-2 was created by the Moving Pictures Experts Group subcommittee of the International Standards Organization (ISO) back in the early 1990s, with the goal of achieving the best image quality possible while using the most advanced compression techniques of the day. That’s different from the committee’s goal when it created the original MPEG, now known as MPEG-1. Then, the goal was specifically to compress video, as well as possible, down to a bit rate that would play off that generation’s single-speed (150KBps) CD-ROM drives.
MPEG-1 and -2 use similar basic compression techniques but differ in terms of target bit rate and image quality. MPEG-2 also supports the field-based interlaced video that is the standard for analog video and television. And, in common practice, MPEG-2 encodes video at its full-frame resolution, while MPEG-1 typically reduces images to a quarter-screen resolution, then zooms them back to full screen for NTSC playback. That almost necessarily yields some image quality compromises. You’re likely to run across MPEG-1 still — including in digital signage applications — because of its smaller file sizes and lower bit rate and because of its ubiquity in the computer industry. It can be compressed and uncompressed very easily “in software” without monopolizing the system CPU.
MPEG-4, also from the MPEG subcommittee, was originally designed for very low-bandwidth Internet video distribution, with bit rates lower than even MPEG-1. However, MPEG-4 has no restrictions on frame size, resolution, frame rate, or bit rate, which means that MPEG-4 can potentially serve both very low- and very high-bit-rate applications.
Because MPEG-4 has had the advantage of about a decade’s worth of mathematical refinement over MPEG-1 and MPEG-2, its increased compression efficiency could allow it ultimately to supplant MPEG-2 as the standard in all of the aforementioned uses. For example, the proposed future formats for high definition-capable DVD discs — Blu-ray and HD DVD — both support the MPEG-4 video standard (although both also support legacy MPEG-2). The MPEG-4 format structure also has the potential to include an “object layer” that can carry metadata within the media stream and include both digital rights management security information and digital signage tracking data.
(There is no MPEG-3. The term “MPEG-3” was used for a proposed specification that was to be for high-definition video and digital cinema. However, the committee ultimately decided that MPEG-2 could be scaled up to higher bit rates and resolutions to satisfy those needs. MPEG-4 was required because scaling MPEG-2 downward to those lower Internet bit rates was less practical due to the manner of compression. MP3 audio is often mistakenly referenced as MPEG-3 audio, but it is actually one of the audio compression “layers” from the original MPEG-1 specification.)
Because it’s new and more bandwidth-efficient, MPEG-4 might seem like the natural solution for all digital signage applications. In theory it is. Indeed, it’s likely that over the coming years most of the motion video content of digital signage will be compressed using MPEG-4. However, as the adage says, “In theory, there’s no difference between theory and practice, but in practice…”
MPEG-4 is a formally accepted specification, but as counterintuitive as it might seem, it’s still an evolving standard. We are still in the early stages of productizing MPEG-4, with independent developers just starting to create encoders and players that work together in the field.
MPEG-2 has been around for more than a decade now, so there is an established infrastructure of products, including both hardware and software encoders and decoders. In fact, most computers today can play an MPEG-2 file with nothing more than the Windows Media Player.
MPEG-4 is available today in both hardware and software configurations, but most are first- or second-generation products. Interoperability remains an issue, as does cost. And while the Windows Media Player and the Apple QuickTime Player both now include a software MPEG-4 decoder, neither is likely to be able to play every MPEG-4 stream out there.
Does that mean you ought to stay away from MPEG-4? That’s not necessarily true either. There are just caveats. MPEG-4 is more efficient compression, and it’s able to achieve the same image quality with much fewer data bits. Therefore, if bandwidth is a serious concern, an MPEG-4 solution might make a lot of sense today. However, you should be prepared to pay more up front for the newer technology, and you’ll probably have to look a lot harder to find acceptable products. MPEG-4 is also really only viable today in a closed infrastructure with full control of all players and viewer locations.
Should you wait for MPEG-4? That depends on the immediacy of your needs. If the last few months are any indication, digital signage is a burgeoning market, and that has little to do with MPEG-4. As I’ve seen from writing about technology for about a dozen years, technology rarely moves as fast as everyone expects. New technology itself is also rarely as important as real solutions to real problems. Technology is just an enabler, and infrastructures can and should be built and rebuilt as products evolve, if and when they increase productivity.