Industry Perspectives: Finding the Bitrate Sweet Spot

Interoperability is an increasingly important issue, and so adherence to standards is becoming crucial. MPEG is a well-established and effective method of video and audio compression, and exists in broadly three forms; MPEG-1, MPEG-2, and MPEG-4. MPEG-1 originally was targeted at compression suitable for PC multimedia use, MPEG-2 was targeted at TV applications, and the more recently ratified MPEG-4 is a scalable compression suite suitable for delivery of content to devices ranging from PDAs to High Definition TVs. The flexibility of MPEG-4 means that it will probably ultimately replace MPEG-1 and -2 in networked applications.

Here’s a breakdown of how each format stacks up:
MPEG-1
• Good for bitrates from 500Kbps to 2Mbps, with a sweet spot around 1.2Mbps.
• Appropriate for Video CD, video clips delivered over the public Internet, video to PCs over networks, and news gathering from remote locations.
• Sweet spot may be lower if MPEG is viewed on a VGA screen.
• If lower quality can be tolerated—for instance, if audio quality is a secondary concern, then more bandwidth can be allocated to the video.
• PC-based software MPEG decoding is possible, obivating the need for dedicated hardware at the point of delivery.
• Suitable for small video monitors, but not for large-format devices such as plasmas or videowalls.

MPEG-2
• Good for bitrates from 1.5Mbps to 8Mbps, with a sweet spot around 3Mbps.
• Appropriate for roadcast applications; high-quality presentations in museums, exhibitions, public spaces, reception areas etc.; DVD-Video; video archives.
• Software decoding of MPEG-2 is CPU-intensive. Unless the PC is only being used for video display, it is advisable to use a hardware decoder to allow other applications to use the CPU.
• Not advised for display within a window on VGA devices as MPEG-1 or -4 is more efficient for this application.
• Sweet spot will be higher for large format applications such as display on plasmas or videowalls.

MPEG-4
• Good for bitrates from 40Kbps to 8Mbps, with a sweet spot that varies from application to application.
• Appropriate for live video over corporate networks; video to PCs over networks or to large format displays such as plasmas.
• More efficient algorithms than MPEG-1 or –2, so ideal for restricted bandwidth or limited-storage applications.
• Benefits from advanced algorithm development resulting in better bitrate quality performance than predecessors. Less prone to "blockiness" in cases where bandwidth is limited.
• Suitable for a wide range of output devices ranging from handheld to large format displays.
• Software available for PC decoding of MPEG-4, with hardware decoders becoming more widespread with growth of DSL applications.

Generally, the bitrate sweet spot also is dependent on the amount of action within the video. For example, a head and shoulders view during an interview will require a lower bitrate than an action scene in which the camera pans over a crowd scene. Another compromise has to be made–high enough bitrate to accommodate the action scenes, but low enough to ensure bandwidth is not being wasted on interview scenes. More sophisticated encoders provide a variable bitrate feature whereby bandwidth is allocated according to the action within a scene.

The goals of business communication channels are to inform, motivate, and entertain. These goals will be achieved by the creation and delivery of imaginative and well-produced content. It is vital that the delivery method is scaled correctly so that it provides a seamless experience for the audience. Careful consideration must therefore be made regarding the audience, the content, the network, display, and compression technologies.