Video has traditionally been a band-width hog. Some applications require multiple video channels with as much as 4 or 5 Mbits/sec for the video link! This has often forced expensive solutions such as separate downlinks for video and data - sometimes with one downlink per video channel. However, advances in video technology now offer alternatives that can radically reduce the bandwidth and the cost.

MPEG-4 compression greatly improves bandwidth efficiency for this traditional band-width hog. Unlike Wavelet, which is proprietary, MPEG-4 is an open standard and codecs are available from many manufacturers. Although results vary from application to application the compression achieved is at least 10 times better than J-MPEG and can be up to 50% better than MPEG-2 (that is equivalent quality for half the bit-rate). It is difficult to give an exact figure as the video content has a huge bearing on the achievable compression.

ACRA CONTROL introduced MPEG-4 to the flight test community in 2005 with the KAD/VID/103. This dual-slot module provides three multiplexed camera inputs (NTSC or PAL) to a single MPEG-4 decoder. The compressed video is transported in an industry standard MPEG-2 transport stream and can be embedded in a PCM stream, encoded as Ethernet packets, or locally stored to a compact clash card.

The KAD/VID/103 has allowed several customers to completely re-vamp the way they handle video. For example, in one site a 4Mbps video link was removed completely as the customer found they could get the same quality with 2Mbps using MPEG-4. This was for a standard resolution, 30 frames per second (NTSC) color image with rapidly moving components.

This allowed them to combine their data (~2.5Mbps) with their video into a single PCM stream and use just one 5Mbps down-link for real-time data and video. In addition, they installed a KAM/MEM/003 module in the on-board system with a 6Gbyte CompactFlash® card to record the video. After the flight they could replay the video and view any information lost due to drop-outs in the telemetry link. The KAD/VID/103 super-imposes a time on the video image to enable cross-correlation with other data.

While this application required high video quality, other application with less stringent requirements can achieve dramatic reductions in video bandwidth. The KAD/VID/103 allows the user to vary the resolution, frames-per-second and the predicted frame ratio to balance the competing requirements of bandwidth versus quality. For example, reducing the frame rate to 15 frames per second would half the necessary bandwidth for viewing images where there was not so much rapid movement.