Video Compression There are no hard drives capable of streaming super high quality video. So how do nonlinear systems claim to be broadcast quality? The answer: Video Compression. Different systems, or compression schemes have been designed that reduce the MB/second ratio of video images. They all work with varying degrees of success.

The Video Toaster Flyer uses a proprietary compression algorithm called VTASC to compress broadcast quality video onto "off the shelf" computer hard drives. VTASC/The Flyer can operate within the limits of current drive technology by limiting the signal we record to broadcast resolution. By law, the FCC limits the frequency response of broadcast video to 4.2 MHz. The Flyer, in HQ5 and Standard Modes, limits frequency response to this figure through a programmable low-pass filter. This first compression step saves data and bandwidth that would be otherwise wasted by including detail which must be discarded during broadcasting anyway.

Standard Mode is based on an assumed drive transfer rate limited to 3.7 MB/sec. HQ5 assumes a drive capable of about 5 MB/sec. Both will deliver lossless D2 video when they can (when complexity, or noise and detail, is low) and both modes will appear identical when complexity permits a data rate of 3.7 MB/sec or lower. If complexity increases, however, Standard Mode will respond with higher compression ratios to keep the data rate down to 3.7 MB/sec or less. The Flyer's VTASC compression system introduces more noise as its compression ratio increases. The VTASC noise resembles random or ordinary tape noise. At the highest compression ratios caused by maximum video noise or detail, VTASC can also reduce the pixel sampling rate to keep the data rate within the 3.7 MB/sec limit. In the worst situations, this can introduce chroma noise.

HQ5 mode also employs variable compression, but the threshold (for the shift to higher compression ratios) is 4.8 MB/sec instead of 3.7 MB/sec. VTASC will introduce less noise, if any, in HQ5 mode even with higher noise and greater video complexity. In fact, with low noise or smooth rather than detailed video, there may be no difference between the two modes.

Extended Mode also uses variable ratio compression, but always employs reduced pixel sampling to reduce data flow. This results in slightly lower resolution Flyer video and increased chroma noise.

All compression systems have difficulty compressing noisy or snowy video. Such video may result from low quality tape, bad heads, multi-generation dubs, or low light shooting conditions. There is no way for video encoding systems to differentiate between noise and detail, so a noisy image acts like an image that requires high compression or a high data rate. Any compression system, including VTASC, must increase its compression when it encounters noisy video. This can result in a lack of detail and even more noise. Noisy video does not compress well. In such cases, HQ5 mode will probably give you far better results than Standard Mode. Video shot in low light usually has very high noise levels. Consumer camcorders often have built-in gain boosters for low-light shots. They allow recording in low light, but they introduce noise from the camera's light-sensitive CCD chip(s) and the video amplifier circuits. Professional cameras usually have an option whether or not to boost gain in low light and can provide some control over introducing noise to a low-light shot.