Grass Valley PROFILE FAMILY v.2.5 - Manual (Page 169)

Setting Compression Presets
Profile Family 169
3. For JPEG settings:
- Min Lum Q sets the minimum compression that can be applied to
luminance in a field to meet the target data rate, typically 0.75, which is
also the lowest rate.
- Max Chroma Q sets the maximum compression that can be applied to
chrominance in a video field to meet the target data rate (typically 60).
- Fixed Q sets a fixed picture quality and ignores variation in field size. Use
this mode for critical multigeneration work. When Fixed Q is selected, the
other fields become fixed as Lum Q (fixed luminance compression) and
Chroma Q (fixed chrominance Q). Useful starting values for Fixed Q and
Chroma Q are 5.00. Verify that there is sufficient available data rate
headroom before using Fixed Q mode. The Bytes per Field target
attempts to meet the Lum and Chroma goals. With Fixed Q mode, Bytes
per Field can increase beyond the system bandwidth, causing interference
with other channels.
- Bytes per Field is the target compression rate. Not every field requires
the target rate, it is simply the average rate the disk recorder tries. The
higher the rate, the better the picture detail. The actual
Bytes per Field
depends on the picture complexity, Min Lum Q , and Max Chroma Q . The
algorithm dynamically changes Min Lum Q and Max Chroma Q to make
the field size meet the target unless Fixed Q is selected.