142 Chapter 6 THE WILD WORLD OF HIGH DEFINITION wide, as measured from the middle of the rise times. The negative and positive portions are each 40 samples wide. The width of the front porch varies from system to system. A specific sync generator is required for all 720-line systems. A standard NTSC or PAL black-burst cannot be used for any 720-line format. Line Structure As mentioned earlier, 720-line video has 750 total lines, con- sisting of an active picture section surrounded by vertical blank- ing (or VANC) sections. Table 6.8 lists the interlaced line structure for 720-line systems. Table 6.8 SMPTE 296M 720p Line Structure Region Vertical blanking Active picture Vertical blanking Lines 1­25 26­745 746­750 Working with High Definition in Avid Since they first supported HD projects, Avid editing systems have been able to freely mix and match SD and HD material in a sequence as long as the frame rate of the two formats matched. Version 3.0 adds the ability to mix not just SD and HD in a sequence, but all HD formats that share a common timeline. This includes both progressive and interlaced formats. For exam- ple, that means that a single sequence can include the following formats: NTSC (29.97 fps), 1080i/59.94, 1080p/29.97, and 720p/29.97. Note, however, that you cannot mix 720p/59.94 as it has double the frame rate (59.94) as the other formats. We cer- tainly hope to see this capability appear in a future release, but we won't include it until we are able to do so in real time with a level of quality equivalent to a high-quality outboard standards converter. When working with a mixed-format sequence, you may need to switch between the different formats supported. This is accom- plished via the Project Type tab located in the Format tab of the Project window. All formats compatible with your current project format will be listed. (We'll talk more about compatible project