e quality and sound fidelity take a hit and accumulate as compression and de-compression cycles occur, the issue becomes how to deal with it on an end-to-end basis. The general approach is to define and quantify the level of picture quality and sound fidelity required at delivery. Once determined, this can drive compression configuration through the food chain based on a trade- off of quality VS. bandwidth cost. Higher payload bitrates require more transport bandwidth, driving transport media cost higher. One final point, which is a source of confusion among broadcasters and network people, has to do with variable bitrate program content and variable bitrate transmission channels. First of all, there are two basic encoding techniques. One fixes the amount of bandwidth used and allows picture quality and sound fidelity to suffer. The other maintains quality and allows the amount of bandwidth to be what- ever it turns out to be. On the transmission channel side, time division multiplexing (TDM) channels such as ATSC A-53, or an E3 or DS3 are rock solid, fixed bandwidth transmission channels, sometime called pipes. Of the five classes of ATM transmission, only constant bitrate (CBR) is fixed. ATM variable bitrate real time (VBR-rt) may or may not be satisfac- tory for fixed or variable rate program content. Most likely it won't. Here's why: VBR-rt channel characteristics are defined in terms of peak cell rate and sustained cell rate (SCR). Carrying fixed rate content on a VBR-rt channel requires SCR to meet or slightly exceed the payload bitrate. Any less bandwidth results in errors in transmis- 1 BASIC FUNDAMENTALS, DEFINITIONS, AND KEY TERMS sion. Carrying variable rate content in variable rate transmission channels requires the peaks in both to match, that is, the peak trans- mission channel bandwidth must match and be available when the peak payload requires it, or errors in transmission occur. There is no known mechanism for synchronizing payload content with network transmission under these conditions. Thus th