BS, DTV, or telecom network. When program content is multiplexed into a single channel, this adds overhead bits. Another source of overhead is forward error correction (FEC). FEC is typically added to the signal after compression but before mapping to the network transport channel. It improves end-to-end performance in terms of fewer errors in the program material caused by the network. The bit rate of the audio or video signal after compression, stated in megabits per second (Mbs), is representative of, and directly impacts the payload bitrate. From the perspective of the network, content clocking information is part of the payload it must transport. The integrity and usefulness of the clock signal is dependent on the network. It must be kept stable within the limits of the decoder's ability to lock to it and perform its decoding function. Any jitter or errors induced into the payload stream, depending on their magni- tude, can impair picture quality, sound fidelity, and validity of closed captioning or metadata included in the payload. Another consideration is use of the content after it has passed through a network. For example, it's one thing to transport content through the network and use it for simple viewing on a TV set or a computer. It's an entirely different matter to use the same content as a feed in a produc- tion operation where it will be mixed with, or inserted into another program source. Two SDTV signals making up frames of 720 X 480 pixels, clocked at 270 Mbs must be synchronized and phased to match to within 740 picoseconds on a pixel-for-pixel, frame-for-frame basis, otherwise image impairment or outright loss of picture occurs. Similar ground rules apply to digital audio. For example, the ordinary CD source runs playback clocking at 44.1KkHz. Most professional audio systems and consoles run at 48KkHz. Using material from a CD without digital analog conversion requires re-clock and synchroniza- tion of the digital signal from the CD. Another issue, or potential concern, is synchronization of 