tandard television systems. Systems labeled as HDTV typically offer at least 1,000 lines of resolution and an aspect ratio of 16:9. header A string of bits in a coded bit stream that is used to convey information about the data that follows. head-end The originating point in a cable television system where all the television and telecommunication signals are assembled for transmission over a broadband cable system to the subscribers. Signals may be generated from studios, received from satellites or conveyed via landline or microwave radio trunks. Head-end sites can be located in the same building as the cable operator's business headquarters or they can be sited close to satellite receiving dishes. They are generally equipped with amplifiers or signal regenerators. hertz Hz. The unit of measurement used in analog or frequency-based networks named after the German physicist Heinrich Hertz. One hertz equals one cycle- per-second. high frequency Electromagnetic waves between 3 and 30 MHz. high-pass filter A device that attenuates the frequencies below a particular cut-off point, a high-pass filter is useful in removing extraneous sounds such as hum and microphone boom. holography The recording and representation of three- dimensional objects by interference patterns formed between a beam of coherent light and its refraction from the subject. The hologram is the holographic equivalent of a phonograph. A three-dimensional image is stored which, if broken apart, can still reconstruct a whole image, though at reduced definition. Holograms require laser light for their Video Communication: the Whole Picture creation and, in many cases, for their viewing. horizontal H. In television signals, the horizontal line of video information that is controlled by a horizontal synch pulse. horizontal blanking The period of time during which an electron gun interval shuts off to return from the right side of a monitor or TV screen to the left side in order to start painting a new line of video. horizontal resolution Detail expressed