lated to a specific frequency slot, where it is then filtered out from the rest of the channels. Varying the oscillator frequency will Parameter Value therefore select different channels. The attraction of the technique is that it deals directly with the available signal without demulti- Linear K ratings plexing; its limitation is the relatively large size of equipment. distortions Other approaches not yet used in cable TV but of potential interest Echoes in the future are: Chrominance/luminance gain inequality 1. Time switching (i.e. moving from one time slot to another Chrominance/luminance 150ns within a digital multiplex). 2. Packet switching, (useful if variable bit rate coding is applied delay inequality taking account of the fact that changes in picture content vary Non-linear Differential gain from time to time). distortions 3. Optical switching, which can itself take the form of space Differential phase 24 degrees switches, frequency (wavelength) switches, or time switches. Luminance line-time non-linearity 57.3.6 Performance standards Chrominance/luminance crosstalk National standards for cable TV are set in many countries, though Noise/ they may only be guidelines rather than mandatory. Although Signal/noise (unified interference broadly in agreement on acceptable levels, there are some variations weighting) between countries, in particular due to the different TV standards Single frequency interference 57dB at < 1MHz (NTSC, PAL, SECAM). Further it may be possible to trade one Crosstalk (undistorted) parameter off against another, as explained below. Digital video coding 57/23 allow for the eye's varying sensitivity to noise according to its can advanced TV (probably now compatible with 6MHz spacing), position in the video signal's spectrum. Also the equivalent car- and the European MAC family (ranging from 7MHz to 16MHz rier/noise figure for VSB-AM signal is 45.6dB. Within reason imps spacing needed according to type and implementation). can be added up, and the total for a good network might be no mo