Conversion of Analog TV Translators for DTV Operation

Abstract

There are approximately 4700 TV translators in use today across the US and it is estimated that about 7-10 Million households receive their broadcast coverage through the use of these translators. As the transition to digital occurs, it will require the conversion of the existing analog translators to digital or the purchase of new digital translators. Where possible, the FCC has indicated that it is desired to add digital translator channels to co-exist with analog translators. In cases where new channels are not available, the more economic scenario to provide digital signal coverage is to convert the existing analog translators to digital operation (assuming the desired coverage and technical performance can be met) rather than purchase new equipment. This paper examines existing analog translator block diagrams and identifies areas of significant concern and/or weak links in the reception/transmission path. Guidelines for minimum performance levels for various portions of the RF block diagram are identified in order to meet a given Signal to Noise Ratio (SNR) level at the translator output. Typical low power translators of solid state design are characterized for DTV performance using modern test equipment. The amplitude and group delay performance of an analog receiver IF filter is examined to determine whether it is suitable for 8-VSB digital signals that are more spectrum efficient. Information on expected performance from heterodyne and demodulation/re-modulation (regenerative) translator configurations is presented with respect to impact on SNR and out-of-band emissions. A two-hop translator system is analyzed to view the impact of cascading distortions and some qualitative information is presented. Specific calculations are provided for adjacent channel scenarios. Specific measurements are made assuming a situation of equal power level incoming adjacent channel interference to determine the level of energy passed through a heterodyne translator. The measured results of typical analog translators converted to digital operation are used to evaluate a hypothetical site consisting of the converted analog translator that is co-located with analog transmitters occupying both adjacent channels. Associated digital power output levels are determined using Threshold of Visibility and Audibility (TOV and TOA) limits.