If the switch to digital television were to occur today instead of one year from Sunday, 13 million households would be staring at blank screens, according to a report by Nielsen research released today (Friday). Another 6 million households would have at least one set that would not be able to receive digital signals. The report suggested that African-American and Hispanic households are the least prepared for the switch -- with 17.3 percent of Hispanic households unable to receive digital signals on any set in their homes. Nielsen released the report at a client meeting in Las Vegas, warning advertisers of the possibility of a serious ratings shortfall unless a major effort is pursued to encourage viewers with analog sets who receive TV over the air to purchase converters.