Terrestrial broadcasters are trying to capture more advertisers by coordinating their commercials — showing the same spot at the same time.Venture coordinator Arnaldo Valsecchi, who launched the campaign this week, said: “With all the channels involved, you avoid zapping. The viewer can’t interrupt the commercial by changing channels.” Such coverage is normally reserved for national political announcements, like a presidential address. But the Assn. Nacional de Television hope this will boost the weak ad market. Terrestrial TV ad spending rose 5.5% in 2000 but figures have dipped 5% this year.