An NBC series that has been one of the network’s few bright spots in the ratings in recent seasons sank to a historic low Thursday night, according to Nielsen overnights. TVbytheNumbers.com reports that “America’s Got Talent” fell to a 1.8 average rating in the key demo of adults 18-49 — down 18% from the previous results episode and an all-time low for a non-recap episode.

Even so, “Talent,” airing at 8 p.m., gave NBC its best number in the time period since March 15. And NBC came out on top for prime time overall, thanks in part to its coverage of the Democratic National Convention. The report notes that the live nature of the convention coverage means figures will be subject to more than the usual adjustments.

Convention coverage in the 10 p.m. hour, when NBC, ABC and CBS all covered the event, saw NBC come out on top with a 2.5 average in the 18-49 demo (with 8.03 million total viewers), well ahead of ABC with a 1.0 (and 3.67 million total viewers) and CBS with a 0.9 (and 3.03 million viewers), all in preliminary numbers.

Elsewhere on the broadcast dial, much of the programming was repeats, but a fresh episode of CBS’s “Big Brother” did a 2.2 in 18-49, while a new installment of ABC’s “Wipeout” settled for a 1.3 (18% higher than last week’s series low).

For prime time overall, NBC came in on top with a 2.0 average rating in viewers 18-49, followed by CBS (1.6 average), Univision (1.3), ABC (1.1) and Fox (0.7) NBC also claimed the top spot in total viewers with 7.2 million, ahead of CBS (5.3 million), ABC (4.3 million) and Fox (1.8 million).