By Jennifer Saba and Liana B. Baker

(Reuters) - Some of the biggest media conglomerates beat earnings expectations on Wednesday, though Time Warner Inc, Viacom Inc and Comcast Corp all found their television businesses under scrutiny in a tough advertising market.

Time Warner posted a profit that topped even the most optimistic Wall Street estimate by 4 cents, and Comcast and Viacom both beat expectations by one penny.

But all three were just shy of revenue expectations. Even though they operate a range of businesses outside of TV, investors focused on ad performance because it is a key revenue source for media companies.

"That advertising number is the first thing people look at when they look at the earnings release," said Matthew Harrigan, an analyst at Wunderlich Securities.

Viacom's stock got the strongest lift from its results, with shares rising 3.7 percent to $66.33 in afternoon trading, after touching a new multiyear high earlier in the session.

The company reported a 6 percent drop in revenue because of a weak slate of movies from its studio Paramount Pictures, but advertising revenue turned positive during the quarter.

Its cable network properties, including MTV and Nickelodeon, posted a 2 percent rise in advertising revenue in the United States.

Viacom has been struggling with a decline in TV ratings, which are the currency for commercials. The children's network Nickelodeon in particular was a drag.

"Viacom had ratings pressure for a long time, so it's significant that they have turned the corner in terms of ad growth, which is now positive instead of negative," said Brett Harriss, an analyst with Gabelli & Co.

For Time Warner, on the other hand, first-quarter ad revenue at cable properties including TNT and news channel CNN fell 1 percent.

UBS analyst John Janedis said in a note to clients he thinks it had to do with weak ad rates and what he characterized as "slow demand."

Part of the decline had to do with the timing of the 2013 NCAA college basketball tournament, a huge TV draw that partially slipped into a different quarter. But ratings also fell, with Evercore Partners estimating a decline of 4 percent for Time Warner's cable networks in the quarter.

The company said second-quarter ad revenue at its networks will be up in the high single digits in percentage terms.

Time Warner shares fell 1.1 percent to $59.12, recouping some of their earlier losses.

Comcast, which unlike the other two companies combines media properties with a cable television system, benefited more from cable than from its media unit.

The company lost more video customers than expected but gained more Internet subscribers than estimated. On the other hand, operating cash flow at NBC Universal was a negative $35 million, much worse than a year earlier.

Revenue at NBC also fell because the network did not have the lucrative Super Bowl this year, TV's premiere advertising event.

Shares rose 1.7 percent to $41.99, giving back some of their earlier gains.

(Reporting by Jennifer Saba and Liana B. Baker; Writing by Ben Berkowitz; Editing by Jeffrey Benkoe)

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