Analysts expect CBS (NYSE: CBS) to release what is likely to be the first of a string of poor quarterly earnings reports on Tuesday, as the broadcaster’s over-reliance on ad revenues will make it difficult to weather an economic downturn, Fortune reports.

To put the situation in context, Merrill Lynch media analyst Jessica Reif Cohen is predicting a 6 percent downturn in ad spending this year overall. With 72 percent of its 2007 revenue derived from advertising, CBS is likely to face some trouble over the next year, even with the cushion provided by ad-related spending pouring in from the summer Olympics and the national political elections. Now consider the contrast posed by other large-cap entertainment companies: News Corp (NYSE: NWS). (44 percent), Viacom (NYSE: VIA) (35 percent), Disney (NYSE: DIS) (23 percent) and Time Warner (NYSE: TWX) (19 percent). That’s why analysts like Zacks Investment Research anticipates CBS’s Q4 profits to come in at $3.74 billion versus $3.88 billion a year ago, a slide of 3.6 percent.

Aside from benefiting from cable subscription revenue, CBS also lacks the ability to tap foreign markets. To be sure, the comparison seems a bit unfair, given CBS’s profile as a broadcaster. But it does highlight the weakness of old-line media corporations versus more diversified companies as the media landscape and economy are set to experience greater turmoil.