Coming up fast on the rails from nowhere in the latest American TV 'sweeps' is outsider CBS - seemingly set for a convincing victory after decades in the doldrums.

The sweeps, something of a mystery to those beyond US borders, take place four times a year - November, February, May and July. During these periods, in which audience sizes are measured to determine future ad rates, the networks stuff their schedules with crowd-pleasing fare in the hope of maximising the inflow of future dollars.

The term 'sweeps' emerged fifty years ago when audience measurement service, ACNielsen, began distributing TV log-diaries to participating homes. The exercise started in the eastern states before "sweeping" westward to the Pacific.

With most of the data already in, it seems certain CBS will snatch the coveted 'most watched' position in the key 18-49 age group - the first time it has done so since 1980.

Understandably over the Moonves is Leslie of that ilk, chairman of CBS and co-president of its parent Viacom: "We're about to accomplish something many thought was impossible," he crows.

CBS is poised to whup its nearest rivals in the critical 18-49 adult demographic by five-tenths of a ratings point. And although that's a fraction of a hairsbreadth to mere mortals, it's seen by Moonves as a "huge margin". One that in this case could rake in many millions more of ad dollars.

But for every winner there's a loser. In this case, incumbent champ NBC which has ruled the 18-49 roost for almost as long as CBS has languished in the doldrums. The General Electric-owned network has for long owed its supreme position to hit series such as Law & Order, now in senile decline. Meantime, replacements such as Hawaii have failed as yet to bust the block.

Moonves predicts CBS's current ratings performance, which is 7% up year-on-year, will translate into an immediate price increase for 'scatter' - last minute ad time purchases.

And looking to the May 2005 'upfront' market, when the bulk of TV advertising is sold, the CBS chairman is confident that "there definitely will be more money coming into us."

Data sourced from Wall Street Journal Online; additional content by WARC staff