Despite a strong first start to the year, terrestrial broadcaster ITV has warned that it is facing tougher comparatives in the current quarter, meaning that the firm could underperform the market in the first half. Nevertheless, revenues for the three months to 31 March grew 11% to £500m, from £450m last year, driven by a 12% increase television advertising, ahead of the market which is up 10%.The group said it has expected the ad market in the second quarter to be difficult, due to a tough comparators from the World Cup last June."As we anticipated...we may slightly underperform the market in the first half. While we expect to outperform the market across the whole of the year, we remain cautious about the economic outlook," said chief executive Adam Crozier.TV net advertising revenue grew by 6% in April, but is expected to be down in May and June.During the first quarter, ITV1's share of viewing increased by 3%, while the digital channels rose by 11%.Net debt increased by 5% from £188m to £197m, as a result of the "timing of tax and pension payments offsetting underlying cash generation," the group said.