Never underestimate the power of a large group of committed couch potatoes. About 90 per cent of Americans pay for television, giving them scores of channels to choose from, but four free-to-air networks they can pick up with a “rabbit ears” aerial still account for 96 of the top 100 primetime programmes.

Audience inertia and brand loyalty built over decades mean that ABC, CBS, Fox and NBC still account for 40 per cent of all primetime viewing. Their unique ability to attract mass audiences, particularly for live sport, has kept TV advertising healthy even as advertising dollars fled other media for Google and Facebook.

Broadcasters once offered signals to cable, satellite and telecoms groups for free, in exchange for perks such as prominent positions in their onscreen channel listings. In the past five years, however, their business has been transformed as they discovered they had the power to demand that distributors pay to retransmit their programming.