The only video recorders at the time were huge Ampex reel-to-reel machines used by television stations to rebroadcast live shows in later time zones.

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Some TV executives and analysts suggest that online video, tablet computers and wider use of digital-video recorders could be pulling prime-time viewers to other places and times that aren't counted in prime-time usage figures, although many also caution it is too early in the TV season to draw any broad conclusions.

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Mr Heilemann (who once worked for The Economist and is now at New York) and Mr Halperin (of Time) have been diligent with their tape-recorders.

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