Technicalities of Regional Variations
Traditionally, regional variations depend on a network or service broadcasting over multiple transmitters. Typically a 'network' feed will originate from a central location, such as BBC Television Centre, and be fed to all transmitters. Local offices or regional contractors would then be said to opt out of this feed when they switch to feeding the transmitter(s) with locally originated content and to opt in when returning to a national feed. Opt-ins and opt-outs were often quite noticeable in earlier days for causing the picture distortion such as jumping and rolling as the feed was switched; such effects are still noticeable today, though less obvious.
Whilst the BBC originated its network feed from the same place (Television Centre) ITV in earlier days would originate its feed from the broadcaster which made the programme.
Satellite services such as Sky Digital often offer regional variations by transmitting duplicate feeds of the same station for each region traditionally covered by groups of transmitters as an arguably costly way to provide regional variations within an area covered by the same satellite. Both the BBC and ITV do this, as do Channel 4 and Five for advertisements. The digital set top box will determine which version of the channel to supply based on a list of post codes corresponding to the details on the user's smart card.
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