[take this with a grain of salt, but probably] because back then one channel = one frequency. Now you have many channels muxed on to the same few frequencies and you need to wait until you've received a "channel table" until you know how to actually decode the mux.
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And my guess would be the code for changing between channels in the same mux (which should be fast because you already have the NIT table) is the same as for changing to any channel: 1) tune to frequency, 2) wait for NIT, 3) pick out the relevant data from the bitstream
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I'm going to go with "keyframes"; it's always keyframes
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The BBC used Dirac for compressed video, and to make sure there was no delay, they just made every frame a keyframe (I-frame).https://en.wikipedia.org/wiki/Dirac_%28video_compression_format%29#VC-2 …
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The question is in regard to digital terrestrial TV, I assume? Generally with digital streams you need to wait for buffers to fill, keyframes to arrive, metadata to load, and some additional stuff like HbbTV also slows things down
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so you have enough mpeg data to decode anything meaningful? (don't know really, the last time i cared about the tv were the 90's)
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Very roughly, not expert. Regarding broadcast television. In the 1990s broadcast signal was analog. Uncompressed information, one full frame at a time. Broadcast is now digital. Compressed information, 99% changes-per-frame only. But the decompression needs many frames to start.
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Digital TV codecs taking a while to fill their buffers and start delivering a signal?
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Other replies (multiplexing, digital encoding, key frames) have part of the why. The other part is that fast-channel-switching has been deliberately disregarded as a feature, presumably because addressing it would cost more: more multiplexers, decoders, buffers, power, code, etc
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But why would you WANT to switch channels with all these great ads to look at? It's a feature.
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