When Does the 6-MHz Channel Disappear?
Here’s something happening in the tech background that rattles the origins of television: The undoing of the 6-Megahertz channel spacing, common to broadcast and cable television since the 1940s.
What’s going on? Progress, in the form of advanced modulation and distribution techniques (here’s that migration to IP again) seeking to wring every literal bit of capacity on communications networks.
But first, a little background. Why are video channels sized at 6 MHz, anyway? Set the Wayback Machine to 1941, when the National Television Systems Committee (NTSC) made a plan for black-and-white TV channel distribution. (Color was added in 12 years later.)
The NTSC’s work at the time was to define how much bandwidth it would take to move broadcast TV from stations to homes. Answer: 4.2 MHz,with an extra 1.8 MHz for modulation (the process of imprinting a video channel onto the carrier that moves it) and guard band (so info from one TV channeldidn’t smear into any adjacent channels).
And here we are, seven decades later, still using “size-six” channel widths. For that reason, 6 MHz is to the video engineer what the inch is to the carpenter: an enormously familiar, tried-and-true unit of measure.
Then, digital happened. Video channels, after being digitized, could be squished down — hello, compression — such that many more fit in the space of the original analog channel width. Ten to 12 in standard definition, two to three in high definition, for video compressed with MPEG-2.
That made the thinking change. If all channels are available digitally, and can be compressed not just with MPEG-2, but with MPEG-4, andbeyond it, H.265/HEVC, and if the carrying capacity for digital is measured in Megabits per second (Mbps) and not so much MHz, is it still relevant to think in those old-fashioned, analog, size-six chunks?
Probably not, but don’t watch for any weird flash-cut to erase 6-MHz spacing. Nor is it likely to anticipate a different official sizing — the 3-MHz channel or the 1-MHz channel.
Instead, and as traditional 6-MHz channels get bonded together to make larger passageways for IP-based services, we’ll wind up with several really big “channels” — 24 MHz, 48 MHz, and so on — with differing “service flows” of voice, video and data running inside them. What’s the ultimate big-size channel? Depends on the upper spectral boundary, but upwards of 700 MHz, anyway.
When does this happen? As with the transition to all-IP, the forces driving it will continue pushing and pulling — until one day, 6 MHz will be gone and we won’t really notice the difference.
Stumped by gibberish? Visit Leslie Ellis at www.translation-please.com or multichannel.com/blog.
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