When Does the 6 MHz Channel Go Away?
Here’s something happening in the tech background that rattles the origins of television: The undoing of the 6 MHz 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 way-back 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 channel didn’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. So, as cable lingo goes, 6 MHz is the good old wagon: Steady, reliable, fundamental.
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 standard definition, two to three high definition, goes the math, 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, and beyond it, H.265/HEVC, and if the carrying capacity for digital is measured in Megabits-per-second (Mbps), not so much MegaHertz (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. 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, they’ll be gone, and we won’t really notice the difference. But your engineers will, and they know how to get you there. They’re already doing it.
This column originally appeared in the Platforms section of Multichannel News.