Ms. Ellis wrote the article almost three years ago, when cable operators were (albeit reluctantly) facing the fact that they would have to carry HDTV, which even with MPEG-2 compression of the digital video stream requires a data rate of 19.2 megabits per second (Mbps). That compares with 3.5 Mbps for a standard-def channel.
Each cable transmission channel — I'll call it a "slot" to distinguish it from a video channel of the type we're more familiar with — has a 6-megahertz bandwidth, but cable operators can squeeze multiple video channels such as HBO or TNT into one digital cable slot. (Not so, analog slots — see below.) And there are many 6-MHz slots on the cable system.
The cable operators — the biggest, such as Comcast, are the "MSOs," for "multi-system operators" — use a technique called quadrature amplitude modulation, or QAM, to do this. Modulation is the way digital video and audio streams are piggybacked onto the carrier frequency for the slot. "QAM" rhymes with either "mom" or "ma'am."
Specifically, the method called "256 QAM" shoehorns a total data rate of 39 Mbps into one 6-MHz transmission channel. That translates to two 19.2-MHz HDTV video channels per slot.
Or in some cases, writes Ellis, "three HD channels can slip into a 256-QAM channel, depending on the source material." Translation: not all HDTV channels make full use of the 19.2 Mbps they are allotted. The reason may be that the video image they are transmitting doesn't contain a lot of detail to begin with. Or it may be that they are filtering out "excess" detail or removing it by means of overcompression.
Ellis compares the temptation to squeeze too many HDTV channels into one 6-MHz slot with
... the early days of video compression, when 24 channels of video were going to fit snugly into one 6 MHz channel ... That much snug affected picture quality, which made content creators grimace. Now, most operators don’t push more than 10 or so SD channels into one 6 MHz channel.
QAM is, as I understand it, not used to modulate the digital signal onto a carrier for over-the-air transmission, just for cable. For OTA a different method is used, called 8-VSB. "VSB" stands for vestigial sideband. It's another modulation scheme entirely.
A digital tuner in a TV or settop box may be designed to receive QAM, or it may be 8-VSB, or in some cases it may be both. Some HDTV sets contain an onboard 8-VSB tuner and also a physical slot that accepts an optional CableCard. The latter is a small card that contains a QAM tuner and allows digital cable reception without a settop box.
Ms. Ellis's entire "Translation Please" archive can be accessed here. These are columns Ellis originally wrote for the Broadband Week section of Multichannel News. Her column titled "Why 6 MHz Channels Take up 6 MHz: Part 1" sheds more light on all this.
If the entire 750-MHz bandwidth of the cable system were a shelf 750 inches long, each 6-MHz slot would occupy 6 inches. The first 54 inches (equivalent to 9 slots, by my calculation) are reserved for miscellaneous uses. Then comes the "analog shelf space," ending at inch 550. This analog-only space can hold around 75 channels. I gather there can be but one analog channel per slot.
Next, the "digital shelf space" consists of around 33 slots, each holding around 10-12 standard-def digital TV channels.
True, when you divide 750 by 6, you get 125 potential 6-MHz slots — and 9 miscellaneous + 75 analog + 33 digital only comes to 117. Ellis doesn't explain that discrepancy. But she does mention that there's really no big reason the digital space needs to be carved up into tidy 6-MHz slots at all, a bandwidth relic of analog days. Digital gurus want to know not about bandwidth in MHz, but data rate in Mbps.
In concept, the "digital shelf space" could be carved up differently, or used as one big source of bitrate. There is something in the works called "DOCSIS 3.0, also known as 'wideband DOCSIS' and 'channel bonding'," that addresses this possibility. Meanwhile, it's still 6 MHz per digital slot.
I should note, in this regard, that my local Comcast outfit has notified its customers that it's about to begin a shift to all-digital. I assume this means that at a date uncertain in the future, there will be no analog slots left. All the current analog channels — many of which are picked up by Comcast from a satellite in digital form and converted to analog — will change to digital.
That will mean every cable customer will need either a digital cable box or CableCard for each TV, I gather. No more running the cable feed directly into the TV's "cable ready" tuner.
I assume the present "analog shelf space" will gradually shrink in favor of more "digital shelf space," in this scheme. Comcast says it will be shuffling the channel lineup around as it converts. I imagine they always want to consolidate the remaining analog channels at the low end of the shelf. The space thus freed up will be converted to digital slots. And I'll bet DOCSIS 3.0 — whatever it actually may be — is providing some sort of road map for Comcast's conversion-to-all-digital process.
Ellis also says, a wee bit cryptically, "A digital channel earmarked for broadband data ... moves at 38 Mbps per 6 MHz channel, sliced according to downstream speed ceilings." I assume the "downstream speed ceiling" for HDTV is 19.2 Mbps. That's why two HDTV channels can go in one digital slot without further tweaks.
It seems intuitively obvious that eliminating the artificial and unnecessary 6-MHz chunks for digital channels, to the extent that that is possible, can do nothing but make it easier to blend more HD and SD digital channels into the one "big fat pipe" (as Ellis calls it), without compromised picture quality. If that's what DOCSIS 3.0 is about, I'm all for it.
And I'm all for cable going all-digital, even if some customers need added hardware to accommodate it. Digital channels simply look a whale of a lot better than analog channels, though they are still just standard-def.
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