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Old 11-30-2012, 01:56 PM   #31
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But SDV really only works for rarely watched channels. With popular channels it provides no bandwidth savings at all. And as more popular channels switch to HD bandwidth is going to become more of an issue. With MPEG2-2 encoding an HD channel requires about the same bandwidth as 3 SD channels. However with H.264 they can squeeze an HD channel into the same bandwidth as 1 current SD channel, and squeeze 2-3 SD channels into that same bandwidth. Which means that by switching everything to H.264 they could upgrade virtually every channel to HD without changing their current bandwidth allocations at all or impacting the user experience by allocating too many channels to SDV.

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Old 12-05-2012, 12:04 PM   #32
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Which means that by switching everything to H.264 they could upgrade virtually every channel to HD without changing their current bandwidth allocations at all or impacting the user experience by allocating too many channels to SDV.
Switching to H.264 just as H.265/HEVC starts to appear. :-)

Smart MSOs who can bide their time with full digital conversion, frequency increases, and/or SDV may skip ahead right to HEVC. That would future proof them for more 3D, 4K, and even 8K content. HEVC is starting to show up in chip samples now and it offers an improvement over H.264 similar to what H.264 offered over MPEG-2. We'll probably see initial adoption driven by mobile devices and Internet streaming services - higher quality while using less bandwidth. The same thing that drove H.264 adoption.
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Old 12-05-2012, 12:17 PM   #33
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Switching to H.264 just as H.265/HEVC starts to appear. :-)
So is that how we are going to get to 4K? Any chance OTA broadcast will go to it (or even H.264)?

Sounds like a good way to sell lots of new hardware in a few years.

Hope all my stuff lasts until it settles out.
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Old 12-05-2012, 02:18 PM   #34
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Switching to H.264 just as H.265/HEVC starts to appear. :-)

Smart MSOs who can bide their time with full digital conversion, frequency increases, and/or SDV may skip ahead right to HEVC. That would future proof them for more 3D, 4K, and even 8K content. HEVC is starting to show up in chip samples now and it offers an improvement over H.264 similar to what H.264 offered over MPEG-2. We'll probably see initial adoption driven by mobile devices and Internet streaming services - higher quality while using less bandwidth. The same thing that drove H.264 adoption.
I don't know about that. The broadcasting industry isn't really cutting edge. A big percentage of them still don't even use H.264 yet, and it's been available for nearly a decade. I don't think they're going to just jump to some unproven new standard just because of bandwidth savings. I don't know much about the specifics of H.265, but broadcasters don't even use H.264 to it's full extent yet. They have to worry about things like entry points for tuning and commerical insertion which limits some of the advantages that H.264 provides. (i.e. long GOPs and large number of reference frames) So unless H.265 provides a way to offer more compression without resorting to super long GOPs or an excessive number of B frames, it's not going to be much use to them anyway.

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Old 12-05-2012, 04:59 PM   #35
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But SDV really only works for rarely watched channels. With popular channels it provides no bandwidth savings at all.
That depends on what you eman by "popular". A typical QAM modulator in use by a majority of MSOs produces 8 QAMs, which can serve 16 HD channels. Three of these units will deliver 48 HD channels on 24 QAM carriers, using up 144 MHz, and leaving 556 or more MHz available for other content. Those 48 channels will easily cover considerably more than 99.5% of viewer selections at any given moment. Anything else is very well served by SDV. The total residual bandwidth available across an entire fairly large city with, say, 3000 nodes would be 1,668,000 MHz, or 556,000 simultaneous unique HD streams. Of course, in the real world, nowhere nearly that much content coud be handled by any CATV system at this time, and not every node is going to have a completely unique mix of content, but the potential is vast. What's more, even with a greatly attenuated nuimber of actually unique streams, with SDV , the CATV system can offer a great deal many more individual channels than they will have to deliver at any one time, making the number of available channels potentially infinite.

(Think about it. If every one of the top 48 channels had precisely the same penetration, then at the very most all of the remaining channels put together could only represent at the very most 2.04% of the number of tuners in use. The reality is #5 - #48 share at most 20% of the number of tuners in use, meaning #49+ all put together share at most 0.47% of the tuners in use.)

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And as more popular channels switch to HD bandwidth is going to become more of an issue.
First of all, all the popular channels are already HD. Secondly, the more channels involved in the mix, the better SDV handles it. Any time a particular channel is not in use on at least 1 node somewhere in the city, SDV makes the delivery of that channel more efficient. If a particular channel stands a significant chance of not being in use for some fraction of the day on a significant fraction of the nodes in town ( typically about #25 or lower in the ratings), then it will save a significant amount of bandwidth if it is deployed over SDV. The existence of "popular" channels makes SDV all that more effective, rather than posing a problem for it.

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With MPEG2-2 encoding an HD channel requires about the same bandwidth as 3 SD channels.
It's much closer to 6. The most common MPEG-II rate shaping in the industry is:

1 QAM = 12 SD, or
1 QAM = 1 HD + 6 SD, or
1 QAM = 2 HD + 1 SD

Some push it further, but tend to get complaints when they do.

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However with H.264 they can squeeze an HD channel into the same bandwidth as 1 current SD channel
Not with anything like acceptable PQ. Four per QAM will be pushing it. Three per QAM should be fine. You know very well the issues involved with excessive compression, especially in real time.

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and squeeze 2-3 SD channels into that same bandwidth.
And with SDV they can potentially squeeze several thousand channels into that same bandwidth.

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Which means that by switching everything to H.264 they could upgrade virtually every channel to HD without changing their current bandwidth allocations at all or impacting the user experience by allocating too many channels to SDV.
SDV does not impact the user experience in any significant way, certainly not as much as over-compressing the data stream, whether h.264 or MPEG-II. It would, however, critically impact every single subscriber who owns a DCR TV or a current generation DVR.

Last edited by lrhorer : 12-05-2012 at 05:12 PM.
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Old 12-05-2012, 05:23 PM   #36
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Perhaps my understanding of how SDV works is flawed. I thought that they took a pool of channels (lets say 16) and assigned them a pool of frequencies less then the number of channels (lets say 8). When a customer tunes one of those channels it is assigned to a free frequency in the pool and their box is told to tune that frequency, if another customer tunes the same channel then they are just directed to that same frequency, but if a customer tunes a different channel in the pool then it has to use another of frequencies. However if all available frequencies in the pool are actively being used then the user is presented with an error that the channel is not available.

Is that not right? If it is right then my comment about the customer experience comes from the possibility that a channel may not be available to a customer when they want it. For a pool of rarely watched channels the chances of this happening is very low, but for a pool of popular channels the chances are much higher.

Now if it worked like U-Verse where the connection to the customer was completely digital and the tuning actually happened at the head end then I could see it being virtually limitless. But I don't think that's how it works with cable.

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Old 12-06-2012, 12:19 AM   #37
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Perhaps my understanding of how SDV works is flawed. I thought that they took a pool of channels (lets say 16) and assigned them a pool of frequencies less then the number of channels (lets say 8). When a customer tunes one of those channels it is assigned to a free frequency in the pool and their box is told to tune that frequency, if another customer tunes the same channel then they are just directed to that same frequency, but if a customer tunes a different channel in the pool then it has to use another of frequencies. However if all available frequencies in the pool are actively being used then the user is presented with an error that the channel is not available.
........
I believe you have the right concept but that a practical number for the number of channels (or bandwidth) in a pool is much greater than 16, more on the order of hundreds, i.e., mulitple QAM modulators devoted to SDV.

From the Wikipedia article on Switched Digital Video:
Quote:
In current HFC [hybrid fiber-cable] systems, a fiber optic network extending from the operator's head end carries all video channels out to a fiber optic node which services any number of homes ranging from 1 to 2000. From this point, all channels are sent via coaxial cable to each of the homes. Note that only a percentage of these homes are actively watching channels at a given time. Rarely are all channels being accessed by the homes in the service group.
My TWC system has bandwidth blocks devoted to three types of video delivery (in addition to DOCSIS internet bandwidth for cable modem):
1. Analog channels (simulcast versions of basic cable channels)
2. Fixed (non-switched) digital channels, presumably the most popular ones.
3. Switched digital (SDV) channels
It carries both SD and HD versions of many digital channels and uses SDV for some channels of both types.

I rarely get the "Channel not availiable" message. It's not quite so rare to lose all SDV channels and have to reboot the TiVo and TA to get them back, but that's just a malfunction of the combined TiVo-CableCARD-TA-CATV system -- not an implicit limitation of SDV.

I don't know the actual numbers for my system but it seems reasonable to guess they may assign three 144 MHz QAM modulaor units, with a capacity of around 144 HD channels, to SDV. With one QAM unit of fixed frequencies adding another 48 "popular" channels, the total capacity is 192 HD channels. Thus (ignoring SD channels just for discussion), if the number of subscribers per node was only 192 (or less), an unbounded (i.e., "infinite") number of channels could be available to every subscriber with no one ever getting the "channel not available" sitiuation. In other words if the channel bandwidth capacity is greater than, or equal to, the number of subscribers per node, an unbounded number of channels can be handled without any denials. I assume that in reality this condition doesn't apply, so there is some small chance of denials. (In a non-switched system the bandwidth must support the total number of channels offered.)
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Old 12-06-2012, 01:09 AM   #38
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If that's the case I wonder what the holdup is in my area? They switched us to SDV like 2 years ago and yet we still have 30+ channels that are analog only and only 29 HD channels. I asked UMatter2Charter about getting more HD and they responded that even with SDV there is some limitation in my area which prevents them from adding more HD channels. They didn't go into more details so I have no idea what that limitation might be. Seems like if they just got rid of the stupid analog channels they'd have plent pf bandwidth for adding more HD even if there is some sort of frequency limiation.

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Old 12-06-2012, 01:35 AM   #39
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If that's the case I wonder what the holdup is in my area? They switched us to SDV like 2 years ago and yet we still have 30+ channels that are analog only and only 29 HD channels. I asked UMatter2Charter about getting more HD and they responded that even with SDV there is some limitation in my area which prevents them from adding more HD channels. They didn't go into more details so I have no idea what that limitation might be. Seems like if they just got rid of the stupid analog channels they'd have plent pf bandwidth for adding more HD even if there is some sort of frequency limiation.
Do you know what frequency Charter is at in your area? They've been known to run at 550MHz, 650MHz, 750MHz, 860MHz, and 1GHz. If they're digital I think it is probably at least 750MHz.

They probably still have analog channels due to legacy users. Charter in my area still has a number of analog channels too - but they're all ADS since they work in my Premiere Elite. But there are still a lot of old cable boxes and users without boxes tuning the basic channels. Charter has been really pushing them to go digital for a few years now, taking channels away, introducing new channels digital only, etc., but they haven't cut them off completely yet.

Still, we have a lot more HD channels in our area and they keep adding more. Nearly all of them, and most of the SD digital channels, are SDV. I know because when I have TA issues most of my channels go away.
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Old 12-06-2012, 01:56 AM   #40
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Our analog channels are not simulcast, so I can't tune them on my Elite and had to keep my regular Premiere just to record them.

That plus the lack of HD channels makes me think they have something seriously wrong in my area. In fact they recently added HBO back as an old school scrambled analog channel. The kind where they have to install a special filter in your box to decode it. They haven't had analog premium channels here for like 6-7 years. They seem to be moving backwards.

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