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Stumped 3 CS professors howabout u?

W

wacki

Senior member
I asked three guys with a Ph.D. in computer science and none of them knew the answer. How does "On DemandTV" get all of it's bandwidth out of copper. With digital cable, I get well over 300 channels and I can select movies from a list of 4-500 and watch them at will. You can pause, rewind, and fast forward as you please. In my home we have 2 TV's that have On Demand TV, and my parent's have three TV's with on demand. If so much as a thousand people, or 500 with 2 TV's in a town use On DemandTV that could mean 1,500 channels, plus regular broadband internet would be transitted across the coax cable in one town. At 11.6 Mbs per channel, regular non-digital cable, digital cable, broadband internet, and every On DemandTV in the area requesting a movie, I don't see how they squeeze that much bandwidth over the coax. Seriously, if coax is that good why are we using Cat 5? But at the same time the stats I see online say coax isn't that powerfull. I just don't get it.
 
Compression? Cable Box decodes the info?

This sounds more like a computer engineer problem than a CS major problem. Good question though. I too would like to know the answer.
 
I think they count on not everyone using it at once. They probably ran some statistical analysis to see the average load of the system so they figured they could support it.

Also, not every digital channel uses the same bandwidth, they can compress some channels more than others..ie they will really compress channels that aren't that popular so that they can fit more channels or give more bandwidth to more popular channels.

Also, the cable providers have a large analog frequency range that they can ride their digital data on top of. CAT5 doesn't ride their digital data ontop of analog frequencies. It's just pulses of electrons.

You try to ask these questions to EE profs that work in the field of communcations, not CS majors....
 
so, you believe that there's only 1 coax cable coming out of the cable company?
I don't know what their exact distribution system is, but imagine they have 2 cables coming out.... many of the channels would be identical, but one half of the town get's one, and the other half gets the other cable. We've just knocked it down from 1500 to 750...

Also, I'd bet that their distribution system is based on reasonable estimates for peak useage, plus a little extra as a safety margin. Thus, I doubt that they could handle it if everyone had an on demand movie going at the same moment. Same thing happens with other types of distribution systems, such as electricity. If every house with 200 amp service was using all 200 amps, etc., there wouldn't be enough power to go around.

(edit: nonetheless, I'm amazed that they're able to provide this programming)
 
Each channel is allocated a frequency range in which they operate on. Your receiver does nothing more than "tune" into the frequency range of interest. Everyone who subscribes to cable get the same signal... your receiver is the one that does all the work.

Edit: For example, lets say you have 10 audio channels that you want to send out through a coax cable. Well, in order to squeeze all 10 channels through a single wire, you will need to modulate each channel on its own frequency. Because audio is band-limited to 22Khz, you need to sample at 44khz to prevent aliasing. If you modulate that to a higher frequency, then the bandwidth required for transmission becomes 88Khz, or roughly 100khz.

Let's say channel 1 gets modulated with a 1Mhz signal.. then its transmission frequency range is 900khz - 1.1Mhz, channel 2 gets modulated with 1.2Mhz and operates on 1.1Mhz - 1.3Mhz.. and so on.

They all get superimposed on each other and transmitted. On the receiving end, there is a bandpass filter that removes all frequencies except for the one of interest. The data gets de-modulated and recovered.

Sheesh, people with Phd these days... !
 
Originally posted by: blahblah99
Each channel is allocated a frequency range in which they operate on. Your receiver does nothing more than "tune" into the frequency range of interest. Everyone who subscribes to cable get the same signal... your receiver is the one that does all the work.

Edit: For example, lets say you have 10 audio channels that you want to send out through a coax cable. Well, in order to squeeze all 10 channels through a single wire, you will need to modulate each channel on its own frequency. Because audio is band-limited to 22Khz, you need to sample at 44khz to prevent aliasing. If you modulate that to a higher frequency, then the bandwidth required for transmission becomes 88Khz, or roughly 100khz.

Let's say channel 1 gets modulated with a 1Mhz signal.. then its transmission frequency range is 900khz - 1.1Mhz, channel 2 gets modulated with 1.2Mhz and operates on 1.1Mhz - 1.3Mhz.. and so on.

They all get superimposed on each other and transmitted. On the receiving end, there is a bandpass filter that removes all frequencies except for the one of interest. The data gets de-modulated and recovered.

Sheesh, people with Phd these days... !
Click to expand...


To defend the Ph.D's, that wasn't the question that I asked, at all. Your talking cable TV in general. I'm talking about Digital Cable, specifically On Demand TV. According to this linkAnd if I'm reading it right, Coaxial cable RG-6 can carry 270 Mb/s for a distance of 415 meters. Since digital cable requires 11.6 Mbs per channel or On Demand TV, I do not see how RG-6 can carry 300 channels let alone On Demand TV to every house that has it.

I understand the whole concept of peak time periods and relay stations (called proxy's and repeaters for internet junkies), but if you look at the number of relay stations you local cable office has, and the number of people per relay, the numbers don't add up. And you can't say they have cable that is super fast now, because the cable that leads to my parents house and entire neighborhood was laid down over 20 years ago and hasn't been dug up since. So were talking about 20 year old technology delivering all this bandwidth. Before you start talking about superimposing frequencies, look up the frequency that digital cable runs at, look at the distance this travels before noise degrades the signal, the number of relays and repeaters then run the numbers. The compression, I do understand, and I don't doubt they use it, but when I watch movies on the High Definition 52" bigscreen they look like DVD quality, so they aren't using very lossy compression, if at all(lossy). DVD's use compression, but they are still 7-14 gigabytes! If not more for movies like Lord of the Rings. Run the numbers and you'll understand, there is something else.

Even with relays, lossy compression (which doesn't look lossy at all in this case on a 52"), time sharing, and so forth, it still doesn't make sense by the numbers that I have.
 
Lossy compression. Proxy cache cascades. Image quality scaling by load.

Don't ask a CS, ask a pro in Communications Technology.
 
Originally posted by: Peter
Lossy compression. Proxy cache cascades. Image quality scaling by load.

Don't ask a CS, ask a pro in Communications Technology.
Click to expand...


Yes, I and the guys I talked to are all well aware of all of these. Does anyone have any statistics that say the ones I have are wrong? RG-6 cable doesn't seem like it could carry all of that even with the tricks everyone has said here.

Links? Statistics?

Info

Remember cable has been in ground for 20 years
 
And if I'm reading it right, Coaxial cable RG-6 can carry 270 Mb/s for a distance of 415 meters. Since digital cable requires 11.6 Mbs per channel or On Demand TV, I do not see how RG-6 can carry 300 channels let alone On Demand TV to every house that has it.
Click to expand...
Now, I'm not an EE or in the communications biz or anything, so I might be wrong. But I would assume that being digital, and especially being 'On Demand', would mean that only one channel would be transmitted to each reciever. This would mean each cable could support about 200 households. I'm sure the cable company has tens or hundreds of cables (depending on the size of their distribution area) leading out from the source and feeding the more local neighborhoods. With repeaters, compression, etc., I don't really see the issue.

But I'm a Mechanical Engineer. What do I know 🙂
 
Perhaps they are using the coax cable as a bi-directional communication line where the receiver sends commands to a base station to tell it what channel to put on the cable.

They might also be using some sort of proprietary communication/data transfer protocol, which is why not much is known about how it works.
 
Digital Cable TV is compressed and frequency modulated according to this:

http://www.ncta.com/images/HDTVKit-Glossary-final2.pdf

ATSC cable television standards:
http://www.scte.org/documents/pdf/ANSISCTE072000DVS031.pdf
page: 21 table 3
256QAM - 54Mhz to 860Mhz range, 6Mhz channel bands, approx 40Mbps per band,

CableLabs Video on Demand Specs:
http://www.cablelabs.com/projects/metadata/downloads/specs/MD-SP-VOD-Content1.1-I02-030415.pdf
page 26 specifies a max bitrate of 3.75Mbps for a video on demand stream

Some more video on demand resources:
http://www.cs.tut.fi/tlt/stuff/vod/VoDOverview/vod.html

Seems to say that backbone is made of fiber anyway with higher bandwidth. Distribution is not broadcast across the entire cable network but more of a point to point approach where cotent is delivered to a specific user from the VoD server. This would make sense since you can use coax for data anyway, just treat every set top box as a node on a network, maybe with an unique ID (IP?).

A search on ACM's digital library portal and IEEE online library turns up a lot of research into Video on Demand research. I am assuming these ideas can also be applied to cable television technology. Some articles on ACM may be free. IEEE is not afaik.
 
As far as I can tell you only use one onDemand channel at a time. So you would only need bandwidth for one channel (or 2 if you have 2 cable boxes). All it does is request a movie from the cable Co's server and buffer it to your box. And the quality is not that high, I can tell a significant difference in watching real TV vs. OnDemand on my LCD projector (about a 60" image). I don't know if I answered your question, but I feel better.

Masul
 
Originally posted by: acx
Digital Cable TV is compressed and frequency modulated according to this:

http://www.ncta.com/images/HDTVKit-Glossary-final2.pdf

ATSC cable television standards:
http://www.scte.org/documents/pdf/ANSISCTE072000DVS031.pdf
page: 21 table 3
256QAM - 54Mhz to 860Mhz range, 6Mhz channel bands, approx 40Mbps per band,

CableLabs Video on Demand Specs:
http://www.cablelabs.com/projects/metadata/downloads/specs/MD-SP-VOD-Content1.1-I02-030415.pdf
page 26 specifies a max bitrate of 3.75Mbps for a video on demand stream

Some more video on demand resources:
http://www.cs.tut.fi/tlt/stuff/vod/VoDOverview/vod.html

Seems to say that backbone is made of fiber anyway with higher bandwidth. Distribution is not broadcast across the entire cable network but more of a point to point approach where cotent is delivered to a specific user from the VoD server. This would make sense since you can use coax for data anyway, just treat every set top box as a node on a network, maybe with an unique ID (IP?).

A search on ACM's digital library portal and IEEE online library turns up a lot of research into Video on Demand research. I am assuming these ideas can also be applied to cable television technology. Some articles on ACM may be free. IEEE is not afaik.
Click to expand...



WOW!!! This is exactly what I was looking for! Thank you very much. How did you find this stuff anyway? I tried everything I could think of on Google.

The links you provided were very helpfull. Although they did fill in alot of holes, I do not think my area follows those standards. I know other people that get On Demand TV and their quality of service seems to reflect these figures. I live in the more well to do part of town, and the service I get is different. I do not get the 1/2 to 3/4 resolution for my On Demand TV as the links stated, I recieve DVD quality video. It's easy to confirm this with one look at the bigscreen, or with the Vivo on my computer providing statistics. However, after a talk with a friend in the communications business, it turns out that there is a fiber station a little over a mile away from my house. From there the signal gets transmitted through 20 year old coaxial cable to all the surrounding houses, including mine. The approximity of the fiber station answers some questions, as well as the links you provided. I'm still curious as to how they squeeze all that info into 20 year old technology, but the majority of my questions have been answered and I think I will be able to fill in the rest of the holes myself. Thank you very much acx.
 
Well I can shed some light on it for you. First they count on only a certain number of users. Second, eventhough your line is cable, the Main line is all Fiber (Much more bandwidth). Also I know that even the line to your house can easily handle 30-50Mbits of traffic (as per a cable guy).
 
HDTV Digital signal Max = 19Mbit/s
256QAM max bitrate @ 6Mhz= 38.8Mbit/s
so two HDTV streams can fit uncompressed in 6Mhz (more if you use compression or if the HDTV signals do not use the max bitrate).

Standard Coax cable can run frequencies above and beyond 750MHz. But let's just keep it at 750MHz to be simple.

The coax coming into your house therefore can carry many (over a 100) 6MHz channels. Each carrying up to 38.8Mbit/s.

A standard digital channel, not HDTV ranges from 2Mbit/s to 8Mbit/s depending on content. For regular movies, 3-4Mbit/s will produce a very good picture.

This gives us the ability to put 10 regular definition channels into a 256QAM 6Mhz channel.

For Video On Demand, the city is split into nodes of about 300 digital customers each. Each node is sent it?s own cable feed with it?s own unique channels. We know that only about 10-15% of customers will be using VOD at any one time. So we only have to send 10-15% of 300 streams = 30-45 streams. These can fit into 1 or 2 6Mhz 256QAM channels.

So in reality, we really only have to allocate 2 or 3 (OK maybe 4 or 5 to be safe) 6MHz channels to VOD. Since the rest of the channels on the cable plant are not ?on demand?, they do not have to be unique to that node. Only the VOD channels need to be unique.

As for 20 year old cable handling all that bandwidth? Yes it can. Sometimes you?ll get more signal loss (signal won?t go as far) but for the most part it works. If it doesn?t, they replace it. The major hurdle for the high frequency stuff are the amplifiers on the old lines, they have to be completely removed and replaced with new models.
 
I read through this post and haven't seen anyone mention a major point of "On DemandTV". Despite what Cable operators and Satellite providers make you think, PPV's(Movies, Special Events, Promos, Etc.) are broadcast to every home. The exact same is said for other pay channels(HBO, CineMAX, Showtime, Etc.). When you purchase a PPV to watch you'll notice its on a specific channel(s) and it repeats over and over. You'll also notice the movie doesn't start when you order, sometimes it will be in progress or waiting to start. Therefore the bandwidth is for 30-60 channels of PPVs. The tuner just sends out for permission to view these channels, a charge is credited to your account, your tuner starts displaying the PPV channel as requested. Anybody who uses a hotbox on cable or programs his own Satellite cards can attest to this.

Digital Cable uses the same compression technology as Satellite as DVD's; MPEG2. DVDs use a 12mb/s VBR. As for the exact MPEG2 bitrate TV providers use, you have to check with them. But keep in mind many people who own DVD writers can copy DVD9 discs onto the standard writeable 4.7gb by reducing the bitrate a little on the original film. Both look quite good. You would be at quite a challenge to tell which movie was recorded at 12mb/s VBR.
 
When you purchase a PPV to watch you'll notice its on a specific channel(s) and it repeats over and over.
Click to expand...

That's just regular PPV, not on-demand.

My local provider has an on-demand system called "i-Command" which lets you view any movie playing on HBO, Cinemax, etc. for that month at any time. If I want to watch a movie at 3:48 AM, I select it from the menu and it starts immediately. I can then fast forward, rewind, and pause the movie whenever I want.
 
On demand pretty much functions as a huge PVR. your box asks for a stream, the headend VOD equipment finds the movie on it's hard drive array and streams it out to the node you live on.
 
Originally posted by: blahblah99
Perhaps they are using the coax cable as a bi-directional communication line where the receiver sends commands to a base station to tell it what channel to put on the cable.

They might also be using some sort of proprietary communication/data transfer protocol, which is why not much is known about how it works.
Click to expand...


Something along these lines would be my guess. Kinda like mainframe computing
 
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