How does switched digital video work

To understand how SDV works, we'll first need a quick lesson on how cable companies traditionally provide service to customers.

In older cable systems, the cable company broadcasts every channel in a single stream of programming to every section of its network all the time. When you use your set-top box STB to tune to a particular channel, the STB searches the stream for the channel's frequency and carries the signal to your television. Because the cable company sends every channel through the entire network, there's not much spare bandwidth available for Internet service or digital video channels.

High definition channels take up more bandwidth than normal digital video, so spare bandwidth decreases as cable companies add HD channels to their lineups. Because HDTVs have become more affordable and more households have acquired them, demand for HD programming has increased. Cable companies have to find ways to meet customer expectations, particularly in light of the fact that HD television providers already offer dozens of channels in HD format.

Switched digital video uses a different delivery system. Instead of combining all channels into one programming stream throughout the network, the cable company selects only the most popular channels for a network-wide stream.

For less popular programming, the company responds to individual customer demands as the customer tunes in to that channel. In other words, the service provider sends only the channels customers are actually trying to watch.

Because the system only sends customer-demanded channels, there's a lot of spare bandwidth left over for other services. According to IneoQuest, a company that offers SDV solutions to cable companies, around 80 percent of all viewers in a given area are watching the same top 20 channels [source: Switched Digital Video Solutions ]. Providing only the requested channel feeds would free up enough bandwidth for the cable company to:.

In this article, we'll examine the architecture and equipment used in an SDV system, and we'll learn about the potential advantages and disadvantages of SDV. We'll also look at some companies and markets that are already using SDV. Aside from bells and whistles like video on demand, digital phone and Internet, a cable company's mission is pretty simple and straightfoward.

This video from HowStuffWorks explains the basics of cable television. There is no standardized way to set up an SDV system, but every system can be categorized into four sections: the headend , the transport system , the access system and the customer network. The headend section of SDV architecture is where the video and Internet feed sources enter the system. It includes the equipment directly connected to the cable company:.

Some elements of the headend only flow one way into the system, such as the cable company's video feed. Others, like the applications servers, communicate back and forth with the network to ensure that everything is running smoothly. Once the headend converts the video feed into MPEG format and encrypts it, it sends the signal on to the transport system of the SDV architecture.

The transport system's purpose is to route the video and Internet feeds from the cable company to the access system. This section consists of nodes , points where cable connections intersect and branch off, and routers.

Nodes and routers redistribute the signal to other nodes and routers so that the original feed covers the cable company's entire customer base. The transport system's path connects the headend to the access system. The access system is where the actual digital switching takes place. The core of the access system is the SDV server.

It's the SDV server's job to keep track of customers' channel change requests. SDV servers are dedicated computers that run software designed to interpret each channel request.

The customer network includes a set-top box that receives and decrypts signals from the access system and a cable modem if the customer subscribes to cable Internet service. Each company uses a different interface between the customer network and the access system.

QAM stands for quadrature amplitude modulation, a frequency division multiplexing technique. The technique allows cable companies to transmit digital signals more efficiently by using a degree phase modulation to send multiple signals across the same line. Switched digital video systems' most obvious advantage is that they address one of the biggest problems cable companies have -- running out of bandwidth.

An SDV system helps cable companies implement a solution without replacing miles of cable, since the system can work on top of an existing infrastructure. This is what you need to do:.

In this example, the cable provider is broadcasting every channel in the lineup at the same time, consuming all the bandwidth, even though the customers in the area are only tuning to a small subset of the channels. In this example, the cable company is sending only the channels the customers are currently tuning to.

Notice that bandwidth is now available for additional channels. Before you can view or record a program on an SDV channel, your box must be able to send channel requests back to the cable headend. A Tuning Adapter is a set top box that provides two-way capabilities, so your box can request SDV channels. Series2 and Series2 DT boxes are unaffected because they can use a cable box to provide two-way communications. There are two Tuning Adapter brands, Motorola and Cisco.

The Tuning Adapter will not have to be customized for the box, however; like a cable box, the installer can simply connect the Tuning Adapter and it is ready to go. Tuning Adapters are in the process of being field tested by major cable providers, and this testing must be completed before these devices are rolled out to the general public.

Some cable providers plan to send installers to connect the Tuning Adapters. Some will allow their customers to pick up the Tuning Adapters from a local office and install them themselves.

If switched digital video is already in your area, or is about to roll out to your area, you will receive a letter from your cable provider telling you what options are available. CableCARDs are still required to decrypt encrypted programming and to decode copy-protected channels. Some will send an installer to connect the Tuning Adapter and either swap out the CableCARD s or call in to get the existing ones reauthorized.

Still others may offer multiple installation options. Your cable provider will give you complete information when they roll out Tuning Adapters in your area. If the Tuning Adapter is not operational because it is disconnected, powered off, in standby mode, or is getting a firmware update, the box uses the channel map on the CableCARD to tune to your channels.

During these times, you will not be able to view or record programming on SDV channels. The RF cable from the wall to the Tuning Adapter to the box carries in-band programming the shows you watch and out-of-band communications, including channel maps for the Tuning Adapter and CableCARD s.

If the Tuning Adapter is disconnected, in standby mode, or powered off, the box still receives the RF signals. If the USB cable is disconnected, you will see this message. When you press Select to continue, the box will begin to use the CableCARD channel map s for all new tuning operations.

You will not be able to view SDV channels. When you reconnect the Tuning Adapter, you will see the following message. Press Select to continue. You will now be able to access all your channels, including SDV channels. If the channel is not currently broadcasting to your neighborhood, an SDV server at the headend sets the channel up on one of the unused frequencies and sends the programming to you.

The time to set up the channel and send the tuning information to the box is so short that you may not be able to tell the difference between this operation and a conventional channel tune. As discussed above a Digital Headend stream all the tv services into the different streams and then transmit the signal in the field.

It means that it is transmitting all the tv services simultaneously in the field using Optical Fiber Cable and Coaxial cable.

Again it means that it is consuming the bandwidth of all channels weather subscribers are viewing all channels or not.

In other words we need to transmit all the tv services to each and every Subscriber even he watch it not. So it consume more bandwidth. There is wastage of bandwidth for the channels that are not watched by subscribers. Thus SDV is a service which delivers the requested channels by the subscribers.

In this manner SDV also save the bandwidth of cable tv. As shown above in the figure a request for a particular channel is made by the subscriber by using the STB or Set Top Box. This request goes into the SDV Server. These parameter are tuned by the STB. This enables the STB to show the request channel to the subscriber.

In other words there is a tuning software installed in the STB.