SDV Explained (Part 1)

Many people have been wondering what SDV is, why it exists and how it works. Here I will try to answer these questions. First I am going to give a basic explanation of how Cable works in general. I will TRY to keep it as simple as possible, but no guarantees :)

Note: Some of the terms I use may or may not be technically accurate. This is just off the top of my head. Also, the numbers are not exact.

Back in the day there was only analog TV. In the US, analog TV channels are defined by NTSC (National Television System Committee) which says TV channels are 6Mhz wide (or was it EIA who made that decision? I can't remember). This means every 6Mhz you get a different channel. When you type in a channel number on your remote, the TV does a mapping from that number to a particular frequency, and then tunes in. This mapping was defined by NTSC/EIA/Someone and then hard-coded into the TVs.

In Cable systems, the range of transmission is roughly between 54Mhz and 864Mhz. If you take the difference and divide by 6Mhz, you get the maximum number of channels that can be transmitted on the Cable, which is 135.

So this is what we have to work with, 135 places to transmit, and with analog TV you get one TV channel per spot, so you get a max of 135 channels. Needless to say this isn't good enough. So now we come to digital Cable.

Currently most digital Cable is transmitted using a technique called QAM256. Using QAM256 a single 6Mhz slot can transmit up to 38.8Mbps (thats bits not bytes). If you take an analog channel and compress it using MPEG2 you get a result which is typically less than 3.88Mbps. This means that each 6Mhz slot which was carrying one TV channel can now carry 10+ channels. We just increased our channel capacity by 10+ times!

Now we start talking about HD channels. All of a sudden our 1350 channels just dropped to 270-405 channels (you can only get 2-3 per 6Mhz slot with HD). This is not taking into consideration the bandwidth used for Cable internet or any of the still existing analog channels. The bandwidth gets used up pretty quick.

So in order to add more HD content and increase overall internet speeds we need to free some bandwidth somewhere.

Enter Switched Digital Video (SDV).

In the past the way TV has worked is that all channels are being sent ALL the time. All channels are sent out, and the user just "tunes in" to the channel they want to watch. The issue is that this is a very bad use of bandwidth. Most of the time most of the channels aren't being watched by anyone, so why send the data!?

In an SDV system, a channel is not transmitted unless someone is watching it. When you change to a channel, your set-top-box (STB) or tuning adapter (TA) will send a signal up to the transmitter and say "hey, I want channel 105". The transmitter then finds a 6Mhz slot with enough bandwidth available to transmit the newly requested channel and then responds to the STB/TA saying "ok, it is on frequency XXX". If someone else was already watching it then the transmitter doesn't have to find a spot to put it, it just tells the STB/TA where it already is. This way you still get to share bandwidth for channels more than one person is watching, but turn off all the channels no one is watching. The transmitter will remember who is watching a channel and not turn it off if there is still at least one person watching it (not entirely true, there are some rare situations where it might turn it off even if people are watching it).

If a device, such as an OCUR, does not have bi-directional capabilities (i.e. the ability to send data to the transmitter) then it obviously can't say "hey I want channel 105". To solve this problem, the Tuning Adapter was created.

A Tuning Adapter talks to the transmitter over the coax cable. It talks to the TV receiver device (i.e. OCUR) via USB. This is why both coax and USB need to be plugged in. The TA and OCUR do not communicate via coax, so there is no requirement for the OCUR to be connected to the "output" of the TA's coax.

In the basic case, your options for connecting the TA are as follows:



This is the simple case, where the receiver is a single device, such as a Tivo, and only one TA is needed. When dealing with MOCURs it gets more complicated.

Next time I will get into details about how TAs and MOCURs work together, including how it works when they are connected through a PC's USB port.