Difference between revisions of "DVB-S"

Revision as of 21:14, 26 December 2008

DVB-S Theory

DVB stands for Digital Video Broadcast - Satellite. At its core, DVB-S is a simple mechanism. Digital transmitters beam sets of channels to an area, and they are received by antennas aimed at the transmitter. The scope of this article is to provide basic understanding of DVB-S technology, dish pointing and peaking, and setup in MythTV. In keeping with the philosophy of the (largely US based) MythTV development and user community, no discussion of prohibited activities (civil or criminal) is welcome at any time. If some activity utilizing DVB-S is legal in your locale, bear in mind that it is likely *not* so in the United States (under whose laws MythTV must exist) and is therefore *still* unwelcome in all official MythTV support channels.

DVB-S and DVB-T (Digital Video Broadcast - Terrestrial) share in common many characteristics. They both share in common a remote transmitter broadcasting a signal through the air to an antenna, with bundles of channels (multiplexes) being transmitted by one or more transponders. In the case of terrestrial broadcasts, the need for precise aiming is less critical as the transmitters are largely omnidirectional and within short distances. In the case of satellite broadcasts, however, the antenna (dish) must be aimed at an object the size of a schoolbus at a distance of 35,000 kilometers. Precise aiming is, to say the least, critically important.

DVB-S satellites are in geosynchronous equatorial orbit. This means that they are in fixed orbital locations over a single spot on the equator. In populated portions of the world, there is a broadcast satellite at approximately every three orbital degrees. It is quite easily to mis-aim the dish slightly and get strong signal from the wrong satellite altogether.

Just as with terrestrial broadcasts, the quality and size of your antenna is directly proportional to the strength and type of signal being received. The two types of digital broadcast commonly in use today are C-Band and Ku-band. C-band is the older format of digital satellite broadcast, bringing to mind gigantic eyesore dishes. Ku-band is the format favored by consumer dish operators just as Freesat, DISH Network, and Bell ExpressVu. While it is possible to tune Ku and C-band signals with large dishes, only Ku-band signals can be tuned with the small round and elliptical "patio" dishes.

MythTV supports *only* unencrypted material, or material that is decrypted through the use of a legitimate provider-supplied CAM (Conditional Access Module). This rules our for-pay services that do not provide CAMs. Attempts to tune DISH network, DirecTV, Bell ExpressVu and similar are unwelcome in the MythTV community as those providers to no supply CAMs.

Lyngsat is an excellent reference for what satellites are in the sky, where they are located, and what channels they contain.

Equipment Needed

• DVB-S Tuner card
  • The DVB-S card must support the modulation and FEC (Forward Error Correction) used by the satellites you wish to tune. If the card does not support these specifications, you will be unable to une your channels! If your provider supplies CAMs, you will need a DVB-S card with an *on-board* (non-USB) CI slot, and the card and CI must be supported by linux.
• Satellite Antenna/Dish
  • The required size of the dish will be determined by the signals you wish to tune.
• One Low Noise Blocking Filter (LNB/LNBF) for each orbital location you wish to tune
• RG6 Coaxial cable
• Optional: A DiSEqC (Digital Satellite Equipment Control) Switch if using more than one Orbital Location/LNB without a motor

Equipment Setup

DVB-S tuner cards provide adequate voltage to power at least one LNB in most cases. Choose a location to mount the dish that is sheltered from the elements (particularly wind) and with as clear a view of possible of the satellites. In the northern hemisphere, this will be a generally southern view and in the southern hemisphere, a northern one. Remember, the satellites are in orbit over the equator.

An extremely helpful site is Dishpointer. This Google Maps mashup allows you to provide a set of coordinates or street adress, select the satellite or satellites you wish to aim, and will provide near-perfect aiming information. It is not uncommon for the aiming information to be within a single degree of peak accuracy for the location.

Mount the dish securely using a tripod, wall mount, or other sturdy means of preventing wind and other interference. Leave the dish slightly loose in the mount for the aiming procedure. Run a short length of RG6 coaxial cable from each LNB (located at the end of the dish "arm") to a DiSEqC switch. If using only one LNB, you can run the coaxial cable straight to your receiver or DVB-S card. If you wish to run multiple receivers/DVB-S cards, you will need a DiSEqC switch with an number of outputs equal to the number of cards/receivers you wish to use, and inputs equal to the number of LNBs/Orbital Locations you wish to aim at. Some DVB-S setups use a single LNB and a DiSEqC-compatible motor to reaim the dish dynamically. This is sophisticated but purely optional.

A flowchart for the simplest type of DVB-S setup would be as follows:

Receiver or DVB-S card-->Coaxial Cable-->Single LNB

When aiming more than one orbital location without using a motor, the flowchart might look similar to:

Receiver or DVB-S card-->Coaxial Cable-->DiSEqC Switch--(Multiple Outputs to..)-->Multiple LNBs

Dish Aiming

Although your provider might supply you with a general chart of aiming instructions, Dishpointer is immensely helpful in showing visual landmarks for aiming the dish. Provide the site with your location using address or Latitude/Longitude, and select the satellites you wish to aim from the drop-down. Satellite aiming consists of three "directions:" Elevation, Azimuth, and Skew. These represent the angle, direction, and rotation of your dish.

Setting Elevation

The most critical element of setting elevation is understanding that most consumer dishes are offset dishes. While you might set the elevation to 45 degrees, it will appear to you that the dish is only aimed at 23 or so degrees. The offset of the dish comes from the fact that the dish arm and LNBs are mounted offset from the center and thus the signal is bounced off from a higher angle and reflected down onto the LNBs. This means that some near-impossible aiming conditions are actually quite possible (see example right).

Dish Peaking

MythTV Setup