Digital Video Compression - MPEG2 MPEG4 Difference
MPEG Stands for "Moving Picture Experts Group." The MPEG group develops standards for digital audio and video compression and their aim is to develop more efficient ways to digitally compress and store audio and video files. MPEG2 and MPEG4 are their two most popular compression technologies for Digital TV bandwidth reduction.
As explained in our DTH Basics page, the amount of data is greatly reduced when the TV picture is split into two parts. One part containing the fixed portions of the picture (called an 'I'-frame) and the other the moving portions of the picture (called a 'P'-frame). The fixed portions are transmitted only once till there are some movements or changes in that portion of the frame. The moving portions are transmitted at the same frame rate as the TV picture. The compressed TV signals are recombined by the satellite decoder STB. The single non-moving part ('i'-frame) is added to each of the moving frames ('P' frame) and the complete frames are sent to the TV by the satellite decoder STB. This method of compression drastically decreases the amount of data to be transmitted by the Satellite transponder. When recombined by the STB satellite receiver or decoder, there is very little loss of quality in the final picture. The two most popular compression and decompression technologies are called MPEG4 and MPEG2. We will make an MPEG2 MPEG4 comparison here.
MPEG2 MPEG4 Comparison
MPEG4 was introduced in 1999 as an improved compression technology over the previous MPEG-2. MPEG-4 is a better compression technology and allows television channels to be compressed more. MPEG-4 combined with the new DVB-S2 (the second generation Digital Video Broadcasting Standard) allows DTH Operators to cut the bandwidth of TV channels by almost half of that used by MPEG-2 with DVB-S1 transmission. This means that with MPEG-4 compression, the DTH company can transmit almost twice the number of channels over the same Satellite Transponder.
Satellite Transponder rent costs are very high, considering the heavy investments in Satellites, Satellite launching by Rockets and high insurance costs due to the high risk of failure in placing a Satellite in orbit by a rocket launch. Rent payments of satellite transponders are the single most expensive item for a DTH operator, may be even more than 50% of his total costs. So being able to reduce this cost by almost half, is a major saving and a big advantage to those DTH companies adopting MPEG-4 technology especially when MPEG-4 is combined with DVB-S2.
Even though the digital compression is more with MPEG-4, the picture quality after decompression, of both MPEG-2 and MPEG-4 are about the same. To the subscriber of DTH, the picture quality they receive at home is the same whether the transmission uses MPEG-4 or MPEG-2. The only problem is that with rising costs of satellite transponders, MPEG-2 DTH operators may be tempted to reduce Satellite Transponder costs by reducing the picture quality of their channels by reducing the bitrate or bandwidth. When bandwidth is reduced, they can have more channels on a single transponder and thus save costs, but sacrifice picture quality.
With the ability to transmit double the quantity of channels at the same transponder costs, the DTH companies adopting MPEG-4 compression with the newer DVB-S2 broadcasting technologies, are in a position to offer more real HDTV & 3D channels and offer other advanced features like interactive services.
Why not change MPEG2 to MPEG4
So the question arises "Why don't the DTH companies using MPEG-2 change over to MPEG-4?" The problem is that MPEG-4 is not compatible with the older MPEG-2 Set Top Boxes or STBs. To change to MPEG4 from MPEG2, all the MPEG-2 Satellite Decoder/Receiver Set Top Boxes (STBs) with its millions of subscribers have to be scrapped. MPEG-4 will only work with new MPEG-4 Satellite Decoder/Receiver Set Top Boxes (STBs).
- Airtel Digital TV in India
- Videocon D2H in India
- Sky+ HD and Freesat in the UK and Ireland
- Saorsat in Ireland
- Dish Network in the U.S.
- Bell TV in Canada
- Cignal Digital TV in Philippines
- TopTV in South Africa