Currently, the DTT in Spain It has around thirty channels in HD. However, the quality with which they broadcast is very limited, and that is noticeable in highly pixelated images. Instead, seeing the Internet TV or through the operators, the quality is far superior. Why is this happening?
The DTT has turned 15 this month. In that time we have lived several changes and modifications both in the emission standards as in the spectrum used through those channels. SD channels came first, and soon after almost all of them began to arrive in HD. However, these channels have always broadcast with questionable quality.
The bitrate in DTT is very limited
The reason is none other than bandwidth limitation. The current DTT uses 8 MHz channels with a bitrate that reaches up to 19.91 Mbps. This rate could be as high as 30 Mbps, but a 2/3 encoding is used, where for every two bits a third of margin is left as redundancy in case one fails to deliver to avoid errors in the image.
Each 8 MHz channel usually broadcasts several channels in SD or HD, with normally between 3 and 6. Thus, most HD channels have a bitrate of around 4.5 Mbps through the cMPEG4 codec. SD channels, which use the MPEG2 codec with the worst compression, have figures of around 3.5 and 4 Mbps. To this must be added that some of these channels have radio stations, program guides, teletext and other services, which also occupies a significant amount of spectrum.
DTT currently uses 224 MHz spectrum, ranging from 470 to 694 MHz. This implies that the bitrate used by all channels is 179.19 Mbps. If all that space were used for 5G, there would be thousands of Mbps of speed, and DTT could be broadcast digitally through this standard, being able to see it also from the Internet on computers or mobiles.
Operators compress the image better
As long as there is no codec change or broadcast system to DVB-T2 and HEVC, the quality of the channels will continue to be very limited. And nothing indicates that it will stop being used MPEG4 for HD channels, although we could see changes in the SD, going to use the space that they leave from 2023 for channels even in 4K, or that the quality of the HD is improved.
In the case of Internet TV such as that offered by operators such as Movistar, Orange or Vodafone, the quality is much higher because they do not have spectrum limitations. In Movistar, for example, we find bitrates of about 12 or 13 Mbps for HD channels, a figure that triples the quality of DTT channels. Thanks to this, watching DTT through the operators’ TV is a better option than doing it from DTT itself, since the image quality is much better.