Iowa Public Television

Tuesday, December 2, 2008

Why Does Digital Reception Behave Differently

As the February 17, 2009 analog shutoff approaches there has been a huge and very successful push to increase the public awareness that something is happening regarding television in the USA. When I first began explaining digital television to viewer in the mid 1990's the focus was on telling them what they would need to do to receive digital services. Television signals, whether digital or analog, essentially travel the same way so how the signal gets to the receive antenna is more a factor of what television channel and band (VHF or UHF) the signal is on rather then whether the content riding on that signal is digital or analog. The real observable difference to the end users is in the performance of the digital decoder in a DTV receiver.

I try to explain it to lay people this way. In analog television there is a direct connection between the quality of the received signal and the quality of the displayed content on that signal. Weak signal means noisy picture. A signal receiving self-interference (called multipath) has visible ghosting or after images in the picture. These and other common signal impairments are easily visible to the end user because although they are impairments to the television signal, they show up as impairments to the displayed content.

What makes DTV behave so differently is that when the content is created, before it is ever placed on a television signal, the content is digitized and from that point on we are not working directly with the content but rather with the numbers that represent the content. Those numbers can be manipulated and shipped around over many different forms of electronic transportation and as long as the number that gets to the decoder (this is what turns the number back into the content) are readable, the picture can be recreated perfectly.

So if we think back the things that impaired the analog television signal and recognize that the digital television signal travels those same paths, it makes sense then that those same impairments would happen to the digital television signal. And just like with the analog service, those impairments show up on the digital content (the numbers) but since the numbers have to go through the decoder before the content can be displayed, the direct connection that existed between the signal and the display of content no longer exists. Instead what happens is the decoder sees the noise from a weak signal but since all it has to figure out what the numbers are, even if they are noisy it will recreate the picture without showing any problems. If the numbers have ghosts it doesn't matter as long as the decoder can figure out the numbers it will recreate the picture.

So when digital service fails, what is really failing is the decoder not the receiver. What has happened is that the impairment to the signal has gotten so great, the decoder can not get enough of the number to recreate the picture. In the analog world this would show up on the screen as a picture that is so noisy and or ghosted that it would be unwatchable. The display would unlock (picture roll) or distort diagonally or be too noisy to make out any images. Digital receivers just can't display that.

So one of the fundamentals that people need to understand about digital reception regardless of whether it is via a new digital television or a coupon eligible converter box is that even though the same things impact the both the digital and analog television signals, the receivers will behave differently and essentially the same things that make analog reception work better (outdoor antennas, high quality wire, etc.) will make digital work better.


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