Iowa Public Television

Tuesday, December 30, 2008

Beware of High Definition Hype

I recently had a question from a viewer who was receiving our digital signal and was quite happy except for the fact that occasionally he had what he described as “digitization” in the picture. Probably anyone that has watched any television for some period of time has noticed this artifact. It is sometimes called “pixelization” or “mosaic tiling” or “blockiness.” The professional term for it is macro-blocking and it is what happens to the picture when the digital decoder in the receiver starts to run out of data. Is it the edge of the abyss we call the digital cliff. The digital cliff is the reason why I stress taking all the steps possible to deliver as clean a signal to the receiver as possible. Those steps increase the margin or distance between the received signal and the cliff and the greater the margin, the better.

The viewer lives in a three story home with an outdoor antenna and uses splitters and cable runs to distribute the signal to five different television receivers. He had done some research regarding splitters and sent me a couple of links to websites that he had looked at and was wondering if there was any merit in the claims made about these splitters and if installing them would improve the reliability of his signal. I visited both websites and one was quite accurate in describing the splitter’s capabilities and function, although I am not sure why a splitter needs a “precision die-cast 24k gold-plated chassis.” Gold-plating the connector contact improves connectivity which gives some improvement in performance but I don’t know how gold-plating the outside of the splitter helps.

The other site was much more worrisome since it was full of misinformation. It started off with this statement “These 1080p rated 2, 3 and 4 Way, 2 GHz RF high definition splitters are solely designed for high definition systems.” Where do I start? An RF splitter’s function is to take an RF signal from an antenna or cable system and divide it equally. 2, 3, and 4 way splitters divide the single input into 2, 3 and 4 outputs respectively. They work with the radio frequency (RF) signal and therefore do not care if the content on the signal is digital or analog, high definition or standard definition. RF splitters have been employed since the very early days of broadcast so it is doubtful that this splitter was solely designs for high definition systems and 1080p is a high definition video display format and has nothing to do with the performance of this or any other splitter. The advertising copy went on to make numerous claims of performance of this splitter and inaccurately relating it directly to high definition television.

I have said it before, but what makes digital television digital is not the RF signal, but the content that is on the RF signal. So until the content is removed (demodulated and decoded), the DTV signal works pretty much just like the analog signal. In the RF domain, there is no such thing as a digital antenna, a digital splitter, or digital wire. These passives components are agnostic to the content and only care about the RF signal. To be sure, having good components will improve reception but the same good components will improve analog or digital reception. Don’t fall for the marketing hype.


Monday, December 15, 2008

IPTV DTV Information Sessions

In the coming months, Iowa Public Television's DTV experts will conduct free information and Q&A sessions about Digital Television. UPDATED JANUARY 23

And remember - if you'd like an Iowa Public Television staff person to speak to your community or civic group, please let us know! Contact Jennifer at 800-532-1290 or at to schedule a session in your community!

Upcoming Free Digital Television Information Sessions

  • Cedar Rapids Public Library - Westdale Mall - Wednesday, January 28 at 6:30 p.m.
  • Ericksen Center - Clinton, Iowa - Thursday, February 12 at 1 p.m.

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.