HDTV Frequencies and Refresh Rates Explained

People often get the term frequency mixed up with the refresh rate when talking about TV’s, especially HDTV.

The frequency is the broadcast information that your cable or satellite box needs in order to tune in to a particular channel. Much like when you used to manually tune an analog TV or video into each signal individually, a decoder does the same on the digital spectrum in order to pick up the broadcasts.

Analog and digital signals are broadcast on the same frequencies, 55-88Mhz VHF low, 174-216Mhz VHF high and 470Mhz-806Mhz UHF. Some channels will still be broadcast in analog, but the other channels, especially HD ones will be broadcast in digital.

Each individual channel is allocated 6Mhz of the spectrum in order to carry the signal. Analog can send the standard 480i picture within this limit, and digital can carry a 1080i or p within it.

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The 480i means that the signal carries 480 horizontal lines of information, the i is for interlaced, meaning each line is refreshed alternately. The 1080 is for the lines, i is for interlaced, but the p is for progressive, which means the lines refresh from the top down. HDTV come in two varieties, 1080i and 1080p. Currently the 1080p sets have an advantage in picture quality over the 1080i ones.

The frequency doesn’t make too much difference to picture quality as it is cleaned up by the set-top box or TV. The refresh rate of the TV is what makes the most difference to the quality of a picture.

When looking at the specifications of a TV, you will often see “60Hz” or “120Hz” in the description. This is entirely different to frequency, this is the refresh rate.

A refresh rate of 100Hz is how many times a second an image frame is drawn on the screen. For example, a movie is broadcast at 24Hz, which is 24 frames per second. A 60Hz LCD TV will convert the frames into 30Hz frames automatically, and refresh them twice every second. A 120Hz TV will repeat the original 24Hz frame five times within every 24^th of a second.

In the real world what this means is that a 120Hz TV will produce a much clearer, crisper picture than that of a 60Hz one because the image is being refresh twice as quickly. Moving images will appear sharper and with less flickering than a 60Hz picture and will have much less motion blur.

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Motion blur is created when an image moves rapidly from one position to another and the eye picks it up. On a 60Hz signal, the image is held for 1/60th of a second the quickly moves to a different position in the next frame. The eye is sensitive enough to detect this and manifests itself as motion blur. As a 120Hz TV refreshes twice as quickly, the in-between frames are less noticeable and the action appears smoother.

The frequency and refresh information is the same for normal TV as well as HDTV. High Definition denotes the amount of lines in a picture, nothing else. So a TV that has 1080i or 1080p is HD because it has a little over twice the information on screen that normal television does.

So with any luck we have decoded the technology enough for you to be able to decide for yourself which is the best option when shopping for a new TV. No more depending on the "expertise" of the guy in the store to help you!