Television broadcast signals

Last week we talked about radio broadcast signals and the difference between AM and FM signals. This week I thought we could take it into talking about television broadcast signals. I am sure many of you in this area have experienced the same things that I have in recent years with broadcast television stations.  If you receive your local news via antenna rather than as a cable subscriber, there is a big difference in the quality of the TV signal since changes in 2009.

I noticed one thing which bothered me a lot; before the digital broadcast switchover in 2009, I could still get KY3 during a heavy storm. The picture had a lot of static and the sound was a little unclear, but I could still hear major weather alerts and be informed. Just recently a small tornado went through the outskirts of Edgar Springs. I heard the tornado warning and then my screen went dark. No signal, and therefore no information. The old analog stations never went completely away like this during a storm.

So what is the difference between analog and digital signals, why does the quality look so great on digital stations all the way up until they just drop off with a no-signal message? With analog it seemed that the quality degraded until you could no longer get the station, but there was never a complete cutoff. It has to do a lot with how the signals are transmitted.

The old analog broadcasts used a dual carrier technique overlaying both AM and FM signals that we talked about last week to transmit both the picture and the audio. The picture is transmitted using AM signals and the sound using FM signals. These signals are prone to noise from interference of other stations or signal bouncing off of walls, tree, and even people. The interference is what caused poor color quality, ghosting, and weak sound quality. The NTSC standard for television broadcast was adopted in 1941 and transmitted 525-lines of image data at 30-frames per second. NTSC worked well and still works today with older analog devices, like VCRs and older DVD players, but because color was not added until 1953 that standard became jokingly referred to by professionals as “Never Twice the Same Color” because of color inconsistencies between broadcast stations.

The new Advanced Television Standard Committee (ATSC) uses the same methods that store video information on DVDs or Blu-ray Discs to transmit the television signal. These methods use a digital signal consisting of a series of ones and zeros, or “on” and “off”. This new standard resulted in better quality images and sound for multiple reasons. The first is that it was designed from the ground up with things like color, surround sound audio, and text transmission taken into consideration.

The digital signal is much smaller now, allowing stations to use the same bandwidth to broadcast multiple stations, or sub-channels in addition to the main channel, using the same broadcast equipment.  Digital signals also allowed for the broadcast of the wide screen format and high definition signals. The only downfall of the digital broadcast is the inability to receive partial information from a weak signal. Digital is an all or nothing type of broadcast, as missing information in a digital signal cannot be interpreted by the receiver, causing errors and the nice “no signal” message to display on your TV.

You can think of a digital transmission as transmitting in code; if a single piece of the code is missing, it cannot be deciphered, resulting in unusable images and sound that cannot be displayed. Analog transmissions transmit the original image and sound, so if pieces are missing, the sound gets static, and the picture gets missing spots, or fuzzy. So even if the picture is clearer with digital, it is less reliable over long distances and in high noise situations like severe storms.