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Channel Changing (and Frequency Follies)
12 years ago
General
Remember when your TV had a dial and you changed channels by turning it to point at the channel number you wanted? Right, right, who am I kidding... probably half of you reading this have never had a TV that tunes channels over the air, let alone one with a dial.
Well back in those days, you clunked the dial around (or if you were lucky, pressed a button on a remote and a servo-actuator clunked the dial around for you) and each position set the receiver to a different frequency. These frequencies are standardized and most often referred to by channel numbers. When you selected Channel 5, the receiver was tuned into 77.25 MHz. Good thing we didn't have to memorize the frequencies ourselves, right?
Now that we've got digital terrestrial TV, things aren't that simple anymore. Each channel is an MPEG-2 Transport Stream carrying a wealth of information way beyond simple audio and video. And one of those streams it carries is the Program and Station Information Protocol, or PSIP. This is where the program guide information comes from, any content ratings and even the exact time of day. Believe it or not... the PSIP carries the channel number!
And this is where things get sticky! Because the PSIP channel number doesn't have to match the actual frequency channel number! Our station transmits on channel 9 and the PSIP channel matches that, but for a while we transmitted on channel 43 (yes, way up in UHF!) while remaining "channel 9". We have a translator in town that rebroadcasts on frequency channel 11.
Which one do you tune in? That's going to depend on your TV. Some TVs will allow you to tune in the real channels, so you could have tuned in 43.1 or go for 11.1 in the examples in the previous paragraph. The TV we use to monitor on-air programming in engineering works like that. Other TVs only deal in the PSIP channels. These are the TVs that won't let you do anything until you do an initial channel scan. It scans all channels and then builds a table of PSIP channels and their associated real channels. And that's awesome, because you know that Channel 9 is channel 9 no matter what real frequency they're actually broadcasting on.
But then you get weird things like the little portable TV we were using to test the aforementioned translator. We did a channel scan, and what do you suppose we get? We've got channel 9.1, channel 9.2... and then channel 9.1 and 9.2 right after that! The first pair were our main 4kW transmitter on 9, the second pair were the 60W translator right next to us. But they are both "9", both transmitting the same thing... it looks like duplicated channels and that the TV is losing its mind.
If you studied the frequency map linked above, you might have had a little "A-ha" moment. Anyone who has used one of those groovy radios that tune in TV bands with an analog dial will remember the TV band was always split in two: Channels 2-6 and then Channels 7-13. And that's because your VHF audio radio bands sit in that gap, and it would be way too unwieldy to map that entire range out onto one band on the tuning dial.
Also, look at Japan's FM radio band. When playing Ridge Racer V and "listening" to Ridge FM on 76.5 MHz... it was tempting to think they just made the number up as something that would fall outside the standard FM radio band. I sure did. And for most of the world, it does too. Except in Japan, it doesn't. 76.5 MHz is a valid FM radio station.
Which leads me to my Sony multi-band radio. Being the international radio that it is, when you select FM band it allows you to tune from 75MHz to 108MHz, to cover all the bases. And guess what I found at 81.7 MHz? The audio for TV Channel 5! I spent many a Saturday morning waking up and listening to the hijinks of Saturday Morning Cartoons... I was familiar with Johnny Test long before I ever saw it on TV.
My own "A-ha" moment came while doing research for my journal on color television. I had realized early on that the audio for Channel 5 was always quieter than standard FM stations, enough that I'd have to turn up the volume control. So when I read that the audio portion of the TV signal is only a third of the bandwidth of standard FM channels, I said, "A-HA!" A third of the frequency bandwidth... frequency modulation... a third of the modulation the radio was expecting... no wonder it was so much quieter!
Last word is about that MPEG Transport Stream. No matter how many sub-channels a digital TV station has (our state-run public TV channel had four at one time) they are still broadcasting on a single channel frequency. One single transport stream is broadcast, and inside that are multiple separate elementary streams (either audio or video) for each sub-channel.
That's why if your reception is bad and you can't get channel 10.1, you're not gonna get channel 10.4 either. You're not picking up the transport stream that carries all the individual elementary streams.
Hoo boy, I could do a whole journal on transport streams. But we'll save that for the future.
Well back in those days, you clunked the dial around (or if you were lucky, pressed a button on a remote and a servo-actuator clunked the dial around for you) and each position set the receiver to a different frequency. These frequencies are standardized and most often referred to by channel numbers. When you selected Channel 5, the receiver was tuned into 77.25 MHz. Good thing we didn't have to memorize the frequencies ourselves, right?
Now that we've got digital terrestrial TV, things aren't that simple anymore. Each channel is an MPEG-2 Transport Stream carrying a wealth of information way beyond simple audio and video. And one of those streams it carries is the Program and Station Information Protocol, or PSIP. This is where the program guide information comes from, any content ratings and even the exact time of day. Believe it or not... the PSIP carries the channel number!
And this is where things get sticky! Because the PSIP channel number doesn't have to match the actual frequency channel number! Our station transmits on channel 9 and the PSIP channel matches that, but for a while we transmitted on channel 43 (yes, way up in UHF!) while remaining "channel 9". We have a translator in town that rebroadcasts on frequency channel 11.
Which one do you tune in? That's going to depend on your TV. Some TVs will allow you to tune in the real channels, so you could have tuned in 43.1 or go for 11.1 in the examples in the previous paragraph. The TV we use to monitor on-air programming in engineering works like that. Other TVs only deal in the PSIP channels. These are the TVs that won't let you do anything until you do an initial channel scan. It scans all channels and then builds a table of PSIP channels and their associated real channels. And that's awesome, because you know that Channel 9 is channel 9 no matter what real frequency they're actually broadcasting on.
But then you get weird things like the little portable TV we were using to test the aforementioned translator. We did a channel scan, and what do you suppose we get? We've got channel 9.1, channel 9.2... and then channel 9.1 and 9.2 right after that! The first pair were our main 4kW transmitter on 9, the second pair were the 60W translator right next to us. But they are both "9", both transmitting the same thing... it looks like duplicated channels and that the TV is losing its mind.
If you studied the frequency map linked above, you might have had a little "A-ha" moment. Anyone who has used one of those groovy radios that tune in TV bands with an analog dial will remember the TV band was always split in two: Channels 2-6 and then Channels 7-13. And that's because your VHF audio radio bands sit in that gap, and it would be way too unwieldy to map that entire range out onto one band on the tuning dial.
Also, look at Japan's FM radio band. When playing Ridge Racer V and "listening" to Ridge FM on 76.5 MHz... it was tempting to think they just made the number up as something that would fall outside the standard FM radio band. I sure did. And for most of the world, it does too. Except in Japan, it doesn't. 76.5 MHz is a valid FM radio station.
Which leads me to my Sony multi-band radio. Being the international radio that it is, when you select FM band it allows you to tune from 75MHz to 108MHz, to cover all the bases. And guess what I found at 81.7 MHz? The audio for TV Channel 5! I spent many a Saturday morning waking up and listening to the hijinks of Saturday Morning Cartoons... I was familiar with Johnny Test long before I ever saw it on TV.
My own "A-ha" moment came while doing research for my journal on color television. I had realized early on that the audio for Channel 5 was always quieter than standard FM stations, enough that I'd have to turn up the volume control. So when I read that the audio portion of the TV signal is only a third of the bandwidth of standard FM channels, I said, "A-HA!" A third of the frequency bandwidth... frequency modulation... a third of the modulation the radio was expecting... no wonder it was so much quieter!
Last word is about that MPEG Transport Stream. No matter how many sub-channels a digital TV station has (our state-run public TV channel had four at one time) they are still broadcasting on a single channel frequency. One single transport stream is broadcast, and inside that are multiple separate elementary streams (either audio or video) for each sub-channel.
That's why if your reception is bad and you can't get channel 10.1, you're not gonna get channel 10.4 either. You're not picking up the transport stream that carries all the individual elementary streams.
Hoo boy, I could do a whole journal on transport streams. But we'll save that for the future.
And yeah, the TV when I was a kid had a dial. I'm ooooold.
And I was thrilled. "Wow, you don't have to get out of your chair to change the channel on the TV!" Damn kids today are spoiled with their 100 button remotes! Back when I was young... ^_~