Digital Broadcast

I've never heard of "Broadcast Safe" but the wiki describes what we used to call a "legal signal," one that complied with broadcast standards. Now it should be obvious why the video signals have to match standards. There has never been a video reason to use black at 7.5 IRE and white at 80 IRE. The higher level for black cutoff does provide latitude for home viewers individual adjustments. But the rules about maximum white levels & audio levels are requirements for broadcasting. And I can teach you to recognize when either level is violated. But we now have to discuss how a TV signal is broadcast.

Just like radio, TV is broadcast on carrier waves. The carrier wave is the channel's frequency and modifying the carrier by adding in the signal you want to broadcast, creates the broadcast signal. For radio, it's pretty much that simple. But a television signal consists of two entirely different elements, audio & video. Now if our video signal is modifying the main carrier wave, how can we add the sound? It was done by adding another carrier wave with a frequency only a tiny bit higher than the video's carrier signal. So you have to imagine the video signal as two carrier wave frequencies traveling together.

So if you imagine each signal looks like an audio waveform, packed closely to each other. Now if the audio gets too loud, the audio waveform will grow in height until it is actually spreading into the area reserved for the video signals. Like two radio stations on the same frequency. So what happens when the audio signal is too loud? The video will have scan lines that pulse in relation to the sound level. You can literally see the sound. Now when the video is too bright, the waveforms get higher and intruded into the area reserved for the audio signal. When video is interfering with the audio, you hear a buzzing sound whose volume relates directly to the quantity of extreme white in the image. So now you know how to recognize an "illegal" or "not Broadcast Safe" signal when it is on your TV set.

Now I've ran through this very quickly, so let me know if it makes sense. But Broadcast Standards are there to solve technical issues with how TV signals are broadcast. Why would this matter to us? Because our editors (both analogue & non-linear) don't have the same limitations. Because during the editing, we never see the signal as will be broadcast. Our edit suites deal with separate signals, video and/or audio travel different routes through our machines. So when you are editing, you really can't tell if your signals are "legal" unless you have meters to adjust the signals. So "Broadcast Safe" issues only arise when the audio & video signals are turned into one signal (with two signals combined.) The resulting signal is called an Radio Frequency signal, or an RF signal for short.

There is only one time in the video process that we deal with RF signals. So playing tapes, DVD's, internet videos and virtually all the ways we watch video at home, either are or can be played back using their separate audio & video signals. But as soon as the signals are combined for RF broadcasting, all the "illegal" settings will suddenly cause problems. And even if you're using an RF converter to send the signal to your TV that doesn't have line inputs, RF signal problems will be created & played back on the TV.

The only way to get out of "Broadcast Safe" standards is to never combine the audio & video into a single RF signal. Which isn't all that uncommon these days. Signals coming off playback devices as line outputs, won't exhibit the buzzing or strobing you'd see if you used the RF connector to view your video. Okay now that we all understand what "Broadcast Safe" means to analogue broadcasting, let's look into the exciting digital broadcast environment.

There is a built-in confusion surrounding Digital Broadcasting. To start with, the broadcast signals are still analogue waves radiating out from the TV tower. They still must contain the separate audio & video signal data and do it within an even narrower channel to send it out. So digital broadcasting signals will look & act just the way analogue TV signals do. The only difference for us is how we decode the signals. I have to admit, I have no information about the construction of a digital broadcast signal. So I cannot tell you how combining the audio & video signals changes them. Or what goes on during the decoding. My guess is that digital broadcasting may have eliminated the old signal crosstalk that caused TV problems. But that's not the end of things.

I'm sure you all have seen the PSA's about the coming transition. And you may recall that some of them mention that if you have cable or satellite TV, you don't have to do anything. That's because converter boxes are going to continue to sending an RF signal to drive your TV set. And the cable companies will continue to use RF signals to distribute programming because they don't have to convert to digital. Besides, the conversion would cost thousands of dollars for new devices to send the signal over the coaxial cable. And in the end, accomplish nothing the consumer could see. So that ain't happening.

What this means for "Broadcast Safe" is that the standards need to be maintained. Not because illegal signals cause TV problems with digital broadcasting, but because the signals are still being converted to RF for distribution. And we expect that to be the case for some time in the future.

And BTW, "Broadcast Safe" production really only matters if you are going to distribute your program via some sort of broadcast. The ill effects of RF conversion don't occur if you distribute via DVD's or web sites. You'll only have playback problems if the line signals have to be combined to move them into a device without line inputs.

But in closing, I would like to add that although you don't have to conform to broadcast standards in your productions, you may want to consider that television screens are optimized to display signals meeting broadcast standards. If you are outside those standards, it is going to be difficult to get a perfect reproduction of your video. Inside the standards and every TV in every situation will have perfect playback reproduction (limited by how the owner has the set adjusted.)

So good luck and stick to those "Broadcast Safe" standards.

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Digital Broadcast