extra data in TV broadcasts

[dpopiz]

you know how they include some extra data along with TV broadcasts, that includes among other things the CC data?

well what all does it contain? I was just wondering if maybe some bit of data contained in it could be used to determine whether a commerical or a show is currently on. cuz then you could write something for a TV tuner to let you know when to switch back to the show you were watching when the commercials end

 

[SirPsycho]

As far as I know, it includes the closed-captioning data, some sort of synchronization signal to keep the audio and video in sync, and a signal that controls the brightness of the display signal. Interestingly, this brightness signal is what Macrovision's copy-protection scheme messes with to keep you from making copies of VHS and DVD movies. I believe they set it to invalid brightness levels that a TV will generally ignore, but that will confuse a VCR. I know that certain VCRs will correctly recognize it as bad data, and will completely ignore it. My Apex DVD player does the same.

Also, I believe that ReplayTV boxes try to automatically filter out commercials from the recordings it makes by looking for the screen to go black for a certain length of time, and it interprets that as being the beginning of a segment of commercials. It then looks for it again when the show starts again to know when to start recording again. I think the networks got wise to it, though, and figured out how to mess with it to the point of making it no longer work.

 

[FrankSchwab]

Closed Captioning and/or teletext data is sent during the Vertical Blanking Interval of each frame of video.

There are no commercial/program indicators, and no video/audio sync indicators. Don't know where SP got that one.

There is other data that can be sent. Look up RFC 2728 - a system to use the VBI for sending TCP/IP. Here's a good start on what a video signal looks like.

There are few effective ways to identify commercials; most rely on subtle clues to guess. For example, the audio on many commercials is more highly compressed than normal program audio, so that it sounds louder. Some systems look for that; this method is only successful as long as it isn't successful (if a large percentage of people filtered commercials this way, commercial audio would strive to match the audio characteristics of the program being watched). You can look for differences in frame rate - film is originally shot at 24 fps, and up-sampled to 30 fps for broadcast, which is detectable. Using this as a filter is possible, if only for movies, but once again is only successfull as long as it isn't successful. You can look for black frames, but that is a very subtle indicator which could be rendered moot by not placing black frames between program and commercial.



/frank

 

[CycloWizard]

Originally posted by: FrankSchwab
For example, the audio on many commercials is more highly compressed than normal program audio, so that it sounds louder.

I always wondered about that... I just figured marketing people made commercials louder to draw attention to them. :thumbsup:

 

[misterj]

Originally posted by: CycloWizard

Originally posted by: FrankSchwab
For example, the audio on many commercials is more highly compressed than normal program audio, so that it sounds louder.

I always wondered about that... I just figured marketing people made commercials louder to draw attention to them. :thumbsup:

me too! but what does compression have to do with audio level?

 

[MrDudeMan]

isnt it also true that you can easily get channels that have been turned off to your house?

 

[The Boston Dangler]

The VBI usually contains a clock, CC, program info, sometimes even a stock ticker. There may be enhanced EBS data too. Analog TV channels that are scrambled can be unscrambled, digital channels can be decrypted and decoded, although "easy" is a relative term. A digital black box frankly isn't worth the expense, given the high materials cost and chance of success. There may also be a trap, a usually low-pass filter blocking transmission of pay channels.

The audio compression in commercials applies to the dynamic range of the audio level. The percieved volume must be between x and y. x is raised so that even quiet sounds fall into the upper parts of the range. You can see the audio carrier in an NTSC picture go from a 10 to 14 dB delta to the video carrier, to something more like 6 - 8 dB. Any higher, and it would bleed into the video carrier of the next channel.

 

[FrankSchwab]

Frankly, there's only one channel in my town (the PBS channel) that supplies time in the VBI, so "Usually" is a pretty strong word. None of the channels in town carry program info or a stock ticker. So, yes, these are possible services but they are nowhere near ubiquitous. Of course, Phoenix is a small backwater so may not have the latest television technology available to it.

BD had it right about audio compression, but a bit technical. When the TV is playing audio, it has to deal with quiet sounds (the tick of the villains boots as he sneaks up behind the hero) and loud sounds (the sound of a gunshot). The audio in the program is set so that the loudest sounds don't exceed the maximum allowed volume level - that means that the "average" volume level is well below this. When you compress analog audio, you raise the volume during quiet moments so that the average volume level gets louder. In the extreme, it means that the tick of the villains boots is just as loud as the gunshot - or the entire commercial is as loud as the loudest momentary sounds during the normal program. So, the station can say "no, we don't raise the volume during commercials" with a straight face, because they don't. Equally, you can say "man, the commercials are so much louder than the program", and be correct also - to you, they sound louder.

/frank