Why do I hear static on my TV when certain text is displayed.

January 16, 2005 6:16 PM

Why does my TV (and most other's I've ever seen) do this?

Why is it that when watching a channel with less than stellar reception over cable I often hear static in the audio during the times when there is text on the screen. This is particularily noticable during commercials, when it says "SALE!!!" in text on the screen the audio static increases greatly and then disappears again when the text is gone, anyone out there in the broadcasting business?

Cos

posted by Cosine to technology (13 comments total)

this is nore apparent when there is red text on the screen, no? Even when there is a really bright red sweater on a character, the same thing happens. I imagine that it is an out of color range thing?
posted by grimley at 6:45 PM on January 16, 2005

nore = more.
posted by grimley at 6:48 PM on January 16, 2005

If memory serves, it has to do with the amount of current being pulled from the power supply by the picture tube. The fully saturated colors require maximum current and those colors occupy a significant proportion of the screen.

If the power supply is not quite large enough to handle that load, the filters cannot remove all of the 60 ~ hum.
posted by RMALCOLM at 6:57 PM on January 16, 2005

Thanks, I don't think that's it though, it sounds like static not like a ground loop hum or such. I can fully saturate the screen with tons of colours when I run my HTPC through it as well, I think it's something in the signal, it only happens on stations that aren't that clear to begin with.
posted by Cosine at 7:04 PM on January 16, 2005

This usually happens when a commercial insertion is being done downstream of the network feed and the commercial being inserted doesn't conform to NTSC standards.

See, to be broadcast properly, the video signal shall only take up so much analog bandwidth and the audio signal shall only take up so much bandwidth. If the video signal is too hot, it bleeds into the audio signal, creating noise.

I say it happens downstream of the network feed because that's usually in spec and the local ad insertions tend to be more loose in their conformity.
posted by tomierna at 8:12 PM on January 16, 2005

tomierna is correct. Specifically, the commerical's creators choose colors that are out-of-range for your TV. I bet the background was a really over-saturated color when you heard the buzz. When the signal is traveling through the "ether," it's all converted into electrical signals. Super-saturated colors bleed into the sound portion of the signal.


TV-stations generally make sure that their programming doesn't use any of these colors. For some reason, commercials often get away with it. Does anyone know why? Is it because the stations don't want to piss off the sponsors? Is it because there's no time to preview the ads before they are aired?
posted by grumblebee at 8:18 PM on January 16, 2005

This discussion talks about the layout of the NTSC color television signal and how it was derived. Apparently, the color (chrominance) carrier sits right below the audio carrier. Efforts were made to prevent the chrominance from affecting the audio. I'd guess that when the signal quality goes down, the tuner starts having trouble and interference becomes more likely. I'm hardly an expert though, so I'm just guessing.

On preview: Sounds like tomierna knows something I don't. :)
posted by knave at 8:22 PM on January 16, 2005

Wow, thanks all, that has the ring of metafilter truth to it. On some stations, (ie: SpikeTV) that are always weak on this cable provider (you can hear the adjacent channel under the audio of Spike a lot of the time) it happens even if it's just fairly plain text on a plain background, I've also noticed it during news programming, the static is there when the reporter's byline is up and fades when it drops.

Note: I have sensitive ears and may be hearing more than some here, no one else has ever noticed this.
posted by Cosine at 9:31 PM on January 16, 2005

knave and grimley's answer doesn't sound right. The chroma is spread out across the spectrum just like the luma, so particular colours aren't what's causing this problem. My guess is that to give text sharp edges you need high frequency components that spread out beyond the luma into the sound carrier, though any decent broadcast system should be filtering that. Another possibility if that the overall frequency of the graphic is low enough to appear in the audible range. The most likely cause though is that its a complex effect of the RF system and crappy hacks used in the decoding circuits of TV receivers, that there's no simple explanation for.

The reason nothing happens on your HTPC is that you're probably putting video and audio through separate cables. You'll only start to get the same effects if you use an RF modulator and an antenna cable.

(Having been trained as a PAL engineer, I take this opportunity to point and laugh in NTSC's general direction)
posted by cillit bang at 3:33 AM on January 17, 2005

This happens on PAL systems too. The higher frequency video signal (going from dark picture to light text) causes rejection in the sound circuitry = noise.

Have words with your cable supplier (they may be able to boost the signal).

Is your cable feed (from the box in your house to the TV) via RF or a direct connection (do they have SCART in the US?). If it's via RF they you may be able to fine tune the channel tuned to your cable box.
posted by i_cola at 4:22 AM on January 17, 2005

Non NTSC safe colors (like pure white) most definitely will produce noise on a tv set, that's why there's an NTSC safe color pallete
posted by m@ at 7:55 AM on January 17, 2005

Ok, getting a lot of data now. Thanks everyone. The problem is program specific, not just continual during a certain time period, and it always requires a stations with a weak signal. I'm not using a cable box, the cable feed goes straight to the TV's receiver. I won't bother talking to the cable co, they are known to suck and are the only game in town so not much chance of getting anywhere with them.
posted by Cosine at 8:01 AM on January 17, 2005

"(do they have SCART in the US?)"

For your information, no. RF, composite, S-Video, and chroma/luma "component" interconnects are common (in roughly that descending order). HDMI pseudo-DVI is gaining traction. Straight-up RGB interconnects are almost completely unheard of in US consumer video.
posted by majick at 10:13 AM on January 17, 2005