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Old 8th January 2003, 05:05 AM   #1
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Default TV noise-ouch!

Hi folks,

Today I was doing some acustical measurements when I realized how loud a TV set is in the area of 15 Khz:
I attach a measurement of a 29" TV set at 4 meters away with audio muted. The noise below is city ambience coming thru the open window.
Big polution, I thought.

Ric
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Old 8th January 2003, 05:43 AM   #2
Wizard of Kelts
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I had heard of the 15K tone a TV makes, but after seeing that chart, all I can say is, "Wow!!".

15K Hz is 40 dB above the rest of the sound.

So if the rest of the sound is coming through at 1 watt, the 15K Hz is being played at 10,000 watts. How can that be?
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Old 8th January 2003, 05:46 AM   #3
Duo is offline Duo  Canada
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This is actually quite normal for a tv set.


A tv's vertical scan(refresh)occurs 30 times a second. There are 525 lines drawn horizonrally every time the vertical scan starts.

The product of 30 and 525 is 15750, which is the exact frequency of the horizontal scan.

The horizontal scan coil of the yoke runs at high current since it is part of the circuit that drives the flyback transformer and it also runs at very high frequency. This causes the amplitude of its signal to be high and thus the sound from it quite loud in some cases.

You might also notice a peak at 30hz and a harmonic at 60 if you are able to pick up sounds right inside the TV set, though I wouldn't suggest trying that since it's a waste of your safety to go inside a tv, let alone just to look for noise...
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Old 8th January 2003, 05:49 AM   #4
Duo is offline Duo  Canada
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I believe it takes less power to produce loud sounds at 15khz than the lower audio regions. The sound from the tv is very high in amplitude but it sounds to the ear as if it were much quieter than it really is since the ear is hardly resonant at such high frequency...
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Old 9th January 2003, 02:07 AM   #5
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An interesting topic indeed...but what caught my attention was the computer program used to capture the image, I see it runs in OS 9, what's it called?

Thanks
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Old 9th January 2003, 03:22 AM   #6
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Default spectrafoo

The name is Spectrafoo. And besides the weird noise it's a great and unique measurement tool.

Ric
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Old 9th January 2003, 05:34 AM   #7
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Default Flyback Operation

Duo,

Thou hast the wisdom of a TV technician. I learned electronic troubleshooting with big 1960s vintage TVs. Ah...I can still hear the big hand-wound flybacks singing, and the warm bakelite smell is a fond memory.

Bob
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Old 9th January 2003, 09:11 AM   #8
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Frankly, the 15k from most TV's has bugged me ever since I was a kid.

Lucky that our new Loewe is a 100hz model and I think scans twice as fast so it's moved out of my hearing band.

My folks never could understand how I could be outside or at the other end of the house through 6 closed doors and still know instantly when the TV had been turned on.

Guess that explains it.

I wonder if sales people in TV stores lose their HF hearing due to the number of sets on all at once?

"Warning, hearing protection must be worn when browsing for home entertainment equipment in this store"

Drew
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Old 9th January 2003, 09:14 AM   #9
dhaen is offline dhaen  Europe
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Default To be pedantic....

Quote:
The product of 30 and 525 is 15750, which is the exact frequency of the horizontal scan.
On the western banks of the pond:
For many years it's been:
29.97Hz (V), 15.734KHz (H).
It was moved from the original 30 and 15750 to reduce sound buzz when colour started. (I could bore you for hours if you want the details).)

On the eastern banks of the pond:
It's 25Hz (V), 15.625(H).
Previously we had some odd ones:
25Hz V, 10.125KHz(H).... ouch!
and
25Hz V, 20,475KHz (H)

Actually, many people develop a notch in their hearing resonse at those frequencies.
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Old 9th January 2003, 09:44 AM   #10
Duo is offline Duo  Canada
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I remember those numebers too. Have looked at some tv's like that. And read service manuals about them. My numbers were just the standard generic numbers that go by default, they're pretty close. I know what you mean but color buzz too, man that's irritating.

DrewP: I have the same problem, most tv's I can hear a mile away. On scary incident was when I was talking to a friend on the phone. He was 1000miles away, literally, and I could hear the horiz. scan on his tv over the phone so loud it was drowning out the speech. Phone systems aren't even supposed to "cover" that range, not nearly. I can imagine it was deathly loud right where he was.

It is true that tv salesmen and technicians get deaf to a region from 15khz - 16khz. Almost every tv person I know that's spent lots of time around tvs has no perception whatsoever of anything in that audio region. Personally I think that would drive me insane, I'm very sensitive to frequencies from 10khz up to 20khz and I'm very used to hearing them in music and everyday life. I'd hate to see what my sense of direction and detail was when I lost that range completely!

It's kind of scary almost!!
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