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How did he figure out cable length given video sync was off by 17 lines?

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JoeKing

Lifer
Oct 9, 1999
10,641
1
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Ok guys please help me out with this one please.

I’m working with a real “white coat” engineer, going over systems for a small television station. As we were aligning sync from one of the camera studios to the actual servers in the operations room, we found the sync of the studio was off by ~17 lines.

Now using just this information the engineer popped open his phone calculator program and was able to quickly calculate there was ~30.8 meters of cable running from the studio to the server room.

Impressed I asked him how he came up with this, but he has a very thick japanese accent and we had more pressing work to do, so I didn’t quite get this geewiz, could be useful information / formula. So I figure my AT brain trust can help me figure it out.

Some things that have been going through my head trying to figure it out:

-Duration of 1 line = 63.556μs
-Sync was off by 17 lines = 1.080542ms
-Distance that light travels in 1ms= 299792.458 meters
-Distance that light travels in 1.080542ms = 323938.342 meters <---very wrong! So I guess light has nothing to do with it.

And that&#8217;s about where my brain takes a poop, I suck with conversions to begin with! I&#8217;m feeling close and that it&#8217;s easier than I think, but I&#8217;m just not seeing it. Did he use the length of a lines sine wave to figure out the distance given time? HALP so I can sleep tonight!

here's a link to some information about video
http://zone.ni.com/devzone/cda/tut/p/id/4750
 
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C

canis

Member
Dec 10, 2007
152
0
0
JoeKing said:
Ok guys please help me out with this one please.

I’m working with a real “white coat” engineer, going over systems for a small television station. As we were aligning sync from one of the camera studios to the actual servers in the operations room, we found the sync of the studio was off by ~17 lines.

Now using just this information the engineer popped open his phone calculator program and was able to quickly calculate there was ~30.8 meters of cable running from the studio to the server room.

Impressed I asked him how he came up with this, but he has a very thick japanese accent and we had more pressing work to do, so I didn’t quite get this geewiz, could be useful information / formula. So I figure my AT brain trust can help me figure it out.

Some things that have been going through my head trying to figure it out:

-Duration of 1 line = 63.556&#956;s
-Sync was off by 17 lines = 1.080542ms
-Distance that light travels in 1ms= 299792.458 meters
-Distance that light travels in 1.080542ms = 323938.342 meters <---very wrong! So I guess light has nothing to do with it.

And that’s about where my brain takes a poop, I suck with conversions to begin with! I’m feeling close and that it’s easier than I think, but I’m just not seeing it. Did he use the length of a lines sine wave to figure out the distance given time? HALP so I can sleep tonight!

here's a link to some information about video
http://zone.ni.com/devzone/cda/tut/p/id/4750
Click to expand...

What is the cause for the out of sync? How is the out of sync corrected? What exactly do you mean when you say "sync of the studio was off by ~17 lines."?
 
J

JoeKing

Lifer
Oct 9, 1999
10,641
1
81
canis said:
What is the cause for the out of sync? How is the out of sync corrected? What exactly do you mean when you say "sync of the studio was off by ~17 lines."?
Click to expand...

The cause for the video being out of sync can be a number of reasons. Primarily the internal sync generator of the remote studio, the length of cable, quality of cable, and more. All of which can be affected by external conditions such as temperature, power, age. And I'm sure a host of other things.

In analog world there are 2 types of sync. 1 is HSYNC and can be known as the blanking interval a line is not active as it's being retraced horizontally across the phosphate of a CRT screen to it's next "on" position to begin it's trace back across the screen at a slightly lower position. The other is VSYNC. This affects the vertical position of the line. It actually moves the line down in preparation of the next line. Through these two sync signals (which are run in the same interval) you keep a smooth picture as frames flash across the screen.

http://en.wikipedia.org/wiki/Video_signal_generator

I may be a little off it's been years since I've done theory on this stuff. And it's hard to articulate without drawing it out.

More or less keeping everything in sync prevents tearing of the picture when you switch from one analog source to another. It gives you that smooth cut from source to source.

Also in NTSC format there are 525 lines per frame.

And we correct the sync discrepancy by varying the timing pulse going to the remote sync generator to better coincide with the master generator in the control room.
 
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C

canis

Member
Dec 10, 2007
152
0
0
Are you in the US? Aren't 720p or 1080i cameras standard in the US?

OK so the cameras you are using are analog. In your experience is there a direct relationship between out of sync and length of cable?
 
M

mutz

Senior member
Jun 5, 2009
343
0
0
it doesn't seem to be speed of light related as it far exceeds the result of ~30m,
iv'e been searching around a bit for a clue,
and these are it's fruits,
i'm not too familiar with these things, yet it might brighten a bit u'r view:
A cable length detection apparatus is provided to detect the length of a cable having at least three pairs of wires for transmitting video signals. The cable length detection apparatus has a signal compressing circuit, a signal converting circuit and a length calculating circuit. The signal compressing circuit compresses an electrical signal into one pair of wires. The signal converting circuit converts the electrical signal transmitted by the pair of wires into a value. The length calculating circuit then calculates the length of the cable according to the value.
Click to expand...

another source discussing skew limits:
http://proavmagazine.com/industry-news.asp?sectionID=1756&articleID=594918

Specifications exist for all types of cabling and vary depending on the cable type you use and the type of network you plan to install. The maximum allowed cable lengths are an important consideration in your initial planning of the network and you should never exceed them. A network tester should allow length testing.

Read more: http://www.openxtra.co.uk/articles/network-cable-testing-0#ixzz0h4CoDdFS
Click to expand...

basically, it seems he's aware at the properties of the cable been used,
and so, can calculate it's length from various variables,
this ain't answering u'r question exactly and u might be aware of some of the things being said,
yet hopefully it gives you some idea or wider perception regarding this "secret" :).
 
S

ScottMac

Moderator<br>Networking<br>Elite member
Mar 19, 2001
5,471
2
0
Propagation velocities are fairly standard for any given type of cable (for example, UTP tends to be ~66&#37;, foam core (high grade) coax is ~84%, fiber optic is ~60-66%).

I don't see this as being any different than calculating "bit time" on network cabling (useful for fault investigation ... Network General used to push this concept a lot).

Of course, he could have just been funnin' you too.

Bulk cable has the length in feet stamped on the sheath; if he happened to be able to read both ends, it's simple arithmetic from there.
 
C

canis

Member
Dec 10, 2007
152
0
0
Perhaps they were using a 100 mile spool of cable and didn't want to cut it. They needed the full length at a later time to route from one end of the Chocolate Factory to another. JoeKing, does Mr. Wonka treat you well? Bitchin' avatar. Just Joking.
 
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Cogman

Cogman

Lifer
Sep 19, 2000
10,286
145
106
For a normal, multipaired cable, the induction/capacitance of said cable will cause issues with the speed at which a signal can propagate through it. However, knowing what the signal propagation speed is, it is just a simple multiplication, propagation speed * sync error.

This propagation speed will depend on a few factors such as number of pairs, how twisted they are, and how close together they are. Apperently, for your cable it is in the range of 28 504 m/s.

OP, you may find this link useful http://en.wikipedia.org/wiki/Wave_propagation_speed
 
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StinkyPinky

StinkyPinky

Diamond Member
Jul 6, 2002
6,977
1,276
126
I suggest you just ask him during a quieter time. Get him to write it down for you.
 
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