# TV on TV **URL:** \ **Category:** Factual Questions\ **Created:** [December 15, 2000, 5:35am UTC](https://boards.straightdope.com/t/tv-on-tv/45701 "2000-12-15T05:35:46Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![jbird3000](https://avatars.discourse-cdn.com/v4/letter/j/aeb1de/32.png) [@jbird3000](https://boards.straightdope.com/u/jbird3000)\ **Post date:** [December 15, 2000, 5:35am UTC](https://boards.straightdope.com/t/tv-on-tv/45701/1 "2000-12-15T05:35:46Z")
Why is it when on TV they show another TV, or computer or whatever, it has moving horizontal lines on it? ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [December 15, 2000, 6:30am UTC](https://boards.straightdope.com/t/tv-on-tv/45701/2 "2000-12-15T06:30:06Z")
The reason is because the field rate (the rate at which the electron gun inside the CRT moves up and down) is not in sync with the scan rate of the video cameras. Thus, you get mucho flickero. The problem is considerably worse with computer monitors, which have very high field rates. ---
**Author:** ![douglips](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@douglips](https://boards.straightdope.com/u/douglips)\ **Post date:** [December 15, 2000, 7:12am UTC](https://boards.straightdope.com/t/tv-on-tv/45701/3 "2000-12-15T07:12:27Z")
It’s the same reason that when you see a helicopter on TV, the rotors seem to be still, or rotating backwards - stroboscopic effects. The picture on the screen is drawn N times per second (on my computer, 72 times per second.) The picture is drawn one line at a time, from the top down (I think) and when it is done, it starts over again. So, if you are taking pictures of this process at regular intervals, you might see a portion of the screen not drawn, or a portion drawn twice, or other weird effects. ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [December 15, 2000, 7:21am UTC](https://boards.straightdope.com/t/tv-on-tv/45701/4 "2000-12-15T07:21:55Z")
> [@](#): > > \*Originally posted by douglips \* > The picture on the screen is drawn N times per second (on my computer, 72 times per second.) The picture is drawn one line at a time, from the top down (I think) and when it is done, it starts over again. Actually, they’re drawn every other line from top to bottom, and the remaining lines from bottom to top. ---
**Author:** ![Zenster](https://avatars.discourse-cdn.com/v4/letter/z/cdc98d/32.png) [@Zenster](https://boards.straightdope.com/u/Zenster)\ **Post date:** [December 15, 2000, 7:42am UTC](https://boards.straightdope.com/t/tv-on-tv/45701/5 "2000-12-15T07:42:51Z")
As a side note, when you see a professionally produced sequence where all of the screens do not roll, they are using a system called “Gen-Lock” which synchchronizes the scan rates of the camera with the display screens. The newer high refresh rate monitors and especially the flat panel screens overcome this problem without all of the electronics. ---
**Author:** ![stuyguy](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@stuyguy](https://boards.straightdope.com/u/stuyguy)\ **Post date:** [December 15, 2000, 7:51am UTC](https://boards.straightdope.com/t/tv-on-tv/45701/6 "2000-12-15T07:51:08Z")
**douglips** wrote: _The picture on the screen is drawn N times per second (on my computer, 72 times per second.) The picture is drawn one line at a time, from the top down (I think) and when it is done, it starts over again._ And **friedo** wrote: _Actually, they’re drawn every other line from top to bottom, and the remaining lines from bottom to top._ They’re both right. TV screens use interlaced scan lines (as **friedo** describes); but computer screens don’t interlace their scan lines – they refresh each whole frame over and over starting at the beginning (as **douglips** describes). ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [December 15, 2000, 1:30pm UTC](https://boards.straightdope.com/t/tv-on-tv/45701/7 "2000-12-15T13:30:52Z")
> [@](#): > > they are using a system called “Gen-Lock” which synchchronizes the scan rates of the camera with the display screens. For film cameras (which are most often used when shooting television monitors) there are two ways of getting rid of the lines. If the camera is running at 24 frames per second (U.S. standard) the shutter can be adjusted to a 144º aperture instead of the fairly-standard 180º. ANother way is to use a “milli-frame controller” to adjust the camera’s speed to 29.(something) fps so that it matches the frame rate of the television monitor, and then use a “phase” button to roll the line off if the image. The advantage of the milli-frame controller is that it can be used to adjust the camera speed for any monitor. Of course if you speed up the camera (“overcranking”, as it’s called), the action gets slower. This might result in problems synching the audio later which can be a pain if you have no budget.