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How are multiple video outputs displayed on various sections of the TV screen?

  1. Aug 11, 2012 #1
    CRT televisions use electron guns to fire electrons at color phosphors on the screen which emit the light of varying frequencies to produce an image. They can scan row by row or column by column depending on the standard of the broadcasting signal though modern models have circuitry that can read the broadcasting codec and adjust the electron gun to match the scanning standard of the broadcast.

    LCD, LED, and plasma screen televisions can either use millions of tiny light bulbs that switch on and off in a sequence to generate the image or focusing mirrors to direct the output of a few light bulbs to certain sectors of the screen in sequence to produce the image. The orientation of the scanning of these flat screen televisions also depends on the broadcasting standard though all flat screen models can be set to automatically adjust to the broadcasting standard.

    But these electronic scanning methods only seem to work if there is only one video signal entering the device. How do some televisions display multiple video signals with each one having their own division in the screen?
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  3. Aug 12, 2012 #2


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    what ... do you mean like eg. picture in picture ?
    if so just the same as a computer screen displaying multiple windows

    not sure if that's what you are asking ?

  4. Aug 12, 2012 #3
    No, the video outputs are arranged side by side like the image below:

    http://img98.imageshack.us/img98/673/multivideo.png [Broken]

    Additionally, how would this work if the videos are coming from TV channels?
    Last edited by a moderator: May 6, 2017
  5. Aug 12, 2012 #4


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    if from different channels then there is a tuner for each display
    its not uncommon to have a shelf in a back room with 6 or more payTV set top boxes on it each one on a different channel
    this is common in pubs and clubs that I have installed these systems

    if its the same pic on all 4 monitors then the signal has just gone through a splitter that takes signal to each monitor .... also a common setup

    if its a single pic spread over a video wall then there is some video matrixing done to the signal
    I have seen it often but havent specifically worked on one

  6. Aug 12, 2012 #5
    In principle it's fairly simple.

    As you rightly say the picture or 'frame' is generated by modulating a scanning process. The modulation can be either analog or digital.

    So consider a single line of the scan - as it scans across the signal modulates the scan.

    Now think of the corresponding line in a second frame.

    If we have a switch that switches between the two frames fast enough we can create a composite scanline that is partly from the first frame and partly from the second.

    If we synchronise the switching with the rest of the system for each line in turn, for the whole second frame, we can substitute frame 2 for part of frame1.

    Alternatively we can feed (switch) the first part of a single line to one monitor and the second part to an adjacent monitor and so on, thus spreading the picture frame over more than one monitor.

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  7. Aug 12, 2012 #6


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    This is a piece of cake for digital signals. (In principle) all you need to do is to read sub-sampled versions of the input pictures into the appropriate parts of a frame store, which is then read out as needed, frame by frame as an output TV signal. This is done all the time for a computer display, as you type, move your mouse, insert movies and graphics.
    It would be a much more difficult process for analogue signals, with analogue techniques alone. (Standards converters for 625/50 PAL to 525/60 NTSC were a total nightmare to design if they were to be 'broadcast' quality and they involved similar processes).

    The scan of the output picture requires the contributions from each input picture (line by line) to be delayed appropriately and to be displayed in half the time of the original, so it would fit on half a line width and half a frame height on the output
    You would need to sample (analogue) each line of each incoming signal, wait for the next field (with a delay line of up to one and a half field period) and then read out the samples (alternate samples or a suitable filtered combination of pairs of samples (or better still, contributions from two input lines would go to make up a single output line) at twice the rate so they would appear half the width of the original. This used to be done with a so-called 'Bucket brigade' (analogue) delay in which the input samples were clocked in to and held in a string of capacitors at one rate and then clocked out at twice the rate - producing lines of half the width.
    Before ADCs, DACs and memory were available to work at TV bandwidths, this was all you could do. No joke, but design engineers used to achieve fantastic results - considering.
  8. Aug 12, 2012 #7
    So in the old system, there were no random access memory cards with enough capacity to hold data for a frame store and the input signals were simply stored in capacitors and then discharged by timed electronic switches.
  9. Aug 12, 2012 #8


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    Field delays were achieved with quartz ultrasonic delay lines and bucket brigades consisted of analogue shift registers in which the signal level was passed along a line of sample and hold (capacitor) stages. Life was tough boyo!
  10. Aug 13, 2012 #9
    Here is a diagram which is an expansion of the previous diagram in post #5 that shows how four video outputs can be compressed into the screen.

    http://img513.imageshack.us/img513/4646/tvsignalswitching.png [Broken]

    To compress each video into a smaller window, some visual information from the signal must be deducted because there are only a limited number of pixels in a smaller window. Ideally the television should have a pixel for every bit of data in the signal but since it is being compressed, the signal can only scan some of the pixels so a circuit must be designed to periodically deduct some bits of data from the signal. After passing through this 'deduction' circuit, signal 1 will be on for a time slice that will enable it to scan half of the first line of pixels and then the fast digital switch will switch to signal 2 to scan the other half while delaying the next scan of data from signal 1 until signal 2 finishes its scan. Signal 1 is then allowed to scan the next line of pixels and the process repeats itself until half of the screen is filled with video from signals 1 & 2. Then, the time delayed switch will switch the connections to signals 3 & 4 to allow them to scan the last half of the screen using the same process used by signals 1 & 2. Meanwhile, signals 1 & 2 will be sent to a time delay circuit that will delay the signals for a very long period of time until the time delayed switch will switch back to the first pair of signals after the electronic scanner has scanned the whole screen and has returned to the first line of pixels once again, but to avoid letting the signals that went through the time delay circuit be cut off, the new bits of data coming from signals 1 & 2 will be sent to the delay circuit while it is the previously delayed signals that will now be allowed to scan the screen.

    The following steps are a summary of this process:

    1.) Signal 1 scans the first half of the first line of pixels while signal 2 is delayed. To prevent data from signals 3 & 4 from being lost, they are both, in the meantime, sent to a delay circuit.

    2.) Signal 2 scans the second half of the first line of pixels while the next scan from signal 1 is delayed.

    3.) The delayed signal 1 is then allowed to scan the first half of the second line of pixels while the next scan from signal 2 is delayed.

    4.) This process is repeated until the first half of the screen has been scanned by signals 1 & 2.

    5.) The time delayed switch then switches the connections to signals 3 & 4 to let them scan the last half of the screen and at this time, signals 1 & 2 are both delayed.

    6.) After the whole screen has been scanned and the scanner once again returns to the first line of pixels, the time delayed switch switches back to signals 1 & 2 but to prevent the delayed data from signals 1 & 2 that did not scan the screen when signals 3 & 4 were scanning the screen from being lost, the new data from the signals 1 & 2 as well as signals 3 & 4 are all delayed to allow the previously delayed data to scan the screen. The new data from signals 3 & 4 then scan the lower half of the screen before the newly delayed data from signals 1 & 2 scan the first half of the screen. During the entire process that the delayed signals from signals 1 & 2 then signals 3 & 4 scan the screen, new data from signals 1 & 2 are delayed.


    Additionally, when an image or video is enlarged beyond its resolution on the screen, the data passes through an 'expander' circuit that multiplies the number of pixels that each bit of data will occupy so that if a certain bit of data is supposed to generate a specific color output, that bit will be duplicated in multiple pixels to enlarge it and this will be done to all the bits of data to enlarge the entire image or video. Of course doing this will make the output look blurred.


    During the age of video tape players and the beginning of home video, the players already incorporated the use of RAM. This is so that if the tape is paused, the pause command will activate the RAM which will store the frame that the video is currently at and a circuit that will continuously transmit the data from the frame by reading the RAM card over and over to maintain the video output at the paused frame. This technology of saving the frame in the RAM card is also the same one used in modern VCD, DVD, Blu-Ray and hard drive players.


    In the case of modern VCD, DVD, Blu-Ray and hard drive players, the video file has information about the time encoded into each frame so that if the player is stopped, the time can be saved in a RAM card or flash drive to be reloaded so that the player can automatically jump to the frame by reading the frames at maximum speed until the matching time frame is reached to almost instantaneously jump back to the time when the video was stopped.
    Last edited by a moderator: May 6, 2017
  11. Aug 14, 2012 #10


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    Once you have the facility for frame (or multiple frame) stores then anything becomes much easier - because you can read in and read out at different speeds. Your "deducting" process is just suitably filtered (and easily achievable) sub-sampling of a full frame picture and storing this in the required place in the final frame, ready to be displayed.
    There can be more involved, of course, if you want smooth merging of multiple picture sources which may not be arriving at the same frame rate. Not difficult to implement with the help of a bit more frame storage, though.

    Your mention of home VCR's reminded me that the mechanical timing of the helical scan VCR was so poor that the technology just had to wait until there was enough, cheap, memory available to allow enough timing correction to make the replayed picture visible. Nowadays, such 'buffering' is just taken as 'a given' but it wasn't always the case.
  12. Aug 14, 2012 #11
    That is correct, the use of VLSI technology to increase the data density of memory devices so that they can perform more data intensive operations is something that people should appreciate more because all of the modern digital electronics conveniences are possible only because of this.
  13. Aug 16, 2012 #12
    Additionally, it is possible to allow multiple television channels to be shown on the screen by having a tuner that can be set to accept multiple signal frequencies at the same time. This is done by having separate sets of variable filters that can each be adjusted separately so that each one can be given an assigned frequency.
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