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terrastudios

LM1881 project

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Hi all,

I'm thinking of playing about with the LM1881 and an AVR but just wanted to run something by someone, hope you dont mind.

I've got 4 video inputs which I want to check to make sure each has a "valid" video signal; i felt the easiest way to do this is to feed the 4 camera inputs into some form of video switch, then the output of the switch to the LM1881. Using the AVR i can then count hsync/vsync pulses to determine if i think a video signal is present.

As the camera inputs go off to another device I was wondering if i should amplify/decouple each video source, i'm not really bothered about the quality of image on my board as i'm just looking at the sync pulses, but i don't want to degrade the signal on the other device.

So how i picture it is (excuse the ascii art!)

Video in 1  --- --------------------------------------------- video mux in 1 ---

                     |                                                                                |

                     -------- other device video in 1                                       |

                                                                                                      | 

Video in 2  --- --------------------------------------------- video mux in 2 ---

                     |                                                                                |

                     -------- other device video in 2                                       |

                                                                                                      |

Video in 3  --- --------------------------------------------- video mux in 3 ---

                     |                                                                                |

                     -------- other device video in 3                                       |

                                                                                                      |

Video in 4  --- --------------------------------------------- video mux in 4 ---

                     |                                                                                |

                     -------- other device video in 4                                       |

                                                                                                      |

                                                              LM1881    ---------------------

                                                                  |  |

                                                                  AVR

Thanks again!

Matt.

Edited by terrastudios

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I was wondering if i should amplify/decouple each video source, i'm not really bothered about the quality of image on my board as i'm just looking at the sync pulses...

There will be no need to buffer the video that is shared by the LM1881. You will need to use a suitable Hi-Z Video Mux design that can deal with the voltage range of the video signal.

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In case you like a shortcut solution, you might consider the MAXIM-ICs "MAX7461

Loss-of-Sync Alarm" and "MAX4311" or "MAX4314" (High-Speed, Low-Power, Single-Supply Multichannel, Video Multiplexer-Amplifiers).

There is a thread in this forum regarding a diversity receiver solution where a similar technique is required. You'll find the thread here:

http://www.rc-cam.com/forum/index.php?show...topic=1324&st=0

At the maxim-ic homepage you can easily order a couple of samples for free (limited quantity per person). If you need more samples of one special device, just ask your friends, relatives, neighbors,... if they might like to order samples for you as well ;-)

Cheers,

Hartwig

Edited by Hartwig

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There will be no need to buffer the video that is shared by the LM1881. You will need to use a suitable Hi-Z Video Mux design that can deal with the voltage range of the video signal.

Thank you for your reply, it is appreciated!

I'm thinking something like a 4066 (as it has quite a high Ron), with a 2.5v bias applied via a simple potential divider, and of course a suitable electrolytic cap on each input to stop the bias effecting the other devices connected to the video input and another electrolytic on the other side of the 4066 to remove the bias voltage as the LM1881 wants a maximum of 2v p-p.

                      5v-->Res (10k)--><--Res (10k)<--GND

                                   |

   Video input  --> || +(47uf 16v) -->---------> 4066 ---> || +(47uf 16v) --> LM1881

Cheers

Matt.

Edited by terrastudios

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In case you like a shortcut solution, you might consider the MAXIM-ICs "MAX7461

Loss-of-Sync Alarm" and "MAX4311" or "MAX4314" (High-Speed, Low-Power, Single-Supply Multichannel, Video Multiplexer-Amplifiers).

There is a thread in this forum regarding a diversity receiver solution where a similar technique is required. You'll find the thread here:

http://www.rc-cam.com/forum/index.php?show...topic=1324&st=0

At the maxim-ic homepage you can easily order a couple of samples for free (limited quantity per person). If you need more samples of one special device, just ask your friends, relatives, neighbors,... if they might like to order samples for you as well ;-)

Cheers,

Hartwig

Hi Hartwig

Thanks also for your reply - i did not see it before i posted the above response. The problem of course with any of these solutions is the need for dual rails unless the video has a dc bias applied to it, but the MAX4314 looks quite promising if i sit this on 0v gnd potential with a ~8v input voltage on VIN, as the maximum video input voltage is VIN - 2.5v - so with a DC bias of 2.5v and 1v p-p video signal i'd be looking at about a 3.5v input.

As its an op-amp i guess it won't have any negative effect on the input video signal which goes to the other device; again i'd have to put a suitable electrolytic before the dc-bias resistors to stop the dc-bias voltage from affecting the camera or the other device. On the output of the MAX4314 i'd put another electrolytic to block out the dc-bias to the LM1881.

Sound reasonable?

Thanks!

Matt.

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Hi Hartwig,

Thanks for the reply - your bottom schematic looks great :)

Do you see any problem in removing the input 75R resistor, as the other device has this already (so the input video signal will already be 1v peak to peak) - and i'll almost T-piece the video input - one will go to the op-amp buffer that you posted and the other side will continue on to an LCD monitor.

Thanks again,

Matt

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Of course, if the signal is already terminated on this side, no second termination should be applied.

You can daisy chain quite a couple of video devices, just make sure the first as well as the last device of the chain is terminated with 75ohms.

For example, a camera has an output impedance of 75 ohms at its video output connector. So when you connect it (for example) to a Monitor, the Monitor needs to have an input impedance of 75ohms to perform proper termination of the transmission line. You can hook up another device between the camera and the monitor as long as this device has a high impedance input >> 75ohms.

Edited by Hartwig

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Of course, if the signal is already terminated on this side, no second termination should be applied.

You can daisy chain quite a couple of video devices, just make sure the first as well as the last device of the chain is terminated with 75ohms.

For example, a camera has an output impedance of 75 ohms at its video output connector. So when you connect it (for example) to a Monitor, the Monitor needs to have an input impedance of 75ohms to perform proper termination of the transmission line. You can hook up another device between the camera and the monitor as long as this device has a high impedance input >> 75ohms.

Thanks Hartwig,

Matt.

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