HDMI connector

Discussion in 'Displays' started by Gary Hunter, Nov 16, 2003.

  1. Gary Hunter

    Gary Hunter Agent

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    I am new to this board and will likely be asking a lot of questions as I am beginning the construction of a new audio/theater room.

    As I am planning for the pre-wire of the room, I am trying to decide if I need to run HDMI to the front projector. I see the advantage of HDMI for connecting source components (which none of mine have yet) to the a/v processor using HDMI, but since the projector is a video only component, would DVI be just as good? I was thinking I'd pre-wire a run both DVI and HDMI just to have it, if HDMI cables are available by the time I do the work, but now I'm thinking it might not be necessary.

    Thanks

    Gary
     
  2. Richard Paul

    Richard Paul Stunt Coordinator

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    Gary, I would highly recommend wiring with HDMI for the simple fact that it can run a much longer distance than DVI. DVI can only go about 5 meters on average while HDMI has been shown to work at up to 25 meters. Another advantage of HDMI is the fact that it can control components using CEC (Consumer Electronics Control). In the future if you ever buy a projector with HDMI it will work far better connected by an HDMI cable. Also you could use an HDMI cable for wiring with HDMI sockets and then connect them using 2 DVI-HDMI cables. This way you could avoid having to run a DVI cable and be fully prepared for HDMI.
     
  3. Adam Gregorich

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    I would run DVI at the minimum. It would be nice to run HDMI as well, but it will probably be a few more years before we see a lot of products using it. Better yet if you could put a conduit with a pull rope to the projector location you would be ready for anything. I would recommend 2.5 to 3 inch, as both DVI and HDMI cables at this point only come preterminated and have plugs that are wider than the cable.
     
  4. MarkDesMarais

    MarkDesMarais Stunt Coordinator

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    Richard-



    This is interesting- where does this come from? The HDMI standard is using the identical TMDS transport medium as DVI. (Same chips from Silicon Image) I don't see how it could work any differently. . . can you provide a reference?

    Thanks

    Markd
     
  5. Gary Hunter

    Gary Hunter Agent

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    Very interesting comment about the conduit as that is what I plan to do to the projector for the very reason. That will allow me to run another cable at some time if I want.

    Why does DVI not work well over 5m?
     
  6. Richard Paul

    Richard Paul Stunt Coordinator

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    Silicon Image, which helped make the DVI and HDMI specifications, showed at CES of this year a HDMI cable 25 meters (82 feet) long carrying a HDTV signal shown on a HDTV. DVI cables have a recommended limit of around 5 meters using common cable though this can be extended with more expensive cabling (no DVI cable though could transmit even close to 25 meters). HDMI's improvement over DVI was done by improvements in the transmitting and receiving circuitry as well as the smaller HDMI connector which is better suited to a proper PCB (printed circuit board) layout and impedance matching.

    The reason DVI has a recommended maximum of 5 meters is that DVI is sending the largest amount of data that copper cabling can carry. As such it's easy for long DVI cables to get errors and such errors are noticeable as random sparkling in the picture. Analog signals have a gradual decay of the picture in the shape of a gentle downward curve. Digital signals have a line that goes straight for a while and then drops down greatly. With DVI it is possible to have a perfect picture at 18 feet and using the same type of cable at 20 feet your picture could become awful.
     
  7. MarkDesMarais

    MarkDesMarais Stunt Coordinator

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    Interesting. . . check this out on their website-

    http://www.siliconimage.com/products...p#transmitters

    Looks like HDMI is limited to a much lower BW than DVI. . . look at the SiI 9190, which is the HDMI transmitter- max of 85MHz pixel rate, vs 165MHz for the other transmitters. They could also be playing other games- if you look, they don't support full sampling rates as DVI does- about the best they do with 24 bit is 4:2:2, which means some sub-sampling is going on, even less data to send.

    I'd bet that most of the increased distance has to do with the lower frequency. The eye pattern at 850MHZ is much easier than the one at 1.65GHz (TMDS runs at 10x pixel clock rate).

    Wow- I had no idea that HDMI was such a step down from DVI! Ok, for DVDs, and even HDTV, probably not a big deal, since the sources are encoding similarly. But if you are an HTPC user, I'd watch out.

    Markd
     
  8. Richard Paul

    Richard Paul Stunt Coordinator

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    Interesting Mark, but before you jump to any further conclusions you should read this HDMI Specification. Also not only are you basing your opinion of HDMI on one transmitter, but your also mistaken. Read the Product Brief on SI 9190 and you will see that it supports RGB at 24 bits along with 24 bit YCbCr at 4:4:4 and even 4:2:2. This means it can even transmit YCbCr with 12 bits for luminance when studio recording still only uses 10 bits for luminance (or 20 bit YCbCr which the SI 9190 can also do). The SI 9190 can also transmit 2 channel PCM, Dolby Digital, and DTS. The reason it's limited to 85 MHz is because it's only meant for 720p and 1080i displays. By next year Silicon Image will most likely make a improved chip capable of 1080p.

    The SI 9190 transmitter is not the best that HDMI can do. The SI 9190 can't send out 1080p at 60hz nor can it do 8 channel PCM at up to 24bits/192Khz (both of which are in the HDMI specification). At the same time though this is the first HDMI transmitter made and attacking HDMI for this transmitters limitations proves nothing. HDMI as an interface is very well designed and is far better than DVI.
     
  9. MarkDesMarais

    MarkDesMarais Stunt Coordinator

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    Ok, I'm embarassed- thats what I get for trying to read too quickly. ;-)

    I did look at the HDMI spec- mostly to see how they were transmitting all the "other" stuff. Good use of porch times. Not really related to video quality or data rates.

    What is the pixel rate for 1080p? Isn't 4:4:4 specified
    as 75MHz, regardless of resolution?

    I'm still curious as to the discrepancy in cable lengths- the base technology is still TMDS. . . why wouldn't they put the same advances back into the DVI designated tx/rx pairs? Also, I couldn't find any info about cable lengths in the HDMI spec- do you know where they are?

    Markd
     

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