Front and back porch, etc

Discussion in 'Projectors, Screens & Video Processors' started by tryingtimes, Aug 8, 2006.

  1. tryingtimes

    tryingtimes
    Well-known Member

    Joined:
    Feb 15, 2001
    Messages:
    4,094
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Ratings:
    +531
    Hi there
    A newbie question this, especially for a crt owner, but....

    What role do Front and Back porches have in modern preogressive displays? And why do we use them in our VPs when it uses valuable pixel clock bandwidth?
    1080x1920x75 = 155MHz
    But once you add in all the Porches etc, you get way over the limit of most VPs and maybe even cables and other peripherals.

    As I said - complete newbie question, so apologies.

    Oh and I know that 75Hz isn't as necessary with progressive displays but it could be again with some of the new display technologies which draw the image line by line.

    Cheers
    tt
     
  2. JohnWH

    JohnWH
    Active Member

    Joined:
    Jan 23, 2004
    Messages:
    1,826
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    48
    Location:
    St Albans
    Ratings:
    +82
    Good question!

    I may be wrong here, but I think its so that non fixed pixel displays could be driven directly by DAC's without a frame store, making it hystorical?

    You also wouldn't have anywhere to hide audio without them.

    John.
     
  3. Welwynnick

    Welwynnick
    Well-known Member

    Joined:
    Mar 16, 2005
    Messages:
    7,274
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    133
    Location:
    Welwyn, Herts
    Ratings:
    +942
    Front and back porches are very necessary with scanning displays, so there is sufficient time for the line retrace, and the new scan can be stablised in time to display the line linearly.

    Fixed pixel displays don't need these dead times, but the analogue interfaces have to be consistent with established standards that support CRT displays.

    Digital interfaces are a different matter entirely, and porches and sync pulses are handled in a different way entirely. There are still frame and line sync periods, but these are signalled digitally rather than analogue.

    I don't believe that audio comes into it; that's multiplexed at a different level.

    Nick
     
  4. JohnWH

    JohnWH
    Active Member

    Joined:
    Jan 23, 2004
    Messages:
    1,826
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    48
    Location:
    St Albans
    Ratings:
    +82
    I'm pretty certain the audio is embedded in "data islands" that live in the non active periods of the video signal, at least thats what it looked like last time I looked at the spec!

    Obviously if DVI had originally been designed to drive fixed pixel displays only there would have been no need for blanking periods, so they would have had to come up with another way of carrying the audio data in HDMI, in fact we may even of ended up with a CE standard interconnect based on something else...

    John.
     
  5. tryingtimes

    tryingtimes
    Well-known Member

    Joined:
    Feb 15, 2001
    Messages:
    4,094
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Ratings:
    +531
    ok - I understand the need for them to exist for analogue connections.

    Nick, you say that they
    So does that mean that they don't actually take up processing bandwidth.
    The only reason I ask is that Crystalio II has a 165MHz max bandwidth and I wondered about 1080p75, whether it's within spec because it's 1080x1920x75 = 155MHz or outside because of the additional porch info.

    It's all theorectical - by the time I need 1080p I'll probably be on a digital display running 48, 50 and 60Hz anyway.
     
  6. JohnWH

    JohnWH
    Active Member

    Joined:
    Jan 23, 2004
    Messages:
    1,826
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    48
    Location:
    St Albans
    Ratings:
    +82
    Yes they increase the required BW, you migth be able to cut the size of the blanking down a bit dpending on your display device, but I'd be surprised if you could do it enough to bring the CII into spec (I think you need ~200MHz)

    Note that I've also found "strange" issues when messing with the timings when the signal is HDCP encoded.

    John.
     
  7. Welwynnick

    Welwynnick
    Well-known Member

    Joined:
    Mar 16, 2005
    Messages:
    7,274
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    133
    Location:
    Welwyn, Herts
    Ratings:
    +942
    Yes, they do eat up some processing bandwidth, but IIRC it's not as much as the 2200x1125 total resolution that VGA uses for example. It's in the DVI and HDMI specs, but take a bit of time to find.

    By the way, I thought that audio was multiplexed into different channels to video within the shared HDMI TDMS channels, rather than being saved up for the blanking periods (which WOULD be nice and efficient). But I'm not sure. Anyone got an HDMI spec and a spare half hour?

    Nick :)
     
  8. JohnWH

    JohnWH
    Active Member

    Joined:
    Jan 23, 2004
    Messages:
    1,826
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    48
    Location:
    St Albans
    Ratings:
    +82
    Had a look at the spec again today, it definitely shows the data islands containing the audio in the blanking periods.

    John.
     
  9. bushwakko

    bushwakko
    Standard Member

    Joined:
    Jan 5, 2009
    Messages:
    18
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    1
    Ratings:
    +0
    Anyone have any idea where I can find info on this on specific models. I'm trying to find these settings for my Samsung LE40A786, so that I can generate a valid 1080/24p setting with 23.976hz on mu mac.
     

Share This Page

Loading...
  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice