Speaker Delay and Video Sync

Discussion in 'Archived Threads 2001-2004' started by Jeremy_Watson, Aug 27, 2002.

  1. Jeremy_Watson

    Jeremy_Watson Stunt Coordinator

    Joined:
    Jul 30, 2002
    Messages:
    249
    Likes Received:
    0
    Trophy Points:
    0
    I know on nearly all H/T receivers you are allowed (and supposed to, I assume) set the speaker distance (or delay, the 1ft = 1ms rule) so that the sound from all of the channels hits you at the same time. It seems logical that the receiver would delay the video signal a few milliseconds as well, especially since light moves faster than sound. Now assuming this is actually how it is done, my question concerns people who use direct video input to their display devices to bypass any noise it may pick up going through the receiver. So, if the audio is being delayed by the receiver in order to synchronize when it hits the viewer/listener, then doesn't the video appear before the audio would arrive?
    Granted, I don't know how this process actually works so please correct me if I'm wrong, but this is how it seems like it works.

    Thanks for the help.
     
  2. John Garcia

    John Garcia Executive Producer

    Joined:
    Jun 24, 1999
    Messages:
    11,571
    Likes Received:
    25
    Trophy Points:
    0
    Location:
    NorCal
    Real Name:
    John
    Running video through the receiver makes zero difference to the timing of the sound vs the video. The receiver would act as nothing more than a passthrough or switch box for the video, it will not add adjust or add any delay.
    I have never run my video through my receiver, and have never experienced any issues with sync.
    Both sound and light travel fast enough that I don't think anyone would really notice anyway [​IMG]
     
  3. Jeremy_Watson

    Jeremy_Watson Stunt Coordinator

    Joined:
    Jul 30, 2002
    Messages:
    249
    Likes Received:
    0
    Trophy Points:
    0
     
  4. John Garcia

    John Garcia Executive Producer

    Joined:
    Jun 24, 1999
    Messages:
    11,571
    Likes Received:
    25
    Trophy Points:
    0
    Location:
    NorCal
    Real Name:
    John
    Your question was with respect to sound vs what is going on on-screen. The delay does not adjust the timing of sound with respect to events occurring on (or off) the screen, that is already in the recording and the decoder does not alter it. As you already noted, it merely adds a delay so that the sound coming from the front is timed with respect to the sound coming from the rears so that sound reaches your ears appropriately phased, or at the right time.

    If your rear speaker were 180 degrees out of phase, you would certainly notice it, as some frequencies may cancel out at the listening position, reducing the overall output of the system. You CAN hear a difference in roughly 10ms of delay, but the end results will be different depending on how far the speakers actually are from you.
     
  5. Bill Kane

    Bill Kane Screenwriter

    Joined:
    Feb 5, 2001
    Messages:
    1,359
    Likes Received:
    0
    Trophy Points:
    0
    ...sounds good to me John, but I like Jeremy's inquiring mind -- mine is now melting.

    All I can figure (d'oh) is sound at sea level is 1.088 ft/ms and
    light (in a vacuum) is 982,080 ft/ms
     
  6. John Garcia

    John Garcia Executive Producer

    Joined:
    Jun 24, 1999
    Messages:
    11,571
    Likes Received:
    25
    Trophy Points:
    0
    Location:
    NorCal
    Real Name:
    John
    I can honestly say, I'd never thought about a question like this until now. [​IMG]
    The recording determines when sound plays through the speakers and images appear on the screen, not the receiver, is all I was trying to say. The purpose of the funuction is to account for room and speaker placement variances.
     
  7. Lew Crippen

    Lew Crippen Executive Producer

    Joined:
    May 19, 2002
    Messages:
    12,060
    Likes Received:
    0
    Trophy Points:
    0
    When I first noticed this post I went through some calculations to prove to myself that the 1ms per foot delay was correct—and its reasonably close at sea level with a temp of 21ºC. But I did not respond as I don’t really know anything about internal delays in receivers and other gear, as well as how DVD players handle this issue (or even if they do). But then it occurred to me that while 10ms, assuming a sitting position of 10 ft from the speakers is no big deal, this is a problem that sound engineers must have overcome in theatres and for outdoor concerts, where uncorrected delays would be routinely on the order of 100ms.
    Anyone know anything. I’m not a sound engineer
     
  8. Bob McElfresh

    Bob McElfresh Producer

    Joined:
    May 22, 1999
    Messages:
    5,182
    Likes Received:
    0
    Trophy Points:
    0
    The adjustment is to sync among the 5 speakers.
    With video, you only have 1 source so you dont have to worry about timing.
    With a 5 speaker array, having a timing adjustment is important for some special effects like making a sound transition from speaker to speaker rather than simply 'jumping'.
    (or so I always thought [​IMG] )
     
  9. Jeremy_Watson

    Jeremy_Watson Stunt Coordinator

    Joined:
    Jul 30, 2002
    Messages:
    249
    Likes Received:
    0
    Trophy Points:
    0
    Bob,
    I understand the need for the delay with the speakers. I was just curious if causing a delay affects the video/audio synchronization, e.g. hearing an explosion in a movie at the same time you see it on screen. Do you know more about this?
     
  10. Bob McElfresh

    Bob McElfresh Producer

    Joined:
    May 22, 1999
    Messages:
    5,182
    Likes Received:
    0
    Trophy Points:
    0
    Some of the others have already given great answers, and no, I dont have any direct knowledge of any compensation done in the studio to keep the video & audio in sync.

    But think about this: many HDTV's will take every frame of an interlace signal, buffer it, de-intelace it, do 3:2 pull-down, add some rows, then ... display the frame of video.

    The TV has no idea that the audio is routed differently. Yet, all this video processing is done without visable delay between the audio and video. Kind of impressive when you think about it.
     
  11. Chris Lanni

    Chris Lanni Stunt Coordinator

    Joined:
    Aug 1, 2002
    Messages:
    134
    Likes Received:
    0
    Trophy Points:
    0
    Real Name:
    Christopher
    OK. So here is something else to really tear into. Lets say that your front speakers are 10 feet away and your rears are 5 feet away. You set the corresponding distances on your receiver. Now you do this so that you can time the sound effects from front to back, right? Ready to start pondering? Speaker wire? The sound has to travel along the speaker wire before it gets to the speakers, right? Well your front speakers might be 10 feet away from you but the wiring is more like 15-20 feet. And the rear speakers might only be 5 feet away, but the speaker wire is 40+ feet long. So doesn't the sound get slowed down by the travel length? If someone understands this I would appreciate an explanation as I am a neophyte in the home theater arena. Wouldn't you be better off setting your speaker distances based off of how much wire is between the speaker and the receiver?

    Chris
     
  12. Jeremy_Watson

    Jeremy_Watson Stunt Coordinator

    Joined:
    Jul 30, 2002
    Messages:
    249
    Likes Received:
    0
    Trophy Points:
    0
    The speed at which the current travels through the wire is orders of magnitude faster than the speed of sound in air. That is why the consideration is for how far the sound travels in the air before it reaches you.
     
  13. Bob McElfresh

    Bob McElfresh Producer

    Joined:
    May 22, 1999
    Messages:
    5,182
    Likes Received:
    0
    Trophy Points:
    0
    Think of a 40 ft pipe. You shove a ping-pong ball into one end and it takes some ammount of time before it comes out the other side. This is a delay.

    Now take the same pipe, but have it stuffed full of ping-pong balls and you shove a new one into the end. Nearly zero delay but the ball that comes out the end is not the same one you shoved into begining. This is like your speaker wire.

    This is why you dont really have to worry about keeping all your speaker wires the same length.

    In truth, there is a very, very small delay. At another fourm they went through the calculations and determined that it takes a DIFFERENCE of about 80 feet of 12 ga speaker wire to cause a 1% delay at audio frequencies.
     

Share This Page