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  1. #1
    Member KennyFeng's Avatar
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    do audio patch cables introduce delay into the path?

    In a TV production system there are video and audio patch bays on which we plug in patch cables of various length. It is known that cables introduce small amount of delay that leads to audio phase shifting and I am wondering if such delay is significant enough to cause Lip-Sync error?Has anybody ever done tests about it? THX.

  2. #2

    Speed of Propagation, analogue audio.

    Hi Kenny, The answer is no, not unless there is a serious imepdance issue caused by a fault. The answer can be found easily. The speed of propagation for electrons in an analogue circuit approaches the speed of light. What you need to calculate is how long does it take for audio frequencies to propagate over a given length of electrical cable, a simple Time = Distance / Speed equation. Transmission-line theory aside, a delay of 50ms would be produced at the end of a wire 15000km long. I doubt if CNN/BBC has a point-point patchbay that long.
    For cables less than a few thousand feet you can ignore any notion of Transmission-line theory and j angles. For cables longer than this then high-frequencies will arrive before low-frequencies due to ZO = [ (R+j2πfL) / (G+j2πfC) ]1/2
    see here for more info. http://audiosystemsgroup.com/TransLines-LowFreq.pdf
    Also, defer I mean refer to John Watkinson for mythbusting Hi-Fi interconnect nonsense.

  3. #3
    Member KennyFeng's Avatar
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    Thank you,buddy. Yeah, apparently,cables are far too short to cause transmission delay,but I still want to know how audio phase is affected in a simple audio production system. Sometimes, we receive SDI video signals embedded with stereo soundtracks that are of wrong phase(e.g.,inverted). How does it happen? Is there anything to do with the transmission path delay?

    P.S. The pdf is a good reference too, which reminds me of my university life.
    Last edited by KennyFeng; 7th Feb 2012 at 12:55.

  4. #4
    Administrator Dave's Avatar
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    Phase and inverted polarity are actually two different things, although you'll often hear them incorrectly used to mean the same thing. If you are receiving an inverted signal, it's because the polarity is inverted, i.e. positive voltage instead of negative and vice versa. This is not the same as a phase problem caused by delay.

    You can "re-invert" the polarity of a signal by using an audio phase inverter
    Dave Owen
    MediaCollege.com

  5. #5
    Member KennyFeng's Avatar
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    Quote
    Quote: Dave
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    Phase and inverted polarity are actually two different things, although you'll often hear them incorrectly used to mean the same thing. If you are receiving an inverted signal, it's because the polarity is inverted, i.e. positive voltage instead of negative and vice versa. This is not the same as a phase problem caused by delay.

    You can "re-invert" the polarity of a signal by using an audio phase inverter
    Thank you,Dave.

    I guess the "polarity" defines if the signal is "hot" or "cold" in a balanced transmission system which I don't think is what I am concerned here. Let me put it like this, let's put video things aside---what happens when delay in time-domain (say, caused by encoding/decoding process) is introduced in one or both of the channels in a stereo environment where L/R sound used to keep a certain constant phase relation?Will the listener sense wrong positions of various sound sources?

  6. #6
    Administrator Dave's Avatar
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    Ah yes, there certainly can be a problem caused by encoding processes. I recall one channel I worked for that had about 4 seconds of delay introduced by various digital processes - it was a real pain to work with. In this case all we could do was delay the video to match the audio. We used to go through a lip-synch procedure at the start of each day in which the video was progressive delayed until it matched the audio.

    I can't remember exactly why the audio was delayed so much more than the video - it had something to do with our comms system.
    Dave Owen
    MediaCollege.com

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