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  1. #1
    Member KennyFeng's Avatar
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    On video and audio isolation transformers

    Hi,I am a rookie in the TV industry. I’ve always had the following questions on video and audio isolation transformers;
    1. When dealing with VBS signal: if an external signal is fed into a system whose reference ground potential level differs from that of the TX end where the signal is originated, a horizontal crawling black bar appears on the receiver’s picture monitor. What’s the reason for this phenomenon?(because out-of-phase?)
    2. Under the same conditions given in topic 1, what would it be like with a digital component signal input? Nothing on the monitor? If so, is it because of that the potential difference between the TX end and RX end is so notable that the receiving amplifier just can’t work?
    3. Some say that the purpose of using a video signal isolation transformer is to “pull the reference level of the input external signal to the same level as that of the receiving system”. How does the “pull” action works? Is it the “ground-canceling” effect when coupling with a transformer?
    4. As with analog audio, the difference of ground potentials of TX and RX systems result in “humming”, is it because that the power lines’ EMF disturbs the audio signal? How? because of ground loop between the two grounding points and is it 50Hz? How to solve it? Using transformer isolator? How does the transformer work?
    Looking forward for your opinions.

  2. #2
    Senior Member SC358's Avatar
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    1. When dealing with VBS signal: if an external signal is fed into a system whose reference ground potential level differs from that of the TX end where the signal is originated, a horizontal crawling black bar appears on the receiver’s picture monitor. What’s the reason for this phenomenon?(because out-of-phase?)
    Hi Kenny, Welcome to Media College! I hope my explanations will make sense to you. If not, just let me know so that I can try to work on being comprehensive.

    If you just record an external signal without a Reference on the record machine AND monitor, it is not out of phase. You can record a tv show (whether vhs or dvd) and there is no sync issues. Reference ground potential does not play a part in a broadcasted signal between TX to RX. Only WITHIN a TX site or RX site could there be potential differences. An external signal needs to be gen-locked to a house system (usually with a Frame Synchronizer) otherwise it would be “out-of-phase” or out-of-sync with the rest of the house (this is my assumption the rest of the house equipment is gen-locked or receiving a reference signal).
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    2. Under the same conditions given in topic 1, what would it be like with a digital component signal input? Nothing on the monitor? If so, is it because of that the potential difference between the TX end and RX end is so notable that the receiving amplifier just can’t work?
    The digital world is a different place. Since it’s all ones and zeroes, the receiver does need to catch a signal. If it doesn’t it’s a zero, therefore you receive nothing. This is what is known in many parts of the industry as, “the digital cliff”, where the signal is there or it’s not. In the analog world, if a signal was degraded, you would see, “something” but you knew a signal was still there.
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    3. Some say that the purpose of using a video signal isolation transformer is to “pull the reference level of the input external signal to the same level as that of the receiving system”. How does the “pull” action works? Is it the “ground-canceling” effect when coupling with a transformer?
    I’ve used a video isolation transformer called a Hum Bucker, which was to remove rolling hum bars in the active video. Sometimes it worked, other times it didn’t. In regards to your explanation, I cannot help confirm or deny it - I’ve never heard it said that way before. I don't know if the term, "pull" is proper usage in conjunction with a transformer, this is why it doesn't make sense to me, sorry.
    SC358
    Relationships are based on compromises - behavior accepted is behavior repeated.

  3. #3
    1. Not sure how the standards translate to China, cuz our power line frequency is 60Hz. Brightness at any point in the display corresponds to instantaneous signal voltage. A ground loop adds AC mains-frequency hum to that voltage, which causes brighter-than-right and darker-than-right bands. In the US, they crawl because the video frame rate is not quite exactly 30Hz, so there isn't quite a 2:1 ratio. If you've ever listened to two instruments that are just slightly out of tune, you'll get an audible "roll" that's equivalent to the video slow crawl.

    So yes, at least in the US it's because the phase of the video's vertical sync is constantly, slowly changing relative to the AC mains. (In the audio seminars I do, I teach my students that when they hear a buzz they should listen carefully - if it stays the same, it's dimmer noise; if is slowly "rolls" it's video interference.)

    2. "Hum bars" of this type usually don't show up with digital because a slight instantaneous voltage shift doesn't matter. AC hum voltage is still added to the data stream, but it doesn't "tell" unless it's high enough to actually swamp the digital 1's and 0's. If you get no video at all, it may be that the AC ground loop voltage is huge - or it may be a standards compatibility thing or something else entirely.

    3. If the video transformer is a true isolation transformer - some aren't - there's a better explanation than "pulling." If you let the monitor do what it wants, it will establish its own reference voltage based on the maximum negative-going part of the video signal, which is the sync pulse. This is historically called "clamping." A low-impedance AC hum source will force the monitor off its reference. An isolation transformer doesn't really "cancel" the hum; it simply disconnects the circuit, thus opening the loop. That may not make sense, but it will in a moment.

    4. A transformer works by converting the electrical signal to magnetic fluctuations, and then using those fluctuations to regenerate the electrical signal. Thus there is no direct electrical connection between one side of the transformer and the other - hence no ground loop.

    Ground hum is generated one of two ways. It is either induced in a signal wire magnetically, by being in proximity to an AC mains circuit which is carrying significant current and thus creating a magnetic field - or it is the result of a small AC voltage difference that often exists between "this" ground and "that" ground. Since the sending unit puts out "X" signal against its local ground, and the receiving unit "measures" this signal against its local ground, from the receiving unit's viewpoint the signal contains the AC voltage difference between the two grounds. When you insert a transformer, the receiving unit only "sees" the voltage across the transformer's output, with no reference to any distant ground.

  4. #4
    1. Not sure how the standards translate to UK, cuz our power line frequency is 60Hz. Brightness at any point in the display corresponds to instantaneous signal voltage. A ground loop adds AC mains-frequency hum to that voltage, which causes brighter-than-right and darker-than-right bands. In the US, they crawl because the video frame rate is not quite exactly 30Hz, so there isn't quite a 2:1 ratio. If you've ever listened to two instruments that are just slightly out of tune, you'll get an audible "roll" that's equivalent to the video slow crawl.

    So yes, at least in the US it's because the phase of the video's vertical sync is constantly, slowly changing relative to the AC mains. (In the audio seminars I do, I teach my students that when they hear a buzz they should listen carefully - if it stays the same, it's dimmer noise; if is slowly "rolls" it's video interference.)

    2. "Hum bars" of this type usually don't show up with digital because a slight instantaneous voltage shift doesn't matter. AC hum voltage is still added to the data stream, but it doesn't "tell" unless it's high enough to actually swamp the digital 1's and 0's. If you get no video at all, it may be that the AC ground loop voltage is huge - or it may be a standards compatibility thing or something else entirely.

    3. If the video transformer is a true isolation transformer - some aren't - there's a better explanation than "pulling." If you let the monitor do what it wants, it will establish its own reference voltage based on the maximum negative-going part of the video signal, which is the sync pulse. This is historically called "clamping." A low-impedance AC hum source will force the monitor off its reference. An isolation transformer doesn't really "cancel" the hum; it simply disconnects the circuit, thus opening the loop. That may not make sense, but it will in a moment.

    4. A transformer works by converting the electrical signal to magnetic fluctuations, and then using those fluctuations to regenerate the electrical signal. Thus there is no direct electrical connection between one side of the transformer and the other - hence no ground loop.

    Ground hum is generated one of two ways. It is either induced in a signal wire magnetically, by being in proximity to an AC mains circuit which is carrying significant current and thus creating a magnetic field - or it is the result of a small AC voltage difference that often exists between "this" ground and "that" ground. Since the sending unit puts out "X" signal against its local ground, and the receiving unit "measures" this signal against its local ground, from the receiving unit's viewpoint the signal contains the AC voltage difference between the two grounds. When you insert a transformer, the receiving unit only "sees" the voltage across the transformer's output, with no reference to any distant ground.

  5. #5
    Member KennyFeng's Avatar
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    Thank you for your clear explanations. I found them very helpful for me to draw the following understandings:
    1. The term “out-of-phase” that I used in my original post is totally wrong. For a VBS signal, which is 625i/50Hz in China(525i/60Hz in the States), frame rate 25Hz, a ground loop adds a power line frequency hum to it, which causes incorrect brightness(according to Karl, brighter-than-right and darker-than-right bands with some crawling,too).
    2. For digital video, there is no such thing. Thanks to error correction techniques, we can still receive the series of 0s and 1s and calculate “the right” series, even if the hum voltage is added to them, unless the hum is so significant that seriously destroys the electrical waveform that is transmitted in the cable.
    3. And there is no “pulling” thing which was my ridiculous imagination. The right term should be “clamping”, which Karl used when explaining to me the process of sync restoration. In our case of a transformer, it just disconnects the ground loop, thus no ground current whose magnetic fields would interfere with our signal.
    4. When connecting two machines, say, a VTR and a monitor, who are powered by different mains, without using a video signal isolation transformer, would it be electrical shock risk when touching the shield of the BNC cable between them? (which is a situation that one of my colleagues had experienced and problem was solved by adding a trans)
    It’s nice of you to answer my questions. I love this forum.
    P.S. I found a site about balanced line transmission. Hope it makes sense here.
    http://www.dself.dsl.pipex.com/ampin...d/balanced.htm

  6. #6
    I have no idea how the duplicate post happened.

    Yes, two cables grounded to two different mains supplies can have a high enough potential between them to give a shock. This is especially true as lighting storms pass through the area. I tell my students that when connecting a cable that originates in another building or farther away, an isolation transformer should always be used.

    I looked at the site you linked to and I highly recommend it to everyone. It is extremely lucid and thorough.

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