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Understanding the Need for Positive Sync Polarities in Sony PVM Monitors

Note: This post has been edited several times during its lifetime, and some questions may have already been answered.

I'm in the process of setting up a Sony PVM monitor using a VGA to BNC cable connected to a computer running CRT Emudriver. My PVM's inputs include RGB and Sync, and after combining the H and V sync signals, I've been advised to enable composite sync mode on CRT Emudriver and ensure both polarities of the sync signals are set to positive in the mod lines.

I'm looking to understand why the Sony PVM specifically requires both sync signals to be positive for correct image display. Additionally, I'm interested in learning about the technical differences between PVMs and regular CRT monitors that necessitate this kind of sync polarity adjustment.

Any insights or technical explanations on why the sync polarity needs to be adjusted this way, and how it affects the display output, would be greatly appreciated. If you have links to resources or documentation that could help clarify this, please share!

Model Number: PVM-14M1J (japanese model)
Manual: PVM M-Series (this is not the exact model manual but for a similar model, unfortunatly I could not get the manual for this exact model)

Connecting to the PVM

I'm using PC CRT Emudrive on Windows 10 with an AMD 5450 graphics card. It's basically a way to output a 15kHz signal from the PC graphics card through a VGA to RGBHV BNC cable and into my PVM.

The H(sync)-BNC & V(sync)-BNC connections are combined via a BNC T-Connector into one and then connected to the SYNC IN connection on the back of the PVM.

After configuring CRT Emudrive to output 15kHz through the VGA port on the graphics card, I was advised to enable "composite sync" (Here is a video I found of a guy using the feature).

From what I gathered, the CRT Emudriver's composite sync function is used to combine horizontal and vertical sync signals into a single composite sync signal. This is particularly useful when dealing with CRT monitors or televisions that only support a composite sync input, rather than separate sync signals, such as my PVM (PVM-14M1J).

The same person who told me about that also told me that I would then need to get into my Modelines and change both H(sync) and V(sync) to positive, as this is the only way the PVM could receive a proper image.

Summing Up

  1. Is it necessary to connect the H(sync) and V(sync) cables together (via a T-Connector) to get an image (whether clear or at all) on the screen?
  2. Is it necessary to change both polarities of the modelines to positive?
    If so, why is it necessary to have both polarities set to positive on a PVM?
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  • I don't think there is any deep technical difference: Hardware can work either one way or the other way, and before there were standards, some hardware worked one way, and some hardware worked the other way (and some could do both). So it was the graphics card that grew to have a control bits to adjust polarity, and for example the X driver modelines had the capability to set this as far as I can remember learning about modelines. The way it affects display output is usually "if your monitor doesn't understand that specific polarity, then no picture, or a garbled picture".
    – dirkt
    Apr 21 at 20:53
  • What I gather from this is that, first, if the polarities are incorrect, adjusting them until the picture looks right is a reasonable approach. However, I still wonder about the preference for positive polarity. Was it chosen simply because Sony thought it was the best option at the time, perhaps for compatibility with other equipment of that era?
    – Tai Alt
    Apr 21 at 21:08
  • I have also been told, though I cannot confirm, that positive polarity in sync signals can be beneficial for signal integrity. Positive sync signals may be less susceptible to noise and easier to differentiate from other types of signal interference in some technical setups. This is crucial in professional video settings where maintaining signal integrity and quality is paramount.
    – Tai Alt
    Apr 21 at 21:09
  • @TaiAlt Please mention which model number CRT it is and what resolution you are using.
    – Justme
    Apr 22 at 4:54
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    @TaiAlt The instructions already went wrong when you were told to connect two outputs together, the HSYNC and VSYNC, with BNC T-piece. I saw such an instruction too and was horrified. First of all, you must not connect digital outputs together. Second, the exact reason to choose composite sync out from your video card is that you don't have to combine HSYNC and VSYNC externally. Third, the reason to choose positive sync applies only to some video cards and drivers because the video card used the other polarity the software tries to set, so generally negative should be OK.
    – Justme
    Apr 22 at 16:05

3 Answers 3

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Summary

  • Do not connect H-sync and V-sync outputs together with a T-connector! (See below for more details.)
  • Your Sony PVM requires negative-going composite sync, and positive-going sync will probably not work.
  • The "set positive-going H and V sync" requirement in some older models of ATI video cards is an artifact of how it's configured to generate composite sync; the positive-going H and V sync settings cause the card to generate a negative-going composite sync.

PVM

Your Sony PVM does not require positive sync. In addition, in general there is no difference between PVMs and "regular" CRT monitors that use composite sync; both take negative-going sync. (Monitors that use separate horizontal and vertical sync, however, do sometimes require specific sync polarities to activate certain video modes, as mentioned in Janka's answer.)

The PVM-14M1J is probably a pretty standard item in the line-up of Sony PVMs. I can't find any documentation on it specifically, but the 'M' indicates that it's a late '90s model (after '50', before early-2000s 'L'), the 1 indicates a particular variant of the model (often lower or higher TVL resolution), and the 'J' suffix indicates the country in which it was sold (Japan).

You can see details of some very similar models in this brochure. In particular, the PVM-14M2U will be very similar to your model: it's a US model within the same screen size and series.

On page 4 of that brochure they give a specification for the sync input (applicable to all models in that brochure): 4Vp-p ±6dB, negative.

Ext sync

Note that this is pretty standard; any PVM that can take a CVBS ("composite") input must understand negative sync, because that's what CVBS signals use. And I in fact have never seen a system that specified positive sync for a composite sync signal (as opposed to separate H and V sync, where positive sync is not uncommon) and I highly doubt that PVMs would even accept positive-going sync (though I've not tried it myself).

If it provides any comfort, I have an earlier Sony PVM, a PVM-9045Q, and I use negative-going sync into the sync input (both TTL and 0.7 V) all the time.

Signal Source

The H(sync)-BNC & V(sync)-BNC connections are combined via a BNC T-Connector into one and then connected to the SYNC IN connection on the back of the PVM.

DO NOT DO THIS. This drives one of your source outputs with another output, and it can damage the outputs or other parts of the source device. (I would hope that it won't cause damage if done for only a short period of time, but it really depends on the exactly design of that circuitry in the source device.)

This also will produce a sync that it so out of spec it may not work (though that depends on the display's sync recovery). This method of combining sync will drop all horizontal sync signals during the vertical sync interval, whereas in composite sync the horizontal sync signals suppress the vertical sync.

If you need to combine H/V sync in hardware, you need an active circuit to do this. A reasonable (if imperfect) hack is to use an XOR gate, but that's a topic for another question.

But, though you need composite sync for that monitor, you don't need to generate it from H/V sync if your signal source also supports producing composite sync itself, which it appears to do. So yes, turn that on, and then connect the appropriate output from your video card (usually the H sync output is changed to send composite sync instead) and you'll be set. This will certainly be sending negative-going sync, not positive going.

Regarding the sync polarity settings, the relevant text from the video you posted is at 1:26:

enter image description here

Composite sync [\.\DISPLAY5]
For legacy cards (pre-HD 5xxx) after enabling c-sync you must set both sync polarities to positive (1) in your monitor preset and regenerate and install all video modes.

Especially given the notation that you must do this only for some cards, and not all, and that those cards are older ones, I am confident that this setting is not producing positive-going composite sync but is required for some trick they're using to make the card produce composite sync.

It's not possible to be absolutely sure without checking the actual composite sync signal being produced, but I would bet a fair amount that if you check it with an oscilloscope (even a super-cheap one will do) you'll find that you have a negative-going composite sync signal there.

(In a previous version of this answer I mentioned that "this whole thing could be something to do not with sending positive sync to the PVM, but an internal configuration of your signal source that still ends up sending standard, negative-going composite sync to the PVM." That's what's happening here.)

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    Isn't this just a lengthy comment? I don't see how this answers the question, which asks about the need for positive sync polarities.. Apr 22 at 6:04
  • @thebusybee There is no such need for the PVM in question, as the bold comment makes clear, and I strongly suspect that postive sync won't even work (though it might; I've not tested it). The manual for my Sony PVM makes no mention of positive-going sync, as best I recall.
    – cjs
    Apr 22 at 6:10
  • Model Number: PVM-14M1J (Japanese model). Since it's the Japanese model, the manual I found was also in Japanese. It could be that this version is very similar to other models, making finding a manual in English a simple task.
    – Tai Alt
    Apr 22 at 7:57
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    @TaiAlt Or I can just read the Japanese manual. For any documentation you've found, you should provide a link to it. (That should go in the question, along with the other missing information I mention.)
    – cjs
    Apr 22 at 7:59
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    @thebusybee The question asked, "I'm looking to understand why the Sony PVM specifically requires both sync signals to be positive for correct image display." ...So, cjs answering "It does not." is perfectly valid.
    – FeRD
    Apr 23 at 2:06
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In older VGA monitors, both hsync and vsync polarity tells the monitor which size it has to zoom vertically. If you use the wrong combined sync polarity for a given vertical resolution the picture is still displayed but too tall or too short.

Non-VGA monitors may require either both negative or both positive syncs depending on their internal circuitry. They ignore that zoom hint anyways.

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    This is a standard TV monitor requiring standard negative sync. It has no modes as it just supports standard TV scan rates.
    – Justme
    Apr 22 at 8:40
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The service manual says the monitor has external sync defined as negative polarity, as expected from composite sync signals for standard 525/625 line TV scan systems, either from composite video input, sync-on-Y input, sync-on-green input, or external sync input.

So if you are told that you should set your program to positive sync settings, it's up to the program what to do with it and what the actual hardware output is. Maybe with other setting it just outputs something that does not work.

So what you select in the program may not have any relation to which TV monitor you use. They should all be sensitive to leading negative edge.

There are multiple ways to combine the H and V sync into composite. Some methods (H XOR V) will swap the horizontal leading edge polarity during vertical sync, and both AND and OR (depending on input polarity) will suppress horizontal pulses during vertical sync.

A proper solution would be to output short pulses for HSync and long pulses for VSync, which it might already do when configured for composite sync output, as it must do that for Sync-On-Green and Sync-On-Y anyway. Even more proper solution would be if the card can output proper syncs for these TV standards, with half-line/double-frequency serrated H and V pulses, but they are not absolutely mandatory, with the assumption of progressive/non-interlaced output.

Some forum posts and release notes about the subject mentioned that some cards had driver problems and in composite sync mode the cards used the opposite polarity of what was requested.

So it is confirmed, driver/card issue. Nothing to do with the TV monitor in question that requires standard negative polarity.

To recap your last update:

  1. No, it not necessary and it is completely wrong to short circuit HS and VS outputs together with a BNC at monitor input. You already selected to output a composite sync from the video card.

  2. Yes, it is necessary to select positive sync if there is a bug in video card drivers or documentation so that only positive sync settings make it output at least something remotely compatible with the monitor which works with standard negative polarity composite sync like any other standard TV monitor.

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