user19085: I have heard of a video-grip for the Panasonic GH5 Panasonic GH4 that supports SDI-output. Off the top of my head a few questions pop up

What is the advantage of using a SDI-output ?
Does this generally allow for higher performance than what the camera alone would achieve ?

I must admit, I have only completely read the Wikipedia entry for SDI in my mother tongue; which is not as detailed as the english entry.

additional notes: 
This was split off from another question about slow-motion recording. Ideally the camera would be used for photography and video recording of all sorts. 

user14504: In short; HD-SDI and SDI are the industry standard for professional applications; and HDMI is for consumers. 
HDMI is highly limited in terms of length; you will need a very high quality cable to go more than 25 feet. 
HDMI is fine for most applications; but it is limited to 4:2:2; you can't do long runs; if you are trying to integrate it with professional equipment you will likely need adapter boxes (such as those by BlackMagic); 
SDI and HD-SDI offer full 4:4:4; you can genlock easier; you can breakout out easier; you can run it a few hundred feet before it needs to be reclocked; you can run it dual link mode for 4K, and so on. 

user2836: SDI is a professional, locking video connection system over coax cable.  SDI itself has more expensive licensing costs associated with it than HDMI, so hardware that supports it will generally be more expensive.  If you don't have hardware that supports SDI, it won't do you any good, so that's the first point.
One of the biggest advantages of SDI is that it can support very high quality video such as the 4k 10 bit 4:2:2 supported by the SDI outputs for the GH4 adapter.  
SDI also supports a stable signal over much longer runs than standard HDMI.  There are ways to boost an HDMI signal that can help it run further, but in general HDMI is limited to around 20-30 ft where as SDI can potentially go hundreds of feet.
The locking connector should also not be undersold.  In any production environment, connections coming loose can result in missed shots or dead air.  Having connectors that lock in to place is a huge must.
The biggest downside to SDI is that it is far less consumer friendly.  HDMI has a relatively small number of backwards compatible versions that are simplified in to some basic version numbering.  SDI on the other hand has not only a variety of different cable qualities, but also a variety of different signal formats that can be sent over the connector.  You need to be aware of the signals supported throughout your environment and properly configure things to exchange video correctly.  It is much more versatile, but also requires a better understanding of what the video formats you are working with actually are.
Overall, for your purposes, if you intend to do live video work I would recommend getting SDI capabilities to make sure you don't lose connections.  If you plan to do post-produced video and want to make use of an external 10 bit prores 4:2:2 recorder, having the SDI output will also be very helpful.  It's a bit limited for recorded video as it unfortunately doesn't support the much more versatile 4:4:4:4 format which is much easier to color grade, but 10 bit color still offers some advantages over any in-built 8 bit recording.  (I forget if the GH4 can support 8 bit recording without third party firmware.)
For the GH4 specifically, if you aren't doing live, I'd take a look at what the third party firmwares now offer.  I think they might support as good or better recording in camera at which point the adapter might be largely unneeded, though the XLR inputs for external audio could be helpful if you want to avoid using an external audio recorder and doing your audio-sync in post.

user14523: General SDI vs. HDMI Information
SDI is easier to connect to other professional equipment than HDMI. It is an all purpose signal, as opposed to HDMI that was originally intended only to connect to consumer display devices. There may be a slight advantage in quality where HDMI can be limited to 4:2:0 subsampling at certain resolutions, where SDI will usually be 4:2:2 or 4:4:4.
There isn't necessarily a performance advantage to SDI over HDMI. Sometimes HDMI outputs don't produce a clean HD signal and you need to put extra equipment in line to clean up the signal.
The disadvantage of SDI is cost. Most all SDI equipment is more expensive than HDMI equivalents. But you usually get higher quality equipment for that price.
In general, you need to weigh the exact specs of the equipment you are looking at for the application you want to use it for. If, in your case, the HDMI output has a 120fps maximum, but the SDI output has a 60fps maximum, you would want to choose the HDMI output.
One other often overlooked advantage of SDI, is that it uses locking BNC connectors (or sometimes other smaller form factor positive locking connectors) as opposed to non-locking connectors of HDMI. If you are going to be in an active environment, it offers a lot of piece of mind to know that your connector won't slip loose.
Why do you need an external output?
External outputs like HDMI or SDI are used for connecting a camera to peripheral devices. Some people use it to connect to an external monitor. External monitors can be bigger, allowing you to see more detail, or allow someone other than the camera operator to see the picture simultaneously.
Some people use it to connect to an external video recorder. External recorders can have larger storage capacity, have higher sustained I/O throughput, or capture a more advantageous format (less compressed, a different codec, etc.) than the internal SD card. For your application, it is the higher sustained throughput that might make a difference. The internal SD card, no matter which card class you get, probably will have a hard time recording high frame rate video flawlessly for extended periods of time. It might drop frames or just stop recording after buffers fill.
Decisions
You need to decide for what you are trying to do and what price/performance trade-offs you want to make. If you are doing work professionally, then I would strongly recommend going the SDI route. If you are just having fun with some short slow motion shots, then you probably only need the internal recorder. If you want to take a step up from there, still in the hobbyist realm, you could consider adding HDMI-based accessories in the future.
Lumix GH5 and the Panasonic YAGH Grip
According to this article, the YAGH Grip is not compatible with the GH5 and there is no planned SDI output.

19085: <p>I have heard of a video-grip for the <s><a href="http://www.panasonic.com/global/consumer/lumix/lumix-gh5.html" rel="nofollow noreferrer">Panasonic GH5</a></s> <a href="https://www.bhphotovideo.com/c/product/1028546-REG/panasonic_dmc_gh4_yagh_lumix_yagh_interface_unit.html" rel="nofollow noreferrer">Panasonic GH4</a> that supports SDI-output. Off the top of my head a few questions pop up</p>
<blockquote>
<p>What is the advantage of using a SDI-output ?</p>
<p>Does this generally allow for higher performance than what the camera alone would achieve ?</p>
</blockquote>
<p>I must admit, I have only completely read the Wikipedia entry for SDI in my mother tongue; which is not as detailed as the english <a href="https://en.wikipedia.org/wiki/Serial_digital_interface" rel="nofollow noreferrer">entry</a>.</p>
<hr />
<p><strong>additional notes:</strong> <br>
This was split off from another <a href="https://video.stackexchange.com/questions/21585/devices-for-slow-motion-video-recording">question about slow-motion recording</a>. Ideally the camera would be used for photography and video recording of all sorts. <br></p>
 14504: <p>In short; HD-SDI and SDI are the industry standard for professional applications; and HDMI is for consumers. </p>

<p>HDMI is highly limited in terms of length; you will need a very high quality cable to go more than 25 feet. </p>

<p>HDMI is fine for most applications; but it is limited to 4:2:2; you can't do long runs; if you are trying to integrate it with professional equipment you will likely need adapter boxes (such as those by BlackMagic); </p>

<p>SDI and HD-SDI offer full 4:4:4; you can genlock easier; you can breakout out easier; you can run it a few hundred feet before it needs to be reclocked; you can run it dual link mode for 4K, and so on. </p>
 2836: <p>SDI is a professional, locking video connection system over coax cable.  SDI itself has more expensive licensing costs associated with it than HDMI, so hardware that supports it will generally be more expensive.  If you don't have hardware that supports SDI, it won't do you any good, so that's the first point.</p>

<p>One of the biggest advantages of SDI is that it can support very high quality video such as the 4k 10 bit 4:2:2 supported by the SDI outputs for the GH4 adapter.  </p>

<p>SDI also supports a stable signal over much longer runs than standard HDMI.  There are ways to boost an HDMI signal that can help it run further, but in general HDMI is limited to around 20-30 ft where as SDI can potentially go hundreds of feet.</p>

<p>The locking connector should also not be undersold.  In any production environment, connections coming loose can result in missed shots or dead air.  Having connectors that lock in to place is a huge must.</p>

<p>The biggest downside to SDI is that it is far less consumer friendly.  HDMI has a relatively small number of backwards compatible versions that are simplified in to some basic version numbering.  SDI on the other hand has not only a variety of different cable qualities, but also a variety of different signal formats that can be sent over the connector.  You need to be aware of the signals supported throughout your environment and properly configure things to exchange video correctly.  It is much more versatile, but also requires a better understanding of what the video formats you are working with actually are.</p>

<p>Overall, for your purposes, if you intend to do live video work I would recommend getting SDI capabilities to make sure you don't lose connections.  If you plan to do post-produced video and want to make use of an external 10 bit prores 4:2:2 recorder, having the SDI output will also be very helpful.  It's a bit limited for recorded video as it unfortunately doesn't support the much more versatile 4:4:4:4 format which is much easier to color grade, but 10 bit color still offers some advantages over any in-built 8 bit recording.  (I forget if the GH4 can support 8 bit recording without third party firmware.)</p>

<p>For the GH4 specifically, if you aren't doing live, I'd take a look at what the third party firmwares now offer.  I think they might support as good or better recording in camera at which point the adapter might be largely unneeded, though the XLR inputs for external audio could be helpful if you want to avoid using an external audio recorder and doing your audio-sync in post.</p>
 14523: <h1>General SDI vs. HDMI Information</h1>

<p>SDI is easier to connect to other professional equipment than HDMI. It is an all purpose signal, as opposed to HDMI that was originally intended only to connect to consumer display devices. There may be a slight advantage in quality where HDMI can be limited to 4:2:0 subsampling at certain resolutions, where SDI will usually be 4:2:2 or 4:4:4.</p>

<p>There isn't necessarily a performance advantage to SDI over HDMI. Sometimes HDMI outputs don't produce a clean HD signal and you need to put extra equipment in line to clean up the signal.</p>

<p>The disadvantage of SDI is cost. Most all SDI equipment is more expensive than HDMI equivalents. But you usually get higher quality equipment for that price.</p>

<p>In general, you need to weigh the exact specs of the equipment you are looking at for the application you want to use it for. If, in your case, the HDMI output has a 120fps maximum, but the SDI output has a 60fps maximum, you would want to choose the HDMI output.</p>

<p>One other often overlooked advantage of SDI, is that it uses locking <a href="https://en.wikipedia.org/wiki/BNC_connector" rel="nofollow noreferrer">BNC connectors</a> (or sometimes other smaller form factor positive locking connectors) as opposed to non-locking connectors of HDMI. If you are going to be in an active environment, it offers a lot of piece of mind to know that your connector won't slip loose.</p>

<h1>Why do you need an external output?</h1>

<p>External outputs like HDMI or SDI are used for connecting a camera to peripheral devices. Some people use it to connect to an external monitor. External monitors can be bigger, allowing you to see more detail, or allow someone other than the camera operator to see the picture simultaneously.</p>

<p>Some people use it to connect to an external video recorder. External recorders can have larger storage capacity, have higher sustained I/O throughput, or capture a more advantageous format (less compressed, a different codec, etc.) than the internal SD card. For your application, it is the higher sustained throughput that might make a difference. The internal SD card, no matter which <a href="http://www.androidpolice.com/2017/05/09/buyers-guide-microsd-cards-classes-speeds-recommendations-spring-2017/" rel="nofollow noreferrer">card class</a> you get, probably will have a hard time recording high frame rate video flawlessly for extended periods of time. It might drop frames or just stop recording after buffers fill.</p>

<h1>Decisions</h1>

<p>You need to decide for what you are trying to do and what price/performance trade-offs you want to make. If you are doing work professionally, then I would strongly recommend going the SDI route. If you are just having fun with some short slow motion shots, then you probably only need the internal recorder. If you want to take a step up from there, still in the hobbyist realm, you could consider adding HDMI-based accessories in the future.</p>

<h1>Lumix GH5 and the Panasonic YAGH Grip</h1>

<p>According to <a href="http://www.newsshooter.com/2017/01/04/panasonic-gh5-internal-10-bit-422-4k-recording-at-400-mbps-and-hd-up-to-180fps/" rel="nofollow noreferrer">this article</a>, the YAGH Grip is not compatible with the GH5 and there is no planned SDI output.</p>
