[Richard Crowley]

[Tom Brown] “disappointed with the built in camera audio “
The built-in microphone on virtually ALL cameras is good for almost nothing. High-end cameras don’t even include a microphone because they know nobody will want to use it.

[Tom Brown] “I have read that it is not wise to combine shotgun and lapel audio”
That only applies to combining two microphones on the SAME subject. As you say, a shotgun mic will have essentially no coverage to the back, so recording shotgun (front) and lav (rear) is successfully done by many people. It would be quite beneficial to record the two microphones to SEPARATE channels so that you have control over gain and balanced in the final mix.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Can I plug a conventional XLR microphone into a wireless transmitter?
Yes. Although only “plug-on” transmitters are likely to have the option of providing phantom power to condenser mics. “Body pack” type transmitters typically are not designed to provide phantom power although you can buy/make a cable that will connect an XLR mic to most any body-pack transmitter if using a dynamic mic or a self-powered (battery) condenser mic. We don’t know exactly what you have in mind by “table mic”, but I am assuming it is something with an XLR output.

Can I use two transmitters with one receiver?
No. Although there are some DUAL-receivers that actually have two receivers inside the same case.

Can I use transmitter and receiver from different product brands?
In some very rare cases this might work, although probably not optimally. But in virtually all cases, you must stick not only with the same brand, but the same model if you want proper operation. It isn’t a matter of the same frequency bands or even the same channels. Different brands use different techniques for processing the audio to improve performance.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Will the instructor be willing to wear a headset (headphones with microphone wand)?
Or does the instructor need to be “hands-free” also?

There are many wired and wireless intercom products of various descriptions on Amazon.
From the kind that people put at their front door, to the kind that drive-through bank tellers use.

https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=intercom

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

But many (most?) laptops have incredibly noisy “line lump” power supplies. Regardless of whether the power mains cord has a ground or not. Since laptops have internal batteries, it is trivial to do the experiment where you completely remove the power cord to see if that causes (or contributes to) the noise problem. The problem is actually larger than simply “ground loop” and I should have stated my previous response better. And then you have the added issue of the ground-reference from the HDMI extender gadgets.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Does this happen when the laptop computer is operating from the internal battery (with the power DISconnected)? Does the audio hum when you listen on the computer internal speakers? Does this happen when you have a LOCAL monitor connected via HDMI?

This has all the symptoms of the chronic problem with many laptop computers. There is a terrible ground-loop problem and is frequently solved by simply using isolation transformers. It could be something as simple as:

https://www.radioshack.com/collections/a-v-couplers-adapters/products/radioshack-ground-loop-isolator

or a pair of these:

https://www.rolls.com/product.php?pid=db25

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Unfortunately, Kopul does not reveal even a simple wiring diagram. So we have no clue how the Kopul cable is wired. If you look at the reference I provided to Hosa, you will see how the adapter should be wired. If you have a DMM meter, you might be able to test your Kopul cable to see how it is wired.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Most XLR to TRS (whether 3.5 mm or 1/4 inch) connect Tip = 3, Ring = 2, Sleeve = 1. However that wiring ends up recording your signal properly on the Left channel, and the INVERSE phase on the Right channel. If you select one or the other channel, you will get a decent signal. But if you take BOTH (Left + Right) channels, the signal cancels itself out to varying degrees. That sounds exactly like what you are describing.

There are two primary solutions to this problem.
1) While editing, select ONLY one side of your “stereo” (actually 2-channel mono) audio track. NEVER combine Left + Right.
2) Use an adapter that is wired properly for this purpose. For example: https://hosatech.com/product/xvm-100f/ or any adapter wired like that.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Sounds very much like a second Rode wireless lav mic would be ideal. Essentially 100% of the news anchors/ newsreaders around the world are wearing a clip-on lav mic there on the news set.

And even on SNL where they traditionally use a boom mic on the dramatic scenes, they are using clip-on lav mics for the “Weekend Update” bit.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Just for comparison, I went and viewed a video from LinusTechTips. He appears to be using a shotgun mic on a boom (at least there was no microphone that I could see). And at a greater distance than you are using.

However, he is working in a MUCH larger space, essentially an industrial warehouse space which is the modern equivalent of the old-school Hollywood sound-stages from 100 years ago. Because they were 3x to 5x larger than your space, and they had exposed wooden building structure (studs, rafters, etc.) they were almost “outdoor-like” in their acoustic properties. There were effectively zero nearby reflective surfaces, and there were many small pieces to “break-up” whatever reflected sound was bouncing around up there. Plus, they were lined with thick sound-absorbing layers on virtually the entire walls and ceiling.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Your space appears to be an office/shop/showroom for public access. So making it acoustically “dead” enough for distant microphone will be expensive if you want to maintain an attractive visual appearance.

Anything that will get the microphone MUCH CLOSER to your mouth will improve the sound by reducing the ambient reverberation vs. your speech. You could experiment by using your NTG4 and the mic stand boom in different positions. For example directly in front of your mouth and no more than 15 cm between the end of the mic and your mouth. Not suggesting that you actually want to record videos with the microphone there, but as an experiment to see how mic position and distance affect the recorded sound.

Or with the mic positioned BELOW the camera and aimed UP at your mouth (to avoid reflections from the desk top). I would put some absorbing material (your foam?) between the microphone and the closest parts of the desktop to avoid reflections there.

While a clip-on lavaliere mic might be better than the distant shotgun, I would still much prefer using a headset microphone. It would put the mic an order of magnitude closer to your mouth than either the boom shotgun or the clip-on lavaliere mic.

———————————————————————————
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.