RE: Inspiron 17 5748 does not recognize external microphone
it is a single combination jack)
I checked the laptop documentation, and it says you can plug in just a headset, just a microphone, or a combination, so I tried an older headset I had that contained two plugs, one for the headphones and one for the microphones. The headphones work fine, but the microphone is ignored.
Hello Charlie. The traditional headset with a separate plug for the headphones and a different plug for the mic will not work in a combo jack, because the mic plug is a TRS (tip/ring/sleeve) and the jack has to have the TRRS (tip/ring/ring/sleeve) type found on smart phone headsets. The headphone's TRS plug is fine to use in a combo jack.
I wonder which document said that you can use "just a microphone" with an Inspiron 5748 combo port? It might be difficult to find a stand-alone mic with a TRRS plug.
If I go to Sound, Recording Devices, the only microphone listed is Realtek High Definition Audio, regardless of whether or not I plug in the headset.
That is normal for Realtek. In Realtek, "Microphone" means the internal mic, unless an external mic is detected in the jack, then it means the external mic jack. That is similar to how the Playback devices are listed. Realtek just lists "Speakers", meaning internal speakers, unless external speakers or headphones are detected in the jack.
I have found some internet comments for similar problems indicating that an "IDT High Definition Audio Codec" should appear on the above list, but I don't know if that is for a specific, alternative audio board and whether it would apply in my case.
The names that appear in the list of devices are derived from the audio drivers for the devices. Earlier models of Dell laptops had IDT audio on the motherboard and used IDT audio drivers. Now most Dell laptops have Realtek hardware and use Realtek audio drivers, which is the case with your laptop. It would be inappropriate and probably impossible to install an IDT audio driver into your laptop, so IDT High Definition Audio Codec will never appear in the list of devices.
["Codec" usually means certain software, but in this context it means the audio hardware on the motherboard. When the Realtek driver is installed, it is called "Realtek High Definition Audio", and when the Windows native driver is installed it is called "High Definition Audio", but both names refer to the same hardware.]
In your next post you said "I have tried installing the Windows native audio drivers as suggested in another post."
That suggestion was incorrect, because the Windows native driver doers not support the external mic jack. It has to be a Realtek driver for the mic jack to work.
I tried the headset (a Logitech h151 with a 3.5 mm plug and four contacts)
That is the right type of headset you need to get the mic function of the combo jack to work, although I don't know anything about that specific headset. It could be that the jack is defective. Or it could be a buggy Realtek driver. It could be that the impedance of the mic is different enough from what the jack sensor is set for by the driver, that it fails to sense the mic. Sorry but it is hard to pin down. You could contact tech support if you want to let them take a stab at it. -- if the laptop is under warranty.
You could get a cheap usb jack like this from Amazon or elsewhere, and use your traditional 2 plug headset with it.
Here is another thing you could try on the software side of things. Go back to Sound>Recording tab. Right click on "Microphone". Click on Properties.
Click the Advanced tab and uncheck the "Exclusive Mode" box if it is checked. Test the headset mic. There is a small chance that this will help, but probably not.