Been doing some experimenting with the kit I have tonight...
This is a USB powered VGA-HDMI converter branded 'eSYNiC' - https://www.amazon.co.uk/dp/B00PAGKZP4 - I paid £11
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This works well, but the output resolution over the HDMI matches the input resolution (i.e. not HD/FullHD but actually 800x600 or 1024x768 etc.). This can cause problems if your HDMI device only expects normal HD resolutions. Because of this, this device didn't always get BIOS screens for example or other slightly odd resolutions.
Next are two Portta branded devices. First one a small plastic 720p and 1080p upscaler. I paid £22 for this and now it's only a fiver!! https://www.amazon.co.uk/dp/B00ATNF346
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This one works well too - the output can be 720p or 1080p (not the native input resolution). The button on the side switches the output between 720p and 1080p when pressed quickly. Press and hold for a couple seconds then release to auto adjust the image (if it's not centred for example). I did have this one once which didn't want to display an output (old Thinkpad in CRT high performance mode - whatever that is!)
This is my favourite one as I have a cheapo eBay HDMI capture card (about £30 seems to be a Timeleak HD72A or a YK762H Video Capture Card) which only really does 720p/1080i (well does a couple of other odd resolutions and 1080p/24 which isn't very useful!) so I can capture anything VGA that goes into this device.
The other Portta branded one is a metal chassis device that will take component or VGA and analogue audio and outputs 1080p (only) HDMI and also digital optical and coax audio. https://www.amazon.co.uk/dp/B008HZU2LM I paid £29 for this.
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This device never failed to display any VGA input from the tests I was just doing on an old ThinkPad 360CS.
They were all similar quality output with the USB powered one probably being the best as my VGA lead seems to be introducing a bit of noise and I can't find another at the moment! They were all decent anyway. For all three DOS seems to end up at 640x480, even in games like Doom.
An example capture from the plastic 720p Portta one is here - https://youtu.be/DKrblR4U1FU (YouTube compression seems to impact the quality a bit)
Hope this helps! If anyone wants anything tested just shout. I recommend that one that's only £4.99 now 😀
Cheers,
Steve