I happen to own an OSSC (Open Source Scan Converter) and wanted to use it to connect my ZX Spectrum Next to my flat panel. The ZX Spectrum Next does have an HDMI output, but it produces a rolling, waving, wobbly picture. So I connected up its VGA output to the OSSC and started tweaking ...
The OSSC is a great device for retro computing fans, it allows you to clean up just about any old RGB (no composite or S-video) or YUV signal and convert it into HDMI. It's not a frame converter, it's a scan converter, so lag is minimal, a few scan lines is worst case and nobody, not even the Street Fighter World Champion, will ever notice. No really, it would be a super human feat. It's just not possible.
I can thoroughly recommend the OSSC if you have more than a couple of retro computers you'd like to attach to a digital flat panel. If you have 8-bit Commodores or other composite/S-video sources, you'll have to get some kind of transcoder as well, I use a Sony YR-1000.
The OSSC is super configurable, with a bit of tweaking you can get a pixel perfect HDMI output. I spent a bit of time with my ZX Spectrum +2 (grey) and ZX Spectrum Next to get pixel perfect timings, and here they are. They should work with the 16k/48k and regular 128k toastrack as well.
With the ZX Spectrum Next, this will produce a crisp, clear, stable HDMI output with perfect timings. It seems like a bit of a roundabout way of getting a nice HDMI signal, but it works with no lag. It seems to me like the Next should be able to output a stable signal, since all the OSSC really does here is clean up the sync. We'll see what happens when the Next sources drop ...
Sampling opt ->
Adv. timing ->
H. samplerate: 912
H. synclen: 65
H. backporch: 96
H. active: 704
Sampling phase: Adjust as needed