The Mayflash HDMI adaptor seems to be capable of producing a decent 480p image, but has a couple of major issues which prevent it from being a reliable solution. Although it is possible to work around these issues in certain circumstances.
Black Crush
Just like the Wii2HDMI and Hyperkin, the Mayflash is also clipping some near black tones — the bottom 10 or so steps above black. This is visible on the 240p Test Suite PLUGE pattern, or manually setting a colour field to hex values to find the point where the field just becomes visible in a pitch black room (in my case 0xA).
Unlike the Hyperkin though, it can be alleviated by increasing the display's black level control (typically called 'Brightness' in the TV's menu). However this means the Mayflash is only going to be usable on displays which have such a Brightness control, i.e not PC monitors.
The Brightness control must also have a large enough range to fully alleviate the black crush. On my Samsung plasma for instance, I must increase Brightness all the way up to 95 out of 100 to restore the correct near-black gamma. If your display has less range of Brightness control than mine, you might be stuck with some black crush.
Another issue is that different displays might use a different implementation for their Brightness control, such that it causes other colour problems when running such an absurdly high Brightness value needed to alleviate the black crush. For all I know you might end up with a raised black floor, near-white clipping, or some weird gamma or saturation tracking.
As I don't have a capture card, the only way I could check colour accuracy after correcting black level at my TV was to measure the colours coming off the screen with my meter (an i1d2 — not an accurate device but perhaps still useful for colour matching). I was surprised to find the colours measure very similar to component after alleviating the black crush:
https://i3.lensdump.com/i/IlLrxx.png
https://i3.lensdump.com/i/IlL1RH.png
https://i.lensdump.com/i/IlLR71.png
https://i.lensdump.com/i/IlLTFc.png
Vertical Lines
There are some vertical lines which appear on areas of uniform colour, according to no particular rules of logic that I could discern. eg. on full field colour patterns in 240p Test Suite, there are no lines whatsoever, but if there are other patterns near the field of colour, these vertical lines start to appear in the field, eg:
https://i1.lensdump.com/i/IlLkIK.png
I tried the following remedies: swap HDMI cable, swap power supply, swap power socket, unplug all other peripherals, wrap foil around connector, wrap ferrite core around cabling, move console to different location. Nothing worked.
Strangely this seems to be partially my Wii's fault (model RVL-101 K01) as even with a component cable I can see the same vertical lines, although they are so faint I didn't even know they existed until now. Whereas the Mayflash exacerbates them to the point where they are visible at normal viewing distances, and this is a dealbreaker for me.
However my other Wii, a brand new RVL-001 CPU-40 taken out of its sealed box only 3 weeks ago, shows no lines on component, and very faint lines on the Mayflash. The lines are so faint that they are identical to my other Wii on component, and therefore not a dealbreaker in my view.
I am not sure if this is due to differences between Wii models or if its an aging issue with my used Wii (capacitors?). You can determine your Wii model here:
https://bitbuilt.net/forums/index.php?threads/revision-identification-guide.863/
I may buy a third Wii to compare.
Update: I got a third Wii for comparison, an RVL-001 CPU-60 (supposedly
rare) and that is showing vertical lines from normal viewing distances with the Mayflash, and extremely faint lines from component.
Update 2: got a fourth Wii, RVL-101 K02 — same result as previous unit.
It appears that as Wiis age, these very faint vertical lines start appearing in the component output? I am not bothered by them as they are so faint, and even close to the screen most people would not see them as they only present on very specific patterns that I still don't understand the logic behind. The Homebrew Channel splash screen is my reference pattern for detecting it.
Update: banding on gradients
I didn't catch this initially as there are no gradient test patterns in 240p Test Suite for Wii, but it seems the Mayflash's conversion to HDMI causes some banding or loss smoothness when displaying colour gradients.
I tried displaying my own gradient test pattern PNG/JPG in Wii's picture viewer and WiiXplorer, but both of those apps seem to create banding in the way they process the image for viewing, and I could never get it to display smooth on component or HDMI.
However the gradient is still smoother on component compared to HDMI, so I believe something is going on with the conversion to HDMI. If you don't have a test pattern, a convenient test is the blue gradient background on the Forecast Channel's splash screen, around the top left quadrant of the frame there are some useful tones.
I'm still a bit skeptical about these results as I have no way of reliably displaying a smooth gradient on the Wii. Perhaps the smoother gradients on component are somehow a function of my TV's video ADC smoothing things out vs the Mayflash's raw ADC. For example I can set my TV's noise reduction filter to minimum on the HDMI input and this smooths out the gradients to be on par with component — perhaps my TV has an always present noise reduction for component signals?
So there you have it, an annoyingly flawed device which if the planets align can produce an acceptable 480p image. Unfortunately the planets are not going to align for many people, so the search for a decent HDMI adapter continues...
edit: for completeness, I decided to measure the Hyperkin vs component/Mayflash. With my TV's black level (Brightness) at maximum (100) the 1-4% shadow detail is actually being resolved with the Hyperkin, however at about 0.2 gamma darker than component/Mayflash. Also measured a moderate roll off of the red and blue channels in the 10-30 IRE range, resulting in a bit of a greenish-yellow hue to the darker tones. The vertical line problem is less pronounced on the Hyperkin. The loss of gradient smoothness is also present, but to a lesser degree.
Component cables with Portta component to HDMI adapter is the Best and cheap option.
This reviewer is quite critical of the Portta.