Realtek Media Centres: luminance takes off; we're not impressed

It's only recently that we've started testing the accuracy of the colours produced by media centres, so we can check that the colour temperature is right and that the gamma curve is where we'd expect.  Shockingly, some of the models we've tested are dreadful!

Our test involves showing a series of thirty colour test cards that we then check using a colour probe.  One of the results is a luminance graph that works in the same way as the one you can find in our Product Face-Offs for TVs and monitors.

After trying this out with a dozen or so different media centres, we realised that those that include Realtek chips have a problem with luminance.

Tech recap: luminance and gamma

Televisions usually encode luminance as a value on a scale from 16 to 235, giving a range of 220 different levels between black and white.  Monitors for computers work differently, and encode luminance on a scale from 0 to 255, leaving 256 different levels from black to white. 

And here's the problem: media centres that use Realtek's chips (the RTD1283 and 1073) are set up using the system for computer monitors, with a luminance scale of 0-255.  That's an interesting choice for devices that are primarily designed to work with a television ...

And in practice?

In practice, these devices will produce non-linear gamma curves.  When you watch a film, areas that should be very dark grey will be black instead.  On the other hand, very pale areas that are't supposed to be entirely white will appear as pure white. 

On some TVs, you can correct for these problems.  If you dig into the advanced options, you need to look for the 'Black settings' option, or better still '0-255 mode'.  Turn this on and the problem will disappear.

Summary: in general, it's important to make sure that your source, whether it's a media centre, Blu-ray player or satellite decoder and your display, which might be a TV, projector or monitor, are in the same mode: either both 16-235 or both 0-255.  If your display doesn't support 0-255 mode, then check that your source can produce a 16-235 signal, or vice versa.

Some media centres that we've tested which use hardware from Sigma Designs, don't have this problem; that includes the PopcornHour C-200, A-200 and version 2 of the WD TV.  All of these models allow you to adjust the luminance encoding in their settings.  We contacted Dvico to ask if it was a problem with Realtek's chips and find out what they thought, as both the Tvix N1 and Tvix 6632N are affected.

It's also worth noting that devices which use the Realtek RTD1073 and RTD1283 chips can't currently handle DTS HD MA and Dolby Digital HD audio as a bitstream.  Some new chips are just rolling off the production line now and should make up for this problem.  There's still a question mark hanging over the current models, as we're still not sure whether or not manufacturers are going to be able to correct the problem using software alone, i.e., by offering a firmware update.

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