Pixel shifting: the false 4K

Projectors with true Ultra HD resolution, i.e. four times higher than Full HD, are rare. And those you think are equipped with UHD, often aren't either. At least, not the real thing, and that's completely understandable from a technological point of view. You wonder why?

I recently took apart two projectors to see how they worked (see article below; in German). Our photographer Thomas Kunz took some pictures. It was then that I realised that although the projected image is very large, where it is created, pure micromanagement prevails. True UHD, a resolution of 3840 × 2160 pixels, is simply impossible.

In short: in a projector, the slab or chip on which the Full HD image with its more than two million pixels is created is roughly the size of a twenty-cent coin. On this surface, manufacturers have to produce a UHD image with eight million pixels.

This means:

• four times as many pixels as Full HD - so eight million pixels

• on the same surface area of around 1 × 1 cm

In this article, I want to explain how manufacturers achieve this almost superhuman feat.

Full HD: this is how the image is created

Compared to a television, the image production surface is tiny. On a 65-inch TV, for example, 145 × 83 centimetres are available for the two (Full HD) or eight (Ultra HD) million pixels.

Drawn out, the principle of proportionality looks something like this:

On the chip, the pixels of the projectors are distributed in a checkerboard pattern. This is often referred to as a "pixel grid". The pixels produce the image that you then see on the projection screen. For Full HD, this is how it works:

1. the image is formed on the grid of pixels
2. in order for the image to be projected, light passes through the pixel grid
3. from the pixel grid, it passes through the projection lens
4. and from the lens, it goes to the projection screen

To show you the concept of the above graphic with real images, I paused a scene from the film "Passengers". For the Full HD image, I paused the Blu-ray, for the other UHD shots, the UHD Blu-ray.

And this is what the top image looks like heavily zoomed in:

Double resolution with e-Shift technology

Tabletop cards. Today, it's not possible for projectors to have true - and therefore native - Ultra HD with a corresponding resolution of 3840 × 2160 pixels. At least, not if the device is to be affordable. The workarounds developed by manufacturers are within everyone's reach. One of these is JVC's e-Shift technology. Admittedly, it doesn't quadruple the resolution, but it does double it. That's not bad.

E-Shift works like this:

1. a special glass is placed between the pixel grid and the lens
2. it shifts each pixel by half an inch diagonally; a second grid of pixels is thus created
3. the glass vibrates 120 times per second (120 Hz), so that the two grids merge to form a single grid

With 120 vibrations per second, your eye is unable to tell the difference between the two grids of pixels. All you see is a common area with double the resolution.

Here's the stopped image:

And here, the heavily zoomed in top image, slowed down and filmed:

There is a difference between "DLP" and "LCD". The individual pixels in DLP projectors have less space on the chip than those in LCD projectors. This means that, on the chip, the two grids of pixels overlap slightly. A true native doubling of resolution can therefore only be achieved with LCD projectors.

Ultra HD thanks to e-Shift DLP technology?

Texas Instruments has developed its own e-Shift DLP technology. As with JVC, a vibrating glass is used, which increases the number of visible pixel grids.

Shift with 2K chip

Here, it's all about the chip. This produces the image with double Full HD resolution before projecting it onto the screen. Compared with JVC's technology, this is a technical breakthrough in the chip itself. Thanks to the vibrating e-Shift glass, the resolution is - as with JVC - doubled.

Here's a freeze frame of a UHD Blu-ray:

And this is what the image looks like if you zoom in hard:

There are two grids of pixels. Both have twice as many pixels as JVC's Full HD pixel grid. Because your eye can't tell the two grids apart, they merge into one Ultra HD grid.

Quadruple Shift with Full HD chip

The other way to generate four times the pixel mass from Full HD is simply to shift the image four times.

1. the pixel grid shifts first to the right
2. then diagonally downwards
3. once again to the right
4. and finally diagonally upwards again to its original position

Here's what the stopped-down image of a UHD Blu-ray looks like:

And here's the heavily zoomed image:

You get four grids of pixels - four times as many pixels as before - which, to you, look like a single grid of UHD pixels.

Ultra HD label despite fake Ultra HD resolution?

Shift technologies don't offer true Ultra HD - or native Ultra HD, as it's known in the trade jargon. I've talked about this before.

I can hear you saying, "No, no, Luca, you've got it wrong. The datasheet clearly says 4K."

Historically speaking, "4K" is a film industry term. In everyday life, it's also used for home cinema, because it's perfectly appropriate for marketing. However, the term itself is not protected by minimum criteria and requirements, as it only describes the number of pixels on the horizontal axis: 4096 × 2160 pixels instead of the UHD resolution of 3840 × 2160. Manufacturers could therefore easily label their products with 4K labels without fear of serious legal consequences.

Ultra HD, on the other hand, is only protected to the extent that a minimum of eight million pixels is required. If we look at the numbers alone, as we saw above, projectors with offset technology are capable of reaching eight million. The difference from true Ultra HD is that the pixels in the pixel grid are not aligned; they are arranged in a zigzag pattern, usually with slight overlaps.

The shifted image does not contain four times as much information as a Full HD image - the information is just copied, shifted and placed next to each other.

However, from a technological point of view, Shift technologies are impressive. Televisions need a surface area of around 71 quarters to create a UHD image, while projectors have to make do with just one. A real challenge.