Face Palm: Our HDMI Cables Produce Blacker Blacks!

This is the first of my Face Palm posts.  In these I'll discuss claims -- almost always Marketing claims -- so divorced from reality the only adequate response is a Face Palm.

Years ago a fun, parody publication, "The Journal of Irreproducible Results", featured a learned article on the recent invention of the Darkbulb.  The Darkbulb, in one practical implementation, looks exactly like a normal Lightbulb.  Except, when you screw it into the socket and turn it on, it ABSORBS light instead of emitting it.  With enough Darkbulbs, of sufficient wattage, you could plunge any size room into inky blackness!

I often think of the Darkbulb when I run into Marketing verbiage to the effect, "Our HDMI Cables Produce Blacker Blacks!"

We'll leave it as an exercise for the reader to show why Darkbulbs are not likely to appear on the shelves of your local Home Depot anytime soon.  Instead, in this post we'll focus on HDMI cables.  And in particular, why they CAN'T produce "blacker blacks" on your TV, no matter how much money you've spent on them!

If you've read my post on Digital Video, you'll recall video on HDMI consists of a stream of pixels, each represented by three numbers defining a brightness and color.

Although there are several different formats used to transmit video data over HDMI, they all share a common trait:  Each such format has a DEFINED VALUE for any pixel which is supposed to be Black.

There's no vagueness about this.  No approximation.  If a pixel has that numeric value then it is a Black pixel.  When the TV receives that value for a pixel, it is the TV's job to render that pixel as a location on the screen which produces no light output -- a Black spot.

TECHNICAL NOTE:  You may also recall video for Home Theater includes the concept of Blacker Than Black pixels.  In a subsequent post I'll explain the reason for those.  But for now it suffices to point out BOTH Black pixels AND Blacker Than Black pixels are supposed to be rendered by the TV as a SINGLE, uniform Black; indistinguishable from one another.

As a practical matter, many TVs can not produce pixels which are truly Black -- i.e., which have no light output at all.  "Emissive" TV technologies -- such as the older, traditional, TV Tube sets, and modern OLED panels, CAN produce truly Black pixels.  Each pixel in such a TV is responsible for generating its own light output.  And that can be "no light at all".

"Transmissive" TV technologies -- such as modern LCD panels, can not -- at least on a pixel by pixel basis.  LCD panels can be thought of as having electronic "shutters" controlling how much light is allowed out from each pixel.  The light itself is produced by something BEHIND the shutters -- a "back light" which illuminates the backside of multiple pixels.  The problem is, the shutters, when closed, are not truly opaque.  Some small amount of light leaks through.  Fancier LCD panels include "local dimming array" back lights -- basically, separate sections of the back light which can be controlled independently.  If you turn off one of these sections of the back light, then ALL the pixels in front of it will be truly Black.  But you can't control that on a pixel by pixel basis.  The controllable sections are bigger than that.

BOTH types of TV also include controls for setting their Black Level.  This is what gets adjusted (when Calibrating the TV) so Black pixels get rendered as close to true Black as possible.  You'd think this would be easy, right?  If the numeric value for a Black pixel comes in on the cable, just don't generate any light output!  But it is complicated by competing goals in the TV, such as ALSO producing the maximum light output for White pixels, and making sure the "response curve" BETWEEN Black and White has the correct shape.  For example, if you adjust your TV to produce higher average scene brightness -- because you are going to use it in a well lit room -- the correct Black Level setting may be different from what you'd choose when setting up the TV for viewing in a dimly lit room.

All of the above is leading to the following key point:  For a given TV, calibrated for given viewing conditions, there's a limit to just how black each pixel can get.  Let's call that "TV Black".  At best, TV Black WILL BE truly Black.  But if it is HIGHER than true Black, it is STILL the closest to Black this TV can produce.

Think about that:

  1. If the pixel coming in on the HDMI cable has the defined value for Black, then the TV will produce TV Black -- the closest to true Black it is capable of producing.  AND
  2. There ARE NO pixel values which get rendered DARKER than that!

Which means there is NOTHING -- quite literally NOTHING -- the HDMI cable can do to cause the TV to produce "blacker blacks".

If the cable is working correctly, Black pixels sent out on the HDMI cable will be transmitted to the other end of the cable as having the pre-defined value for Black.  And thus, the TV will produce TV Black for them -- the closest to Black it can get.

If the cable is NOT working correctly, there is NO WAY for it to transmit a pixel value "darker" than Black.  So all of the pixels will still be either the value for Black -- or something BRIGHTER!  And again, the TV can't produce a darker result for any given pixel than its own, inherent, TV Black.

You could conceive of a comparison between an expensive cable which works and a cheap cable which transmits incorrect values.  You could point at the cheap cable and say, "See?  It is not transmitting proper Blacks!"  But the way digital video works on HDMI, the comparison is not likely to be between an image with good Blacks and something yielding milky grays or the like.  It is, instead, going to be between having an image and having NO image at all!  Because cables that faulty end up failing the HDMI handshake and Copy Protection checks.

Or to put it more simply, EVERY working HDMI cable will transmit CORRECT Blacks.  And no HDMI cable -- whether or not it is working correctly -- can transmit Blacks which are any "blacker" than that!

OK, I lie.  There is one way:  If the HDMI cable has an electrical short which fries the TV, then the TV WILL end up producing a screen of nothing but true Black.  Urh, most likely after a brief interval of brilliant, white light, a short, sharp "SNAP!" noise, and the release of a lingering puff of smoke.

In a later post I'll detail the theory:  All electronic devices work because of the smoke contained within them.  If you let the smoke out . . . . they stop working.

But for now:  If you come across an HDMI cable labeled as, "Produces Blacker Blacks!", RUN -- do not walk -- to a different brand.  One which has, we'll hope, invested more in product quality than in fanciful Marketing.