Opinions, Observations and Ramblings about media technology.

Monday, March 11, 2013

4K TV is equivelent to Bay Bridge Lights. I can prove it.

My recent post on the new Bay Bridge Lights had several nice comments.   The lights are a new enhancement to our landmark San Francisco Bay Bridge, and have been it's been very entertaining to watch the modulated light patterns each evening.  One colleague on Linked In said (presumably tongue in cheek) "who in their right mind would want 4K in their living room when they can have 0.025K out their front window and get it commercial free!"

That got me thinking, and it turns out, it's a good point.   By some measures, the bridge and your new Ultra HD television provide the same visual performance!
Coming soon to your living room:  Ultra high definition TV,
with four times the resolution of 1080p.
In a previous post, I babbled on about the inevitability of 4K television, more accurately termed Ultra High Definition, with the "UHDTV" terminology likely driven by lawyers rather than engineers.  This new standard was prolific at the CES show in January, and several models will become available for purchase over the next few months, with prices likely to start below $4,000 for a 55-inch model.  UHDTV has four times the spatial resolution of the 1080p high definition that you are now used to.  Can you really tell the difference?  The answer is absolutely yes -- but only with proper content and viewing conditions.

Human vision has a remarkable ability to resolve detail.  The fovea of the human eye (the part of the retina that captures details in the center of your field of view) contains about 140,000 sensor cells per square millimeter.  This means that if two objects are projected with a separation distance of more than 4 microns on the fovea, a human with a normal visual acuity (20/20) can resolve them.  On the object side, this corresponds to 0.2mm at a distance of 1 meter, equaling one minute of arc.   This is the reason that the Snellen eye chart has special font characters, called optotypes, made from lines that are exactly thick enough to subtend an angle of one arc minute when viewed from 20-feet away.  That's where the term 20/20 comes from.  The optotype font is five line-widths high (notice the funny looking E at the top?).   As with all things, humans are a diverse lot, with many variables, so this is just a ballpark characterization.  Actual results may vary.

The Snellen eye chart Optotypes (line 8) subtend an
angle of five arc minutes vertically, when viewed from 20 feet

Based on this understanding of human vision, the International Telecommunications Union sector on Radiocommunication (ITU-R) recently issued a report on the present state of Ultra High Definition TV.  It concludes that the optimal viewing distance for UHDTV (3,840 x 2,160 resolution) is 1.5 times the screen height.   A 55-inch television, measured as diagonal screen size, has a picture that is about 27inchs, or 68.5cm high.  One and a half times this height means the optimum viewing distance is just a tiny bit over 1 meter.  Further away, a viewer with 20/20 vision would not benefit from such high detail, so why bother with the nearly 8 million pixel display?   Closer, and an astute viewer would start to see individual pixels, breaking the magic spell of Downton Abby's storyline with distracting dots.

Now, lets turn our attention to the Bay Bridge Lights, which use 250,000 LED lights grouped in 25,000 nodes, which we will consider pixels.  After some digging, I was able to determine the pixel spacing is 12-inches vertically.  But this is not a normal display -- the horizontal dot pitch is 30 feet, since the lights are affixed to the 306 northern support cable bands securing the bridge deck (roadway) to the huge suspension cables spanning over the towers.  In TV terms, this would imply a pixel width ratio of 30:1, far from the 1:1 "square pixel" that we are accustomed to.   As computer graphics wizard Alvy Ray Smith so eloquently puts it: "A pixel is not a little square".   This proves him right -- in this case, a pixel is really a huge rectangle!   (Yes, I know this is exactly the opposite of Alvy's point, so you don't need to remind me.  I just couldn't resist...)

The Bay Bridge has 612 cable bands supporting the deck from the suspension cable,
each with for wire ropes, for a total of over 42 miles of rope used in the 9,260-foot western span.
So, based on normal human visual acuity of 20/20, what is the optimal viewing distance of the bridge lights -- with a focus on the vertical resolution of pixels 12-inches apart?   The answer is one kilometer (actually, 0.9873 km, but who's counting).  That is to say, the vertical resolution perceived by a viewer 1 meter away from a 55-inch UHDTV (4k) television is the same as the vertical resolution perceived by a viewer 1 kilometer away from the bridge.  

Next, I had to find such a viewing location to check it out.  The Bay Lights Web site includes a map with recommend viewing locations.  Among the spot most used for photographs is Pier 7, just North of the Ferry Building.  Google maps teaches us that the end of Pier 7 is one kilometer from the second tower (W2 pier) of the bridge.   

It was a clear, cool evening here in San Francisco, so I paid a visit to Pier 7 this evening.  When viewing the bridge lights from that location, the vertical rows appear almost continuous, without visible pixelation.  This seems to reinforce the theory in calculating the optimal viewing distance.   Viewing the bridge from a closer location, such as Pier 1, could allow viewers to see individual lights more prominently.  Viewing from further away, perhaps from Coit tower or Pier 39, would look great, but not benefit from all 25,000 LED nodes -- the same effect would be achieve by fewer lights (and lower electric bills). 

Bay bridge lights viewed from Pier 7 achieve the limit of 20/20 visual acuity
(Photo: James Tensuan, The Chronicle)     

However, I'll admit it's not fully definitive.  First, when viewed from that distance, the lights appear to shimmer from atmospheric refraction.  Also, the pixel fill factor on the bridge is very low:  on a typical LCD panel, there is a very small space between the light emitting LCD cells, so that roughly 90% of the planar area of the panel is emitting light.  By contrast, the bridge has 1.25-inch LED nodes spaced on 12-inch centers, resulting in a very low pixel fill factor of 10%.  This probably contributes to imperfect viewing -- so it's not fully comparable to a $25,000 Ultra HD television.

Please consider this as you shop for your next television at Best Buy.   If you plan to watch your UHDTV from more than five feet away, you'd better spring for the 84-inch model to achieve that optimal viewing distance.

1 comment:

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