S Stock internal describer puzzle

[class411]

I hope this is the correct place to put this.

Observing the internal train describer on an S-Stock unit today, I noticed (it's only taken me a few years) that the display, whilst dot matrix, does not have symmetrical horizontal/vertical dot placement. Whilst all columns are present, every alternate line is missing.

For a few seconds I thought it was just cost saving. Then I had an idea.

Obviously, as I wasn't party to the design of the system I cannot know for certain why they did this, but I have a pretty good idea and it really is quite clever of the designers.

So, the puzzle is: Why are alternate lines missing?

To give everyone a chance, please don't post the (possible) answer(s) until Thursday 9:00am. If you like, you can PM me with your answer to register the time you though of it.

[rincew1nd]

Admin comment

Given that this isn't really a quiz (the question-setter, by their own admission, doesn't have the answer) I've moved it here as more of a discussion topic.

[johnlinford]

Would it be possible to post a photo of the train describer? It's been a while since some of us have been on one!

[class411]

Jun 1, 2021 23:02:39 GMT johnlinford said:

Would it be possible to post a photo of the train describer? It's been a while since some of us have been on one!

Stupidly, I didn't think to take a photo (and I won't be in London for a couple of weeks). Unfortunately, none of the S-Stock internal photo's I found through Google show the describer clearly - you can see what it says, but not the detail of how it's made up. (You would need a manual focus camera unless you were very close).

[humbug]

I work for a train manufacturer, and this comes under my purchasing area.

Bounced this off an engineering colleague, and neither of us can think of a practical reason for alternate rows of the PIS (Passenger Information System) display being turned off, though this function is user programmable.

Interested to see the answer though.

[Chris L]

commons.wikimedia.org/wiki/File:London_Underground_S_Stock_interior.jpg

I still don't get the question

[Chris M]

Indeed, that photo certainly appears to show a full grid the same dot pitch horizontally and vertically with all lines in use.

[class411]

Yes, the photo does, indeed, show all lines in use. It does not look like the last one I saw. I'm wondering if they are trying a new display format and that is what I saw. It did seem odd that I'd never noticed the missing lines before. (This also fits with the information that I was PM'd from someone who worked on the system). Of course, it could have been a fault on that train and my observation was serendipitous. As it was the last S-Stock I travelled on before leaving London, I couldn't compare it with other examples.

Notwithstanding all that, the question remains: Why would they disable alternate lines, be it for a trial or a permanent change)?

And my proposed answer remains the same.

[t697]

With the last Thursday's date long past I thought to post a public comment;
Having worked on the project, I'm sure the LED spacing is the same vertical and horizontal on the saloon internal and the external side displays. Further, if you take a close look at the display when it's off (e.g. during the few seconds after the station approach message while it blanks awaiting the station stop) you'll be able to see the individual surface mount LED devices in their grid and you'll notice that the grid is regular with the same spacing horizontal and vertical. Zooming in on the picture here redirect.viglink.com/?key=71fe2139a887ad501313cd8cce3053c5&subId=1060900&u=https%3A//commons.wikimedia.org/wiki/File%3ALondon_Underground_S_Stock_interior.jpg
that someone else posted, you can see the individual dots and confirm the equal spacing.

The effect noted on the internal scrolling displays is an effect of the scrolling technique used. As well as shifting across from right to left, there is an intentional jitter as the LED that's on shifts to the left with the original and next one switching to and fro a little in the change to the next dot to the left and so on, continuously.
The human eye perceives this as smooth horizontal scrolling due to persistence on the retina.
The closer you are to the screen the more noticeable the effect becomes in appearing to be horizontal lines spaced vertically from each other. This is visible to an extent in the photo mentioned above with the 'jitter' in action as perceived by that camera.

By the way, the cab front display actually does have closer horizontal than vertical dot spacing. This is to get the letters to RVAR compliant height while having sufficient horizontal dots within the available width between carbody structures inside. The font on the cab front display is more comparable to Johnston Condensed in appearance, although the actual dot allocations are the same as the other displays with equal horizontal and vertical dot spacing where it fairly well represents Johnston in the 16 dot high display.

The original poster went on to PM me that they had actually seen a display with alternate rows actually off. That must have been an actual defect causing alternate lines to be off. There's certainly not been a trial of running them like that.

[Chris L]

It's such a shame that the displays were just before LED internal displays became widely available.

Those used on the Crossrail trains are so much better and more flexible.

[class411]

Sorry I didn't get back earlier. When t679 kindly PM'd me to say that there were no trials running, I realised that it must have been a faulty unit. It only takes the LSD (Least Significant Digit) to go open (or closed) circuit somewhere along the line to produce the effect described.

I have to say that I was even more impressed with the attention to detail that went into the design and development of these displays from what t697 said.

As to the answer to the original 'puzzle', now rephrased as: 'why might someone design a display with alternate lines 'missing'', look at the following:

       aa
       aa
     aa  aa
     aa  aa
   aaaaaaaaaa
   aaaaaaaaaa
 aa          aa
 aa          aa
aa              aa
aa              aa



 
       

       aa

     aa  aa

   aaaaaaaaaa

 aa          aa

aa              aa




Note that the fully populated 'A' appears to have stepped diagonals (unavoidable for diagonals that are not 45 degrees), whereas, in the one with the missing lines, the diagonals are not stepped.

Unfortunately, I didn't get a photo of the units in question, partly because I had assumed that the effect had been there all along, and I simply hadn't noticed it. It looked as if the characters were extremely well formed (because of the lack of 'steps', and displayed in a stripy font. (Others may disagree - I suspect it might depend on the individual.)

[d7666]

And, at the back of my mind, alternate line displays also have something to do with the ability to produce what the Mk.1 eyeball sees as scrolling italics, which they can, even though all LED are in a vertical array. As posted above, it is all to do with how the eye perceives these things.

[brigham]

The 'intentional jitter' was required because of the fast reaction time of LEDs.
Filament lamps provided this smoothing effect naturally.

[class411]

Jun 10, 2021 7:35:10 GMT brigham said:

The 'intentional jitter' was required because of the fast reaction time of LEDs.
Filament lamps provided this smoothing effect naturally.

Indeed.

The 'jitter' has the effect of a dimmer using PWM (Pulse Width Modulation).

As an aside, it's interesting to note, that you cannot dim an LED smoothly to absolute darkness because of the enormous range of the human eye. Even if it's only turned on for one ten millionth of a second every second, you can still see a faint glow.