Screening your calls, and calling your screens!

by Peter_vdL Motorola 07-08-2011 03:37 PM - edited 09-19-2011 11:18 AM

They say that smartphone software is “all about the screen,” meaning appearance and usability is the single most important part of every app.  Given that emphasis, let’s review the various technologies used for smartphone touchscreens.

Resistive screens are one of the oldest touch screen implementations.  The screen has two flexible sheets of resistive material, separated by microdots or an air gap.  When you press on the screen, the voltage travels from the first sheet to the second, and the x,y coordinates can be measured as analogs of the voltage change.  Resistive screens respond to anything that presses on the screen, such as a fingernail or a stylus.  In practice, if a device uses a stylus, it usually (but not always) has a resistive screen.

With a stylus, touch events can be accurate down to the level of individual pixels, allowing complicated or fine symbols to be drawn. As a result, resistive screens have enjoyed popularity in Asian regions that use ideograms rather than letters.  However, resistive screens need recalibration after each power cycle, typically by pressing a stylus in the corners and center of the screen.

We have several Android devices with resistive screens. The most recent one, XT806 (with Android v2.1), was released in Fall 2010.

Capacitive screens are an alternative to resistive screens.  The screen has a glass insulating layer, coated with a transparent conductor.  When the user touches the screen with something that is also an electrical conductor (like a finger), it changes the electrostatic field on the screen.  This can be measured as a change in capacitance, hence the name “capacitive screen”.  Most of our current products, such as the XOOM tablet and the ATRIX smartphone, have capacitive screens.

A quick rule of thumb to tell the difference between capacitive and resistive screens is that resistive screens work with a poke from anything, and capacitive screens only work when poked by something that conducts electricity.  There’s a story that, in cold weather, some people didn’t want to take their gloves off to operate their capacitive touchscreens.  They discovered that they could use a sausage as a stylus.  It’s hard to believe that story, as they would have to remove their gloves anyway, to hold the sausage in their bare hands so it would conduct electricity.  But let’s not spoil a good story with undue skepticism. We don’t want to believe the “wurst” of anyone.


Would a banana work, if you don't have a sausage?

Screen Output
As well as accepting input, touchscreens have to display output.  It’s a bit beyond the scope of this note, but there are two major screen output technologies, and several minor variations and refinements.  

The two major screen output technologies are LED and LCD.  Liquid Crystal Displays don’t produce any light, but use polarizing crystals to block or allow light from a source underneath the screen. Thus LCD displays always require some form of backlighting. Sometimes the backlighting is done by LEDs.   In this case, some manufacturers choose whether to describe the screen as LED or LCD, according to what the manufacturer thinks the market currently prefers!

Light Emitting Diode displays, as the name suggests, do generate their own lighting, and thus are not backlit.  So LED screens can display darker black colors. The most popular technology for smartphone LED screens is OLED - Organic LEDs.  OLED screens use a layer of organic (carbon-based) semiconductor material.  The organic material and one or both of the electrodes connected to it are usually transparent.

AMOLED screens
OLEDs come in two varieties - Passive Matrix OLEDs, and Active Matrix OLEDs.  Active Matrix OLEDs (AMOLEDs) use a thin film transistor layer to switch each pixel on and off individually and keep it in that state.  The TFT backplane is an enabler for higher resolutions and larger screen sizes.

The different screen technologies have different characteristics.  When you choose one product over another, you should evaluate them (ideally, side-by-side) for aspects like battery life, performance in sunlight, contrast, viewing angle, color accuracy, switching speed, image sticking/retention, water resistance, cost, and longevity.  There is a lot more to screens than just the brightness!

Screen Size
Screen size is always specified as the distance across the screen diagonally. Wikipedia claims this is a historical holdover from when the first CRT screens were manufactured on the bottoms of glass bottles of a certain diameter. When screens were later made in a square format, the screens continued to be measured diagonally, to remain comparable with the older round screen format.

Let’s finish by reminding everyone of Android terminology relating to screens.  To Android, screens come in 4 generalized sizes, and also in 4 resolutions (dots per linear inch):

Characteristic Resource qualifier Description Example Motorola Products
screen size small 2 to 3.5 inches CHARM, FLIPOUT
  normal 3 to 4.5 inches DROID X2, ATRIX
  large 4 to 7 inches CLIQ 2
  xlarge 7 inches and larger XOOM tablet
screen density ldpi ~120 dpi CHARM, FLIPOUT
  mdpi ~160 dpi XOOM, XPRT
  hdpi ~240 dpi DROID X2, ATRIX
  xhdpi ~320 dpi no current products in this category
  nodpi resources tagged with this density are never scaled, regardless of device dpi.  

The sweet spot is the normal/hdpi screen, accounting for over 75% of devices accessing the Android Market.  Use this Android class to get the screen size and dpi values:

Next time you are thinking about screening your calls, you will also know what to call your screens.



Peter van der Linden                                   Brent Gossett
Android Technology Evangelist                Android Developer Advocate 


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