CCD and CMOS
Oct 1, 2008 12:00 PM, By Steve Mullen
Understand old and new camera sensor technology.
After two decades use, we are all familiar with basic CCD
technology. During this period, the most recent development
has been CCD sensors that support HD. CMOS sensors, like CCDs,
are made from silicon. However, as the name implies, a
complementary metal oxide semiconductor manufacturing process
is used. Today CMOS is the most common method of making
processors and memories. More on that later. First, we'll look
at SD CCDs.
Interlaced scanning
Obtaining interlace video from a CCD employs a process called
line-pair summation, which increases sensitivity by one-stop.
The process may be implemented by a circuit external to a CCD
or within a dual-line CCD. In the former case, all rows are
read out each field time. In the latter case, alternating
fields of odd or even rows are read out each field-time.
In either case, a two-line window slides down through the rows
in increments of two lines. The window starts with the top row
for odd fields and the second row for even fields. Each pair
of rows within the window is added, thereby increasing
sensitivity by +6dB.
The window acts as a filter that reduces vertical resolution
by about 25 percent, thereby minimizing interline flicker and
interline twitter. The filter creates an interlace coefficient
of about 0.75, which means a 480-row CCD can output only about
360 lines of effective vertical resolution.
Progressive scanning
When progressively scanned video is required, a CCD is read
out at the frame rate. However, some dual-line CCDs can output
all rows if they are clocked at half-speed (for example, 30Hz
rather than 60Hz). These CCDs nicely support cameras that
record 60i and 30p video.
Progressive scanning does not employ line-pair summation;
therefore, progressive video has a 1.00 interlace coefficient.
Without line-pair summation, sensitivity is not increased.
HD CCDs
An SD, dual-line CCD with a resolution of 720 × 480 outputs
173kb per readout. At 60i, the bandwidth required is 10Mb/s.
An HD camera uses CCDs that output at a significantly higher
data rate. For example, at 60i, a 1440 × 1080 CCD requires a
bandwidth of 47Mb/s.
CCDs running at very high clock rates consume a great deal of
power. Small CCDs — typically those under 2/3in — cannot
easily dissipate heat produced by high-power consumption.
When, for example, a camera captures full HD at 60i and 30p,
plus 720p60 and 720p30, one option is to use 1920 × 1080 chips
running at 60Hz. Unfortunately, each chip requires a whopping
125Mb/s bandwidth. One solution: CMOS sensors that consume far
less power and thus generate far less heat.
Most CMOS imagers use active pixel sensor (APS) technology,
which requires at least three transistors per pixel. Each
pixel incorporates a photodiode to collect light, a reset
transistor, a row select (column bus) transistor and an
amplifier transistor.
An image capture begins when a trigger is sent to all reset
transistors within an addressed row to prepare the row's
photodiodes to capture light. Upon the reset trigger, the
transistor closes and pulls up the photodiode to VDD, thereby
clearing any photodiode charge. All photodiodes in a row are
reset at the same instant. The reset begins an integration
period. During this period, the amount of light falling on a
photodiode determines how much charge accumulates in its
potential well.
The integration period ends when a signal is sent to all row
select transistors in an addressed row. Row select transistors
pass the accumulated photodiode charges onto column busses.
Each amplifier transistor acts as “source follower” that
allows a photodiode charge to be sampled as a voltage.
All photodiodes in a row are simultaneously connected to all
column busses. Each column bus terminates in a bus transistor
that charges a sample-and-hold capacitor. The capacitor stores
the voltage while a column waits for its turn to be output.
When it is time to output a column, transfer transistor passes
the stored voltage to an operational (OP) amplifier.
It is not possible to instantaneously output each row after
sending it down the column busses. The time required to read
out all pixels in a row is called row readout time (RRT).
Rows, of course, are read out from a chip in a top-to-bottom
sequence.
Adding additional ports to a CMOS sensor reduces RRT by
simultaneously outputting multiple samples from a row.
Exposure control
When a camera operator sets a shutter speed, he or she is
setting the integration period. For example, with a shutter
speed of one-sixtieth second, the integration time is 16ms.
A CMOS chip's top-to-bottom exposure process operates much as
a vertical focal plane shutter does in a modern film camera. A
curtain (red) is released to start its downward travel at the
beginning of the exposure time.
When the first curtain has completed its travel, the film
frame is fully exposed. When the exposure time ends, a second
curtain (blue) is released to begin its downward travel to
close-off the film.
When a film camera is set to a short exposure time, the first
curtain will not have traveled far before the second curtain
starts chasing it down the frame. The narrow traveling slit
formed by the gap between the two curtains exposes the film.
When a scene contains little motion, the difference in time
between exposing the top and bottom of a frame creates no
harm. However, when the scene contains motion, the time
difference between the top and bottom exposure is captured on
film as a rolling shutter artifact.
Because a CMOS sensor's rows are processed — reset and output
— with an offset equal to RRT, a CMOS sensor also exposes each
frame in a top-to-bottom pattern. The row exposure offset
creates a rolling shutter skew that matches the direction of
the object's movement.
The row exposure offset also is responsible for wobble (a
stretchy look when a camcorder is subjected to sudden motion),
partial frame exposure (from flash cameras) and fluorescent
flicker (bands of shifting scrolling color or flickering dark
lines).
Noise reduction
The inherent design of CMOS chips is responsible for many
types of picture noise. Dark current noise is one type of
fixed pattern noise (FPN). Each photodiode has its own unique
level of charge even when there is no light falling on it.
Therefore, a matrix of photodiodes has a matrix of noise
values. FPN is also introduced by tiny differences among
amplifier transistors. These amplifier differences introduce
errors as photodiode charges are converted to voltages.
FPN noise can be attenuated by a noise reduction technique
called correlated double sampling (CDS). CDS requires two
readouts of each photodiode. The first readout is performed
immediately after a row is reset and is a measure of a pixel's
noise. The second readout occurs after the integration period
and is a measure of a pixel's noise plus signal.
At the bottom of each column, one sample-and-hold stores the
noise voltage. After the integration period, a second
sample-and-hold for a column stores the signal-plus-noise
voltage.
To output each column, every column is addressed one-by-one.
The two stored signals are fed to an OP amplifier. Each OP
amplifier subtracts the noise voltage from the
signal-plus-noise voltage, thereby yielding a signal voltage.
Image readout
A CMOS device has the unique capability to employ both row and
column addressing to create a window within the photodiode
matrix. A small window can be used to capture slow-motion
video. For example, a 960 × 360 pixel window within a 1920 ×
1080 sensor requires less than 17 percent of the entire sensor
be read out, allowing the readout to occur up to 6X faster.
For a CMOS camera to support interlace video, its chips are
clocked at the field rate. Then:
•An external circuit performs line-pair summation that
converts each frame to a field. Through this process,
sensitivity is increased +6dB, and the interlace coefficient
drops to 0.75, thereby minimizing flicker and twitter.
•Cameras that employ vertical pixel-shift or vertical
interpolation can simply discard alternate fields and record
the field not discarded. Line-pair summation is not required
because the image is inherently slightly soft. Sensitivity is
the same for both interlace and progressive operation.
All column signal voltages are loaded into an analog shift
register where pixel information is shifted to output port(s).
In this design, analog-to-digital (A/D) conversion is external
to the CMOS chip. (See Figure 5 on page 46.) To decrease image
noise, internal A/D conversion can be employed. In this
design, a CMOS chip's output path is digital rather than
analog. (See Figure 6.)
Sony Exmor technology employs a massive amount of circuitry
between each column bus OP amplifier and the chip's output
port(s). Each column has its own A/D converter. From this
point forward, the data path is digital — not analog — thereby
reducing image noise.
Key CCD/CMOS differences
The obvious difference between these technologies is that CMOS
sensors inherently have no vertical smear. Therefore, when
using CMOS-based cameras you can shoot just as you would were
you shooting film. However, the latest CCD chips significantly
decrease the appearance of vertical smear.
Clearly, rolling-shutter artifact is the Achilles' heel of
CMOS technology. This artifact can be eliminated by
implementing a global shutter. However, this requires an
additional transistor within each pixel that decreases a
chip's fill factor — thereby reducing its sensitivity or
forcing a reduction in the number of pixels.
CMOS sensors typically offer a maximum exposure time of
one-fiftieth or one-sixtieth second. To obtain greater
sensitivity, as is done with CCDs, video gain can be
increased. When even greater sensitivity is required, some
CMOS cameras offer the option to specify a number of
accumulated frames. Frame accumulation defines total
photodiode integration time as a multiple of the current
interframe interval.
CMOS accumulated modes do not suffer from increased image
noise, but can have significant blur on moving objects. The
image in was shot using 16-frame accumulation.
CCDs typically obtain super high sensitivity by engaging hyper
gain. The result is a significant increase in image noise.
Motion, however, remains clear.
These high sensitivity modes are quite different and support
different shooting goals.
Forecast
Both CCD and CMOS sensors will be used for years to come.
Moreover, both technologies will employ multiple
implementation strategies to meet performance goals and price
points. |
