21
FN6760.0
September 12, 2008
Technical Highlights
The ISL98003 provides all the features of traditional triple
channel video AFEs, but adds several next-generation
enhancements, bringing performance and ease of use to
new levels.
DPLL
All video AFEs must phase lock to an HSYNC signal,
supplied either directly or embedded in the video stream
(Sync On Green). Historically this has been implemented as
a traditional analog PLL. At SXGA and lower resolutions, an
analog PLL solution has proven adequate, if somewhat
troublesome (due to the need to adjust charge pump
currents, VCO ranges and other parameters to find the
optimum trade-off for a wide range of pixel rates).
As display resolutions and refresh rates have increased,
however, the pixel period has shrunk. An XGA pixel at a
60Hz refresh rate has 15.4ns to change and settle to its new
value. But at UXGA 75Hz, the pixel period is 4.9ns. Most
consumer graphics cards (even the ones with “350MHz”
DACs) spend most of that time slewing to the new pixel
value. The pixel may settle to its final value with 1ns or less
before it begins slewing to the next pixel. In many cases, it
rings and never settles at all. So precision, low-jitter
sampling is a fundamental requirement at these speeds, and
a difficult one for an analog PLL to meet.
The ISL98003's DPLL has less than 250ps of jitter,
peak-to-peak, and independent of the pixel rate. The DPLL
generates 64 phase steps per pixel (vs the industry standard
32), for fine, accurate positioning of the sampling point. The
crystal-locked NCO inside the DPLL completely eliminates
drift due to charge pump leakage, so there is inherently no
frequency or phase change across a line. An intelligent
all-digital loop filter/controller eliminates the need for the user
to have to program or change anything (except for the number
of pixels) to lock over a range from interlaced video (10MHz or
higher) to UXGA 60Hz (162MHz, with the ISL98003-165).
The DPLL eliminates much of the performance limitations and
complexity associated with noise-free digitization of high
speed signals.
Automatic Black Level Compensation (ABLC™)
and Gain Control
Traditional video AFEs have an offset DAC prior to the ADC,
to both correct for offsets on the incoming video signals and
add/subtract an offset for user “brightness control” without
sacrificing the 8-bit dynamic range of the ADC. This solution
is adequate, but it places significant requirements on the
system's firmware, which must execute a loop that detects
the black portion of the signal and then servos the offset
DACs until that offset is nulled (or produces the desired ADC
output code). Once this has been accomplished, the offset
(both the offset in the AFE and the offset of the video card
generating the signal) is subject to drift (the temperature
inside a monitor or projector can easily change +50°C
between power-on/offset calibration on a cold morning and
the temperature reached) once the monitor and the monitor's
environment have reached steady state. Offset can drift
significantly over +50°C, reducing image quality and
requiring that the user do a manual calibration once the
monitor has warmed up.
In addition to drift, many AFEs exhibit interaction between
the offset and gain controls. When the gain is changed, the
magnitude of the offset is changed as well. This again
increases the complexity of the firmware as it tries to
optimize gain and offset settings for a given video input
signal. Instead of adjusting just the offset, then the gain, both
have to be adjusted interactively until the desired ADC
output is reached.
The ISL98003 simplifies offset and gain adjustment and
completely eliminates offset drift using its Automatic Black
Level Compensation (ABLC™) function. ABLC™ monitors the
black level and continuously adjusts the ISL98003's 10-bit
offset DACs to null out the offset. Any offset, whether due to
the video source or the ISL98003's analog amplifiers, is
eliminated with 8-bit accuracy. Any drift is compensated for
well before it can have a visible effect. Manual offset
adjustment control is still available (a 10-bit register allows the
firmware to adjust the offset ±511 codes in exactly 1/4 ADC
LSB increments). Gain is now completely independent of
offset (adjusting the gain no longer affects the offset, so there
is no longer a need to program the firmware to cope with
interactive offset and gain controls).
Finally, there should be no concerns over ABLC itself
introducing visible artifacts; it doesn't. ABLC functions at a
very low frequency, changing the offset in 1/4 LSB
increments, so it can't cause visible brightness fluctuations.
And once ABLC is locked, if the offset doesn't drift, the DACs
won't change. If desired, ABLC can be disabled, allowing the
firmware to work in the traditional way, with 10-bit offset
DACs under the firmware's control.
Gain and Offset Control
To simplify image optimization algorithms, the ISL98003
features fully-independent gain and offset adjustment.
Changing the gain does not affect the DC offset, and the
weight of an Offset DAC LSB does not vary depending on
the gain setting.
The full-scale gain is set in the three sets of registers
(0x12-0x13, 0x14-0x15 and 0x16-0x17). Each set of gain
registers is divided into an 8-bit MSB register (0x12, 0x14
and 0x16) and a 2-bit LSB register providing a 10-bit gain
value that both allows for 8-bit control compatible with the
8-bit family of AFEs and allows for the expansion of the gain
resolution in future AFEs without significant firmware
changes. The ISL98003 can accept input signals with
amplitudes ranging from 0.35V
P-P
to 1.4V
P-P
.
ISL98003
