Abt ISCAN VP50 Owner's Manual - page 33
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• 48Hz Lock source locked 48Hz
• 60Hz Lock source locked 60Hz
•
72Hz Lock source locked 72Hz
• Unlock unlocked
if you chose ‘unlock’, press Enter again to specify the desired output frame rate. use the
p
or
q
button to increase or decrease the frame rate. press Output Select again to exit the menu and
complete the output frame rate conversion setting.
Locked mode (1:1): This means that the iscan’s output frame rate tracks the input rate - i.e., the
output rate is locked to the input rate. In the 1:1 mode - i.e., 60 Hz source-locked to 60 Hz, and 50
Hz source-locked to 50 Hz - there is always exactly one output frame for every input frame/field. This
mode has the advantage that there are never any dropped or repeated frames (unless the source
itself does this). One disadvantage of this mode is that when the video source is changed, the iScan
has to re-lock to the new input signal timing. This causes a disruption in the iscan’s output signal
while the locking process occurs, and can cause many displays to loose sync to the iscan’s signal and
temporarily blank.
Unlocked mode: This means that the iscan’s output frame rate is independent of the input rate.
Regardless of any variations in the input, the output frame rate will be fixed at the specified value.
This mode results in the most stable output signal as it’s independent of any changes in the input
signal. Unlike the locked mode(s), the iScan’s output signal timing will not be disrupted when the
input source is changed. However, it’s guaranteed to cause dropped or repeated frames as the input
and output frame rates are not the same. note that regardless of what number the unlocked output
frame rate is set to, it will never be identical to the input rate in this mode as the output signal timing
is independent of the input timing. These dropped/repeated frames can result in visible irregularities
in smooth motion, sometimes called motion judder or stutter.
Locked mode (2:2 or 3:3): For standard definition interlaced sources the iScan can detect 2:2 and
3:2 pulldown cadences and convert these to either a 2:2 or 3:3 frame repetition rate. The main ad-
vantage of this is for 60 Hz sources using 3:2 pulldown, as the irregular 3:2 pattern can be converted
to 2:2 at 48 Hz or 3:3 at 72 Hz. In these modes, each original film frame is repeated exactly 2 or 3
times, respectively, and motion is therefore smoother. Note that these modes are only useful when
the video source is film-based - i.e., 3:2 pulldown for 24 Hz film on 60 Hz sources, and 2:2 pulldown
for 25 Hz film on 50 Hz sources. If the source is not film-based, then the end result is a lot like
unlocked mode without that mode’s stability advantages.
The iscan Vp50 has a built-in motion-judder test pattern to help you determine which frame rates
your display supports. This is the test pattern with the vertical bar which pans horizontally back
and forth across the screen. note that this test pattern is used to determine the capabilities of your
display, and does not provide any indication of motion smoothness (or lack thereof) in the iScan’s
output. The test pattern is used with the unlocked output mode of the iScan. You set the (unlocked)
output frame rate to be the one you’re testing your display for, and observe if the back and forth
panning motion of the test pattern is smooth or if it jerks or stutters. if the former, then the display
supports that particular resolution and frame rate without performing any frame rate conversion of its
own; if the latter, then the display is performing frame rate conversion on the signal.
Examples. These configuration options provide a lot of flexibility to deal with differing source and
display characteristics. Let’s look at a few examples:
A. The display only supports 60 Hz input. In this case, the iScan must produce a (nominal)
60 Hz output. For 60 Hz sources you can run in either 1:1 source-locked mode, or
in unlocked mode with the output frame rate set to approximately 60 Hz. You might
choose unlocked mode if the display takes a long time to re-sync to new inputs and you
channel-surf a lot. Locked mode will provide the smoothest motion and guarantee no
dropped or repeated frames. For 50 Hz sources, you’d set the iScan’s output to unlocked
with a (nominal) 60 Hz frame rate. This will result in some motion irregularities as the
iScan periodically repeats frames to convert the 50 Hz input to the 60 Hz output rate,
but since the display only supports 60 Hz inputs there’s not much you can do.