Texas Instruments OMAP36 Series, Technical Reference Manual, Page: 1216 / 3738

x[n] 8 M
LPF
w
c
= min
Æ
L M
,
É
p p
C L
y[n]
camisp-E123
x[n] 8 C P
y[n]
H(z)
P RSZ x
256
P =
º ½
8, = 64 ~ 512 when RSZ
4, = 513 ~ 1024 when RSZ
camisp-E124
x[n]
C
C
P
y[n]
H(z)
P
256
P =
º ½
8, = 64 ~ 512 when RSZ
4, = 513 ~ 1024
when RSZ
256/P
boxcar
RSZ 8
camisp-E125
w
c
= min
Æ
P
,
É
p p
Å È
P RSZ x
256
camisp-E126
Public Version
Camera ISP Functional Description
www.ti.com
The resizer module uses the same resampling algorithm for the horizontal and vertical directions.
The resizing/resampling algorithm uses a programmable polyphase sample rate converter (resampler).
The polyphase filter coefficients are programmable so that any user-specified filter can be implemented.
Figure 6-88 shows a general sample-rate converter where the resampling rate is equal to L/M.
Figure 6-88. Camera ISP VPBE Resizer Typical Sample-Rate Converter
L phases are used in a typical polyphase implementation. The resizer module, however, fixes the number
of phases to 8 for a resizing range of 1/2 ~ 4 (RSZ = 64 ~ 512), or 4 for a resizing range of 1/4 ~1/2 (RSZ
= 513 ~ 1024). In this way, the upsampling value (L) is fixed to either 8 or 4, and the downsampling value
(M) is based on RSZ. To achieve a resizing ratio of 256/RSZ, the downsampling value (M) is equal to
(PxRSZ)/256, where P is the number of phases.
Figure 6-89 shows the resizer functionality.
Figure 6-89. Camera ISP VPBE Resizer Functionality
Figure 6-90 shows a model of the resolution of the noninteger downsampling ratio. The interpolated output
from the filter is upsampled and replicated 256/P times before it is downsampled by the RSZ factor.
Figure 6-90. Camera ISP VPBE Resizer Approximation Scheme
This implementation means that a resizing ratio with 256/RSZ times the input size can be obtained.
However, each output pixel is rounded to the nearest interpolated output of 1/P input-pixel precision. The
polyphase filter coefficients are programmable so that any user-specified filter can be implemented. It is
recommended that coefficient sets be chosen to implement a sample rate converter where a low-pass
filter is used, with a cutoff frequency as shown in
Figure 6-91 .
Figure 6-91. Camera ISP VPBE Resizer Cutoff Frequency for Low-Pass Filter
If polyphase resampling is used, all upsampling factors can share the same set of coefficients. However, a
different coefficient set is required when changing between 8-phase and 4-phase modes, and with
different downsampling factors.
32 programmable coefficients are available for the horizontal direction (registers RSZ_HFILT10 to
1216 Camera Image Signal Processor SWPU177N – December 2009 – Revised November 2010
Copyright © 2009–2010, Texas Instruments Incorporated