GC4116 Datasheet, PDF (17/57 Pages) List of Unclassifed Manufacturers – MULTI-STANDARD QUAD DUC CHIP

GC4116 MULTI-STANDARD QUAD DUC CHIP
DATA SHEET REV 1.0
The resampling ratio for each channel is determined by
setting the 32 bit RATIO control in addresses 16 through 31
of the resampler ratio page. The value of RATIO is defined
as:
RATIO = 226_{N----N-D---D-E----EL----AC----Y--} = 226_{O----I-U-N----TP---P-U---U-T----T--S---S-A---A-M---M--P---P-L---LE---E---R---R-A---A-T---T-E---E---}
Up to four ratios can be stored within the chip. A ratio
map register (address 23) selects which ratio is used by each
channel.
The three spectral plots shown in Figure 13 illustrate the
steps required to resample the channel data. The first
spectral plot shows the data just after zero padding. The
sample rate after zero padding is NDELAY*F_S, where F_S is
the sample rate into the resampler. The second spectrum
shows the shape of the QTAP filter which must be applied to
the zero padded data in order to suppress the interpolation
images. The last spectrum shows the final result after
decimating by NDEC.
3.7.3 The Resampler Filter
Figure 13(b) illustrates the spectral shape requirements
of the QTAP filter. If the desired signal bandwidth is B, then
the filter’s passband must be flat out to B/2 and the filter’s
stop band must start before F_S-B/2. The user designs this
filter assuming a sample rate equal to NDELAY*F_S. Section
7.6 contains example resampler filters coefficient sets. Other
passband and stopband responses can be used, such as
root raised cosine receive filters, as desired. The resampler
filter can also be used to augment the CIC, CFIR and PFIR
filters’ spectral response.
The number of filter coefficients, QTAP, is equal to
NMULT*NDELAY, where NMULT is the number of multiplies
available to compute each resampler output, and NDELAY is
either 4, 8, 16, 32 or 64 as will be described later. The
maximum filter length is 512. The user specifies NMULT in
address 17 of the resampler control page.
The filter can be symmetric, or non-symmetric, as
selected by the NO_SYM_RES control in address 17 of the
resampler control page. The symmetric filter is of even length
which means the center tap repeats.
The 12 bit filter coefficients are stored in a 256 word
memory which can be divided into one, two, or four equal
blocks. This allows the user to store one symmetric filter of
up to 512 taps, two symmetric filters of up to 256 taps each,
or four symmetric filters of up to 128 taps each. The number
of filters is set by NFILTER in address 16 of the resampler
control page. The filter used by each channel is selected
NDELAY Images added by zero padding
Power
...
-3F_S
-2F_S
ImabgyeFsilrteedriuncged
-F_S
-F_S/2
F_S/2 F_S
a) Spectrum after zero padding
QTAP Filter
Frequency Response
Power
...
-3F_S
-2F_S
-F_S
-F_S/2
F_S/2 F_S
b) Spectrum after filtering
In-band aliasing caused by decimation by NDEC
Power
...
Freq
2F_S
3F_S
...
Freq
2F_S
3F_S
-F_O
-F_O/2
F_O/2
Freq
F_O=(NDELAY/NDEC)F_S
b) Final Spectrum after decimating by NDEC
Figure 13. The Resampler’s Spectral Response
© 1999−2001 GRAYCHIP,INC.
- 12 -
APRIL 27, 2001
This document contains preliminary information which may be changed at any time without notice