SingMai Electronics
PT9 User Manual Revision 0.3
Page 12 of 38
PT9_Subcarrier_gen.v
The input clock drives a 32-bit ratio counter which generates a precise subcarrier frequency for the
selected standard. An 11-bit phase word from this ratio counter addresses 12 bit wide Sin and Cos
look-up tables. To save on memory only one quadrant of the Sin and Cos tables are stored and the
addressing and data is modified to generate the other three quadrants.
The resulting Sin and Cos data is then multiplied by the U and V data during the active video period
and a shaped Burst Gate pulse is used to insert the colour burst.
The colour frame output of the SPG ensures the burst is correctly inhibited whilst the PAL switch
output of the SPG switches the 135deg and 225deg phase increments in the correct sequence. For
NTSC it is possible to add in a phase offset (hue).
PT9_Chroma_modulator.v
The Cb and Cr chroma from the BT656 receiver is interpolated from its 6.75MHz sample rate to the
27MHz output sample rate. The interpolation filter has a 1.3MHz (-3dB) frequency response with a
Gaussian response (see Figure 4).
Figure 4 Chroma modulation filter.
The filtered Cb and Cr signals are then scaled, multiplied by the sin and cos waveforms from the
subcarrier generator and added to generate a composite chroma signal. The resulting U.sin
(2πfsc.t) and V.cos (2πfsc.t) data is added together to create the chrominance signal which has
blanking added as well as programmable gain.
PT9_ROM.v
This module provides the 512 x 12 bit sine and cosine lookup tables for the subcarrier generator.
PT9_output_proc.v
The output processing module combines all the components of the video output into one
composite output signal.
The luma signal is interpolated from 13.5MHz to 27MHz – the filter response is shown in Figure 5.
The composite sync signal has an approximation to a 2T raised cosine wave shape added to ensure
rise/fall time conformance to the video specification.
The luminance signal is then passed to the comb filter module to provide the cross-colour
reduction. A description of this mode can be found in Chapter 7.
The luminance, chrominance and composite sync are then added together to generate the 10-bit
27MHz CVBS signal.
Inphase Filter Frequency Response
Frequency in MHz
M
a
g
n
it
u
d
e
0
1
2
3
4
5
6
7
8
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
