Texas Instruments | TVP5160 IF Compensation Calibration Procedure | Application notes | Texas Instruments TVP5160 IF Compensation Calibration Procedure Application notes

Texas Instruments TVP5160 IF Compensation Calibration Procedure Application notes
Application Report
SLEA111 – May 2011
TVP5160 IF Compensation Calibration Procedure
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1
Introduction
The TVP5160 supports IF compensation. This feature compensates for attenuation at higher frequencies
from the transfer characteristics of television tuners or due to channels that are not correctly tuned. This
feature can also correct for asymmetrical color subcarrier sidebands resulting in incorrect and uneven UV
demodulation.
2
Technology Overview
The television tuner IF stage can attenuate the upper sideband of the chroma, which results in unnatural
fringe artifacts in the color boundaries. This block compensates for the resulting crosstalk between the
demodulated chroma components u and v and the attenuation of the upper sidebands. A block diagram of
the IF compensation is shown in Figure 1. The crosstalk IF compensated or crosstalk plus gain
compensated outputs can be selected. Variable gains Gdiffu and Gdiffv control the amount of crosstalk
compensation that are applied to u and v. Variable gains Glow_u and Glow_v control the amount of peaking
applied to the lower band of frequencies of the compensated u and v, while Ghigh_u and Ghigh_v control the
amount of peaking applied to a higher band of frequencies of the uncompensated u and v.
Enable High
Frequency
Ghigh_u
0
Peaking
_
Glow_u
LPF
Gdiff_u
+
upeak
+
uout
Peaking
uin
vin
ucomp
Crosstalk
Compensation
vcomp
vout
Peaking
vpeak
+
Gdiff_v
_
Glow_v
LPF
Select
Output
Peaking
Ghigh_v
+
0
Enable High
Frequency
Figure 1. IF Compensation Block Diagram
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Technology Overview
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Figure 2 shows the crosstalk compensation. A differential filter (coefficient weights are 1, 0, -1) outputs
pulses corresponding to transitions in its input (first derivative of input). An adjustable gain is applied to the
differential filter output and the result is added to the delayed u and subtracted from the delayed v.
ucom
Equalization Delay
u
Differential Filter
Gdiff_u
Differential Filter
Gdiff_v
_
v
Equalization Delay
vcom
+
Figure 2. Crosstalk Compensation
Figure 3 shows the peaking block and its frequency characteristics. It amplifies high frequencies by an
adjustable gain and then adds the delayed input.
G
Peaking
Filter
In
Out
Delay
Figure 3. Peaking Amplifier
2
TVP5160 IF Compensation Calibration Procedure
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Technology Overview
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Figure 4 shows the frequency characteristics of the low pass filter (LPF). It limits the bandwidth of the u
and v to 1 MHz and establishes the low-frequency and high-frequency bands.
Figure 4. Low-Pass Filter
With reference to Figure 1, equations for upeak and vpeak in Equation 1 show the low and high frequencies
can be amplified by the gains.
upeak = LPF × (Glow_u × ucomp – Ghigh_u × uin) + Ghigh_u × uin
vpeak = LPF × (Glow_v × vcomp – Ghigh_v × vin) + Ghigh_v × vin
= LPF × Glow_u × ucomp + (1 – LPF) × Ghigh_u × uin
low frequency
= LPF × Glow_v × vcomp + (1 – LPF) × Ghigh_v × vin
low frequency
high frequency
high frequency
(1)
This is an important feature that applies when u and v do not have the same bandwidth. If u and v have
the same bandwidth, then both have the same crosstalk and gain attenuation. Therefore, the same
differential gains and the low-frequency and high-frequency gains are applied to u and v. If u and v have
different bandwidths, then the symmetry of the IF compensation changes. Figure 5 shows an example
where u has a greater bandwidth than v.
x
y
u
v
fsc
Tuner IF
Figure 5. UV Frequency Spectrum (u Bandwidth Greater Than v)
The frequency attenuation of the tuner IF stage on the baseband chroma is also shown. There is more
crosstalk from u into v and less crosstalk from v into u. The differential gains Gdiff_u and Gdiff_v are first
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Programmable I2C Control Registers
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adjusted to eliminate the crosstalk. The low frequency gains Glow_u and Glow_v are then adjusted to
compensate for the attenuation of the crosstalk compensated ucomp and vcomp in the region x. Then
frequency region y of the uncompensated u can be boosted by the gain Ghigh_u. The controls that enable
high-frequency gain of u is set to 1 and enable high-frequency gain of v is set to 0. Table 1 shows the
different possible control settings.
Table 1. Control for Different UV Bandwidth
Combinations
3
uv Bandwidth
Enable
High-Frequency
Gain of u
Enable
High-Frequency
Gain of v
Same
0
0
u greater than v
1
0
v greater than u
0
1
u equal to v
0
0
Programmable I2C Control Registers
Table 2. IF Compensation Control (subaddress = 8Dh)
Bits
Description
Default
0
Enable IF compensation
0
1
Select crosstalk and gain compensated outputs
0 = crosstalk compensated outputs only
1 = crosstalk and gain compensated outputs
0
2
Enable high-frequency gain of v
0
3
Enable high-frequency gain of u
0
Recommendation
For SAW IF stage, set to 0.
For non SAW IF stage, set to
1.
Table 3. Differential Gain (subaddress = 8Eh)
Bits
7-4
Description
Label
u differential gain
Gdiff_u
Default
Comment
Recommendation
2
Actual gain = n/4
as n varies from 0 to 8
3-0
v differential gain
Gdiff_v
2
IF Stage
Distortion
Setting
Low
1
Medium
2
High
3-4
Table 4. Low Frequency Gain (subaddress = 8Fh)
Bits
Label
Default
7-4
u low frequency gain
Description
Glow_u
4
3-0
v low frequency gain
Glow_v
4
Comment
Actual gain = n/4
as n varies from 0 to 8
Table 5. High Frequency Gain (subaddress = 90h)
Bits
4
Label
Default
7-4
u high-frequency gain
Description
Ghigh_u
0
3-0
v high-frequency gain
Ghigh_v
0
TVP5160 IF Compensation Calibration Procedure
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Comment
Actual gain = n/4
as n varies from 0 to 8
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Calibration Procedure
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4
Calibration Procedure
Perform these steps to perform IF compensation calibration.
1. Enable IF compensation.
2. Select crosstalk compensated outputs only and set differential gains to 0.
3. For color bar test pattern determine whether or not u and v bandwidths are same by looking at
crosstalk pulses around color bar transitions (undershoots and overshoots).
4. Adjust differential gains to eliminate crosstalk pulses (default is 2).
5. Select crosstalk and gain compensated outputs and set low and high-frequency gains to 0.
6. Adjust low frequency gains for optimum picture (sharper edges in color transitions); if the gain is too
high undershoots and overshoot will reappear (default is 4).
7. Set enable high frequency controls based on Table 1 (default is 0).
8. Adjust high-frequency gains for optimum picture (sharper edges in color transitions); too high a gain
may distort edges.
9. CTI can also be used to sharpen edges.
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