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SPT9210SCS

ANALOG VIDEO PROCESSOR

Signal Processing Technologies 

ANTI-ALIAS FILTERS

Both luminance and chrominance video signal paths have an

anti-alias filter with a cut-off frequency of approximately

4.8 MHz. The cut-off frequency can be changed by an

external resistor value. The relationship between the external

resistance value and the cut-off frequency is shown in the

Typical Performance Curves section.

AUTOMATIC GAIN CONTROL

The SPT9210 performs automatic gain control (AGC) of the

composite/luminance and chrominance signals. The hori-

zontal sync signal level is used as a reference for control of

the signal gain. The chrominance signal gain is slaved to the

luminance gain value.

The gain circuitry can operate over an input voltage range of

VIN = -6 dB to +3 dB (where 1 VP-P = 0 dB). The AGC will

maintain a 2.5 VP-P amplitude on the composite/luminance

signal output and 0.7 VP-P amplitude on the chrominance

signal output. The AGC settling time can be adjusted via an

external capacitor. SPT recommends using a 0.47 µF capaci-

tor for most conditions. In cases where extreme fluctuation is

possible, a diode inserted between pins 7 and 16 will restrict

the maximum control voltage. This will serve to reduce

recovery time.

FINAL DC CLAMP AND GAIN STAGES

After the clamp, low pass filtering and automatic gain control

functions are performed. Each signal path is clamped to a fixed

DC value and amplified to the proper voltage range for input

into the SPT7852 analog-to-digital converter. The analog sync

signal is retained in the output of the SPT9210 and passed on

to the SPT7852 and SPT2110 NTSC/PAL video decoder.

For the composite/luminance output, the horizontal sync

level is clamped to +1 V, and the full-scale amplitude of the

composite luminance signal (including sync tip) is set to

2.5 VP-P amplitude by the AGC. (See figure 2.) The generated

signal is optimized for SPT7852 performance (+1.0 to +3.5 V

input range). The chrominance output signal is biased to

+2.5 V and a 0.7 VP-P full-scale amplitude is maintained by

the AGC.

The output drive circuit is optimized for interface to the

SPT7852. When driving loads other than the SPT7852 (which

have a capacitance >TBD µF), it may be necessary to insert

a series resister between the output and the load so as to

avoid oscillation.

ON-CHIP VOLTAGE REFERENCES

In addition to performing the analog processing of the video

signal before data conversion, the SPT9210 also provides

the voltage reference sources (force and sense for the top

and bottom of the reference ladder) required by the SPT7852.

This eliminates the need for external reference sources. A

source of +3.5 V is provided to the top of the reference ladder

and +1.0 V is provided to the bottom.

Pin 7 is the VL reference force pin and is tied to pin 4 of the

SPT7852. Pin 13 is the VH reference force pin and is tied to

pin 1 of the SPT7852. A 240 Ω resistor should be tied between

pin 7 and pin 13 on the SPT9210. Pin 8 is the VL reference

sense pin and is tied to pin 3 of the SPT7852. Pin 8 is the VH

reference sense pin and is tied to pin 2 of the SPT7852.

The values of VL and VH can be adjusted by changing the

external resistor values at pins 9 and 11, respectively. The

curves in the Typical Performance Curve section show the

voltage reference output values versus the external resis-

tance values. The typical values for nominal +1.0 V and

+3.5 V operation on VL and VH are approximately 10 kΩ and

25 kΩ, respectively.

The VRef clamp pins need decoupling capacitors as shown in

figure 3. Each pin should have a 0.1 µF and 10 µF capacitor

connected in parallel for proper decoupling.

OTHER INFORMATION

The SPT9210 is available in a 20-lead SOIC package and

operates over the commercial temperature range. It requires

a single +5 V supply and dissipates 620 mW of power.

Figure 2 - Composite/Luminance Signal I/O Relationship

Input Signal

1.0 Vpp

Composite

Signal

SPT9210

Pin 19

Pin 14

3.5 V

Composite/Y

Output

Y Signal

Pin 2

1.0 V

Output Signal

0.0 V

SPT

4

SPT9210

11/7/97

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