Welcome to Oscilloscope Use 101
How to Calibrate the GeForce graphics card

The Scope:

To insure measurements are correct, it is necessary to perform a calibration check on the scope. Almost all scopes have a Cal Check post on the front control panel. The following procedure is for my Tektronix 100 MHz lab scope with a 0.300 mV / 300 mA / 1000 Hz Calibration check post. If your scope has a different voltage/frequency for the Cal check, different settings will be required and you should follow the instructions in the operations manual for your scope.

The scope CRT grid system (graticule) is used in combination with the Volts/Div and Time/Div controls to determine voltage levels and frequency. A typical scope graticule consists of eight- (8) vertical and ten (10) horizontal squares that form a graft pattern. Some squares within the grid may also be subdivided to aid in determining the levels.

Calibration Check Procedure:

1. Warm up scope for 30 minutes with the bright line (trace) intensity reduced. The warm-up time is needed to allow the deflection circuits to reach operating temperature and the trigger circuit to stabilize.
2. Connect a 1x probe to the channel 1 input and with the Input Coupling Switch set to ground (GND), center the bright line on the center vertical grid line. This line is now our Zero Level reference line.
3. Adjust the bright line intensity to mid-point. Adjust the trace rotation to center the bright line on the horizontal grid line. Adjust focus as needed to maintain a sharp image on the bright line.
4. Set the Vert Mode switch to CH-1, Trigger Source switch to CH-1 (or NORM), Trigger Mode switch to Auto, Volts/Div to 0.1V, Time/Div to 0.5mS
5. Attach probe to Cal post and Input Coupling switch to DC.
6. If all is well with the scope calibration you should see a perfect square wave. It may be necessary to adjust the trigger level slope to sync with the positive slope of the signal.

GeForce Grayscale Linearity Calibration;

The default settings on the GeForce cards produce a perfect VESA waveform with peak white voltage level of ~700 mV and black level of 0 mV. However this is not the waveform we need for NTSC encoded DVDs. We need a NTSC type signal with peak white at 714 mV (IRE 100) and black level (setup) at 53 mV (7.5 IRE) and blanking at 0 mV (0 IRE). For the NTSC system the area below 7.5 IRE is called blacker than black (or below black). Because the DAC voltage is fixed at ~700 mV, we can never achieve the 714 mV level, but, we can get close.

With the default VESA settings we lose details in the dark areas of the video and when we increase the brightness setting we force the peak white into clipping (remember fixed voltage) loosing details in bright areas. The objective of our calibration is to raise the black level from 0 IRE to 7.5 IRE (setup) and reduce the peak white to below clipping.

Scope Setup;

1. Disconnect the probe from the Cal post.
2. Position the bright line to the lowest line on the grid. This is now our zero voltage reference line.
3. Set Volts/Div to 0.1 V, and Time/Div to 2 micro Sec.

GeForce Setup:

1. Set the brightness and contrast settings to default for the video overlay.
2. Start Avia DVD and go to Title 1, Chapter 103 (Horizontal Gray Ramp).
3. Set the grayscale ramp to full screen and select pause in the DVD player.

The VESA Waveform Measurement:

1. Disconnect the VGA cable from the graphics card.
2. The RGB output is from hole 1, 2, and 3 (#1 is marked) on the 15 pin VGA connector.
3. Terminate the output by bending the leads of a 75 ohm, 1% resistor 90 degrees and inserting one lead into hole 1, 2, or 3 of the VGA connector and the other lead in the retaining screw head. Connect the probe to the lead inserted in the RGB hole and the ground clip to the other lead. Because this is a measurement of the wide band RGB signal a termination resistor equal to the characteristic impedance of the video signal cable is required to prevent measurement errors.
4. A small adjustment may be needed to return the zero level to the reference line on the scope.
5. What does the waveform look like? From the zero reference line the trace will make a 90 degree vertical turn until reaching ~ 700 mV and will make a sharp turn downward on a slope equal to the change in value of the horizontal gray ramp until reaching the zero voltage reference level and will then continue at 0 mV until the next positive pulse. The waveform looks like a reverse right triangle with the short side missing. The sloping ramp should be uniformly strait (linear) without any bumps or dips.

NOTE: A better procedure is to use a 75 ohm, 1% termination resistor adapter connected to the RGB BNC connectors at the end of the video cables.

The 7.5 IRE (Setup) Waveform Measurement:

1. Remove the termination resistor and reconnect the VGA cable to the display device.
2. Close the DVD player and set the graphics card Brightness setting to +28 or +128 depending on the numbers used for the default in the drivers.
3. Restart the horizontal gray ramp pattern on Avia.
4. Disconnect the cable and reinsert the termination resistor.
5. What does the waveform look like? At the 700 mV level the waveform will most likely have a flat or rounded spot (clipped) for one or more micro seconds at the transition to the descending ramp and will then continue in a linear manner until reaching a voltage level greater than 53 mV at which point the trace will vertically descend to the zero reference level.
6. Lowering the Contrast setting in the graphics card video overlay reduces the clipping. Reduce the contrast to 97 and recheck the waveform. If the voltage level for black drops below ~50 mV increase the Brightness setting by one point. Keep working between Contrast and Brightness settings until you can get a waveform that has a very sharp point (transition) at the 100 IRE level (700 mV) and does not drop below 50 mV for 7.5 IRE.

This procedure should get you very close to a reference standard player and provide good details in both light and dark areas of the video image. Once you get the video overlay properly calibrated it becomes easy to use Avia to calibrate the contrast and brightness of your display.

Best Regards, Cliff

 

Special Note on scope probes and test patterns:

While the use of a 1x probe (10 MHz) will provide for a non-distorted waveform for the grayscale ramp the same can’t be said for other test patterns on Avia such as the multi-burst and other high frequency related patterns. To prevent high frequency rolloff of the waveform (and misinterpretation of the test results) caused by the probe, a 10x, 100 MHz (minimum) bandwidth probe is mandatory.