Optimizing NTSC PAL DAC Output on DM64xx

From Texas Instruments Embedded Processors Wiki

Introduction

The video DAC output for the DM6446 and DM6437 EVMs can be optimized through a combination of hardware and software changes which are described below.

EVM Hardware Changes:

• On the DM6446 EVM, the OPAMP feedback resistor can be changed to 1.3-Kohms to boost DC gain from 5.2V/V to 5.6V/V. The value of the feedback resistor can be tuned to achieve 100 IRE white level.

• On the DM6437 EVM, the output termination resistor can be changed from 75-ohm to 57.6-ohm to boost DC gain from 5.2V/V to 5.6V/V. The value of the termination resistor can be tuned to achieve 100 IRE white level.

• On both EVMs, an equalizer circuit can be added across the output termination resistor to boost the high frequency response. The circuit can be tuned to boost the color burst amplitude to match the sync-tip amplitude.

• DM6446 EVM Equalizer Circuit:

• DM6437 EVM Equalizer Circuit:

Software Changes:

• The VENC’s CVBS.YCDLY bit can be set to 1 to delay the Luma by 40ns to more closely match the Chroma delay

• The VENC’s 27MHz upsampling feature can be turned on by setting CVBS.YUPS = 1 and CVBS.CUPS = 1. Note: A bug exists in the default upsampling filter that can result in ringing on the video signal. The alternate test-mode filter should be used instead. The test-mode filter can be selected by setting bit 0 of the VENC register at offset x25F4 to 1. The test-mode filter is simpler in design and attenuates the higher-frequency video signals more than the default filter. This attenuation may or may not be acceptable, depending on the application. A VM700T screen capture of the multiburst pattern is provided with upsampling both enabled and disabled.

• The VENC sync/white levels were set to 286mV/714mV for NTSC and 300mV/700mV for PAL by programming the CVBS.CVLVL bit

• The 7.5 IRE pedestal can be disabled in the TVP5146 video encoder by setting the “Pedestal not present” bit of the “Luminance Processing Control 1 Register” = 1, and also disable in the in the VENC by setting CVBS.CSTUP = 0

• The ITU-R BT.601 coding range should be be used throughout the signal chain by setting “Output Formatter 1 Register”, field “YCbCr code rage” = 0 in the TVP5146 and setting VDPRO.ATCOM = 0 in the VENC

Measurements

Below are NTSC/PAL measurement results taken from both the DM6446 and DM6437 EVM DAC outputs. The test setup for these measurements was as follows:

• A Tektronix TG2000 NTSC/PAL signal generator was connected to the EVM Composite input.
• The EVM Composite output was connected to a VM700T video measurement set
• The VM700T was terminated with a 75-ohm resistor.

Media:DM6446EVM_1p3KFeedback_256pFEqualizer_6ft_NTSC.zip

Media:DM6446EVM_1p3KFeedback_256pFEqualizer_6ft_PAL.zip

Media:DM6437EVM_57p6Ohm_200pF_Equalizer_6ft_NTSC.zip

Media:DM6437EVM_57p6Ohm_200pF_Equalizer_6ft_PAL.zip

Leave a Comment