Support -2IRE?

Hi Eddie,

If the CSM is being used to map ITU 601 code range to the full-scale DAC output, code 64 (16 with 8-bit input) will be shifted down to the bottom range of the DAC output.  If your-2IRE extends below code 64, try disabling the clip/shift function in the CSM.  The DAC output should then be raised up allowing the -2IRE to pass.

Hello Larry,

It works by disabling the CSM block. However, the output waveform seems not to be same as the original signal during the end of active video and start of sync. I tried to increase the the distance from the end of active to start of sync (dtg1_spec_b, reg. 0x26), and it will pull up part of this interval back to blanking level. But, this cannot cover the whole interval (reg. 0x26 set to 0xFF). Do you have any idea of how to solve this problem? Btw, this symptom only happens in signal formats with 50Hz, like 1080/50i, 720/50i, 1080/25p, and 575/50i.

PS. Is this a limitation of the THS8200 that you mentioned in previous post?

Channel 1: Output of THS8200

Channel 2: Original signal

Eddie,

The REG26h limitation will only occur for the 1080p25 fromat and formats that have long front porch intervals.

It looks like the THS8200 blank level needs to be adjusted down.  The sync tips are not symetrical.  Try lowering the dtg1_y_blank setting.

Larry 

Hi Larry,

I have tried to lower the blank level, but all active area will be shifted up and the amplitude of this interval seems to be same as that of -2IRE. By the way, I checked the SMPTE 296M and 274 standard, the signal formats whose frequencies are 50Hz, 25Hz, and 24Hz have long front porch. For example:

1080p25 & 1080i50: f-p+1=2404-1919+1=486; 1080p24: 2514-1919+1=596

1080p60, 1080i60, 1080p30: 46; 720p60: 72

They are all wider than the range of REG26h and cannot be covered by setting this register?