ADV7181C Letterbox detection issue

Hello,

I'll check with an expert on this part. It's not a feature that gets used much that I'm aware of.

Dave

Hello,

Sorry for the delay-- we're going to run some tests in the lab on this.

Dave

Hi,

We ran some tests in the lab. We plugged in a PAL CVBS signal with a wide screen format from a DVD player into an evaluation board, and could observe the black bars at the top and bottom of the picture, and read back the number of black lines at the top and bottom with LB_LCT and LB_LCB.

We used the ":AUTODETECT CVBS IN NTSC/PAL/SECAM, 8-Bit 422 encoder:" script from the ADV7181C_ADV7181C@_ADV7341-VER.3.2c.txt script file available here: https://ez.analog.com/docs/DOC-1621

We observed that if the reserved bits (bits 5, 6 and 7) in Reg 0xDC were overwritten and set to 0 when changing the LB_TH value (Reg 0xDC[4:0]), we would no longer get the number of lines in LB_LCT and LB_LCB. They would always be 0x00.

Did you overwrite the reserved bits in register 0xDC (Reg 0xDC[7:5]) when you get the issue? Or did you leave them to their default values (i.e. 0b101)?

Hi,

1. We've checked the ":AUTODETECT CVBS IN NTSC/PAL/SECAM, 8-Bit 422 encoder:" script from the ADV7181C_ADV7181C@_ADV7341-VER.3.2c.txt  -  we are using the same script for ADV7181C.

The only one difference in our case is that we use 27MHz Crystal for ADV7181C instead of 28MHz Crystal in script. So we've changed:

     42 1D 47 ; Enable 28MHz Crystal

to

     42 1D 07

to work with 27MHz.

All other values are 100% the same...

2. I leave the reserved bits in register 0xDC (Reg 0xDC[7:5]) to their defaults.

To set LB_TH I read register 0xDC at first and then change only LB_TH bits (Reg 0xDC[4:0]) and write the value back to register. I've checked it also... The value I read always contains 0b101 in place of these bits.

I've also tried to write 0xA0 | LB_TH_value for sure but it did not help...

The values of LB_LCT and LB_LCB are always 0x00.

Maybe the problem is in LB_SL and LB_EL (Reg 0xDD)? I've searched the docs from the link (https://ez.analog.com/docs/DOC-1621) but did not found any description of how to use it in case of PAL signal. I've met only something like :

     Example: 0100: 23/286 (NTSC)

     Example: 0101: 24/287 (NTSC) etc

Can you please explain the usage of LB_SL and LB_EL in case of PAL (correspondence of the Reg values and the line numbers etc). And what may be the other possible reasons of the issue?

I'll also run some tests with some other PAL CVBS signal sources for sure.

Thanks in advance.

Hi,

1. We tested this using a 27MHz crystal, still using the ":AUTODETECT CVBS IN NTSC/PAL/SECAM, 8-Bit 422 encoder:" script from the ADV7181C_ADV7181C@_ADV7341-VER.3.2c.txt, but with the crystal set to 27Mhz (42 1D 07).

We could still observe the black bars at the top and bottom of the picture, and read back the number of black lines at the top and bottom with LB_LCT and LB_LCB. We left LB_TH, LB_SL and LB_EL set to the default values.

2. For PAL, the values for LB_SL and LB_EL correspond to the following:

LB_SL:

0100 = line 23 for Field 1/line 336 for Field 2 in PAL mode

0101 = line 24 for Field 1/line 337 for Field 2 in PAL mode

etc...

LB_EL:

1101 = line 310 for Field 1/line 623 for Field 2 in PAL mode

1100 = line 309 for Field 1/line 622 for Field 2 in PAL mode

etc...

3. Do you know the number of black lines per field provided by the video source you are using for your tests at the top and at the bottom of the active video?

4. Can you experiment with LB_SL, LB_EL and LB_TH based on the characteristics of the video source you are using for your tests?

5. Can you check the status register to make sure the decoder is properly locked to the CVBS input (IN_LOCK, Reg 0x10[0]; LOST_LOCK, Reg 0x10[1], INST_HLOCK, Reg 0x13[0])?

6. Is your hardware different from the ADV7181C evaluation board (see evaluation board schematic available here:https://ez.analog.com/docs/DOC-1621)?

7. Do you have an evaluation board? If yes, can you try your source with an ADV7181C evaluation board?

8. Can you try different sources?

Hi,

Thanks a lot for you help!

We've tested letterbox detection with several other PAL signal sources and found that it works for ATV signal from TV broadcaster's cable with the default values (we've tested before with signal from our head station and letterbox was not detected in spite of the fact that we can see black lines on screen - perhaps the ATV modules of our head station are working not correct).

So, thanks once more for the excellent support and kind regards!