LMH1981 staring field of the frame

Hi Nikoo,

We are looking into this case, the Applications engineer of this product will respond soon.

Regards

TK Chin

Hello Alan,

Thank you for your reply. Please consider that we only have two fields, odd and even. please refer to the link below for an explanation of what I mean by field dominance.

http://en.wikipedia.org/wiki/Field_dominance

The confusing point for me is that in some resources, including the above link, each frame starts with an even field, while in the datasheet it is considered that each frame starts with an odd field.

I am wondering which one is correct, and whether it is correct to consider that the OEOUT pin will be high on the first filed and low on the second field, in both odd dominant and even dominant fields.

I do not concern about the odd or even field detection, and I want to use OEOUT, as the input to another circuit for frame detection to trigger another camera.

Regards,

Nikoo

The LMH1981 looks at the vertical serration pulses to determine the odd/even signal.  Figures 3 & 4 (525i, similar for 625i) and  Figure 7 & 8 (1080i) illustrate the relationship of the vertical serration to the OEOUT timing.  The LMH1981 assumes that Odd Field (Field 1) contains the first half of the total scan lines (Lines 1, 2, 3, ..., Total_Line_Count / 2 ).  It seems that field dominance can get mixed up and result in color artifacts when video is edited/mixed without maintaining proper sync to the output color frame sequence.  See http://en.wikipedia.org/wiki/Color_framing. 

Regards,
Alan

Hello Alan,

Based on what I understand, the standard NTSC, which is 525i, starts with an even field and the PAL, which is a 625i, starts with an odd field. But in the datasheet for a 525i, NTSC signal, the starting frame is odd. Regardless of the color framing and field, I will be thankful if you can clarify this point.

At the time, I wont use the sync separator for video editing, so I do not concern about the color frame and color field. All I need is to detect the start of each 525i NRSC frame, to trigger the other camera at the start of each NTSC frame. I will be thankful if you can verify the logic level of OEOUT pin at the start of the frame, and its transition at the first and the second field.

Regards,

Nikoo

NTSC color field 1 & 3 ("odd field" in LMH1981 datasheet): OEOUT = Logic high (transitions low to high).

NTSC color field 2 & 4 ("even field" in LMH1981 datasheet): OEOUT = Logic low (transitions high to low).

NTSC color field number sequence is defined per ITU-R BT.470-6, figure 5c.

Alan

Hello Alan,

Thank you for your reply. Based on what I understand, each color frame consists of two video frames. Hence, the first and the second color fields form the first video frame, and the third and the fourth fields are in the second video frame.

As I only want to detect the video frame, and not the color frame, please consider that I do not want to differentiate the first color field from the third one; as well as the second color field from the fourth one. Then, may I consider that the rising edge on OEOUT determines the start of the first video filed and the falling edge is the start of the second video field? i.e., the rising edge on OEOUT is the start of video frame?

Regards,

Nikoo