TVP5146, overscan or underscan?

The TVP5146 always samples the entire image width/height and does not underscan.

601/square relates to the number of pixel samples horizontally in the entire image, but the entire image is always captured.

BR,

Steve

One thing to understand is the the sampling mode only changes the sampling frequency and not the region which is sampled. What I mean here is that square sampling still samples the same time period (i.e. the same image line length) but takes fewer samples over the same line.

The analogy here is... draw a line 5 inches long, 601 sampling will divide the line into 720 pieces, square pixel will divide the same 5 inch line into 640 pieces.

Now, if your source is from a 640x480 sensor there are 2 possibilities for how this is composted into a video signal. It can spread the 640 'pixels' across the entire NTSC line time, in which case you would then prefer to sample the entire line into 640 samples, i.e. square pixel sampling. Alternatively the camera could window the 640 pixel image by adding black to the left and right sides such that 601 sampling would align the sample points on the middle 640 points correctly. In this case you would want 601 sampling even though your source image is really 640 samples, since it would have an extra 80 samples added to 'make it fit'

Now, to confuse things even more many if not all TVs do what is called over scan, where they scale the image up and crop off the outer border of about 10%. What this means is that even if your source takes the 640 image and adds black borders to maintain a 601 sampling requirement the black borders may not be visible on the TV, incorrectly implying that square sampling is being used.

The easiest way to check is to see if your TV has a mode called something like "native" or "full image" etc... which may disable the overscan hence allow you to see all the image sent by the camera.

Hope this helps a little?

BR,

Steve

Just to add to this...

If your ultimate objective is to receive a normal NTSC signal and display it on a 640x480 LCD directly then you have 3 options.

1) Use square pixel sampling in which case your LCD will display then entire source image on the entire LCD.

2) Use 601 sampling but crop the image horizontally. This will look much more like a regular TV in the horizontal direction since most TVs overscan, but it will not over scan vertically. The aspect ratio may not be correct though and depends on the pixel aspect ratio of your LCD. The horizontal cropping can be achieved either by changing registers in the TVP4146 (SAV and EAV) or in the back end processor.

3) Use a scaler in the back end processor to crop and scale the 720x480 source so that it is correctly over-scanned to your 640x480 LCD

BR,

Steve