[FFmpeg-user] YUV->RGB conversion. What is the default matrix used?

[Tim Nicholson]

Although we now have the "colormatrix" filter to convert between
different YUV  colorspace standards it occurs to me that it is not clear
what matrixing is used by default when converting between YUV and RGB.

A quick test using:-

ffmpeg -i bars-601.mov -t 1 -an -r 1 %04d-601.png

and an inspection of the resultant png shows RGB values of the bars to
differ from the values expected if using the 601 matrix values, and even
a grey scale has significant colour casting at certain levels.

So it begs the question, what coefficients are being used by default?
-- 
Tim