Re: Which data rate to choose for capturing video for web

Hi Christian,

You are correct that a format like XDCAM EX operates at 35Mbps, or HDV at 25Mbps. The issue is that these formats use Long-GOP MPEG-2 with 4:2:0 color space.

While this works well for acquisition, these are not very robust editing formats - they are meant to create SMALL files for convenience of recording to memory cards or DV tape.

The Matrox MPEG-2 I-Frame HD codec uses 4:2:2 color and as the name implies, is an I-Frame format. That means that each frame is individually compressed, and includes all the data to uncompress (and thus playback) that frame, with more color information than 4:2:0.

The Long-GOP format actually saves just two I-Frames per second; the rest of the frames are recorded using only the differences between several frames following and preceding the current frame.

That is the reason that a "drop out" or glitch in an HDV recording typically wipes out 15 frames at a time - they are all tied together!

So, the default 100Mbps used by Matrox is not unreasonable given the quality of the recorded HD image. You have the option to lower the quality slider, but that is up to you to try and see how it works.

You mentioned working with green screen keying, and you will get better key edges using the Matrox codec than using the native footage. I have tried this myself with 1080i HDV and there is a BIG difference in quality when I capture to the RT.X2 using Component video captured to the Matrox codec versus capturing native HDV via Firewire.

Since you are recording the green screen footage in a studio, your best workflow would be to connect the camcorder via HMDI to the MXO2 and capture LIVE during the shoot, simultaneously recording in-camera as well for safety.

Most camcorders with HDMI output send out an uncompressed 4:2:2 signal direct from the image sensor prior to the circuitry that compresses the video for recording. By capturing this signal direct to the Matrox MPEG-2 I-Frame HD codec before it gets compressed in the camera, you get a much better quality and color fidelity, with the 4:2:2 being very beneficial for keying.

Jeff Pulera
Safe Harbor Computers