Thursday, February 26, 2004

MJPEG Capture Data-Rates and Quality

Did some test capture off some VHS source material to check data rates (earlier listing and recent list). Capture settings were 720x480 30fps with 48kHz 16bit stereo audio.

RAW - 20Mb/sec - 72 Gb/hr
Q20 - 7.8-8.5 Mb/sec - 27.4-29.9 Gb/hr
Q19 - 2.6-3.2 Mb/sec - 9.1-11.2 Gb/hr
Q18 - 1.7-2.1 Mb/sec - 6.0- 7.4 Gb/hr
Q17 - 1.5-1.7 Mb/sec - 5.3-6.0 Gb/hr
Q16 - 1.2-1.4 Mb/sec - 4.2-4.9 Gb/hr
Q10 - 0.7-0.9 Mb/sec - 2.5-3.2 Gb/hr

At Q10, it was very easy to see pixelation around things like text or hard edges on the screen. Q16 and Q17 were pretty good, but with a semi-noisy VHS source it's hard to see a difference between that setting and a higher quality setting. Normally, I capture at Q19, which is a good trade-off between space/quality. To save more space and to make encoding easier, I can capture at 352x480 which chops my data rate by roughly 30-40%.

If I was doing a PVR-type setup where I knew that I'd be processing the video prior to putting it out to DVD (which means I don't want to capture straight to MPEG2), I'd consider Q17 or Q18 as a way to save even more space.

Right now, I'm using (5) VHS tapes to do my weekly TV episode captures (8 hrs/wk or so)... which is not quite enough for me if I should screw up a capture and not catch it until after I make the DVD at the end of the process. Usually, I do the capture / encode / burn around 3-5 days after I record the episode off the air. That process can take 24-48 hours depending on what I have in the filtering / encoding queue. A more comfortable buffer would be (15) tapes where I'd only use each tape around once every 3 weeks, giving me almost 2 weeks of buffer time to go back and catch a goof.

Why figure that out? Because I'm trying to estimate disk requirements should I switch to recording straight to disk. 30 hours at Q19 is 315Gb (ouch) and that needs to be multiplied by 2 with an additional 4Gb/hr of scratch space to account for working files. (30 * 10.5Gb/hr * 2 + 4 * 30 = 750Gb) Switching down to Q18 and 352x480 (4.5Gb/hr) cuts that about in half (30 * 4.5 * 2 + 4 * 30 = 390Gb). Still a heck of a lot of space, but finished clips could be off-loaded to a 2nd disk.

posted by Wuphon's at 12:28 AM