I, P, & B Frames?

[PlasticDVDr]

I've just picked up a HP 200i DVD+RW recorder and a ATI AIW VE card so that I can use my pc as a vcr.

I'm a little confused with the I, P & B frames when capturing...

Setup 1 (Lots of dropped frames, cpu maxed out)
MPEG-2 8000 Mbps
Audio 48k Stereo PCM Uncompressed
29.97 FPS
720x480
4 P frames, 2 B frames

Setup 2 (No frames dropped, 60-75% cpu usuage.)
MPEG-2, 8000Mbps
Audio 48k Stereo PCM Uncompressed
29.97 FPS
720x480
I frames only

After reading the forums and doing some research, I've noted that I frames need the most cpu usuage and to only use the B and P frames if I encounter lots of dropped frames. But for some reason I only lose frames when I capture using P & B frames...

Can some one clearify this for me?

Computer Specs:
-------------------------------------------
15' NEC LCD Monitor
1.7Ghz Celeron
384MB 2100 DDR Ram
Intergrated 16-bit sound card
40GB & 20GB 7200rpm HDD's
Radeon 7500 64MB PCI Vid Card
HP 200i DVD+RW
Yamaha CRW-F1 CD-RW

[i_am_dave]

I am probably restating what you have already learned, so don't be annoyed.....but,

I suspect you are running out of CPU power. It is true that I-frames contain more data, and therefore require more throughput to your hard drive.
However, in the first case you listed, the P-frames and B-frames take extra CPU time to generate.

Your capture card is feeding the encoding software raw video frames, I am not familiar with this type of card, so I don't know if it is feeding the encoder raw frames or DV format. All the encoder does is quantisize these frames to generate an I-frame.

When generating P-frames and B-frames, it must first generate the differences from the neighboring frames and then encode the frame. This is an extra step.

Have you tried disabling all non-essential software on your computer? Like virus scanners and so on? You might also look at increasing the priority of your encoder.


One thing you might try if you have enough disk space and your video card supports it, is to capture to DV format and do the encoding later with another application like TMPGenc.

Hope this helped!

[PlasticDVDr]

Thanks alot, that makes more sense now...

So in basic terms if I understood properly, the direct feed is encoded to "I" frames and if "P" & "B" frames are added it is then taking the "I" frame looking at the similarities and differences between each frame so that certain blocks and groups of colours can remain in the video until say the next major scene change instead of adding a new colour pallete to each frame...

Which is the reason why a video encoded with only "I" frames requires a higher bitrate and more diskspace as opposed to using "I", "P" & "B" frames....

I believe the only reason I get dropped frames is because either I have a celeron, it's only 1.7Ghz or both.... I never leave anything that isn't needed running...

[i_am_dave]

I am running a 1.8 Ghz P4 and can barely encode on the fly without dropping frames.
I always capture in DV format and encode later, sure, it takes about 12 Gig per hour of video, but I purchased a 120Gig drive that I have dedicated for video. This lets me capture up to 5 hours and still have room to edit and encode.

[bmiller]

I have a 1.2 khz. AMD,and capture mpeg2 720x480 at 8000kb/sec vbr with little or no dropped frames.The ati mmc 7.7 indicates less than 1% frames dropped.the video turns out great,no artifacts or jerkiness.
I'm lucky i guess.I just use the default settings in mmc7.
I'd be interested in any mmc tuning,or if anyone has compared power vcr captures to ati mmc captures.
Brad

[jessieqwert]

This is a great post.

1. I think the original question has already been covered plus I don't know the answer for sure.

2. Ppl have brought up several things I'm curious about

a) Which produces better quality?

i. Capturing in DV format at 720x480 and exporting to an MPG encoder with a high bitrate

ii. Or using a medium quality card like an ATI and using MMC to capture directly with a high bitrate

Lets assume I'm using a D8 camcorder with both DV and analog out.

b) Since both MMC and Power VCR record in real time speed is not an issue. However, which produces a better quality picture given a set file size
and bit rate?

Thanks to all for their replies!

[txpharoah]

I have a 1.2 khz. AMD,and capture mpeg2 720x480 at 8000kb/sec vbr with little or no dropped frames.The ati mmc 7.7 indicates less than 1% frames dropped.the video turns out great,no artifacts or jerkiness.
I'm lucky i guess.I just use the default settings in mmc7.
I'd be interested in any mmc tuning,or if anyone has compared power vcr captures to ati mmc captures.
Brad

It probably just drops the first few and no more. Double click (or is it right-click?) the 1% marker and it will change to shoing the actual number of drops.