overcranking bitrates at DVinfo.net

Rob Stowell · January 3rd, 2012, 05:06 AM

Hi- I'm looking for a camera to do some slow-motion work, and it's exciting to see 50 and 60p everywhere now. I see people shooting everything at 50p, and it seems a lovely option, to be able to use slow-motion at will. But I do wonder: doesn't shooting 50p, say (I'm in PAL territory) mean you are effectively halving the bit-rate of the recording? And if the maximum quality is 24 Mbs and you're shooting twice the frame-rate, does anyone notice any serious lowering of quality?
I've shot 50 fps on a jvc HM100, and it looked very clean, but it's shooting at 35Mbs, and it was 720p so less info. I have also tried the 50 fps option on a jvc HD201 recording 720p hdv at 19 Mbs. It looked fine for tripod shots, slow moves etc, but fast motion tended to break up the detail- and what is an acceptable blur at full-speed doesn't look so good slowed down.
So how does the AVCCAM codec handle halving the bit-rate per frame? At 720 or 1080? I'm thinking of using it for commercials, where the image needs to stay clean. Anyone looked at this with a critical eye?

Bruce Watson · January 3rd, 2012, 01:21 PM

Quote:

Originally Posted by Rob Stowell

... doesn't shooting 50p, say (I'm in PAL territory) mean you are effectively halving the bit-rate of the recording?

Not really. Depends on the hardware and especially the CODEC. A modern long-GOP codec like AVCHD will give you about the same bit rate, and almost no loss in video quality. The reason is that it's mostly recording the difference between the video frames. And with double the frames, you get about half the difference between the frames. So it works out pretty well.

For example, the specs on AVCHD give you a max bit rate of 24Mbps at the highest quality setting (30 fps). With AVCHD 2.0, it's raised to just 28Mbps, but with double the frame rate (60 fps). This is just grabbing numbers from memory -- if I'm off I'm sure someone will correct me.

Phil Lee · January 6th, 2012, 03:45 PM

Hi

The above is exactly right, and on Panasonic camcorders at least, the 28Mbit/sec bitrate can easily peak greater than 30, I've seen peaks of 35 before now giving good head room for difficult to encode scenes.

Also progressive footage is easier to compress than interlaced as these modern codecs really want to work with progressive footage, and as you go up the quality scale (Levels) of something like AVC/H264, it doesn't support interlaced footage at all.

Also as pointed out, the more frames per second gives the encoder greater opportunity to find similarities between groups of frames and track motion from one frame to another.

Regards

Phil

January 3rd, 2012, 05:06 AM	#1
Rob Stowell Regular Crew Join Date: Jun 2008 Location: Christchurch, New Zealand Posts: 55	overcranking bitrates Hi- I'm looking for a camera to do some slow-motion work, and it's exciting to see 50 and 60p everywhere now. I see people shooting everything at 50p, and it seems a lovely option, to be able to use slow-motion at will. But I do wonder: doesn't shooting 50p, say (I'm in PAL territory) mean you are effectively halving the bit-rate of the recording? And if the maximum quality is 24 Mbs and you're shooting twice the frame-rate, does anyone notice any serious lowering of quality? I've shot 50 fps on a jvc HM100, and it looked very clean, but it's shooting at 35Mbs, and it was 720p so less info. I have also tried the 50 fps option on a jvc HD201 recording 720p hdv at 19 Mbs. It looked fine for tripod shots, slow moves etc, but fast motion tended to break up the detail- and what is an acceptable blur at full-speed doesn't look so good slowed down. So how does the AVCCAM codec handle halving the bit-rate per frame? At 720 or 1080? I'm thinking of using it for commercials, where the image needs to stay clean. Anyone looked at this with a critical eye?

January 6th, 2012, 03:45 PM	#3
Phil Lee Major Player Join Date: Feb 2010 Location: London Posts: 302	Re: overcranking bitrates Hi The above is exactly right, and on Panasonic camcorders at least, the 28Mbit/sec bitrate can easily peak greater than 30, I've seen peaks of 35 before now giving good head room for difficult to encode scenes. Also progressive footage is easier to compress than interlaced as these modern codecs really want to work with progressive footage, and as you go up the quality scale (Levels) of something like AVC/H264, it doesn't support interlaced footage at all. Also as pointed out, the more frames per second gives the encoder greater opportunity to find similarities between groups of frames and track motion from one frame to another. Regards Phil