Format Wars - Intraframe vs Long-GOP

[Gints Klimanis]

Originally Posted by Dan Keaton

It is my opinion, if one is using any of the modern Non-Linear Editors, the ones that are typically discussed on this forum, if a cut is made in the middle of a Group of Pictures, a new Group of Pictures will be created starting with the first frame after the cut.

Dan, you are correct. I should have added that only *some* recompression would occur during the start and end cuts if either is not on a GOP boundary, but not the GOPs in-between.

[Alister Chapman]

Changing a codec from Long GoP to I frame only IS transcoding.

Avid doesn't need to transcode because it works natively with HDV and XDCAM, as I said it can edit material directly of the media, nothing is imported so it MUST be working with the native material in it's native codec.

The AMA import system works by using decoder scripts written by either Avid or the camera manufacturer. In the case of XDCAM the decoder script was written by Sony. The decoder plugin allows Avid to work with the material Natively.

[Bob Willis]

Hi Mark,
You stated ...."I know Avid has changed the way it handles MPEG once again, since version 3.5.x, because it is no longer necessary to make a video mixdown or transcode to output Long GOP based HDV projects. Now you can use a Quicktime Reference File to export to DVD or Web encoders, such as Sorensen Squeeze."

This is not correct. You cannot export a quicktime reference of Long-GOP material in ANY version of Avid. As far as I know you cannot do this in any NLE. Apple's QT does not support the MPEG structure in a way that would allow this. This is not an Avid issue, but a Quicktime issue.

You may be confusing export of HDV material in some other fashion, but you CANNOT export a quicktime reference of Long-GOP material unless you first mixdown or transcode the material first.

The Avid forums have several current threads discussing this issue and I have done tests in Avid 4.0.2.

[Mark Job]

Hi Bob:

I'm quoting from the Avid Media Composer Readme ver 3.5x

"Long-GOP Media Creation
Avid editing applications now allow full media creation and playback support for all of the
specified Long-GOP formats and modes (XDCAM HD and XDCAM EX). This includes
support for video mixdown using Long-GOP HD formats."

....In the Avid forums they said this means the mixdown is performed automaticly and it is not necessary to transcode or mix it down automatically to export it out via QT Ref. I will have Avid Media Composer ver 4.0.2 this week and I will test it and tell you. EDIT: They specifically said HDV projects can now be directly exported in their native format via QT Ref without performing a video mixdown. Perhaps this is *Not* the case for the Sony Long GOP formats ?

[Bob Willis]

Mark,
After detailed testing with Avid 4.0.2 and 4.0.3 and conversations with Avid there remains no way to export ANY Long-GOP file as a Quicktime reference file. Again, as I stated before this is due to Apple Quicktime not supporting this feature. Until Apple does something with Quicktime program you will never be able to export ANY Long-GOP file from Avid as a Quicktime reference file.

Steering this back to Convergent Designs, it does look promising that they will be able to provide support for many of their various recording codes that are not now supported in Avid through an AMA plugin.

[Mark Job]

Hi Bob:
You wrote: "Mark, After detailed testing with Avid 4.0.2 and 4.0.3 and conversations with Avid there remains no way to export ANY Long-GOP file as a Quicktime reference file. Again, as I stated before this is due to Apple Quicktime not supporting this feature. Until Apple does something with Quicktime program you will never be able to export ANY Long-GOP file from Avid as a Quicktime reference file."

.......Oh crap ! Well, this is what I was told from an editor who's opinion I trust. I suppose my good buddy got this info wrong. (??) Can AMC 4/0/3 output Long GOP as a full Quicktime Movie instead, Bob ?

[Rafael Amador]

Originally Posted by Bob Willis

Hi Mark,
This is not correct. You cannot export a quicktime reference of Long-GOP material in ANY version of Avid. As far as I know you cannot do this in any NLE. Apple's QT does not support the MPEG structure in a way that would allow this. This is not an Avid issue, but a Quicktime issue.

You may be confusing export of HDV material in some other fashion, but you CANNOT export a quicktime reference of Long-GOP material unless you first mixdown or transcode the material first.

I don't know in a PC, but in a MAC no problem to export Reference movies with XDCAM or NANO footage.
rafael

[Mark Job]

Hi Rafael:
Are you referring to Avid Media Composer running on a Mac, or are you referring to FCP ?

[John Mitchell]

Originally Posted by Dan Keaton

1. Some feel that when using Long-GOP, the only real, true frames are the "I-Frames" inside of the group of pictures.

For example, I was told that if a fast moving object did not appear in the initial I-Frame, inside of a group of pictures, that the object would not appear in the video.

The example was given that in a hockey game, a fast moving hockey puck could not be seen in Long-GOP.

I was able to quickly produce a video, demonstrating that Long-GOP handles fast motion, and the person was completely satisfied.

A funny way to put it -just to be clear, because of the I frame/predictive nature of long-GOP compression bit starved long-GOP codecs do suffer from poor motion rendering. For example, most of the cheaper handycams that use 19Mb/s or 24Mb/s fall apart in complex motion scenes or fast moving action. Even the standard 35Mb/s XDCamEx codec can fall apart. The nano and XDR use very high bitrate Long-GOP (in some cases higher than many intraframe codecs) so they render motion very, very well.

Originally Posted by Dan Keaton

3. Another strongly held belief is that Long-GOP is harder to edit, or editing system will be less responsive, a belief from the HDV days.

We are in a fairly unique situation in that our Long-GOP is always "Full Raster". For 1080, this means 1920 x 1080, even if a camera's sensor is 1440 x 1080.

This means that the NLE does not have to do any "heavy lifting" to work with our footage, in that the footage does not have to be converted to 1920 x 1080 from 1440 x 1080.

True, full raster takes less computing power to decompress, but an NLE still has to decompress the GOP "on the fly" to individual frames, and recalculate and render either in the background or on the fly to new GOPs when the cut does not fall on an I frame. Clearly this takes more computing power than the intraframe codec. With todays modern PC's it only becomes an issue when you have multiple streams of video to decode. So Long-GOP definitely = less real time layers and effects. Just as compressed intraframe codecs mean less RT streams than uncompressed - this is less noticeable because you start to run into other botllenecks with uncompressed like hard drive throughput.

In practice would you notice it? Well I think you definitely would in multicam editing. Dan is right about one key point - it is easier than spatially compressed HDV1 which has the dual hit of MPEG long-GOP decompression and spatial decompression to handle. But not all HDV is spatially compressed - JVC's 720P HDV2 is full raster and actually uses only a 6 frame GOP - so technically it is easer to edit, but less visually pleasing:)

Don't get me wrong, I agree 100% with Dan's overall point - that these units are fantastically flexible, and get around the limitations of many cameras making them far more versatile tools.

[Rafael Amador]

Originally Posted by Mark Job

Hi Rafael:
Are you referring to Avid Media Composer running on a Mac, or are you referring to FCP ?

Hi Mark,
I was talking about FC.
rafael

[Dan Keaton]

Dear John,

Thank you for posting.

I agree completely with all of your points. There are certainly cases where I-Frame Only should be used or considered.

We are scheduled to release 280 Mbps I-Frame Only to add to our 100/140/160 and 220 Mbps bit-rates that we already support.

But, overall, it is really hard to beat our 100 Mbps Long-GOP for many uses. Our users have the options to select what is best for their needs.



It would be nice if someone would run a test, using multiple streams of I-Frame Only at 220 or 280 Mbps (due soon) versus multiple streams of 100 Mbps Long-GOP.

While the computer always has less workload with I-Frame Only, the 2.2 to 2.8 times as much disk I/O (input/output operations) can also create problems.

One stream of 100 Mbps Long-GOP equals 12.5 Megabytes per second (ignoring overhead).
One stream of 220 Mbps I-Frame Only equals 27.5 Megabytes per second.
One stream of 280 Mbps I-Frame Only equals 35.0 Megabytes per second.

So, theoretically, one can have more streams of Long-GOP before reaching the throughput limit of one's disk subsystem. But, this does not become an actual problem, unless one reaches the throughput limit of one's disk subsystem.



John, I always enjoy your posts. It is obvious that you have a great deal of experience and expertise.

[Greg Boston]

Dan,

That aspect is what initially sold me on XDCAM HD. It was the first to offer a tapeless workflow with the benefit of not requiring a dedicated capture card/RAID setup to work in HD. My lowly G5 1.8Ghz iMac could actually work with this stuff since the data rate was only slightly higher than DV25.

The trade-off has traditionally been...

More compression = Higher CPU load, less I/O overhead

Less compression = Lower CPU load, higher I/O overhead

The units offered by CD allow people to place the sweet spot of the trade-off where they need it by using different bit rates.

-gb-

[Bob Willis]

Originally Posted by Rafael Amador

I don't know in a PC, but in a MAC no problem to export Reference movies with XDCAM or NANO footage.
rafael

If the material is I-frame material that is true.

i.e. Pro-res material. FCP also has to convert the material before you can export a QT reference file.