MPEG2 encoding – using only I frames?

[Curious Turtle]

GOPs will as a rule produce a higher overall picture quality. An I-frame is the least “efficient” frame type because it has a large amount of data to compress in comparison to the information contained in B- & P-frames, so it has to throw more stuff out to fit into your datarate.

I-frame only is extremely useful when you need to do something else with the footage, such as editing it. But as a flat-out delivery format you’re cheating yourself.

I’m trying to think of a colourful analogy to describe it, but I’m one cup of coffee too light at the moment. :o) So if anyone else wants to chime in with something flowery….

Cheers,
Ben

Curious Turtle Professional Video
Training | Editing |Support

http://www.curiousturtle.com

[Daniel Low]

Ignoring, for a moment, the fact that you need the material on DVD, I-Frame only at a high enough bitrate (e.g. 50Mb/s) will give you the best quality. – e.g. IMX 30/40/50. Generally speaking these formats are 4:2:2, DVD spec is 4:2:0

Taking into account the fact that you need the material to playback from DVD then Long-GOP (Max 9.8Mb/s) is the only way to go to get high quality at a bitrate that falls within the DVD spec.

[Jason Brown]

Great info!!!

Thanks so much for the clarification.

So my next logical question is…what is a good GOP structure if file size isn’t a factor?

I frames = 6? 12? 15? 18? (those are my only options)
P frames = depends on my I frame selection

using sorenson. I’m guessing the answer I’ll get is, “depends what the source material is”. The video is a blended…”dissolvey” historical video with old images mixed with moving animated backgrounds. Needs to be keyframed often, I would guess.

Thanks in advance for the advice…you’ve been very helpful so far!

-Jason

[Curious Turtle]

Well put, Daniel.

Jason, your best bet is to do some experimenting. I would keep the GOP structure set to 12 for PAL or 15 for NTSC. Start with a high-bitrate CBR encode. If your picture is breaking up at specific areas add a compression mark. Lather, rinse, repeat.

Your encoder is going to be the make or break here. Using a different encoder is going to show much greater differences than fiddling with the GOP length IMHO. Not saying that Sorensen won’t give you a decent result, but it’ll be different than a 24-pass encode using Cinemacraft.

Cheers,
Ben

Curious Turtle Professional Video
Training | Editing |Support

http://www.curiousturtle.com

[Daniel Low]

Agreed, messing about with GOP structure is a great way of totally wasting many hours! Squeeze default settings are fine. As Ben points out, if you need better quality than Squeeze can produce, you really need to look at another application.

[Adrian George]

Well said, but you can also do something better, the only trouble is that you need to know exactly what setting is standing for what action.
So, you open the file you want to encode, when opened, you need to right click on the file and choose Edit. Click on and a new window wil pop out with thousands of settings.
You scroll down the page where the settings are and choose Load, and here you have two options: either to Load a .cvs or .txt file with your settings of choose from the preseted ones. As far as I know is beter to have a file .txt with your settings, is better, but you need someone to give it to you! 😛

Anyone, can someone who knows better to write dons a step by step tutorial for “dummies”?

Thank you!

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