Why 50 Mbps?

[Tom Gresham]

I have an XF-105, and was hoping to get a better understanding of something here.

What is the benefit of recording at 50 Mbps? Obviously, it is "better," but how so?

At 35 or 25 Mbps, recording time increases.

What is gained when increasing the data rate? Sharpness, color, ? What do you see in the image that makes 50 Mbps better?

If this is covered in another thread, just point me in that direction, please.

Thanks. Trying to learn this camera (which I really like so far).

[Nigel Barker]

I haven't even bothered to compare recording at different bit rates as I always prefer to record in the highest quality mode just like I always shoot photographs in RAW rather than JPEG. Unless you are short on Compact Flash cards I cannot see the point in recording a lower quality.

[Tom Gresham]

Nigel, maybe it's as simple as you put it. Why shoot in anything less than top quality?

I shoot RAW in my Nikons. Same thing. Even if I don't plan to need the higher quality, it would be disappointing to need it later and realize you don't have what you need in that shot.

It's why I started shooting in HD several years ago, even though we were putting videos online in compressed Flash. Now, I have hundreds of hours of HD video I can use for various projects, including broadcast TV.

File storage is cheap these days.

[Pete Bauer]

The 50Mbps rate gives you Constant Bit Rate 4:2:2 sampling, which is a significant increase in color resolution and will tolerate color correction, green screening, etc much better than 4:2:0. I can't confirm it, but maybe someone else (in addition to the BBC) has done an analysis to show that even with the extra information stored for the 4:2:2 color, there's probably enough additonal data rate to reduce compression artifacting as compared to the lower bit rates.

Page 46 of the English language XF100/105 manual has a little table showing the bit rates and color sampling at each frame rate and resolution combination that the camera can record.

There's a lot of choice here. On the extremes, if you're doing a once in a lifetime live event that demands top quality, or you know you'll be doing a lot of post on the footage, you might want to use 50Mbps dual slot recording. With 64GB CF cards, that'll give you 160 minutes of recording time without changing cards. If you shoot a lot of straight-to-web footage, maybe you might choose 25 Mbps (1440 x 1080 CBR 4:2:0) without dual slot and you'll be able to record over 10.5 hours without needing to offload a CF card.

Personally, I'm in Nigel's camp. I have yet to shoot anything other than 50Mbps with my XF105.

[David Heath]

Originally Posted by Pete Bauer

I can't confirm it, but maybe someone else (in addition to the BBC) has done an analysis to show that even with the extra information stored for the 4:2:2 color, there's probably enough additonal data rate to reduce compression artifacting as compared to the lower bit rates.

For a progressive frame, we're talking about 1920x1080 luminance samples (2,073,600) and 2x960x540 chrominance samples (1,036,800) for 4:2:0, 2x960x1080 (2,073,600) for 4:2:2 - so 33% more samples overall for 4:2:2 than 4:2:0.

Simplistically, you'd therefore expect a 422 system to need 33% more bandwidth than 420 for the same quality, and 133% of 35Mbs is about 46Mbs.

But this ignores the fact that the "extra" samples of 422, cf 420, can have a first guess made at their value (by interpolation) from the 420 set, so only these DIFFERENCES need to recorded. The result is that whereas 422 may contain twice the chrominance samples of 420 (all else equal), you don't need twice the data rate to code it.

Hence yes, 50Mbs XDCAM doesn't just give 4:2:2 as the advantage over 35Mbs, but better compression artifacting as well.

[Bruce Watson]

Originally Posted by Tom Gresham

What is gained when increasing the data rate? Sharpness, color, ? What do you see in the image that makes 50 Mbps better?

All other things being equal -- same codec, same chroma subsampling, etc. what you get for a higher bit rate is smoother, cleaner motion. For example, lets say you are covering a basketball game -- panning the floor as the athletes run from one end to the other. With insufficient bit rate, the checking in the wood floor turns to "mush" as you pan. I've seen this happen; for me it's almost unwatchable. With enough bit rate, you'll see the checking in the floor, which is a much more "natural" look.

In a different example, say you are filming a sit-down interview with a locked down camera. Even a lot bit rate will probably be sufficient, and as you raise the bit rate you'll see little if any change, because there's not enough motion to show it.

That said, often more changes than just the bit rate itself when comparing between cameras. Comparing an XF100 to an AVCHD camera takes you to another codec and a different chroma subsampling. To many things change -- such a comparison is problematic at best.

[Philip Lipetz]

Perhaps I missed it but the thing that everyone who has posted knows, but has not explicitly stated, is that ONLY at 50bps do you get 4:2:2 color, all other bit rates are 4:2:0 ccolor.

[Pete Bauer]

Yep, true. Page 46 of the manual.

[Derrick Williams]

I actually ran into this problem this weekend with my XF-300. I was shooting an all day event on only had a 64GB & 32GB card and wanted to only use 50Mbps. I was going to transfer the data to my laptop during a break, but my laptop crapped out...So I had to make the "tough" decision to go with the 35Mbps. Bottom line 35Mbps got me through the entire day with an extra battery.

Haven't had chance to go through the footage yet to compare it to the 50Mbps, but I'm hoping I don't lose too much data from using 35Mbps.

[Robin Davies-Rollinson]

Originally Posted by Derrick Williams

Haven't had chance to go through the footage yet to compare it to the 50Mbps, but I'm hoping I don't lose too much data from using 35Mbps.

I wouldn't have thought so - EX1/3 users have to put up with it all the time ;-)

[Vincent Oliver]

I have both the EX3 and XF305, can't see much difference between 35mbs and 50mbs. I suspect most people are still downconverting footage to write to DVD or are using material on the web. In both cases you needn't worry about 35 or 50mbs.