FORUMS: list search recent posts

XDCam EX data rate

COW Forums : Sony XDCAM - EX & Related

<< PREVIOUS   •   VIEW ALL   •   PRINT   •   NEXT >>
Robert Hutchings
XDCam EX data rate
on Mar 13, 2009 at 9:24:26 pm

Is 35Mb/sec for XDCam EX an average or a peak data rate?


Return to posts index

Don Greening
Re: XDCam EX data rate
on Mar 14, 2009 at 12:44:14 am

It's an average rate. When using over-cranking for example, the data rate can go much higher, according to those in the know.

- Don


Return to posts index

Noah Kadner
Re: XDCam EX data rate
on Mar 14, 2009 at 4:20:38 pm

Really? I've never seen it go higher than 35MBps.

Noah

Check out My My FCP Blog and my new RED Blog. Unlock the secrets of the DVX100, HVX200 and Apple Color.
Now featuring the Sony EX1 Guidebook,
DVD Studio Pro and How to Light Interviews.
http://www.callboxlive.com


Return to posts index


Don Greening
Re: XDCam EX data rate
on Mar 14, 2009 at 5:27:07 pm

[Noah Kadner] "I've never seen it go higher than 35MBps. "

Indeed. I've actually never looked nor am I too worried about what the the data rate is from one moment to the next. My primary concern is whether the picture looks good or not. The belief that higher data rates come in to play is most noticeable when recording over-cranked footage, for example shooting HQ 720 30p @60 frames per second. This shooting mode takes up twice as much space as normal speed. Which begs the question: is it just 2 times the stored information and does that mean the data rate is higher as a result? Or is it as simple as having a file size that's just twice as big? Sony calls is it variable bit rate for a reason, I guess. Normally the idea is to have more bits assigned to more frame motion.

I'm just trusting that those who say the data rate is higher are more knowledgeable about such things than I.

- Don



Return to posts index

Noah Kadner
Re: XDCam EX data rate
on Mar 14, 2009 at 8:36:06 pm

Yes it's 'variable bit rate' but the variable is the image quality not the data rate- in my experience the actual data rate at a given resolution is always the same no matter what the content is. And that's pretty much the same in DV, HDV, DVCPROHD etc. I've always assumed the camera takes the maximum image quality possible and then on the fly fits it into the current data rate. If that means a reduction in quality to hit the rate, that's what happens. If it means padding the data to accommodate a simple scene- that happens too. So yeah in the camera itself the data rate may be variable but what actually gets recorded to tape or memory is a constant bit rate.

-Noah

Check out My My FCP Blog and my new RED Blog. Unlock the secrets of the DVX100, HVX200 and Apple Color.
Now featuring the Sony EX1 Guidebook,
DVD Studio Pro and How to Light Interviews.
http://www.callboxlive.com


Return to posts index

Craig Seeman
Re: XDCam EX data rate
on Mar 14, 2009 at 11:14:15 pm

I've seen the bit rate vary from about 29mbps to 36mbps. The low bit rates were from files I received that had large areas that were blownout (not shot by me of course). Many of my own files seem be around 36mbps.



Return to posts index


Don Greening
Re: XDCam EX data rate
on Mar 15, 2009 at 4:28:39 am

[Noah Kadner] "in my experience the actual data rate at a given resolution is always the same no matter what the content is. And that's pretty much the same in DV, HDV, DVCPROHD etc."

Since I'm a proponent of the 'Ockham's razor' theory I find the above statement quite logical. There must be different levels or flavors of Long GOP MPEG2 recording then, because the end result of a DVD encode is fluctuating VBR numbers when the data rate is called up on a screen during playback.

I would be very interested in seeing a comparison of the the same picture at 35VBR with the higher XDCAM HD 50 Mbit rate. Perhaps one day.

- Don







Return to posts index

Robert Hutchings
Re: XDCam EX data rate
on Mar 16, 2009 at 8:41:32 pm

Thanks all, for taking the time to respond.

Our tests confirm that the 35Mb/sec is a peak rate and not an average, in that the data rate never spikes above 35Mb/sec (without audio). Data Rate readings for various XDCamEX clips in Final Cut are consistently 4.3 MB/sec with two channel audio and 4.5 MB/sec with four channels of audio no matter what the content.

An exception to this is a clip of black video that we generated that predictably came through with a very low data rate. This is a VBR codec after all. Interestingly and also predictably a clip of black in XDCamHD 422 50Mb/sec, which is CBR, came through at a full 7.1 MB/sec.

We also were able to lower the data rate of the XDCamEX codec by greatly blurring via a blur filter all the detail out of what was a highly detailed still image. This blurred clip weighed in at 2.6 MB/sec.

As part of the test, we also edited another clip together going frame by frame to ensure there was a radical change ie. each frame was a unique frame radically different than the preceding frame. This clip came in, true to specs, at the 35Mb/sec.

As to comparing 35Mb vs 50Mb: Our tests show very little difference between the two. I suspect the difference would become more apparent with rapid motion etc. XDCamHD 422 at 50Mb noticeably outperforms HDCam. I believe this is most significantly due to the scaling to 1440 of HDCam as opposed to the full raster XDCamHD 422 where no scaling is necessary. HDCam really suffers from this scaling and alternately I assume XDCamEX benefits greatly from not having to change frame size.


Return to posts index

Bjoern Adamski
Re: XDCam EX data rate
on Mar 19, 2009 at 9:44:04 pm

I-frames are usually higher than the average bit rate of 35 Mbps.

If you have the chance to rewrap to a MPEG file container you can use MPressionist to analyze the data rate: http://www.digigami.com/mpressionist/

XDCAM HD422 50 Mbps I-frames for example go up to 100 Mbps.



---------------------
Product Manager
MXF4mac
http://mxf4mac.com


Return to posts index

<< PREVIOUS   •   VIEW ALL   •   PRINT   •   NEXT >>
© 2017 CreativeCOW.net All Rights Reserved
[TOP]