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Slow motion on Sony a7s + Atomos Shogun


Trias
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When the A7s operates at 120fps, it only uses the APS-C crop portion of the sensor (not the full frame area) and it only resolves at 720p.  That means your image will be cropped at 1.5x compared to the Full Frame image and you will likely see some amount of moire and aliasing. 

 

When operating in that mode, the actual video file that is created on the internal SD storage card is 720p. 

 

Interestingly, when operating in that mode, the camera does output a 1080p signal that can be displayed on a consumer monitor (that does not support 120fps) -- meaning, the signal is successfully displayed at either 1080p/30 or 60i.  I don't know for sure, but I suspect the consumer monitor is handshaking with the camera over HDMI and telling it what frame rates it supports -- and the camera is subsequently giving the monitor what it needs.  But purely from a raster point-of-view, the signal is obviously being up-converted from 720p to 1080p in-camera.

 

I have not yet conducted comparative tests to determine if the in-camera resolution conversion is any better or worse than doing it as a post process.  I also don't know if a device like the Shogun, that supports 1080p/120 (and probably conveys that support over HDMI), can coax 120fps of realtime video out of the camera's HDMI port.  Maybe someone else knows that.

 

But to answer your question, even if it can, you would still be recording a 720p signal that had been up-converted in-camera to 1080p.  

 

In spite of that, the advantages of that approach would seem to include;  baking the up-conversion to 1080p into your video files, recording said video files at 4:2:2 instead of the internal 4:2:0, having your choice of a preferred codec/bitrate combination and, finally, having your media on a storage device that is intrinsically higher performing (SSD) and conceivably directly usable by your NLE for editing, without the requirement of importing or transcoding.  In aggregate, assuming there is no qualitative image penalty to the in-camera conversion, those represent some compelling advantages.

 

It's a bit inconclusive at this moment in time, at least for me.  Others might know more and I welcome their input.  I suspect we'll know definitively at the beginning of October (Shogun's expected release date).  In the meantime, I hope that helps a little bit.

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I got an official response from Atomos:

************************************************************

Hi Daniel,

For the Shogun to capture anything, the signal must first be sent over HDMI or HD-SDI by the camera.

I don't believe the A7s is capable of outputting a 1080p120 signal over HDMI, so it wouldn't be possible to record it--since it was never there to record.

Hope this info helps!

Nathan

=========

Technical Support

Atomos Inc.
************************************************************

Bummer.

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This document seems to support that premise:

 

Clean HDMI Output
1080 60i compatible device:  3840 × 2160(30p/24p) / 1920 × 1080(60p/24p) / 1920 × 1080(60i),YCbCr 4:2:2 8bit/ RGB 8bit

 

If that is true, it is unfortunate, as it means that 120fps on the A7s will always be limited to 4:2:0 and 50Mbit compression.

 

It would be nice to get corroboration on this.

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This document seems to support that premise:

 

Clean HDMI Output
1080 60i compatible device:  3840 × 2160(30p/24p) / 1920 × 1080(60p/24p) / 1920 × 1080(60i),YCbCr 4:2:2 8bit/ RGB 8bit

 

If that is true, it is unfortunate, as it means that 120fps on the A7s will always be limited to 4:2:0 and 50Mbit compression.

 

It would be nice to get corroboration on this.

If found to be true, hopefully it could be amended in future firmware update...

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