50fps is a misdirection though isn’t it. The transcoder from the HD and SD broadcasts is configured to write a 50fps file. That’s just the way the Aura is built. I agree of course that as the source (from SD anyway) is 25fps that transcoding to 50fps makes little sense (though encoders with duplicated frames often have very efficient coding to make sure that that the duplicated frames do not take up much space), but I can see why HD broadcasts in 1080p at 50fps are being transcoded into 50fps files.

But Gordy’s problem is simlpy that he changed the bitrate. That explains why he’s got bigger file sizes than he had previously, and for ease of use on his tablet the easy fix is to set that back to 2Mbps on the Aura and ignore the 50fps issue as there’s little point in faffing about with reducing framerate just to achieve even smaller file sizes.

There’s a separate point about the transcoder and perhaps having the option on the Aura itself to render 25fps files, but that’s a feature request that ought to go into Humax.

OK just had a quick play. I recorded Coast on BBC4 as a 6 minute programme to rapidly convert.

If you use the Web Controls IF page and go to “settings” there is a field called “Refresh rate” which should be set at 50000. I changed this to 25000.

I then transcoded the Coast file at both the 50000 setting and the 25000 setting. And guess what? With it set at 50000 the downloaded file had 50fps and when set at 25000 the downloaded file had 25fps.

So that’s where you can make the change to the 25/50fps setting if that bothers you.

I should also add that the 50fps file was 110.0Mb in file size, the 25fps was 108.9Mb file size, so as expected there’s very little difference from the duplicated frames.

I don’t believe this setting is editable from the front end of the Aura though so it needs editing on the WebControlIF page.

larkim – 1 hour ago  » 

50fps is a misdirection though isn’t it. The transcoder from the HD and SD broadcasts is configured to write a 50fps file. That’s just the way the Aura is built. I agree of course that as the source (from SD anyway) is 25fps that transcoding to 50fps makes little sense (though encoders with duplicated frames often have very efficient coding to make sure that that the duplicated frames do not take up much space), but I can see why HD broadcasts in 1080p at 50fps are being transcoded into 50fps files.

But Gordy’s problem is simlpy that he changed the bitrate. That explains why he’s got bigger file sizes than he had previously, and for ease of use on his tablet the easy fix is to set that back to 2Mbps on the Aura and ignore the 50fps issue as there’s little point in faffing about with reducing framerate just to achieve even smaller file sizes.

There’s a separate point about the transcoder and perhaps having the option on the Aura itself to render 25fps files, but that’s a feature request that ought to go into Humax.

There are no 50fps broadcasts. They are either 25fps interlaced. Interlaced transmits the signal in 2 parts sequentially Upper field first Lines 1 3 5 7 etc with the lower field next 2 4 6 8 etc. When combined in a digital TV frame store you get a single 1/25 second frame. Alternatively Freeview-HD can do 1080p25.

The whole frame is transmitted progressively also resulting in a 25fps broadcast.

As to the extra frame not taking up much space as it’s a duplicate of the previous it takes exactly the same space as the others.

Note the file Video Bitrate and the actual video bitrate

[attachment=86537,1791]

I think I’ve demonstrated that the extra frames don’t take up (materially) more space. A 50fps version of a 25fps source will (through the encoding algorithm) effectively say “Frame 1 – ABCDEFGHIJ”, “Frame 2 – as Frame 1”, “Frame 3 – EFGHJILK”, “Frame 4 – as Frame 1” so it is much more effectively compressed.

Point taking about the 1080i HD broadcasts, that was casual laziness on my part.

Anyway, the point is that you can control the frame rate and bitrate from the WebControl page and you can conrol bitrate only from the Aura itself.

Changing the framerate between 50 and 25 doesn’t make a material difference to file size (at least, not in the programme I tested) though as noted the 25fps is faster to encode (I just tested that).

There is one way I can show this. IF export jpeg images from adjacent frames. Pen them in Photoshop.

Add a new layer under the frame 1 image. Copy the frame 2 image to the lower layer any then reduce top layer opacity so you can see the underlying frame any differences should be clearly visible.

larkim – 11 mins ago  » 

Yes, but modern compression / encoding uses algorithms to keep file sizes small and efficient. I know I’m over-simplifying the process but a duplicated frame doesn’t take the same space on the disc that the initial frame does, far from it. Anyway, i think I’ve demonstrated that the 50fps files are not significantly larger than the 25fps files, no matter how they do it.

I agree Frame 2 will be a duplicate of Frame 1 in a 50fps output from a 25fps source (normally); but it doesn’t mean that Frame 2 takes the same space within the file because that’s the way the various encoding / compression algorithms work.

That’s clearly wrong. The original HD video is compressed using H264/AVC AKA AVC.

The downloaded file also uses H264/AVC. There is no way the aura is recoding each frame using H264/AVC. It has to taking each Group of pictures as is.

A GOP contains one Iframe (the only one with a full image) with the rest containing lossy difference data. The H264/AVC decoder in the device playing back the video stream recreates the whole number of frames in the GOP. It can only do this for a complete GOP.

Video redo can be set to only jump from Iframe to Iframe. So you can jump from onr GOP to the next.

larkim – 4 mins ago  » 

Just tried a larger file (30 minutes) and same result. 25fps file 526Mb, 50fps file 535Mb.

I'm now wondering though if that is more because I'm not halving the bitrate despite halving the framerate.

Confusing myself… !

The average total bitrate and the duration of the recording are directly related to the final size of the file.

The Video track will of course be larger than the audio ones

Basically how much smaller does the 50fps file get if you recode it to 25fps using something like Vidcoder. You can retain the resolution and change the framerate in vidcoder.

[attachment=86551,1794] [attachment=86551,1795]