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File size and movie formats

After spending the weekend working on other overdue projects, I managed to sit down last night and do some editing.

I started working on the behind the scenes documentary from my first independent shoot, where I handled lights, camera, production, direction and performance without any tech crew and only Tom (who has no more experience than me) on hand to help. Unfortunately, once the project file reached a certain size/length, I kept running into a persistent problem I'm having with my editing software, and needed to put it down and work on something simpler.

I ended up editing of the clip Tom and I filmed that day, a consensual office spanking scenario inspired by some of our favourite novels. I left it rendering overnight and went to sleep in Tom's room so I wouldn't be disturbed by the loud whirring of my poor struggling PC's fan.

The render completed successfully overnight, leaving me with a full res (1920x1080 - the resolution my camera shoots at) .wmv copy. It felt good to get something finished. That's not much use for my site, though, so today I set about re-exporting the clip for the web.

There are several overlapping considerations when selecting file formats for downloadable content:

  • Operating system (Windows / Linux / Mac OS)
  • Internet connection (ADSL / mobile broadband / wifi / 3G / dial-up)
  • Usage pattern (streaming / download)

Slower internet connections and streaming users require smaller, compressed files. Users with faster broadband who are downloading prefer larger, higher-res files - but there are still upper limits on file size. As a user in the latter category, I would say that 100-300MB is a good download size for a web clip, with bigger file sizes forgiveable in longer clips. For a half hour clip I'd be happy with 500MB - for a full length feature film I'd want it to be somewhere between that and 900MB. Personally, I wouldn't want to download any file that came in over 1GB (I recently bought a feature film which was around 2GB, and it took four attempts over two weeks to successfully complete the download).

For streaming, file sizes need to be considerably lower; and viewing in a web browser rather than your desktop, it makes sense for films to be lower res too. The smaller version will double for streaming and for users with slower internet connections. I would say that in most cases these files should be under 100MB, but that up to 200MB is permissible for long films. (Would you agree?)

So I need a high-res and a low-res version of each file format. What formats are essential if I want people to avoid having to spend ages installing codecs before they can view my content?

  • .wmv - optimised for Windows users with relatively low file size. Some Windows-using site owners think this is all you need, but they're not straightforward to play on Linux or Macs.
  • .mov - optimised for Mac OS but playable on all systems with the right software installed.
  • .avi - good compression/quality balance. Most people can play .avi files (depending on the codec¹), and they're very easy to burn onto DVD. This may or may not be an advantage; it's a file format favoured by filesharers.
  • .mp4/.m4a - playable on all operating systems, but relatively high file sizes.

Other factors: my editing software exports directly to .wmv and .mov, but not to .avi or .mp4 - I would need to convert the file after rendering. I think there's greater support for streaming .wmv and .mov, but I'm not sure.

So to cater to the highest number of users with the smallest number of files, I could offer one avi (low res) and one mp4 (high res); one large and one small .mov, one large and one small .wmv; or some other combination. I'd be very interested in hearing which file formats you, as a user, prefer.

In the meantime, I'm experimenting with compression, format and file size. Here are my results so far for my 12 minute clip (25fps, 48khz):

  • 1920x1080 .wmv: 233MB. Bit blurry, still in intro fairly pixellated, motion not always smooth.
  • 960x540 .wmv: 211MB. Same visual issues as above, but not noticeably lower file size.
  • 960x540 .mov: 967MB. Really nice video quality - smooth, sharp, no pixellation - but very high file size.
  • 640x360 .mov: 734MB. Again, great quality but file size too big for that visual resolution.

Interestingly, when I accidentally exported 640x360 .mov without the audio, the file size came out at 173MB, so it seems that most of the bulk is in the audio track. I'm currently exporting the same resolution with the audio bitrate dropped from 64 to 32kbps, to see how big it is and how it sounds.

Lots of experimentation still to do before I hit on the right compression guidelines. Each export/conversion takes between 1-2 hours, so it's a slow process. But it's all useful data, and I'll get there in the end.

1. I think all of this depends on the codec actually. I'm still operating somewhat in the dark when it comes to codecs.

Comments

Linux

Linux is generally fine with .wmv these days, although it does depend on the codec to some extent - same goes for .avi - a really modern codec might not be supported, but the older / more widely used ones are fine - DivX is a popular choice from what I'm told.

Thanks for this. I guess anyone using Linux is likely to be happy installing plugins/codecs - it's the off-the-peg Windows and Mac users who want everything to Just Work TM without any customisation.

I got some useful feedback on my other blog on this question, and have decided to provide one each of high res and low res MP4/WMV for each film.

Out of interest, what formats do embedded video players tend to prefer...? :)

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