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Video Website Datasheets #1: YouTube

January 4, 2008

Vista busy cursor I’m starting a new series looking at video hosting websites, very much focusing on video quality. I’m approaching this from the point of view of someone wanting to embed video clips in blog posts, but not impressed by the quality of YouTube and looking for something better.

I have already made some comparisons, but that was purely qualitative. To get anywhere we need to be rather more scientific, so I have been conducting technical analyses of the main video websites.

In this and future posts in the Video Datasheets series I shall be publishing the results of my analyses in the form of technical datasheets and embedding the corresponding videos.

My method is to start with a decent quality video which I use as a standard source for test purposes. I chose a 2m 50s video which started life as an AVI file in DV format (PAL, 720 x 576, 25fps, PCM audio 48kHz) and which has been encoded as H.264 (qp=26) in an mp4 container, 448 x 336, video bitrate 700 kbps, 25fps with AAC audio.

For encoding details see method here. You can download the test .mp4 file here, from the vimeo website (Download Quicktime version 17.02MB).

I have uploaded this video file to the various websites under review, which invariably re-encode it and convert to a Flash Video file (.flv). I play back the converted videos and retrieve the downloaded flv files from the browser cache for analysis using a range of software tools.

I’m starting with YouTube. What else?

YouTube

A. Output Quality

File Format: flv (Flash video)

Video Codec: Sorenson H.263 (FLV1)

Video Resolution: Resizes to 320 x 240

Video Bitrate: 320 kbps

Max. GOP: 250 frames

Frame-rate: As source (in this case 25fps)

Always re-encoded: No. See comments below

Audio Format: MPEG-2 Audio layer 3

Audio Channels: mono

Audio Sampling Rate: 22.05 kHz

Audio Resolution: 16 bit

B. Input Flexibility

Input File Formats include: MPEG, MOV, WMV, AVI, MP4, RA, RAM, ASF, 3GP, FLV

Input Video Codecs include: H.263, H.264, VC1, DivX, XviD, SVQ3, DV

Input Audio Codecs include: AAC, MP3

Max. Video Length: 10 minutes

Max. File Size: 100Mb as single file or 1Gb using YouTube Uploader

C. Comments

  • Files uploaded in flv format (FLV1 compliant – using Sorenson H.263 codec) will not be re-encoded provided the overall bitrate (including audio) is strictly under 350 kpbs.
  • YouTube philosophy appears to be about immediacy and reliability of the user experience. Low bitrate is used to ensure that as far as possible video starts playing without delay and continues to play without interruption, even if user has modest broadband connection. YouTube have though recently (December 2007) suggested that from a future date users with higher bandwidth will be able to enjoy better quality (subject to source material).
  • YouTube videos have a long GOP (Group of Pictures) setting. That is, gap between key frames (which are encoded in full) may be as much as 250, corresponding to 10 seconds of video at 25fps. Intervening frames only hold information about changes in pixels from frame to frame, greatly reducing file size but at the expense of quality. For example, typical GOP settings for a DVD (encoded using MPEG2) would be 12 or 15. The long GOP has been a masterstroke for YouTube in terms of reducing bitrate, and the idea seems in general to have been copied by their competitors. The forced resize down to only 320 x 240 is also vital in bitrate control.

Summary of just the key data affecting video quality for all websites analysed can be found here.

For all Video Datasheet posts click here.

[youTube=http://youtube.com/watch?v=l-jP9Wrqjcw]

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One comment

  1. One of the best ways to embed media is through the use of a module or widget.

    One of the fastest growing online web applications to offer mass video embedding is VideoSwiper which now supports wordpress and over 30 video search engines.

    You can now embed media at the click of your mouse and transfer video to your websites fully automated.

    http://www.videoswiper.com



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