This is the first part in a series of articles I will be writing on encoding Flash Video (flv files) using the Adobe CS3 Video Encoder. I intend to cover all aspects of flv encoding to help you to attain the best results possible. If you are using another software to encode your flv files, such as Adobe After Effects or Final Cut Pro, many of the principles will be the same.

Why Flash Video?

The main reason I would recomend using flash video as a format is that you can build / modify your own player using Flash and that you can integrate videos into flash sites / applications in a way that is not possible with other formats. Flv compression provides good results, equal to mp4 for instance, and is cross browser / platform to anyone who has the flash plugin installed. More users have Flash than other players such as Real Player, Quicktime or Windows Media Player.

The source

The most important thing to start with is the quality of your source material. Videos that have a lot of fast motion, flickering, strobing etc. will always be harder to get good results with compared to ones that have a steady image. For this reason cartoons with large blocks of matte colours always tend to encode well, whereas video footage with fast motion, camera shake, fast editing tend to encode badly.

Footage that is grainy and has low contrast, such as VHS footage, is always difficult to encode as is footage that has already been heavily compressed such as low / medium quality mpeg, AVI or WMV files.

The best file format to encode from is Quicktime Movie (mov) using Motion JPEG codec. The field settings should be set to single field (This will be explained in more detail later).

Frame Size

The natural frame-size for PAL video is 720x576 pixels or 1024x576 for widescreen. However, I have found that encoding flv to any size above 500 pixels wide gives mixed results, depending on the source. Half size 360x288 is a good starting point. I have found I get better results if I create my source file at the size that I will be encoding to rather than downsizing my video during compression.

Frame Rate

The frame rate for PAL video is 25fps. This is often too high for flash video. I would recommend dropping the frame rate to 15fps.

Fields

PAL video splits each frame into two fields, upper and lower (odd and even). This means that for each frame first all the odd lines of the frame are displayed, then all the even lines, which is what gives it a smooth playback when watched on a television. For playback on a computer, and especially when compressing video for the web, this is not a good thing. A symptom of incorrect feild settings is horizontal lines appearing in areas of high contrast and fast motion.

I find it better to de-interlace my source movie first, so it is displaying both fields for each frame, and then also select the de-interlace option in the Flash Video Encoder.

Data rate

The maximum data rate or 'Quality' setting for your movie dictates the overall image quality for your movie. However, higher data rates do not always mean better quality. If you preview a high data rate movie from your desktop the results will be very good, but if you upload the same movie to your server you will often find that the movie drops frames and plaback is stuttered and buffering time is too long. This is because your bandwidth is not high enough to deliver the full data rate of the movie.

The total data rate for an flv is the sum of its video data rate and its audio data rate. The rate that you choose to encode at depends on the quality of the source material and the bandwidth of your target audience. Around 250kbps should work well.

Audio

The audio data rate should be set no higher than 96kbps. Make sure the audio of your source is mixed loud but not too loud, peaking at -6db is ideal. It can be a good idea to create the source with mono sound to keep filesize and data rates down.

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FLV Data Rate and Bandwidth... Demysitifed.