How SlingSync encodes files (for advanced users)


	How SlingSync encodes files (for advanced users)

If you want to know what goes on "under the hood" when SlingSync encodes files, here you go. If this is all gibberish to you, that's okay ... this is for advanced users.

SlingSync converts files into Windows Media Video (WMV9) with Windows Media Audio. Depending on the source file resolution, frame rate, aspect ratio, and other parameters, SlingSync automatically determines the best output file format that can be played back by SlingCatcher. For more details on conversion parameters, see the Related Information links below.

Video

Codec

SlingSync converts files this way: media container is ASF, video codec is WMV9.

Frame rate (FPS)

If the source frame rate is less than or equal to 30, the transcoded frame rate is rounded to nearest integer. For example 29.997 is rounded to 30.
If the source frame rate is greater than 30, the transcoded frame rate is restricted to 30.

Multiple streams

Only the first video stream is transcoded.

Resolution

Width and height are rounded down to nearest multiple of 4.

Aspect ratio

The correct aspect ratio is maintained by adding black borders, if necessary, in the transcoded video. The Pixel Aspect Ratio (PAR) of the transcoded file is always 1.0 (i.e. SAR and DAR are always equal).

SD/HD clip classification

Any clip with product of width and height greater than 720x576 is arbitrarily classified as a HD clip. (This is just an internal classification, to determine transcoded bit-rate and other parameters.)

SD clips

Bit rate conversions

If the source bit rate is greater than 256 kbps and less than 5 mbps, the transcoded bit rate is same as the source bit rate.
If the source bit rate is less than 256 kbps, the transcoded bit rate is 256 kbps.
If the source bit rate is greater than 5 mbps, the transcoded bit rate is 5 mbps.

Resolution conversion: same as source

HD clips

The transcoded bit rate is always 4 mbps.

For clips with a frame rate <= 25 fps:

Clips with width <= 1280 and height <= 720, no change in resolution
Clips with width > 1280 or height > 720 are scaled down with the correct aspect ratio until their width <= 1280 and height <= 720

For clips with fps > 25:

Clips with width > 1280 or height > 720 are scaled down with the correct aspect ratio until their width <= 1280 and height <= 720
A “scale factor” is computed, S.F. = clip_width*clip_height*fps /(1280*720*25)

If the Scale Factor <= 1.0, no further scaling is done.
If the Scale Factor > 1.0, clip width and clip height are divided by square root of the Scale Factor, then rounded to nearest multiple of 16.

Some common HD resolutions and their transcoded resolutions:

Source resolution Transcoded resolution

960x544, 24 fps Unchanged

960x544, 25 fps Unchanged

1024x768, 30 fps 960x720, 30 fps

1264x704, 29.997 fps 1184x656, 30 fps

1280x720, 24 fps Unchanged

1280x720, 25 fps Unchanged

1280x720, 29.997 fps 1168x672, 30 fps

1280x544, 23.976 fps 1280x544, 24 fps

1280x688, 30 fps 1200x656, 30 fps

1440x576, 25 fps 1280x512, 25 fps

1440x1080, 29.970 fps 960x720, 30 fps

1920x800, 23.976 fps 1280x544, 24 fps

1920x1080, 29.970 fps 1168x672, 30 fps

1920x1080, 24 fps 1280x720, 24 fps

Audio

Audio is always transcoded to WMA 9, 2ch, 128 kbps, 48 KHz.

Multiple tracks

Only the first audio track is transcoded. Other tracks are currently ignored.