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How can I get stable, reliable MPEG playback?
Hardware FAQ Solution #17, MPEG (last modified 12/04/2007):
Best Practices for encoding / transcoding Standard Definition content
Note: While 30 fps video playback is easily achieved with InfoChannel 5 Release 2, it can be challenging to reliably achieve high quality 60 fps playback on most hardware. Scala is making significant changes to our video playback so as to deliver higher quality 60 fps playback on a wide range of hardware. This will be available within the next few months.
In order to obtain the best visual results in InfoChannel 5, it is necessary to encode video clips with the post-processing that is often done by dedicated video playback devices implemented at the encoding stage.
Encode / transcode the video file using these recommendations:
Audio encoding:
· Use MPEG-1, Audio Layer-II.
· Down-mix 5.1 channel source material to 2 channel stereo.
· It is generally recommended to keep the audio sample rate the same as the source material. However if you want to change it, for 48 KHz audio use either 192 Kbps or 224 Kbps CBR. For 44 KHz audio use: 128 Kbps, 160 Kbps or 192 Kbps CBR.
Video encoding:
· Use MP@ML
· 720x480 @ 59.94 fps for NTSC. 720x576 @ 50 fps for PAL
· Bit Rate: 6 to 8 Mbps CBR. You can use up to 9.8 Mbps CBR for very high-motion content.
· Aspect Ratio: 4:3
· DC Component Precision: 10-bits.
Note: If you want to reduce the effective bit rate required while maintaining visual quality you can try lowering the resolution of the horizontal scan line from 720 to 480 or 352 pixels and keep the 4:3 Aspect Ratio setting. You will often find that 480x480 4:3 NTSC video can be effectively encoded at 3-5 Mbps CBR with very good results.
De-interlace:
Apply a temporal-deinterlace filter. If the source material was originally film converted to NTSC video, apply an Inverse-Telecine deinterlace filter; this option is available on some video encoders / transcoders.
Time base Corrector (TBC):
If the source video is from a device that does not have a TBC, there will often be a flickering half-scan line at the top or the bottom of the video frames as captured with a TV tuner card. This causes two problems:
· It is distracting / objectionable
· It reduces the efficiency / video quality of the video encoding process.
To resolve this issue, place a crop and expand filter to the video.
For example: A video with a 1/2-scan line at the bottom of the frame - crop the 720 x 480 video to 720x [1..478] and then resize the 720x478 frame back to 720x480 with Lanczos or Bi-Cubic Interpolation.
High-Frequency Video Noise
Apply a filter to minimize high-frequency video noise. This is needed especially if the source video is from older tape stock or has poor lighting conditions. This step often dramatically improves the quality of the resulting encoded video.
High-Frequency Audio Noise
Apply a filter to minimize high-frequency audio noise. This reduces tape hiss, circuit hum or wind noise.
Best Practices for encoding / transcoding High Definition content
Using MPEG-2 is still the most efficient from a system resource standpoint. The processing requirements are significantly less than that of comparable quality high definition Windows Media or H.264 / MPEG-4 AVC.
Items that should be considered when using High Definition content in InfoChannel 5:
· Frame Rate Correction: Convert 24, 25, 29.97, 50, or 59.94 fps to whatever is the native frame rate of the destination display device or an integer multiple of the display device's frame rate.
· Producing CBR streams from what are often Variable Bit Rate (VBR) sources. Most 720p content should be encoded at between 10 Mbps "mostly still images", and 16 Mbps "for Sports", CBR, MPEG-2, MP@HL. (Audio as with normal NTSC video).
Encoding profiles typically used for each resolution:
MP@HL
· 720p - Content is produced at 1280 x 720p
. 24 fps for film transfers or HDV / HDCAM source material
. 50 fps for DVB-S/T/C as used in Europe
. 59.94 fps for ATSC as used in the United States.
. Convention limits the bit rate used to 40 Mbps. The profile permits up to 80 Mbps. Actual bit rates used in common industry practice are 10, 12, 16, or 20 Mbps.
· 1080i - Content is produced at either 1440 x 1080i or 1920 x 1080i
. 25 / 50 fps for DVB-S/T/C as used in Europe
. 29.97 / 59.94 fps for ATSC as used in the United States.
. Content that originates from film stock, 24 fps progressive, is encoded at 25 / 50 fps interlaced for Europe, which results in a +4% speed-up distortion. For ATSC / United States it is encoded using 3:2 Telecine Pull-Down to obtain a 23.976 fps when displayed using 29.97 / 59.94 fps.
. Convention limits the bit rate used to 40 Mbps. The profile permits up to 80 Mbps. Actual bit rate used in common industry practice are 20 to 40 Mbps.
· 1080p - Content is produced at either 1440 x 1080p or 1920 x 1080p
. 24 fps for film transfers or HDV / HDCAM source material
. 25 fps for DVB-S/T/C as used in Europe. When the source material is film stock, a +4% speed-up distortion results.
. 29.97 fps for ATSC HDV / HDCAM source material
. Convention limits the bit rate used to 40 Mbps. The profile permits up to 80 Mbps. Actual bit rate used in common industry practice is 30 to 40 Mbps.
MP@HL1440
· 1080i - Content is produced at 1440 x 1080i
. 25/50 fps for DVB-S/T/C as used in Europe
. 29.97 / 59.94 fps for ATSC as used in the United States.
. Content that originates from film stock, 24 fps progressive, is encoded at 25 / 50 fps interlaced for Europe, which results in a +4% speed-up distortion. For ATSC / United States it is encoded using 3:2 Telecine Pull-Down to obtain a 23.976 fps when displayed using 29.97 / 59.94 fps.
. Convention limits the bit rate used to 20 Mbps. The profile permits up to 60 Mbps. Actual bit rates used in common industry practice are 16 or 20 Mbps.
· 1080p - Content is produced at 1440 x 1080p
. 24 fps for film transfers or HDV / HDCAM source material
. 25 fps for DVB-S/T/C as used in Europe. When the source material is film stock, a +4% speed-up distortion results.
. 29.97 fps for ATSC HDV / HDCAM source material.
. Convention limits the bit rate used to 40 Mbps. The profile permits up to 60 Mbps. Actual bit rate used in common industry practice is 20 to 30 Mbps.
Best Practices for Color and Detail for NTSC Video
· Keep the black level 35 points off of baseline (RGB - 35, 35, 35 or higher).
· Keep the white level 15 points off peak (RGB - 240, 240, 240 or lower).
· Don't over saturate red. Keep it 35 points off peak. No more that 40 Red / Blue or Black / White transitions per scan line. Remember that NTSC red is different than what you see on a television.
· Keep any vertical lines at least two pixels wide. Any horizontal lines should be at least three pixels thick. Try to avoid more than about 170 points of color component change in adjacent pixels.
Potential Issues
Judder
Judder is an artifact that appears to the viewer as uneven motion or a periodic hiccup in the motion of what should be smooth panning or translation scenes.
One common cause of judder is a mismatch between the video file's encoded frame rate and the monitor's refresh rate. This symptom is often confused with motion compensation artifacts.
Examples when this can occur:
1. Film source, 24 fps, displayed at 29.97 fps, with poor, or no, 3:2 pull-down correction. Poor meaning a video file created from multiple sources - each of which had 3:2 pull-down applied independently and the resulting video has multiple mismatched 3:2 sequences.
2. For NTSC, a frame-rate of 29.97 or 59.94, video displayed on a computer monitor at 60, 70, 72, 85 fps.
3. For PAL, a frame-rate of 25 fps or 50 fps video displayed on a "Computer Monitor" at 60, 70, 72, 85 fps.
Solutions:
· Use an advanced transcoder with "blending" frame rate correction. This product converts the video's frame rate to the desired destination rate. An example encoder that does this: Canopus / Grass Valley ProCoder 3.
· For NTSC, use a display device that can natively display 59.94 Hz.
· For PAL, use a display device that can natively display 25, 50, 75, or 100 Hz.
If you do not have the ability to set the display refresh to match the above recommendations, it is better to transcode the NTSC or PAL source material to the appropriate intended native frame rate and to set the video card on the computer to match that rate exactly.
Variable Bit Rate Encoding
Variable Bit Rate (VBR) encoding is attractive from an encoding efficiency / resulting file size perspective. For example: Setting the "typical" bit rate to, 6 Mbps and the encoder can use up to a predefined (+/-) bit-rate % for simple and / or complex scenes. As a result a 6Mbps VBR file will often have the appearance of a 12Mbps CBR file but is only half file size.
The problem with VBR video decoding is that it uses substantially more CPU than the equivalent CBR video. In cases where decoding a video pushes the CPU to its performance limits, decoding a VBR video may interfere with other running components in the system, but decoding an equivalent CBR video will work just fine.
The default settings for many encoders is to permit up to a 10:1 swing in bit rates from one Group Of Pictures (GOP) to the next group of pictures (up to the profile maximum with no real minimum value). This can result in a 2 Mbps VBR video going from 1 Mbps to 10 Mbps.
The solution to this is to use Average Bit Rate (ABR) encoding instead. This provides controls for the amount of bit rate variation by setting the maximum and minimum bit rate to be no more than 50% of the desired ABR. For example: An ABR of 6Mbps with an 8Mbps ceiling and a 4Mbps floor.
Regards,
Scala Technical Support
