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Forum Discussion
The WDTV is a fantastic Video Streamer
I have a Readynas Pro 6, Virgin TIVO box and 2 Apple TV's and 3 Logitech Squeezeboxes I have loads of movies and music stored on my ReadyNas Pro. I have all my music stored in iTunes with the ac...
Audio / Video sync requires accurate timestamps in the container (above the codec layer). There are several scenarios where it is lost.
-bugs in conversion software used (either remuxing or transcoding) which results in the original audio/video offset getting lost. Sometimes simple remuxing will fix it. Most muxing tools let you enter a manual adjustment, which you can also use. However it can be time-consuming (and usually takes experimentation to find the right offset).
-errors in the media bitstreams which cause unequal amounts of audio/video to be discarded. If you are making over-the-air recordings, this can occur at the very beginning, since the video decoder waits for an I-Frame (which might not be the first frame it sees). Transcoding often resolves this, but not always.
-If an audio A/D converter is in the recording path, the clock rate of the A/D is not perfect. With a nominal 48000 samples per second stream, consumer devices actually typically clock at 47956 Hz to 48048 Hz. Even a 1 sample per second error will create a 100 ms delay in about 80 minutes. If audio/video is captured by the same device, it is usually avoided by locking the audio and video clocks together (so there is an identical video frame rate error). However, if was not done in the original recording, it is not very easy to solve it later. Some transcoding tools allow you to resample the audio to a different sample rate, though figuring out what the right value is takes experimentation. Also, the actual clock rate drifts.
-playback equipment (particularly surround sound receivers and TVs) have their own processing chains, and need to share information on their delays. In new equipment this is done over HDMI, and sometimes fails. Older non-HDMI connections need to be configured manually.
Might be going a bit off topic here though...
-bugs in conversion software used (either remuxing or transcoding) which results in the original audio/video offset getting lost. Sometimes simple remuxing will fix it. Most muxing tools let you enter a manual adjustment, which you can also use. However it can be time-consuming (and usually takes experimentation to find the right offset).
-errors in the media bitstreams which cause unequal amounts of audio/video to be discarded. If you are making over-the-air recordings, this can occur at the very beginning, since the video decoder waits for an I-Frame (which might not be the first frame it sees). Transcoding often resolves this, but not always.
-If an audio A/D converter is in the recording path, the clock rate of the A/D is not perfect. With a nominal 48000 samples per second stream, consumer devices actually typically clock at 47956 Hz to 48048 Hz. Even a 1 sample per second error will create a 100 ms delay in about 80 minutes. If audio/video is captured by the same device, it is usually avoided by locking the audio and video clocks together (so there is an identical video frame rate error). However, if was not done in the original recording, it is not very easy to solve it later. Some transcoding tools allow you to resample the audio to a different sample rate, though figuring out what the right value is takes experimentation. Also, the actual clock rate drifts.
-playback equipment (particularly surround sound receivers and TVs) have their own processing chains, and need to share information on their delays. In new equipment this is done over HDMI, and sometimes fails. Older non-HDMI connections need to be configured manually.
Might be going a bit off topic here though...
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