What does bit rate do? [closed]

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I want to make a video transcoder as a holiday project in Java. I was looking into the basics of video files and came across terms like containers, bit rate, bit depth and so on.

I have questions regarding bitrate.

I know bitrate is the amount of data that is contained in the video per second. Sound also has a bit rate but is considerably lower than that of video, obviously.

So, say there is a video that is of 8 Mbps (YouTube HD 720p) and the user wants to transcode it to a lower bitrate of 4 Mbps.
Will this cause the sound to go out of sync?

I am not doing the transcoding myself, I am using Xuggler for it which contains a lot of codecs like H264 and others.

Also, if, by an accident the user decides to convert a 4 Mbps video to a 8 Mbps video, what will happen ?
This situation is possible if the user gives a video captured from a phone camera and decides to store it in DVD quality.

Also, there are other things to take into consideration like frame rate right ? Because a low capacity device can not handle a higher frame rate. is frame rate related to bit rate?

Answer 1

There are several possibilities of what will happen, depending on the decoder and so on. I'm not familiar with Xuggler, but:

The sound should not go out of sync if you drop video bitrate with a proper software. It will not shorten the video or anything like that. Depending on what you do to it, either the frame-rate will drop ( it will discard every 2nd frame) or each frame will be more compressed.

Answer 2

The audio and video are generally independent, so changing the bitrate of one will not affect the other. In the case of changing to a higher bitrate, the transcoder will either throw an error, or produce a larger file at the same quality as the original.

The frame rate is not directly related. The bit rate is just a measure of how many bits are being used to encode one second of audio or video.