What bitrate to use during video conversions

[mgray]

I'm converting various formats (avi, mpeg, wmp etc) to mpeg4 to put on my ipod. Obviously high bitrates result in large files, and vise versa. What I'd like to know is if the bitrate of the input means the same after conversion.

Such as, does an avi with a bitrate of 256 means the same as a mpeg4 with 256? What about going to H.264?

I don't want to make the videos unnecessarily large, nor do I want to degrade the quality any more than it already is. I just want to match the input quality to the output.

Thanks

[admin]

AVI is just a container, so it can contain any codec, including H.264. Most common is DivX/XviD (MPEG-4, Simple Profile or Advanced Simple Profile, but usually SP for portable devices). When people talk about MPEG-4 files, they usually refer to MPEG-4 SP.

H.264 is the new codec which is a bit more efficient than MPEG-4 SP/ASP, which means you can get a better quality picture using the same bitrate (or the same quality picture using lower bitrates). There is no rule to say how efficient it is, but I like to say that you can achieve the same quality as MPEG-4 SP/ASP at 80% of the bitrate (so 256 kbps MPEG-4 SP/ASP => 200 kbps H.264, for example). It's not an exact science, and probably some experimentation on your part might be needed to see what is acceptable for you.

As with most types of video conversion, even if you match the input video's bitrate, the output's quality will reduce if you are using the same codec. It's due to the lossless nature of the codec.

[mgray]

I appreciate the help, but I still have a few Q's.

H.264 sounds like the best option then for my aplication since iPod seems to support it?

If I match the bitrate of the output to the input then I should be fine, or I could even use the next option down?

I notice tho that when I look at the bitrate for .mpg files, I usually get really high numbers like 1.4mbits/sec, 715.60kbits/sec or sometimes no bitrate is listed. Choosing something like 256kbps seems like a huge drop. Obviously there is something else going on?

What would you sugest is the easiest and best method to convert my various formats and also how to determine the original bitrate? Right now I use SUPER to do my converting.

Thanks again for your knowledge.

[paglamon]

how to determine the original bitrate?

Use Mediainfo.
H264 or XviD encoded files are usually smaller than mpg files at the same quality because they are better compressors(i.e they can use a lower bitrate to produce a reasonably good quality). SUPER,which you are already using,is one of the easiest and best in the business.

[admin]

If I match the bitrate of the output to the input then I should be fine, or I could even use the next option down?

Yes, something like that. Bitrate is really just another way to represent the file size. For example, 128 kbit/s divided by 8 (8 bits in a byte) => 16 kBytes/s * 60 => 960 kBytes/minute, so that's nearly 1 MB per minute of video. Most videos these days user variable bitrate, so the bitrate you select is only an average figure, but the above calculations will get the file size pretty accurately (assuming the encoder is a decent one). Older style video codecs used constant bitrates, so the calculations above are exact to almost the KB.