XviD undersized files?

[Mdoodm1000]

So I've been trying to convert my entire DVD collection to XviD with all 30 minute shows being 175mb 1 hour 350 and so on. I haven't had a problem until recently. I tried to convert an episode of south park from dvd to xvid with VirtualDub, and the file size was way lower than what I specified in the XviD settings. I then changed the quantizer restrictions to min 1 and max 31, and now the file is 150mb, still not what I wanted. Is there any way I can make the file 175mb? I know I supposedly "saturated" the codec, but I've been using these same settings for a while and haven't had a problem until I tried to convert this show. Thanks in advance!

[redwudz]

Most any codec can be 'saturated' at some point where increasing the bitrate will not increase the quality. I would take a look at the output of the encoder and check it's quality. If it is similar to the input quality, maybe that's all that you can do with it.

South Park is very limited as to colors and resolution. It may not need that much bitrate.

[Mdoodm1000]

Thanks for the info! Hmm..I just re-encoded the video and it's now 175mb lol I guess changing the quantizer restrictions is what did it..Does anyone know if there is any harm in leaving all the minimums at 1 and maximums at 31 for all my encodes?

[manono]

Quant 1 is just there so you can make the size. Just as redwudz speculates, I also think you saturated the codec because South Park is about as simple as animation gets. If I were you I'd either lower the size you want (to 150 or even 125 MB) and be happy, or raise the resolution, use better quality audio, not use B-Frames, or a variety of other things to get them to the 175 MB. Me, I wouldn't ever use Quant 1 in my encodes. I restrict them to between 2 and 6, but that's just me.

[jagabo]

Why bother with 175 MB at all? Just use constant quality encoding and let the episodes come out whatever size is necessary to achieve the quality you asked for.

[Mdoodm1000]

Quant 1 is just there so you can make the size. Just as redwudz speculates, I also think you saturated the codec because South Park is about as simple as animation gets. If I were you I'd either lower the size you want (to 150 or even 125 MB) and be happy, or raise the resolution, use better quality audio, not use B-Frames, or a variety of other things to get them to the 175 MB. Me, I wouldn't ever use Quant 1 in my encodes. I restrict them to between 2 and 6, but that's just me.

So you recommend I set all the minimums to 2 and maximums to 6? I'm a little confused by what the quantizer restrictions even do.. Do they effect the quality?

[jagabo]

The exact relationship between min/max quantizers and overall quality is hard to predict. The quantizer, in a simplified sense, controls how much information is throw away. The higher the quantizer the more information is discarded. During a bitrate (file size) based encode Xvid will use different quantizers on different frames. On a particular frame it may decide you won't notice if a lot of detail is thrown out so it will use a large quantizer. On another frame it may decide to use a low quantizer to keep lots of detail. To meet your requested bitrate it will have to take away bitrate from shots that don't really need it, and give it to shots that do need it. When you limit the quantizer range you are constricting its ability to apportion bitrate to different frames and shots.

At a quantizer of 2 frames will look almost identical to the source. A quantizer of 1 will be more accurate but you would be hard pressed to see the difference unless you flipped back and forth between enlarged still frames. So it does make sense to limit the min quantizers to 2. Except in the case where you are trying to get arbitrarily high bitrates for the material you're compressing. Without using a quantizer of 1 on some frames the codec won't be able to get the bitrate up to your requested value.

Demanding 175 MB files really makes no sense unless you have to put four videos on a CD. Otherwise it's just an arbitraty number. It's like saying you heard that 175 horsepower is good for a family sedan so every vehicle you buy must be 175 horsepower. Well, it would be silly (though maybe fun) to have 175 horsepower in a moped you use just to get to work in downtown traffic. 175 horsepower would be very wimpy in a Humvee. 175 horsepower in the jet that transports the space shuttle would never get the thing off the ground.

[Mdoodm1000]

The exact relationship between min/max quantizers and overall quality is hard to predict. The quantizer, in a simplified sense, controls how much information is throw away. The higher the quantizer the more information is discarded. During a bitrate (file size) based encode Xvid will use different quantizers on different frames. On a particular frame it may decide you won't notice if a lot of detail is thrown out so it will use a large quantizer. On another frame it may decide to use a low quantizer to keep lots of detail. To meet your requested bitrate it will have to take away bitrate from shots that don't really need it, and give it to shots that do need it. When you limit the quantizer range you are constricting its ability to apportion bitrate to different frames and shots.

At a quantizer of 2 frames will look almost identical to the source. A quantizer of 1 will be more accurate but you would be hard pressed to see the difference unless you flipped back and forth between enlarged still frames. So it does make sense to limit the min quantizers to 2. Except in the case where you are trying to get arbitrarily high bitrates for the material you're compressing. Without using a quantizer of 1 on some frames the codec won't be able to get the bitrate up to your requested value.

Demanding 175 MB files really makes no sense unless you have to put four videos on a CD. Otherwise it's just an arbitraty number. It's like saying you heard that 175 horsepower is good for a family sedan so every vehicle you buy must be 175 horsepower. Well, it would be silly (though maybe fun) to have 175 horsepower in a moped you use just to get to work in downtown traffic. 175 horsepower would be very wimpy in a Humvee. 175 horsepower in the jet that transports the space shuttle would never get the thing off the ground.

Thanks! You actually explained it all fairly well