bitrates and stuff

[dante22]

I want to know about bitrates. Where do I find it and how can I change it if it is possible to do so. Any responses would be appreciated.

[SaSi]

Your question is quite vague, and I suppose that reflects the fact that you are struggling to understand what's up. So I'll try to shed some light. Apologies if I confuse you further instead

So here goes the impromptu Bitrates 101.

Bitrate is the measure of the data USED to represent information for each second. As such, it is a property of the audio or video stream. Several factors determine the bitrate used for every stream. The main two are the encoding scheme (if used) and the selection made by the one who created the video or audio stream.

For example, an uncompressed full colour PAL resolution video stream has 720 x 576 pixels for every picture and 3 bytes for every pixel. That makes 720x576x3 = 1.244.160 bytes per picture. Since there are 25 pictures for every second, the bitrate required for this uncompressed video would be: 1.244.160 x 25 = 31.104.000 bytes / second, or roughly 30Mbps.

This bitrate is the result of the data representation in this video, and cannot be changed unless the information is transformed in some way. Either by re-sizing the picture (to reduce the number of pixels and hence the bitrate), or by encoding (which means to transform the pictures in a way that they require less data to represent the information).

There are several encoding methods (DivX, MPG, ASF, MOV are some of the most popular extensions denoting the encoding method). Each encoding method examines the video (and audio) streams, take advantage of certain "features" of the video and audio and the equivalent features of our sensors (i.e. eyes and ears), and reduce the amount of data required to store the image information.

To avoid getting into technical details that are hard to understand and will only obscure the issue, I will only state that the same principles used to convert a huge BMP picture into a tiny Jpeg picture that your eyes view as identical to the original are used in the video encoding methods.

What the encoding does is to reduce the data requirements for each frame (using similar and different techniques) so that the bitrate is reduced.

Hence, an encoded video requires (always) less bitrate compared to the original, un-encoded one.

Most encoding methods - algorithms, take the step to remove some of the information contained in the uncompressed video based on psycho-acoustical and "psycho-visual" findings. Simply put, they remove what you can't really see or listen. By a factor. The more they remove the less bitrate is required. And if they remove too much, you will be able to start to see and listen the difference.

Different encoding algorithms can achieve different levels of compression (of the bitrate) without visible - and audible - loss of content. MPEG-1 doesn't do too well, MPEG-2 is better and MPEG-4 is even better. There are others, like DV, RLE based and others, that are in-between in terms of bitrate compression.

Different encoders are designed around different uses and needs, and therefore achieve different results.

RLE based encoders can achieve a 2.5:1 ~ 5:1 compression, taking the 30Mbps down to 6Mbps ~ 10Mbps. These are low compression ratios, but the prioroty here is speed and not to loose ANY content.

MPEG-2 can easily achieve a 20:1 ~ 25:1 compression, without significant loss in quality. This makes the bitrates 1.2Mbps ~ 1.5Mbps (which by the way are the ones used in SVCDs). For DVD Quality, MPEG-2 encoders are tuned for a 5:1 ~ 10:1 compression, resulting into 3Mbps ~ 6Mbps rates.

All video streams generated through the encoding processes now inherently posess a bitrate property. It's because the video content in them has this data-rate requirement so that it can be viewed in real-time.

Some video file formats have the bitrate (average and maximum) as information fields in the file, for ease of access. For other stream types bitrate can only be calculated by scanning through the file and measuring data size for each 25 frames (for PAL).

For further understanding, you can read the specifications or FAQ for each encoding scheme. You can do a search for "bitrate" and for "specifications" or "specs" in the DVDRHelp forums. There is quite a lot you can find and read and understand.

[dante22]

Thank you I have learned a lot! So are you saying that if I get it at a certain bitrate then I can't change the bitrate at all.

[Quigonsean]

Wow SaSi that's probably one of the easiest, yet most complete explanation I've heard. (Heard, what am I saying,err typining I read it not heard it) Anyhow they should post it in the guides section. If your listning Baldrick I would put this in the guides section, it is quite easy to understand but quite pregnant with information.

Sean

[ZippyP.]

...pregnant with information.

Now that's a new one on me.



Sasi's info is so good, I thought it was from a text book!

[CogoSWSDS]

Dante22 wrote:

So are you saying that if I get it at a certain bitrate then I can't change the bitrate at all.

No this is quite not the case. For example, 10 minutes of uncompressed 320x240 x 30 frames per second video runs at right around 2 gig on my computer. If I then recompress it using HuffyUV (a compression algorithm that keeps all the data but makes it take up less space), I can get it to take up a bit less than 1 gig. If I recompress it to VCD standards (1150 kbps) it takes up about 100 meg. It's the same video, but different transformations give me different bitrates and thus different file sizes.

Hope this helps,
CogoSWSDS

[dante22]

Oh Ok now I have it.