quick question on MPEG2 bitrate conversion

[SpiralDragon]

ok if i understand this corectly Mpeg 2 for DVD bitrate maxes out at 9.8Mbits... right... ok the question is how much does that translate into Kbits.... i know its not exactly 9800KBits now ... or is it? and we are talking constant bit rate.. not variable for DVD right? ... and WTF is GOP and GOP size? .... i am very new no encoding video.... can some one please give me a rundown... i am using cleaner XL... origonal files are PAL QT Ref digital master using AVID codec.....

thanx guys

[Brian Ellis]

Good question! I'd guess at multiplying by 1024, but there is so much crap around, who can tell? According to ISO, 1 M = 1000 k, but this falls down with bytes so I assume it would with bits as well, because 1 byte is normally 8 bits. I must admit units and the way they are expressed, especially in IT, is a nightmare. For example, you use "9800KBits" in your message. Irrespective of the the value, you made four VERY common mistakes, according to ISO:
1. The abbreviation of kilo is k, not K
2. The abbreviation of binary unit is bit, not Bit (capital letters are reserved for abbreviations derived from proper nouns, such as V for volts, from Mr. Volta)
3. You omitted the space between the value and the unit (depending on the type face or font, 3lm could mean 3 lumens or 31 metres)
4. You pluralized the unit. This is NEVER done with abbreviations, lest the s be mistaken for a multiplier of seconds
The correct way of expressing it would have been "9800 kbit". Even more correctly would have been kbit/s or, even better, kbit.s^-1 (sorry, can't put superscripts here!).

Apart from 9.8 Mbit/s being the max, it is not really practical. I think 8000 kbit/s is about the practical limit. However, I have done careful checks with a clip with much movement at 8000 CBR, 6000 CBR and 6000 VBR. On a TV, I could detect no difference: the picture starts to deteriorate below 6000 kbit/s, but even at 5000 kbit/s, it's still very good. Similarly, I cannot hear any improvement with AC-3 stereo (2/0) encoding higher than 196 kbit/s. I've always said that one can be more royalist than the king.

DVD encoding with CBR is fine if you can fit your project into 4+ Gbyte at 6000 kbit/s. If you can't and you have to use a lower bitrate, I still use CBR if there is not much movement in the image, but I switch to VBR if there is a lot of fast movement, especially fast pans or zooms, F1 cars or suchlike.

When burning the DVD, the MPG file is split up into separate video and audio and combined with menu, sub-title, separate audio track files. The video is also split up into sizeable chunks of about 1 Gbyte max. All this is managed by various files which serve to combine them back as desired when the DVD is played. I haven't analysed what each file does, but a typical DVD may have a total of a dozen or more files split into two or more directories. I just accept it as a fait accompli!

I use DVD Workshop 2 for authoring.

[SpiralDragon]

so if i encode at 9800 kbits/s will it still work on a regular DVD player... film is only about 8min... would like to have it at best qulity setting... and what about GOP? wht is that

[Hulk]

Dimensional analysis:

9800kbits/sec x 1 byte/8 bits x 1MB/1000kbytes = 1.23MB/sec

A useful conversion to keep in mind is that 8000kbps encoding yields 60MB/min for storage requirements.

9800kbps is the max bitrate for the DVD specification but that includes both audio and video streams.

I wouldn't encode the video higher than 8000kbps since many DVD players will stutter if the bitrate is too high. Also, unless you are very high motion video or water scenes bitrates over 6000kpbs aren't generally noticeable. Of course this depends on the quality of your encoder, but it's a good general rule. Also, the most important determining factor on final output quality is your input source. There is no use to encode a VHS captured source at an extremely high bit rate.

My advice, encode the video at 8000kpbs VBR and the audio as LPCM since storage isn't a concern for this project. Otherwise use AC3 audio encoded at 192kbps.

GOP stands for Group of Frames. In short MPEG-2 is a temporal encoding scheme. That means that some frames B(idirectional) and P(redictive) need adjoining or following frames, respectively to reconstruct the current frame. I frames contain all of the data required to display a frame. A GOP is an arrangement of I, B, and P frames. It can vary depending on the type of video content you are encoding. Standard settings are fine unless you are a compressionist.

Now, constant bitrate encoding allocates the same amount of bits to each GOP in the video stream while Variable Bit Rate encoding allocates more data to harder to code, high motion scenes and less for easier to encode or more static scenes. So it achieves higher quality video given the same storage requirements as constant bit rate encoding.

Why would you ever use CBR? Well there aren't many reasons anymore but the explanation would require time I don't have right now.

BTW, I also use DVD WS2. Great program, it's the only one I know of that does real time previews, compositing within the application, and has playlists.

- Mark

[Brian Ellis]

For short projects, I always use CBR 6000 kbit/s.

Advantages:
- static scenes are encoded to the same quality as fast movements
- no visible quality improvement possible with VBR or higher bitrates when viewed on a quality TV
- faster and more reliable encoding
- more set-top players will play it without glitches than with higher bitrates (even momentarily with VBR)

I agree with Mark that VBR is advantageous for projects requiring a lower bitrate for the reasons he states.

[Hulk]

Brian is right, here's some additional info.

Due to not having a variable readout rate from the drive using CBR, the disc can spin at more or less constant speed so the buffer doesn't empty or overrun, either of which would lead to stuttering.

This issue has to do with the minimum readout rate setting when sending a DLT for mass replication of DVDs and is why you don't want huge swings in bitrate when using VBR, although this is more of a problem with older drives. Still, if you're looking for maximum compatibility...

Brian, I agree. Unless the camera is REALLY good and there is A LOT of motion, CBR 6000 is generally sufficient.