Bit Rate vs Compression

[Guy_Fawkes]

I have been capturing video from a VCR and converting it MPEG2 file, eventually creating a DVD. I realize the quality of the VCR source is not the best to start with. I have played with adjusting the bit rate they fit on a single layer DVD. The results have been fair.

I have also captured at bit rates auggest for a DVD9 disk and used Cpmpression, such as found in Shrink and Anydvd. Those results have also been fair.

I find that an hour in the about that be burned on a DVD5 disk at best quality. I like to put each 2 hour tape on a seperate disk. DVD9s are still a little pricey for me.

Both version look pretty close to the same on 27 to 31 inch TVs. Does and have suggestions as to which will give the best result if they dvds are viewed on a large screen TV.


thanks

[mats.hogberg]

Generally, encoding at the correct bitrate to begin with is better than encoding at a random too high bitrate and then transcoding (shrinking) down to size.

/Mats

[FulciLives]

If you want 2 hours on a single layer DVD recordable then it boils down to simple math ... you need to use a BITRATE CALCULATOR

Below is a bitrate calculator set up for 2 hours and using 256kbps audio



As you can see the bitrate calculator says to use a VIDEO bitrate of 4820kbps and doing a CBR encode that is your bitrate. If you are doing a 2-pass or multi-pass VBR you would set the AVERAGE bitrate to 4820kbps and then you need to set a MAX and a MIN and the bitrate calculator is telling you to set the MAX no higher than 9537kbps although in the real world it is best not to set the MAX above 8000kbps although some people will set it as high as 9000kbps (I usually use 8000kbps myself). If you were to select a MIN I would, based on the results here, go with 2000kbps. Some people though will set it as low as 1000kbps or lower. A lot of people fight about what are the best MIN and MAX values to use with a 2-pass or multi-pass VBR but a good rule of thumb is no higher than 8000kbps and no lower than 2000kbps.

Please bear in mind though that does not leave a whole lot of room for the DVD authoring stage. It all depends on how simple or complex you want to make your menu but I would suggest using 100MB - 200MB for the ISO option instead of the 25MB default. For a simple menu 100MB is good for a more complex menu go with 200MB (a complex menu would be a menu with background video or chapters that are video instead of still images etc.)

Here is the bitrate calculator again but this time I've entered 200MB for the ISO option.



The bitrate reported is now 4617kbps instead of 4820kbps ... remember the lower the bitrate the smaller the end result so now there will be more room left over for the DVD Authoring stage.

Also you will notice that the bitrate calculaor asked for an audio bitrate (I used 256kbps) but did not ask what format the audio will be ... why is that? That is because it doesn't matter. A 256kbps MP2 audio file will be the exact same size as a 256kbps AC-3 audio file. The difference is not size but quality. The 256kbps AC-3 audio file will sound better than the 256kbps MP2 audio file because AC-3 uses compression better than MP2 does. As for the bitrate choosen I went with 256kbps because that seems to be the "sweet spot" for AC-3 audio. Going higher will generally NOT increase quality unless you have multi-channel audio (like 5.1 AC-3 which of course you will not have with video captures). Other popular options for 2 channel mono/stereo audio include 224kbps and 192kbps but since 256kbps seems "best" that is what I use ... plus to be honest you save very little by going as low as 192kbps so might as well go 256kbps.

OK so we got the bitrate thing down but what about quality?

The DVD format can handle multiple resolutions. The two most popular are Full D1 (720x480 for NTSC or 720x576 for PAL) and Half D1 resolution (352x480 for NTSC or 352x576 for PAL). You can also used what is called "cropped" D1 which is 704x480 NTSC or 704x576 PAL but generally standard Full D1 is favored over "cropped" D1.

Now things get a bit "sticky" here ...

VHS is a low resolution source and it has pretty much been proven that Half D1 resolutioin is enough for standard VHS but some still feel that Full D1 looks better. In reality I do find that Half D1 is a bit "softer" looking than Full D1 but Half D1 can and does still look very good for a VHS source.

So which to pick?

Generally speaking the "rule of thumb" is to use Full D1 when the bitrate is a CBR of 4000kbps or higher ... or if doing a 2-pass or multi-pass VBR you would want the AVERAGE to be 4000kbps or higher. If the bitrate is lower than 4000kbps then the "rule of thumb" says to use Half D1 resolution.

Sometimes though it is not that simple. The quality of the video you are encoding affects the MPEG encoder. The "cleaner" the video the easier it is to compress by the MPEG encoder but the reverse is also true ... a very "ugly" source will not compress so well by the MPEG encoder and will need more bitrate than a "clean" source.

VHS is a "nosiy" video source especially if the LP or SLP/EP speed was used for the recording. Also home made camcorder footage tends to have a lot of movement due to the camcorder usually being handheld so the more movement the harder it is to do the MPEG encoding.

So the line between Full D1 and Half D1 is a fuzzy one. If we go with the 2nd bitrate calculator example of a video bitrate of 4617kbps will we find that with some sources that will be fine if we go with Full D1 ... but with a "bad" or "ugly" or "noisy" video source we might need to drop down to Half D1 to get a good quality image.

MPEG-2 as used by the DVD Video format can introduce "compression artifacts" and the more "noise" in the video the more bitrate we need to avoid the "compression artifacts".

However Full D1 has more "information" to process than Half D1 so Half D1 needs less bitrate to look good. Yes it is true that Half D1 has less resolution but consider this ... would you rather have a slightly "soft" image with less resolution but one that is baby smooth ... or would you rather have a "sharper" image but one that might have some "compression artifacts"?

That's the trick and the trade-off.

So what I am saying is that most of the time Half D1 is fine for VHS but if you want to use Full D1 just beware that you might end up with some "compression artifacts" depending on the image quality of the source and how easy or difficult it is to encode to MPEG format.

Also in this example if you do Full D1 the bitrate is low enough that you definately would want to do a 2-pass or multi-pass VBR to get optimal quality.

However 4617kbps is "high" for Half D1 so you could get away with a CBR encode which will save a lot of time! Also if you capture direct to MPEG-2 then a CBR at Half D1 makes more sense than a real-time VBR encode at Full D1 ... at least at the bitrates we are talking about in order to get 2 full hours on a single layer DVD recordable.

Another thing you can do to improve the image quality is to use some sort of "video noise" filter. Such a filter attempts to remove "video noise" by smoothing the image. The trick is not to "over do it" or you end up destroying too much real detail along with the "video noise". The benefit of applying a "video noise" filter is that it makes the MPEG encoding more effecient since the image is "cleaner" i.e., less video noise.

Obviously I have too much time on my hands to type all this ... LOL ... so I am stopping now. I think I more-or-less made the point and I hope you understand it all and can use it.

The best way to "get this" and "understand it" is to do it.

Take a VHS tape and trying doing it Full D1 and Half D1 and try to learn about video noise filters and play around with them until you find a combo that you feel comfortable with.

If you have any questions feel free to ask ... also it would help if you gave more details on your capture hardware and how you capture and with what programs etc.

- John "FulciLives" Coleman

[ronnylov]

That is a very good post FulciLives!!!

[Guy_Fawkes]

I'm glad you have too much time on your hands, thanks for the useful info. I never gave much thought of Hald D1. Now I will. Much thanks

[redwudz]

Great explanation, John! I'm taking notes.

I use 1/2 D1 a lot with old VHS conversions, especially with my LP tapes.

[slacker]

Be careful...

ASSUMING that 1/2 D1 is going to be better than FULL D1 when using low bitrates in all cases is ill advised. With Vegas 6.0 I consistently see better results processing DV-AVI using FULL D1 when putting 2.5 to 3.0 hours of high motion video on a dvd. Always run your own tests to be sure.

[edDV]

FulciLives - Excellent post!

Some additional advanted topics to explore later

- VCR calibration and/or Proc Amp (analog level adjustments) before A/D minimizes clipping and reduces the need for digital levels filtering.

- TBC smooths field to field motion making true MPeg2 motion detection more accuarte. Tape jitter gets detected as motion reducing effectiveness of interframe MPeg motion compression.

- IVTC (NTSC inverse telecine) of film sourced material can cut 20% of frames, allowing 20% less compression