Re: How to determine VOB encoding quality?

From: Zarax (zarax999_at_hotmail.com)
Date: 01/12/05 

Date: Wed, 12 Jan 2005 09:12:05 +0100

It is not easy to answer such a question.
Aside from pure bitrate, encoding techniques can influence quality to various degrees.
You can usually obtain a better (or at least homogeneus) average quality per bitrate ratio by using VBR, as the stream will be better tailored for the content (IE more bitrate for high motion scenes).
About quality and bitrate estimates they are based on WMV higher encoding efficency, which allows the same visual quality at half to one third of MPEG2 bitrates.
Finally, quality mught also depend (but it's usually not a major factor) from your MPEG2 decoder, as some have extra features such as postprocessing.
A good place to discuss about WMV encoding is www.microsoftusernetwork.com/forum where you can also find tutorials and a small knowledge base for player/encoder issues.
  "Tagman" <Tagman_2000@hotmail.com> wrote in message news:O53jqpG%23EHA.2568@TK2MSFTNGP11.phx.gbl...
  Alessandro - Thank you for the reply -

  I understand your audio explanation.
  I'm still trying to get a handle on the video explanation regarding the various compression options.

  I understand the need to calculate the bitrate to use when using a time constrant (trying to fit the video onto a specific disk size) - but if there is no capacity constrant (hard disk) - then video quality becomes the constraining factor. While the rule of thumb is that a higher bit-rate will normally result in a better picture - I'm assuming that it is possible to select a bitrate in excess of what is required (based upon the video quality of the source).

  In your example - you stated that a DVD with a 5.4 mbps could be encoded with a wmv rate of 2.0 mbps with similar results. How did you calculate the 5.4 figure? How did you determine the 2.0mbps suggestion?

  1) I guess my question should be "how can I tell which bitrate would be the appropriate selection to encode a vob file into a wmv file without incurring "Noticeable" video and audio degradation?"

  2) Once I determine what that setting should be - should I always stick with it - regardless of the source video - or do I need to constantly tweak the encoding bitrate each time by checking the bitrate of the source?

  ***************************************

  Here's my latest experiment....

  I have a 23 minute 46 second video clip (1,426 seconds)
  The existing VOB size is 1,032,394kb

  How would I calculate the source mpbs rate in the case? My results don't make logical sense...

  I ran the file through a program to analyze the bitrate with the following results:

  Num. of picture read: 69014
  Stream type: MPEG-2 MP@ML VBR
  Resolution: 720*480
  Aspect ratio: 4:3 Generic
  Framerate: 29.97
  Nom. bitrate: 9201600 Bit/Sec
  VBV buffer size: 112
  Constrained param. flag: No
  Chroma format: 4:2:0
  DCT precision: 9
  Pic. structure: Frame
  Field topfirst: Yes
  DCT type: Field
  Quantscale: Nonlinear
  Scan type: Alternate
  Frame type: Progressive
  Peak Bitrate: 10279
  Average Bitrate: 6329

  Based upon this small 60 second sample - the average bitrate is calculated as 6329
  Do I need to take the peak bitrate of 10279 into account?

    "Alessandro Angeli [MVP::DigitalMedia]" <nobody@nowhere.in.the.net> wrote in message news:udkTxv59EHA.2876@TK2MSFTNGP12.phx.gbl...
    Tagman wrote:

> Is there a tool (or some other way) available which would
> allow me to learn the 'quality' of the encoding used to
> create the vob? (a 1 hr video can be 3-1/2 g or 7g - I'm
> assuming the larger file size indicates that a better
> encoding rate was used to create the vob)...
>
> I know that when I check the right click to see the
> properties of a WMV file - I can learn the video and
> audio specifications - seeking the same info for the vob
> file.
>
> I thought if I had this info, I could match the Audio and
> Video encoding rates on the MS MWV encoder settings so
> that I could achieve a smaller file (no sense in using a
> better audio or video setting for WMV than the source
> encoding in the VOB as I see it).

    The size only depends on the average bitrate (size = bitrate
    * duration). It is not a measure of the quality of an
    encoding but it is a rough estimate: if anything else is the
    same (stream format and encoding parameters), the higher the
    bitrate the higher the quality (but not always!). There is
    no way to measure the quality once the encoding is done
    without comparing to the original. If you are referring to
    the quality setting in the encoder properties, that does not
    really specify the visual quality but only the compression
    factor to be applied and, in some cases, it also affects the
    bitrate distribution curve. You can not really infer this
    parameter from an encoded file and it would have no meaning
    going from MPEG2 to WMV.

    Notice however that an encoder will notice you are wasting
    bitrate and produce a smaller size if it doesn't need all
    that space. All the more so if you encode in 2
    bitrate-costrained passes.

    Last, 3.5GB for 1.5h means an average 5.4Mbps. With WMV, you
    should be able to get the same result (quality-wise) with
    only 1.0-2.0 Mbps or even less, but it strongly depends on
    the video content and whether you use 1 or 2 passes and CBR
    or VBR and quality- or bitrate-costrained encoding.

    The audio part is a lot simpler: my rule is to use
    56Kbps/24KHz (or 56Kbps/22KHz) for stereo voice or
    128Kbps/48KHz (or 128Kbps/44KHz) for stereo music (with
    music, if I have space, even 192 or 224 KBps), with CBR
    encoding. Unless you are an audiophile, I don't think you'll
    notice the difference form the higher bitrate VOB's audio.

    --

    // Alessandro Angeli
    // MVP :: Digital Media
    // a dot angeli at psynet dot net

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