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Our intention is to give very simple explanations of the
concepts of MPEG encoding. We will avoid to use complicated formulas or standard
descriptions.
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Bitrate
This parameter (first of all) refers to the compression level.Generally the basis
is in Kbps, to say kilobit-per-second. The number means how much space needed to store (or
send) one second long material. The lower number means less space or bandwith. The
alternate concept of the bitrate is the speed of MPEG stream, so the bitrate =
speed of the stream. Several standards declares limits or exact speed in relation
of streams. If the encoder exceeds or drops these limits or rules the result will be not
standard or the decoders will not be able to playback such kind of streams. To be more
precise some of decoders can playback them some of others do not. Generally the PC
players, software or hardware decoders can playback any kind of stream in relation of
bitrate. The success depends on the player (decoder) realization: |
- Maximum bitrate, usually this is between 10000 and 25000 Kbps.
- The storage device's speed. A 4x CD-ROM can playback a stream of 5000 Kbps as max. bit
this is a bit risky. A 2x speed DVD-ROM can playback a 20000 Kbps stream.
The MBps (megabit-per-second) also often used. 1 MBps = 1024
Kbps, but the manufacturers frequently use the 1 MBps = 1000 Kbps formula. Here are some
examples about the standards:
The numbers are just informative
|
average Kbps |
minimum. |
maximum |
| Video-CD |
1124 |
1100 |
1150 |
| Super VCD |
any |
any |
2600 |
| DVD-VIDEO |
any |
any |
9800 |
| Satellite broadcast |
8000 |
8080 |
7920 |
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Quantization
scale
This number can describe the quality level. The higher number declares stronger
compression - worse quality, the lower one means lighter compression - better quality.
Some of encoders use the standard linear quantization tables, others uses internal or
custom, non-linear tables. The basic measurement of quantization numbers is between 1 and
31. Is not worth to compare just by the quantization scale numbers a linear and a
non-linear quantized stream, even two non-linear quantized streams can cause significally
different result in quality, because the real quantization depends on the custom
quantization table stored in the stream. |
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CBR
Constant BitRate. This way of encoding
declares that the bitrate must be constant independenlly of the picture
content/complexity. Some of standards claim very strictly to hold the bitrate, such as the
Video-CD, satellite broadcasting, net-streaming. By this rule if the picture complexity is
too high, the quality can be very bad, and vice-versa. Here is a good example for a real
constant bitrate stream:
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Figure 1. The bitrate
is constant, the quantization level (therefore the quality) is variable |
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At the setting of the encoder parameters just the bitrate can
be set. No other parameter needed. Depending on the encoder realization, the bitrate will
be very close to the set value. If the encoder varies the bitrate too much, some of
decoders or devices won't play this kind of stream. But the PC encoders will playback this
stream certainly, btw.
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VBR
Means Variable BitRate. The bitrate
will depend on the complexity of picture. The encoder tries to find some compromise
between the bitrate and quality. Neither the bitrate nor the the quantization level will
be constant. This needs a very complex (and intuitive) calculation while encoding. The
figure 2. shows a typical VBR stream. The source is the same as of the CBR example:
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Figure 2. The bitrate
and the quantization level (therefore the quality) is variable |
The effectivity of VBR depends on the encoder
realization. You can see on the graph, that is there some compromise in the quality,
because a typical encoder is not able set always the right bitrate, not to mention is
there a very inportant rule, that the encoder has to hold the average bitrate of a such
stream. To set a VBR stream at least two parameters are needed: the average bitrate and
the peak bitrate. Some of encoders are more refined asking about the average quantization
level and/or the average bitrate time interval, too. This last parameter could be very
important, because if the encoder can handle short time, the bitrate can be very low and
this saves space or the bitrate can be very high for a moment if the source is too
complex. Is true in this case the bitrate oscillation can be too high. The most of
encoders can't utilize the possibilties, so the bitrate will never reach the peak usually
even if the pircture content claims this and even the bitrate will be never low enough
although the picture is very simple. Some of encoders use dual-pass encoding to try to
create the best VBR stream.
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CQ
CQ = Constant Quality, to say the quantization
level will be constant by all means, and who knows how much will be the peak or average
bitrate, they are unpredictable. Some encoder developers confuse the VBR and the CQ and
claim that their encoder can create VBR although this is just partially true. Yes,the
bitrate will be not constant but does not meet the expectations of a normal VBR stream.
This kind of method used by the BBMpeg and by the WISmpeg2i 1.1, btw. The problem is that the
stream created by such encoders can easiliy exceed the bitrate limits of some standards,
even - regaring to our test - the bitrate can reach the 30000 Kbps value too, which is an
extremelly high bitrate and the most of decoders can't playback this file. Not to mention
that the result will be a large file.
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Figure 3. The
quantization level is constant, the bitrate is unpredictable. |
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The most important disadvantages of the CQ method is
1. The average and peak bitrate is unpredictable and it is
possibile that the user will be forced to encode a couple of times the stream to get the
expected file size. This can be very time consuming.
2. To get usable stream, sometimes the quantization level will be too high constantly,
this means the quality of the simple pictures will be not so good as possibile.
Of course the encoder waits just only one parameter, the
quantization level. The intelligent encoders implement either the CQ or the VBR methods.
The Ligos, Tsunami or Cinemacraft encoders can create both kind of streams.
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Dual pass
encoding
To create the best VBR stream some expensive (or sometimes freeware) encoders
implement the dual (or more) pass encoding method. In the first pass the encoder examines
the source and sometimes applies some basic encodings too. In the second pass using the
first pass's results, the encoder now knows well what compression can be used to get an
optimal VBR stream. Is there a significant difference between the realtime and the offline
encoders. The realtime encoders claim frame accurate VCR cotrol too, without this
possibility the dual pass encoding is impossibile. |
To be continued...
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