Case item

The factors that cause a gear to mesh tightly are (1) tolerance on concentricity of shaft hole with pitch diameter, (2) tolerance on center distance, (3) tolerance on quality, (4) coefficient of thermal expansion, and (5) change in dimensions due to moisture absorption, which is a consideration/detriment in some materials. The first three apply to gears of any material. Item 4, and in some cases Item 5, for plastic gears deserves special consideration. 

The data needed are the rate equation, energy of activation, heat of reaction, densities, heat capacities, thermal conductivity, diffusivity, heat transfer coefficients, and usually the stoichiometry of the process. Simplified numerical examples are given for some of these cases. Item 4 requires the solution of a system of partial differential equations that cannot be made understandable in concise form, but some suggestions as to the procedure are made. 

As in the case of MPEG-1, there are three versions of the multichannel extension called Layer 1, Layer 2 and Layer 3. Layer 1 and Layer 2 MC extensions basically both use a bitstream syntax similar to Layer 2. As in the case of MPEG-1, Layer 3 is the most flexible system. As one special feature, MPEG-2 MC Layer 3 permits use of a flexible number of extension channels. While the original idea behind this was to alleviate the dematrixing artifact problem for some worst case items, this idea can be used to do simulcast of two-channel stereo and 5-channel extension without the artistic restrictions of a fixed compatibility matrix. 

The first branch that has a case item whose value matches the value of the case expression is selected. A case item may be a constant or a variable. 

A case statement behaves like a nested if statement, that is, the value of the case expression (Op) is checked with the first case item (ADD), if it does not match, the second case item (SUB) is checked and so on. The equivalent i f statement for the above case statement is shown next. 

In a casez statement, the value z is considered as a don t-care when it appears in a case item expression. The character can also be used alternatively for the character z. Values z and x are not allowed in a case expression. Additionally, value x cannot appear in a case item expression. Here is an example of a casez statement. 

The casez statement is equivalent to the following i f statement (note that the character in a case item denotes a don t-care value). 

In a casex statement, the values x and z ( for a z is ok too) in a case item expression are considered as don t-care values. These values, for synthesis purposes, cannot appear as part of the case expression. Here is an example of a casex statement used to model a priority encoder. 

In the previous section, we saw that a latch may be inferred for a variable that is not assigned a value for all possible values of a case expression. Sometimes it is the case that the designer does not expect the case expression to have any value other than those listed in the case items. Here is an example. 

Verilog HDL semantics of a case statement specifies a priority order in which a case branch is selected. The case expression is checked with the first case item, if it is not the same, the next case item is checked, if not the same, the next case item is checked, and so on. A priority order of case item checking is implied by the case statement. Additionally, in Verilog HDL, it is possible for two or more case item values to be the same or there may be overlapping case item values such as in casex and casez statements however, because of the priority order, only the first one in the listed sequence of case items is selected. 

What if the designer knows that all case item values are mutually exclusive In such a case, a decoder can be synthesized for a case statement control (the case expression is checked for all possible values of the case item values in parallel) instead of the priority logic (which could potentially be nested deep depending on the number of branches in the case statement). 

The information that all case item values are mutually exclusive needs to be passed to the synthesis tool. This is done by using a synthesis directive called parallel case. When such a directive is attached to a case statement, a synthesis tool interprets the case statement as if all case items are mutually exclusive. Since the synthesis directive appears as a comment in the Verilog HDL model, it has no effect on the language semantics of the model. This implies that no priority logic is synthesized for the case statement control instead decoding logic is used. Here is the case statement with the parallel case directive. 

In Verilog HDL, it is possible to have a non-constant expression as a case item. This is shown in the following example of a priority encoder. 

When value x is used in a case item of a case statement (not casex, casez), the branch corresponding to that case item is considered never to execute for synthesis purposes. 

In this case, no latches are inferred for Z and NextState since the full case synthesis directive states that no other case item values can occur. However, the preferred style for not inferring latches is to use the default branch. 

Researchers can also use item misfit (when an item seems to cause idiosyncratic responses) to improve a test and to evaluate a data set. An item with misfit may be an easy item unexpectedly missed by a number of very high performing students (those students who correctly answer almost all of a test s items). A different scenario can also cause item misfit A test may include a difficult item for most test takers, but a number of students who did very poorly on the test may correctly answer the item. In this case, item misfit may also indicate a problem in item wording. 

Fibres may be submitted in a tape lift format, which is a piece of sticky adhesive tape called J-Lar . This tape is less adhesive than cellotape and is used to collect extraneous fibres from clothing or from the upholstery inside vehicles, for example. Any fibres that are different from the background fibres can be removed later for subsequent examination and comparison with fibre from other items involved in a case. Items of clothing and hair combings are also frequently submitted for examination for the presence of fibres. 

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