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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-B4C05C46-F2C9-57FA-AD85-EFE6479C2FF1" xml:lang="en"><title>Design</title><shortdesc>When you start a port, you must make a decision if the driver must

    13 provide record, playback, or both playback and record.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <p>If the PDD is to support audio transfer in a single direction, either record

    15 or playback, then a conventional PDD implementation is required. The PDD opens

    16 only a single driver channel and the PDD factory creates either a record or

    17 playback PDD object. </p>

    18 <p>If the PDD is to support audio transfer in both directions then it must

    19 be implemented to open two units, one playback unit and one record unit. For

    20 each unit the PDD factory must create the appropriate PDD object. </p>

    21 <p>One complication in this configuration is the need to co-ordinate access

    22 to the single audio hardware device from the two separate PDD objects. This

    23 configuration needs coordination when accessing the hardware device from two

    24 separate PDD objects, detecting and preventing situations where the handling

    25 of a PDD function for the channel in one direction conflicts with the channel

    26 setup in the other direction, specifically: </p>

    27 <ul>

    28 <li id="GUID-076CE21B-E350-5D87-A486-8AFC120E8DE9"><p>preventing the setup

    29 of conflicting audio configurations between the record and playback channels. </p> </li>

    30 <li id="GUID-27C1DD90-DE14-51CD-A723-86C7696D4C4F"><p>preventing the channel

    31 in one direction from powering down the audio hardware device while it is

    32 being used for data transfer by the other channel. </p> </li>

    33 </ul>

    34 <p>The solution is to move the code that controls those aspects of audio hardware

    35 setup which are shared between the two driver channels into the PDD factory

    36 object, as this object is shared. </p>

    37 <p>The porting process focuses on implementing a PDD that supports both record

    38 and playback as this is the most common situation. The template port Sound

    39 Driver is setup for this configuration. A PDD that supports audio transfer

    40 in a single direction only omits the implementation for the direction not

    41 supported. </p>

    42 </conbody></concept>