DVD Rate Change Property Set

The DVD Time Stamp Rate Change property set enables you to change DVD playback rates by modifying time stamps between input and output pins on two filters.

Time stamp remapping is a linear relationship given by the equation y = r(x - xi), where y is the new time stamp, r is the rate (or slope), x is the original time stamp, and xi is the x-intercept. This is shown graphically in the following diagram, with the time stamps that are input to the filter shown along the x-axis and the output time stamps shown along the y-axis.

Variable definitions for the time stamp conversion graph:

• x = input time stamp
• y = output time stamp
• xi = current x-axis intercept
• xs = start time of the current rate
• xi2 = x-axis intercept for new rate
• xs2 = start time of the new rate
• r = current rate
• r2 = new rate

The decoder filter handles DVD rate changes, by reading the original time stamp on the media sample (x). The remaining variables, r and xi, must either be supplied or internally calculated to determine the new time stamp for the sample. The Time Stamp Rate Change property set supports two mechanisms for setting these parameters:

• The AM_RATE_ExactRateChange property accepts the OutputZeroTime parameter as the x-axis intercept for the new rate, and the Rate parameter as the new rate (xi2 and r2). However, you must supply the upstream filter setting this property with the equation (y = r(x - xi)) that the downstream filter is using to do the rate change.
• The AM_RATE_SimpleRateChange property specifies two parameters that an upstream filter already knows: the new rate (r2) and the start time of the new rate (xs2). This requires the filter doing the rate change to calculate the x-axis intercept (xi2). You can calculate the new x-axis intercept with the following formula: xi2 = (r/r2)(xi - xs2) + xs2. Now that you have the input time stamp (x), the new rate (r2), and x-axis intercept (xi2), you can plug these variables into the original equation y = r2(x - xi2) to calculate the new time stamp.

Specifying the AM_RATE_SimpleRateChange property is the easiest way to change the rate, but requires the filter to do some additional calculation.

The AM_RATE_MaxFullDataRate property enables an upstream filter to query for the maximum rate at which it can process the data. It uses the AM_MaxFullDataRate data type, which is a LONG value.

The following information presents the necessary constants and data types to use for this property set in calls to IKsPropertySet methods. It provides values for the GUID (guidPropSet), property ID (dwPropID), and property data type (pPropData) parameters.

Property Set GUID: AM_KSPROPSETID_TSRateChange

The following list provides the Time Stamp Rate Change property set IDs and descriptions:

AM_RATE_ExactRateChange

Use the AM_ExactRateChange structure to change rate.

AM_RATE_MaxFullDataRate

Use the AM_MaxFullDataRate Long value to process at maximum rate.

AM_RATE_SimpleRateChange

Use the AM_SimpleRateChange structure to change rate.

AM_RATE_Step

Indicates the number of frames to step over. (Obsolete. See DVD Framestep property set.)

Use the IKsPropertySet interface to set, retrieve, and query for support of these properties.

This property set uses the following data types.