Frequency Overrides

Microsoft DirectShow 9.0

Frequency Overrides

A significant amount of effort was spent to ensure that the broadcast frequencies and color standard assignments are correct for each country/region. Even so, there will be situations when the frequency tables are not sufficient, contain errors, or become obsolete. To address this problem, the frequencies listed in the TV Tuner filter's frequency tables may be selectively overridden by using the registry.

Frequency overrides are grouped into application-defined "tuning spaces," which are identified by number. The following example shows an example override:

HKEY_LOCAL_MACHINE\Software\Microsoft\TV System Services\TVAutoTune\TS0-1
"12"=dword:04022750

In this case, TS0-1 indicates Tuning Space 0 for cable frequencies. The first number identifies the tuning space. The second number is either 0 for broadcast frequencies or 1 for cable frequencies.

The subkey named 12 overrides the frequency value for the frequency at index 12 in the current frequency table. The value of the subkey is a DWORD that specifies the frequency in Hertz (Hz). In this example, the frequency is set to 67.25 MHz. Overrides can be defined for any channel numbers in the range of 1 to 999, inclusive. If the tuning hardware does not support a given frequency, the tune request will fail.

This mechanism can also be used to create new channel numbers outside the existing range in the frequency table. The IAMTuner::ChannelMinMax method will return the extended channel range. For example, if the original channel range was 1 to 158, and a channel override of "200" is added to the registry, the ChannelMinMax method will return 200 as the maximum channel. In this case, channel numbers in the range of 159 to 199 will have no frequencies assigned to them, so any tuning requests in that range will automatically fail.

The IAMTuner::put_TuningSpace method allows the application to choose which set of overrides and fine-tuning information to use. Tuning space numbers are arbitrary. It is the application's responsibility to maintain the relationship between the tuning space and the frequency table. The most straightforward approach is to use the country/region code as the tuning space number. Then, every time the application switches to a new country/region code, it also switches to the same tuning space (in that order).