Video Capture Devices with Radio Tuners

Microsoft Windows XP and later, and Microsoft DirectX 8.1 and later provide support for video capture devices that include FM radio tuners.

A video capture minidriver for a device with an FM tuner should support the KSPROPERTY_TUNER_STATUS property. This will allow user-mode clients to retrieve a KSPROPERTY_TUNER_STATUS_S structure that describes the progress of tuning operations.

Minidrivers can support one of three tuning strategies:

1. Tuning by PLL offset.

   If your FM tuner hardware supports tuning through PLL offset, your minidriver should set the Strategy member of the KSPROPERTY_TUNER_MODE_CAPS_S structure to KS_TUNER_STRATEGY_PLL.

   If your FM tuner hardware does not provide PLL support, the minidriver should emulate PLL support by using the native signal strength indicator. The system-supplied FM tuning logic in KsTvTune.ax is enabled only if the minidriver specifies that it supports the KS_TUNER_STRATEGY_PLL strategy.

2. Tuning by Signal Strength.

   If the minidriver sets the Strategy member of the KSPROPERTY_TUNER_MODE_CAPS_S structure to KS_TUNER_STRATEGY_SIGNAL_STRENGTH, KsTvTune.ax still attempts to use the PLLOffset member of the KSPROPERTY_TUNER_STATUS_S structure. Consequently, this is not a valid option for future compatibility.

   In addition, the minidriver should set the SignalStrength member of the KSPROPERTY_TUNER_STATUS_S structure to -1, 0, or 1, depending on whether an acceptable frequency is currently selected. Vendors decide what Receiver Signal Strength Indicator (RSSI) or decibel millivolt (dBmV) level above or below the baseline voltage constitutes an acceptable signal for FM reception.

3. Tuning performed manually by the minidriver.

   Set the Strategy member of the KSPROPERTY_TUNER_MODE_CAPS_S structure to KS_TUNER_STRATEGY_DRIVER_TUNES to control tuning logic in the minidriver.

In FM mode, KsTvTune.ax steps through the 200-kHz band around a frequency (100 kHz on either side), using the minidriver-specified TuningGranularity member of the KSPROPERTY_TUNER_MODE_CAPS_S structure as a step size. The search stops when KsTvTune.ax has searched the entire 200 kHz band, or when the minidriver determines that a good signal has been found, whichever occurs first.

Tuning takes much longer if the minidriver always specifies a PLLOffset value of -1 or 1. In this case, the tuning logic in KsTvTune.ax retries overlapping frequency ranges. The minidriver should specify a PLLOffset of -1 or 1 only on the first tuning request, or when the tuner is within eight steps of the best signal. For more information about tuning requests see Recognizing the First Tuning Request.

The tuning process always starts at the center frequency, as requested by an application, and steps up no higher than 100 kHz above the center. However, if the PLLOffset becomes 1 near the upper 100-kHz limit, the tuning logic steps beyond the 100-kHz band.

If the tuning process does not find an acceptable signal in the upper range, it tries below the center frequency, stepping up from no lower than 100 kHz below the center and ending at the center frequency if it still has found no acceptable signal. Again, if the PLLOffset becomes 1 near the center frequency, tuning steps beyond the center frequency before finally returning to it.

A PLLOffset member value of -1 or 1 on the first tuning request causes KsTvTune.ax to enter fine-tuning mode. Fine-tuning mode consists of tuning requests in rapid succession at step intervals specified by the TuningGranularity member of the KSPROPERTY_TUNER_MODE_CAPS_S structure in the direction indicated by PLLOffset.

KsTvTune.ax stops its tuning attempt if it is unsuccessful after eight fine-tuning steps in either increasing or decreasing frequencies. After KsTvTune.ax is in fine-tuning mode, if PLLOffset changes direction from -1 to 1, or 1 to -1, or becomes 0, the tuning request is considered successful. Both the fine-tuning and the search through the 200 kHz band stop at that point.

However, if PLLOffset is greater than 1 or less than -1, fine-tuning either does not start, or is abandoned. Fine-tuning mode is independent of the search through the 200 kHz band around the center frequency, although both use the step size specified in TuningGranularity (thus the caution against always returning a PLLOffset of -1..1).