Issuing Raw AV/C Commands

The IAMExtDevice, IAMExtTransport, and IAMTimecodeReader interfaces work by translating the method calls into commands for the driver, and then interpreting the driver's response and returning it through an HRESULT or an output parameter. However, some device functions may not be accessible through these methods. Therefore, MSDV supports sending raw AV/C commands to the device, using the IAMExtTransport::GetTransportBasicParameters method.

You should keep the following points in mind when using this feature:

• The command is passed directly to the device, with no error checking or parameter validation. For this reason, you should issue raw AV/C commands only when the DirectShow interfaces do not implement the functionality you need.
• All raw AV/C commands are synchronous. The thread issuing the command blocks until the command returns.
• Only one command can be given at a time. While the command is being processed, the device will reject any additional commands.

To send an AV/C command, format the command as an array of bytes. Then call the GetTransportBasicParameters method, passing in the ED_RAW_EXT_DEV_CMD flag, the array size, and the array. You must cast the array address to an LPOLESTR* type, because the original purpose of this parameter was to return a string value.

BYTE AvcCmd[] = { ... }; // Contains the AV/C command (not shown)
long cbCmd = sizeof(AvcCmd);
hr = pTransport->GetTransportBasicParameters(
    ED_RAW_EXT_DEV_CMD, 
    &cbCmd,
    (LPOLESTR*) AvcCmd);

The contents of the array are passed directly to the device, so you must be careful to format it correctly. A command can target the unit (camcorder) or a subunit (tape or camera). The relevant standards are available from the 1394 Trade Association Web site.

• AV/C Digital Interface Command Set General Specification
• AV/C Tape Recorder/Player Subunit Specification

The former describes how to format AV/C commands and lists the unit commands. The latter specification lists the subunit commands.

The GetTransportBasicParameters method may return one of the following error codes:

Note   These errors are returned as Win32 error codes, not HRESULTs, so you should test for these values directly, rather than using the SUCCEEDED and FAILED macros.

If the method returns S_OK, the response from the device is copied into the array. The response payload might be larger than the command, so you must allocate a buffer large enough to hold it. The maximum payload size is 512 bytes. Note that a return value of S_OK does not always mean the device successfully carried out the command. The application must examine the response payload to determine the status.

The following example shows the command to search for an absolute track number search:

// Set up the ATN search command.
BYTE AvcCmd[] = 
{ 
    0x00,   // ctype = "control"
    0x20,   // subunit_type, subunit_id
    0x52,   // opcode (ATN)
    0x20,   // operand 0 = "search"
    0x00,   // operand 1 = ATN
    0x00,   // operand 2 = ATN
    0x00,   // operand 3 = ATN
    0xFF   //  operand 4 = D-VCR medium type.
};
// Specify a track number.
ULONG ulTrackNumber = track_number; // Specify the track number here.
// Shift over by 1 (LSB of operand 1 is a 1-bit blank flag)
ulTrackNumber = ulTrackNumber << 1; 
// Plug this number into operands 1 - 3.
AvcCmd[4] = (BYTE) (ulTrackNumber & 0x000000FF);
AvcCmd[5] = (BYTE)((ulTrackNumber & 0x0000FF00) >> 8);
AvcCmd[6] = (BYTE)((ulTrackNumber & 0x00FF0000) >> 16);