Flex Radio Systems

1
Flex Radio Systems

• A DirectX sound-card interface in Microsoft
  Visual Basic.
• 16 bit stereo card 44 kHz Bandwidth
• For high-performance applications, it is
  important to select a card that offers a high
  dynamic range.
• Full duplex - Digital signal processing using a
  PC that allows to capture blocks of digitalized
  inputs, process those signals and return them to
  the soundcard outputs in real time.
• 2 applications programming interface
• Limitation

2
Flex Radio Systems
3
Circular Buffer Concepts

• The DirectX API internally manages all low level
  buffers and their respective interfaces to the
  sound-card hardware
• Code has to manage the high level DirectX buffers
  (Direct SoundCaptureBuffer Direct SoundBuffer)
  to provide uninterrupted operation in a
  multitasking system
• - Direct SoundCaptureBuffer (double buffering)
• Stores the digitized signals from the stereo A/D
• Converter in a circular buffer
• - Direct SoundBuffer (quadruple buffering)
• Used to output data to the D/A converters

4
Iterative Structures

• Full duplex DirectX implementation is
  accomplished in the following steps
• Installation of DirectX Runtime and SDK
• Download DirectX driver Software Development
  Kit
• Add a reference for Visual Basic type library to
  DirectX 8.0
• Register DirectX SDK with Visual Basic
• Define variables, I/O Buffers, and DirectX
  objects.
• Article provides code for the necessary variable
  declarations in general sections of code.
• Implement DirectX events and event handles.
• Events implemented in variable declaration using
  Implements keyword.

5
Creating the Audio Devices with DirectX

• Create Audio Devices
• DirectSound Objects set up format of the capture
  and play buffers
• Process
• 1. Create DirectSound and Direct SoundCapture
• 2. Check for errors
• 3. Set cooperative level DSSCL_PRIORITY
• 4. Set DirectSoundBuffer Description
• 5. Set Primary Buffer to that of Secondary
  Buffer
• Set the DirectX Event
• DirectSoundCapture Buffer Divided into two
  blocks read from one while capturing to
  the other
• Starting and Stopping Capture/Playback

6
Processing the Direct-XEvent8 and Testing

• Direct-X Event8 will be executed after the Direct
  Sound Capture Buffer started.
• Performs checking before writing to the buffer
• Automatically reset when the processor is
  overloaded
• Write the outBuffer() array to the next
  StartAddress to EndAddress
• Subroutine calls to StartTimer and StopTimer
• Testing the Driver
• Connect an audio generator
• Mute line-in on the mixer control panel to
  prevent direct audio from operating system
• Running the Sound application