AN2113 Freescale Semiconductor / Motorola, AN2113 Datasheet - Page 12

• Manufacturer Part Number: AN2113
• Description: AN2113 Multichannel Voice Coding System on the RTXC Operating System
• Manufacturer: Freescale Semiconductor / Motorola

System Overview
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6.2 Application Software
The application software manipulates four independent audio data streams or channels: ch0, ch1, ch2
and ch3. Each audio channel is processed on the basis of one of four user-selected modes:
To help manage this system, a C data structure is defined that contains the information necessary to
manage an individual audio channel, as follows:
The channel identification parameter, id, indicates which channel (ch0, ch1, ch2 or ch3) is defined by
the C structure parameters and points to a time slot in the ESSI frame (see Section 7.2.2) that contains the
audio data associated with the channel. The channel processing mode, mode, defines which mode is
currently used to process the channel (Off, Pass, Delay, or IS96a). The remaining elements in the
CHANNEL_CONFIGURATION C structure implement a double buffer mechanism that manages the frame
buffers in the Delay and IS96a modes. The IS-96-A voice coder is obtained from SASL.
A channel is a data stream of audio data that is accessed using the DSP56307 Enhanced Serial
Synchronous Interface (ESSI). The ESSI first initializes the hardware daughter board described in Section
6.1, Target Hardware, on page 10 and then receives and transmits the audio data sampled by the
multichannel board. The system connects to a host computer to allow a user to enter commands and
change a channel’s processing mode via a command-line interface (CLI) that provides status information
and allows configuration of the four audio channels. The Serial Communication Interface (SCI) on the
DSP56307 communicates with the host computer via an RS232 serial communication (COM) port.
Finally, the RTXC RTOS, version 3.2d, provides multi-tasking support, MIPS usage information, and
snapshot views of the RTXC state and objects (the latter two supplied via the CLI). Use of a multitasking
operating system makes it easier to integrate the various software modules (tasks, ISRs, and drivers) and
allows reuse of the code.
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Off. If the channel is Off, the received audio input sample is simply ignored.
Pass. In Pass mode, each received input is immediately transmitted without any further
processing.
Delay. In Delay mode the data received is used to fill an input ‘frame’ buffer that is then copied to
an output ‘frame’ buffer for transmission. A certain amount of delay is introduced into the audio
stream that is proportional to the size of the frame buffers being used.
IS96a. In IS96a mode the operation is similar to Delay mode except that rather than copying the
data from input frame buffer directly to the output frame buffer it is encoded (compressed) and
immediately decoded (de-compressed) by the IS-96-A voice coder.
struct CHANNEL_CONFIGURATION
{
}ch0,ch1,ch2,ch3;
int _Y id;
int _Y mode;
_fract_Y *rx_fbuff_base; // receive frame buffer
int _Y rx_fbuff_ptr;
_fract_Y *in_fbuff_base; // input frame buffer
_fract_Y *out_fbuff_base;// output frame buffer
_fract_Y *tx_fbuff_base; // transmit frame buffer
int _Y tx_fbuff_ptr;
int _Y rx_fbuff_full;
int _Y tx_fbuff_empty;
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// channel identification
// processing mode
// receive flag
// transmit flag