Audio Unidirectional

Description

The Audio Unidirectional application uses the SPARK Wireless Core to transmit a stereo audio stream with 48kHz/24-bit depth from the Coordinator to the Node. When the link quality degrades, it will automatically switch into fallback mode which involves an audio stream with 48kHz/16-bit depth instead. The audio stream will revert to 24-bit depth once the link quality returns to normal.

The application can either use the analog audio codec of the EVKs or the USB audio interface to receive and transmit audio data. Different compilation flags are required to select the desired audio interface. A USB audio Coordinator can communicate with an analog audio Node and vice versa.

To compile the application for USB audio, set the “USB_AUDIO_ENABLED”: “ON” flag in the application’s CMake preset cacheVariables section.

     {
         "name": "audio-unidirectional-sr11xx-quasar-usb_audio",
         "inherits": ["base","quasar"],
         "description": "Audio unidirectional example for SR11xx with Quasar USB AUDIO",
         "cacheVariables": {
             "APP": "Audio-Unidirectional",
             "TRANSCEIVER": "SR1100",
             "USB_AUDIO_ENABLED": "ON"
         }
     },

Behavior

Audio Configuration

The application can be configured using the sac_cfg.h file located in the config folder.

/* **** Main Channel Settings. **** */

#define MAIN_CHANNEL_SAMPLE_RATE_HZ 48000
#define MAIN_CHANNEL_SAMPLE_COUNT   13
#define MAIN_CHANNEL_CHANNEL_COUNT  2
#define MAIN_CHANNEL_BIT_DEPTH      24
#define MAIN_CHANNEL_LATENCY_MS     5

/* Calculated values. */
#define MAIN_CHANNEL_SWC_PAYLOAD_SIZE \
    SAC_CALCULATE_PAYLOAD_SIZE(MAIN_CHANNEL_SAMPLE_COUNT, MAIN_CHANNEL_CHANNEL_COUNT, MAIN_CHANNEL_BIT_DEPTH)
#define MAIN_CHANNEL_I2S_PAYLOAD_SIZE \
    SAC_CALCULATE_PAYLOAD_SIZE(MAIN_CHANNEL_SAMPLE_COUNT, MAIN_CHANNEL_CHANNEL_COUNT, I2S_DMA_BIT_DEPTH)
/* Size of the latency queue used by the Audio Core for the main channel. */
#define MAIN_CHANNEL_LATENCY_QUEUE_SIZE                                                                    \
    SAC_CALCULATE_LATENCY_QUEUE_SIZE(MAIN_CHANNEL_LATENCY_MS, CODEC_LATENCY_MS, MAIN_CHANNEL_SAMPLE_COUNT, \
                                     MAIN_CHANNEL_SAMPLE_RATE_HZ)

Audio Interface

The application receives audio from the line in of the Coordinator, utilizes the audio codec to transmit and receives over the I2S line using DMA transfers, and outputs the audio to the headphone out of the Node.

The audio core pipeline processes audio samples coming from the I2S to format them to be sent over the air using the SWC. The I2S end of transfer callback and the SWC transmission success callback will trigger the audio processing to synchronize the I2S production rate and the SWC consumption rate.

Audio application timings

Figure 11: Audio application timings.

Audio application processes

Figure 12: Audio application processes.

When a packet transmission fails, the audio packets will accumulate in the audio consumer queue until the wireless core can transmit it’s payloads and free a space in its queue.

Audio application timings

Figure 13: Audio application timings.

The application receives audio from the USB audio interface at either 1 kHz (USB Full-Speed) or 8 kHz (USB High-Speed) depending on the capabilities of the device.

Note

When using USB audio, the host computer controls the playback. If audio playback stops on the host computer, no audio samples are receives on the device. This means that the audio connection will not send any packets.

These frames are then accumulated and splitted into audio packets to be sent to the audio pipeline for processing before being sent OTA using the SWC. When using The USB interface, the audio samples are already formatted to packed 24-bit.

On the receiving side, the audio packets received OTA are processed and sent to the USB audio interface where a FIFO will accumulate the samples.

Note

When using USB audio, the host computer controls the recording. If the audio recording stops on the host computer, no audio samples are read from the device. This means that the USB audio transmission FIFO will not be depleted and will eventually overflow. Once the recording resumes, the FIFO will be consumed again and will stabilize to the correct level to match the expected latency.

In addition to transmitting a unidirectional audio link, the application allows for the transmission of various data such as the status of a button and the link margin. The link margin makes it possible for the Coordinator to track the quality of the connection, thereby switching between fallback and normal modes as needed.

The wireless schedule allocates 49 timeslots for the audio transmissions and 1 timeslot for the data transmissions. The schedule is designed to reserve more bandwidth for transmitting audio stream instead of data. When the Node acknowleges the data transmissions, the reply also includes the data to be transmitted from the Node to the Coordinator.

In the UWB frequency range, the wireless core alternates between 5 different frequency bands at each transmission.

Application Schedule

The following diagram illustrates the general structure of the schedule used in this application.

Audio Unidirectional schedule

Figure 14: Audio Unidirectional Application’s Schedule.

Pairing

On boot, devices attempt to pair automatically. If unsuccessful, manual pairing can be performed afterward to use the application.

Auto Pairing:

  • On boot, devices automatically start the pairing process. RGB blinks blue twice to indicate that pairing is in progress.

  • If the pairing procedure is unsuccessful, RGB blinks red twice after 10 seconds.

  • Once successfully paired, RGB turns magenta, and the devices begin exchanging application data.

Manual Pairing:

  • Press the SW1 button on both devices to start the pairing procedure; RGB blinks blue twice when doing so.

  • If the pairing procedure is unsuccessful, RGB blinks red twice after 10 seconds.

  • Once successfully paired, RGB turns magenta, and the devices begin exchanging application data.

Unpairing:

  • When paired, pressing the SW1 button again unpairs the device; RGB blinks red twice to indicate that unpairing has been completed.

  • Unpairing causes the devices to forget their learned addresses. The devices are no longer able to exchange application data.

  • To start a new pairing procedure, both devices must be unpaired first.

Note

LED behavior changes in pairing mode versus normal execution mode. During pairing mode, LEDs indicate pairing status as described below. Once paired, LEDs indicate packet transmission and reception and fallback mode status.

Auto Pairing:

  • On boot, devices automatically start the pairing process. LED0 blinks twice to indicate the pairing is in progress.

  • If the pairing procedure is unsuccessful, all LEDs blink twice after 10 seconds.

  • Once successfully paired, LEDs turn on sequentially, then turn off, and the devices begin exchanging application data.

Manual Pairing:

  • Press and hold the SW1 button on both devices for 3 seconds to start the pairing procedure; LED0 blinks twice when doing so.

  • If the pairing procedure is unsuccessful, all LEDs blink twice after 10 seconds.

  • Once successfully paired, LEDs turn on sequenstially, then turn off, and the devices begin exchanging application data.

Unpairing:

  • When paired, press and hold the SW1 button for 3 seconds to unpair the device; All LEDs blink twice to indicate that unpairing has been completed.

  • Unpairing causes the devices to forget their learned addresses. The devices are no longer able to exchange application data.

  • To start a new pairing procedure, both devices must be unpaired first.

Fallback

The Coordinator continuously monitors the link quality. If the link quality degrades, the system will enter fallback mode. If the signal quality remains strong for 5 seconds and the CCA try count stays below a defined threshold, the system will revert to normal mode. The thresholds for entering and exiting fallback mode differ, creating hysteresis to prevent the system from toggling frequently in conditions of marginal signal quality.

Clock Drift Compensation (CDC)

Clock Drift Compensation (CDC) is used to adjust the timing difference between the two devices. The Coordinator is the reference clock. Therefore, for the stereo audio stream, the Node can use its internal PLL (when hardware allows) to adjust its clock frequency to the Coordinator’s. For more information regarding CDC, refer to Clock Drift Compensation.

Coordinator User Interface

The Coordinator user interfaces includes LEDs, RGB and button switches.

Table 4: Quasar Coordinator User Interface Description

Interface

Name

Description

SW1

Pairing process

Pair, unpair, or abort the pairing process depending on the current state.

SW2

Button activity

Turn on the LED state of the remote device.

SW3

N/A

N/A

SW4

N/A

N/A

LED1

Transmission success

Toggles every time an audio packet is successfully transmitted.

LED2

N/A

N/A

LED3

Fallback status

LED is OFF in Normal mode and ON in Fallback mode.

LED4

Button state

Turn on upon receiving button state.

RGB

Pairing Event

Indicate pairing event as described in the pairing section.

Note

  • LEDs assigned to transmission or reception status are almost always on because of the high packet rate.

  • Audio volume can be adjusted by the Node, allowing to decrease or increase the sound it receives. This control is managed through the SW3 and SW4 buttons.

The Coordinator user interfaces includes LEDs and button switches.

Table 5: EVK1.4 Coordinator User Interface Description

Interface

Name

Description

SW1

Pairing process

Press and hold for 3 seconds: Pair, unpair, or abort the pairing process depending on the current state.

SW2

Button activity

Turn on the LED state of the remote device.

LED0

Audio transfers success

Toggles every time an audio packet is successfully transmitted or received.

LED1

Fallback status

LED is off in Normal mode and on in Fallback mode.

LED2

Button state

Turn on upon receiving button state.

Note

  • LEDs assigned to transmission or reception status are almost always on because of the high packet rate.

  • Audio volume cannot be adjusted using this hardware as only two buttons are available.

Node User Interface

The Node user interfaces includes LEDs, RGB and button switches.

Table 6: Quasar Node User Interface Description

Interface

Name

Description

SW1

Pairing process

Pair, unpair, or abort the pairing process depending on the current state.

SW2

Button activity

Turn on the LED state of the remote device.

SW3

Volume up

Raises the received audio volume (default at 100%).

SW4

Volume down

Lowers the received audio volume.

LED1

N/A

N/A

LED2

Reception success

Toggles every time an audio packet is successfully received.

LED3

Fallback status

LED is OFF in Normal mode and ON in Fallback mode.

LED4

Button state

Turn on upon receiving button state.

RGB

Pairing Event

Indicate pairing event as described in the pairing section.

Note

  • LEDs assigned to transmission or reception status are almost always on because of the high packet rate.

The Node user interfaces includes LEDs and button switches.

Table 7: EVK1.4 Node User Interface Description

Interface

Name

Description

SW1

Pairing process

Press and hold for 3 seconds: Pair, unpair, or abort the pairing process depending on the current state.

Volume Level

Press for less than 250 ms: Increase output jack volume level.

SW2

Button activity

Turn on the LED state of the remote device.

Volume Level

Press for less than 250 ms: Decrease output jack volume level.

LED0

Audio transmission success

Toggles every time an audio packet is successfully transmitted or received.

LED1

Fallback status

LED is off in Normal mode and on in Fallback mode.

LED2

Button state

Turn on upon receiving button state.

Note

  • LEDs assigned to transmission or reception status are almost always on because of the high packet rate.

Certification

This application has the ability to run in Certification Mode.

Different levels of certification are required to test the fallback modes and data connection.

  • Depending of which mode is requested, the max_payload_size of the audio connection will be changed.

  • The order in which connection are initialized and their priority will be swapped if the data connection needs to be tested.

Activating the Certification Mode

Table 8: Certification Mode Activation for the Coordinator

Certification Mode

Activation Process

24-bit Audio

Hold SW2 at startup until the LED 1 blinks once, then release.

16-bit Audio

Hold SW2 at startup until the LED 1 blinks twice, then release.

Control Data

Hold SW2 at startup until the LED 1 blinks three times, then release.
Table 9: Certification Mode Activation for the Node

Certification Mode

Activation Process

Control Data

Press and hold SW2 at startup. LED 1 will blink once to confirm the selected certification mode.

Terminal

The following figure shows the terminal output of this application.

See Viewing the Terminal for details on how to view the terminal output.

Audio Unidirectional Serial Interface Output

Figure 15: Audio Unidirectional Serial Interface Output (from left: Coordinator & Node).

See Audio Core API - Statistics and Wireless Core API - Statistics for more details.