SA728F33 Full-Duplex Wireless Audio Application Board: Features and Field Range Tests

The SA728F33 full-duplex wireless audio board delivers 2 W RF power, 48 kHz audio, and 16 simultaneous talkers. Field tests at up to 2.9 km show how sample rate and concurrency affect range. Includes hardware setup, PC software configuration, and UART/I2S interfaces.

November 13, 2026

The NiceRF SA728F33 full-duplex wireless audio application board offers 2 W RF transmit power, supports audio sample rates up to 48 kHz, and allows up to 16 devices to transmit simultaneously. How do different sample rates and concurrent-transmission settings affect communication range? We conducted field tests at four distances—797 m, 1.4 km, 2.3 km, and 2.9 km. This article also explains how to connect the hardware and introduces the supporting development resources to help you get started with testing and development.

What Is the SA728F33?

The SA728F33 is a full-duplex wireless audio application board built around the SA718F33 module. The “F33” in its model number indicates 33 dBm of RF transmit power, equivalent to approximately 2 W. The board integrates analog audio interfaces, an I2S digital audio interface, a UART interface, and onboard controls. It can connect to a host controller and audio devices for full-duplex voice and wireless data communication.

Figure 1. Front and back views of the SA728F33 application board.

For an initial intercom prototype, connect a suitable antenna, power supply, and audio devices. You can then configure parameters and test communication using the PC software. After connecting a host controller, you can read and write parameters and control the module over UART. The onboard power, channel-selection, volume, and PTT buttons also make on-site operation easier.

Figure 2. Onboard controls and audio input/output connectors on the SA728F33.

What Has Improved Over the SA628F22?

Compared with the SA628F22, the SA728F33 increases RF output power and the maximum number of simultaneous transmitters, adds more audio sample-rate options, and supports AES256 encryption, 2-byte Device ID and Group ID fields, and a low-power mode with a configurable idle timeout.

Comparison | SA628F22 | SA728F33

RF output power | 22 dBm | 33 dBm

Maximum simultaneous transmitters | 8 devices | 16 devices

Audio sample rates | 8 kHz | 8, 16, or 48 kHz

Audio frequency response | 60 Hz–3.8 kHz | With LINE IN: 20 Hz–3.2 kHz at 8 kHz; 20 Hz–6.3 kHz at 16 kHz; 20 Hz–19 kHz at 48 kHz

Higher RF Power and More Simultaneous Talkers

The SA728F33 increases RF transmit power from 22 dBm on the SA628F22 to 33 dBm, or approximately 2 W. See the field test results below for communication range under different sample-rate and concurrent-transmission settings.

The maximum number of simultaneous transmitting devices increases from 8 to 16, making the board suitable for full-duplex intercom systems that need more people to speak at the same time.

Three Sample Rates for Different Audio Sources

The SA728F33 offers three selectable sample rates: 8, 16, and 48 kHz. With LINE IN, the frequency response reaches 20 Hz–19 kHz at a 48 kHz sample rate, allowing a wider-band audio source to be transmitted. For primarily voice-based intercom applications, 8 kHz or 16 kHz can be selected based on listening tests and coverage requirements.

MIC IN is intended for microphone voice input and has a frequency response of 200 Hz–3.5 kHz. LINE IN can accept external wideband audio sources when used with an appropriate sample rate.

Encryption, Device Management, and Low-Power Settings

The SA728F33 includes AES256 encryption, configurable through the PC software or the serial communication protocol. When both communicating devices enable encryption and use the same key, they can exchange encrypted voice and data.

Device ID and Group ID have been extended to 2 bytes. Device ID identifies an individual device, while Group ID defines a communication group. Together with Audio-Target ID and Source ID, these fields allow users to select transmission targets and accepted sources for multi-device communication. Device ID can be set from 1 to 65535. Setting a target or source ID to 0 addresses all devices within the group. The concurrency setting refers to the number of devices transmitting at the same time; it is different from the range of available Device ID values.

Low-power mode can be enabled or disabled using the low-power switch. Idle Timeout determines how long the device waits without transmit or receive activity before entering low-power mode.

Figure 3. The Extend tab in the PC software, showing low-power mode, Idle Timeout, and AES256 encryption settings.

SA728F33 Field Range Tests

The tests were carried out in an area with roads, vehicles, and nearby buildings. We observed communication performance at four distances: 797 m, 1.4 km, 2.3 km, and 2.9 km.

Figure 4. Environment at the field test site.

Test Equipment and Settings

Item | Setting

Test devices | Two SA728F33 boards

Antenna | SW433-ZB172, mounted vertically

Test frequency | 440.125 MHz

Transmit power setting | Power Level 7

VOX | OFF

MAX-Concurrent-Num | 1, 2, 4, 8, 16

Sample Rate | 8, 16, 48 kHz

In the tables below, “Concurrent TX Limit” refers to the MAX-Concurrent-Num setting. A value of 1 allows only one device to transmit at a time and corresponds to “Half_duplex” in the software. Two modules were used in these tests to evaluate communication under different maximum-concurrency and sample-rate settings.

Figure 5a. Test distance: 797 m.

Figure 5b. Test distance: 1.4 km.

Figure 5c. Test distance: 2.3 km.

Figure 5d. Test distance: 2.9 km.

Communication Results at the Four Test Points

At 797 m and 1.4 km, communication was checked in all directions. At 2.3 km, communication was assessed after selecting a favorable orientation. The 2.9 km point was used to check reception at the range limit. The table below records normal communication, interruptions, and reception outcomes for each setting.

Concurrent TX limit | Sample rate | 797 m | 1.4 km | 2.3 km (oriented) | 2.9 km

Long-Range Performance at 48 kHz

At 797 m and 1.4 km, communication was normal at 48 kHz with concurrency limits of 1, 2, 4, or 8. At 2.3 km, communication remained normal with limits of 2 or 4 after selecting a favorable orientation.

When transmitting an external wideband audio source, 48 kHz with a concurrency limit of 2 or 4 is a practical starting point. Test communication while moving or turning the devices within their intended installation locations and operating range.

Choosing a Sample Rate for Higher Concurrency

At the 2.3 km test point, all concurrency settings at 8 kHz and 16 kHz communicated normally after selecting a favorable orientation, including the setting that allows up to 16 simultaneous transmitters. These two sample rates can be prioritized for long-range intercom applications requiring higher concurrency.

Receiver sensitivity for the different sample-rate and concurrency settings is shown below.

Concurrent TX limit | 8 kHz sensitivity | 16 kHz sensitivity | 48 kHz sensitivity

2 | −113.5 dBm | −113.5 dBm | −113.5 dBm

4 | −113.5 dBm | −113.5 dBm | −110 dBm

8 | −111 dBm | −110 dBm | −107 dBm

16 | −110 dBm | −105 dBm | −102.5 dBm

With a concurrency limit of 2, receiver sensitivity is the same at all three sample rates. At limits of 8 or 16, lower sample rates correspond to better receiver sensitivity. For systems requiring more simultaneous talkers, first choose the necessary concurrency limit, then compare the audio quality and field coverage at 8 kHz and 16 kHz.

Reception at the Range Limit and Coverage Verification

At 2.9 km, occasional reception was observed with two configurations: a concurrency limit of 16 at 8 kHz and a limit of 8 at 16 kHz. See the table above for the observed behavior.

Interfaces and Hardware Connections

For an initial test, connect a microphone and speaker to each of two SA728F33 boards and verify two-way voice communication. You can then add an external audio source, host controller, or I2S audio device as required. All pin numbers below refer to the SA728F33 application board.

Power Supply and Basic Intercom Connections

Figure 6. SA728F33 application board pin numbers and connector locations, corresponding to the wiring table below.

Connect a suitable antenna, power supply, microphone, and speaker to each application board. If you use the pin headers, refer to the wiring table below.

External component | SA728F33 connection | Wiring

Power supply | Pin 14 VCC; pin 15 GND | Connect the supply positive to VCC and negative to GND. Supply range: 3.3–5.5 V.

Microphone | Pin 5 MIC+; pin 6 MIC− | Connect the microphone positive and negative leads, respectively.

8 Ω, 2 W speaker | Pin 3 OUTP; pin 2 OUTN | Connect the two speaker terminals to OUTP and OUTN.

External PTT button | Pin 24 PTT; GND | Connect the button between PTT and GND. Pressing it pulls PTT low to enter transmit mode.

Antenna | Onboard antenna connector | Connect an antenna matched to the operating frequency band.

Figure 7. Typical SA728F33 connections, including power, speaker, and MCU/button-control wiring.

The board has an onboard PTT button that can be used directly for bench testing. At a 4.2 V supply, RF output is approximately 1.5 W; at 5 V, it is approximately 2 W. Actual output depends on frequency band and supply conditions. When selecting the power supply, check the power and current table for the intended operating conditions and allow adequate margin for transmit-current peaks in the power circuit and wiring. The listed currents are average values; peak current is more than twice the average.

Configure and Test Intercom Communication with the PC Software

After completing the wiring, connect both application boards to a computer using USB cables. Select and connect to the corresponding serial port for each board in the PC software. You can first click DEFAULT on both boards to restore the default parameters, then apply the desired intercom settings.

Figure 8. Basic PC software settings. The screenshot shows a half-duplex, 8 kHz example; use the table below for a direct connection between two boards.

For the first direct communication test between two boards, use the following settings.

Parameter | Setting on both boards

Working Mode | Node

Max Hops | 0 NoMesh

MAX-Concurrent-Num | 2

Sample Rate | 16 kHz

Group ID | Set the same value, e.g., 0001

Channel and frequency | Use the same channel and confirm its corresponding frequency matches.

Audio-IN | MIC&Line In

Audio-OUT | Line Out

VOX | OFF; use PTT to control transmission

Source ID / Audio-Target ID | ALL (0000)

Priority | Set both to Disable

AES256 Enable | For the first test, set both to OFF. To test encrypted communication, set both to ON with the same key.

These settings provide a convenient way to check basic intercom operation. The sample rate and concurrency limit can be adjusted afterward. After configuring the parameters, click SET and confirm that “Successfully saved” appears so the settings are retained after power-off.

Press and hold PTT on one board and speak. Check the speaker output on the other board, then reverse the direction and test again. With a concurrency limit of 2, you can also hold PTT on both boards simultaneously to verify two-way voice communication. Then compare audio performance and coverage at 8, 16, and 48 kHz as needed.

When selecting a sample rate and concurrency limit, consider the number of simultaneous talkers, audio performance, coverage, and communication latency together. Typical SA728F33 latency values at concurrency limits of 2, 4, 8, and 16 are 281, 365, 525, and 992 ms, respectively. These figures can help guide the setting selection.

For applications using Mesh relays, configure the parameters according to the number of hops and the actual number of simultaneous transmitters shown in the PC tool.

Connecting an External Analog Audio Source

To test music or other external audio sources, connect the source output to pin 10 (LINEIN) and pin 11 (GND), and set Audio-IN to Line In. On the receiver side, use the onboard LINE OUT connector or connect external audio equipment to pin 8 (LINEOUT) and pin 9 (GND). The board has LINE IN and LINE OUT jacks, so you can use either the jacks or the pin headers depending on your audio equipment.

The maximum undistorted LINE IN input level is 1.2 Vrms under the condition of maximum volume on the receiving device. After connecting an external audio source, start with a low output volume and increase it gradually. For wider-band audio, set the sample rate to 48 kHz; the LINE IN frequency response at this setting is 20 Hz–19 kHz.

Connecting a Host Controller via UART

A host controller can configure the board through UART. Cross-connect the transmit and receive lines and connect the grounds together.

Host controller | SA728F33 application board

UART TX | Pin 25 RXD

UART RX | Pin 26 TXD

GND | GND

The UART signal level is 3.3 V. The default serial format is 115200 bps, 8 data bits, no parity, and 1 stop bit. Commands are sent as hexadecimal bytes. The following commands can be used for initial debugging.

Operation | HEX bytes to send

Read firmware version | AA FA 08

Read basic parameters | AA FA 01

Save current parameters | AA FA 23

Start by reading the firmware version and basic parameters to confirm that the serial connection works. You can then implement parameter-setting functions according to the communication protocol. After changing parameters, send the save command. A successful response is 4F 4B 0D 0A, representing “OK” followed by carriage return and line feed.

I2S Audio Signal Directions

The I2S interface can be used when an MCU, DSP, or audio codec needs to process digital audio. Clock and data signal directions on the application board are as follows.

SA728F33 signal | Pin number | Direction and connection

MCLK, LRCK, SCLK | 16, 17, 18 | Clock outputs from the application board; connect to the corresponding clock inputs on the external device.

SDIN | 19 | Audio data input to the board; connect to the external device’s data output.

SDOUT | 20 | Audio data output from the board; connect to the external device’s data input.

GND | GND | Connect to the external device’s ground.

I2S signals use 3.3 V logic levels. For digital audio input, set Audio-IN to IIS (the software label for the I2S option); for digital audio output, set Audio-OUT to IIS. Configure the external device’s bit depth, data alignment, and clock ratios according to the I2S format supported by the firmware in use.

NiceRF provides the SA728F33 product datasheet, PC configuration software, software user guide, and serial communication protocol. These resources support hardware connections, audio communication testing, parameter configuration, and host-controller development.

Development resource | Main purpose

SA728F33 Datasheet V1.0 | Electrical specifications; power and current consumption; audio performance; pinout; typical connections; and mechanical dimensions.

SA7x8_PC_V1.0 Software | Configure frequency, power, concurrency limit, sample rate, audio interfaces, IDs, low-power settings, and encryption; test text-message data transmission and reception.

SA718/SA728 PC Software User Guide V1.0 | Step-by-step instructions for connecting devices, setting parameters, saving settings, and testing.

SA7x8 Communication Protocol V1.0 | Develop UART parameter reading/writing, device control, and wireless data transmission and reception.

You can first use the PC software to test the device and determine the required operating parameters, then implement parameter read/write, device control, and data transmission/reception functions in the host-controller program using the communication protocol.