Testing the USB2Rig Interface with APRSdroid

Digital interfaces are incredibly useful in modern amateur radio. They allow radios to communicate with computers and smartphones for modes like FT8, RTTY, Winlink, and APRS. One of the most well-known compact interfaces in this category is the DigiRig Mobile.

Recently I came across a device called USB2Rig, which appears to be a near-identical clone of the DigiRig concept. Curious about how well it works, I decided to try it with APRS using my Android Tablet and the APRSdroid application.

🔌 What is USB2Rig?

The USB2Rig compact digital interface

USB2Rig is a small USB audio interface designed for radio digital modes. Functionally, it behaves almost exactly like the DigiRig Mobile.

Interface Capabilities

✓ USB sound card functionality
✓ CAT/PTT control capability
✓ Audio input and output between radio and computer/phone
✓ Standard radio-specific cable connections

In practice, the cable wiring and pinout appear to be identical to DigiRig, meaning most DigiRig-compatible cables should work with it.

💡 Compatibility Note: Because of this compatibility, many existing setups designed for DigiRig can be used without modification.

🎵 Note About the Audio2Rig Model

The simplified Audio2Rig model

USB2Rig is not the only interface in this product line. There is also a simplified model called Audio2Rig.

The Audio2Rig is designed as a basic audio-only interface, meaning it provides:

USB sound card functionality
Audio input and output between the radio and computer/phone

⚠️ Important Difference: Unlike the USB2Rig, it does not include a serial interface for CAT control or PTT via serial commands.

Because of this design, the Audio2Rig appears conceptually similar to the DigiRig Lite, which focuses on simple soundcard-based digital modes without the additional serial control features.

✅ For APRS Users: For modes like APRS using APRSdroid, an audio-only interface can still work perfectly well since the application handles modulation & demodulation and PTT through the sound card.

Operators who need CAT control may prefer the full USB2Rig model instead.

🔧 Wiring Diagram

Detailed wiring diagram for USB2Rig connections

⚙️ Serial Interface Mode Selection

Similar to the DigiRig design, the USB2Rig PCB includes a 3×3 solder pad matrix used to configure the serial interface mode. This allows the interface to support different radio control standards by changing the electrical signaling levels.

The 3×3 solder pad matrix for serial mode configuration

The available modes typically include:

Mode	Electrical Level	Typical Use
CMOS / TTL	0–3.3V or 0–5V logic	Logic-level CAT control used by radios such as Yaesu, Xiegu, and some Baofeng handhelds
RS-232	±6V to ±12V	Older radios with standard RS-232 serial ports, such as some Kenwood models with DB9 connectors
CI-V	Open-collector serial bus	Icom radios using the CI-V control interface

🔧 Configuration: The desired mode is selected by bridging specific solder pads on the PCB, which routes the serial signals through the appropriate circuitry.

  Default Setting: By default, the interface is configured for logic-level (CMOS/TTL) serial, which is suitable for many modern radios that expose CAT control signals at 3.3 V or 5 V logic levels.

🧪 Test Setup

For my test, I kept the setup simple and portable.

📻 Equipment Used

📱 Android Tablet

📡 APRSdroid

🔌 USB2Rig interface

📻 Amateur radio handheld

🔋 USB OTG adapter

📲 Note on the APRSdroid Version Used

🌟 NA7Q Community Fork

For my testing, I did not use the standard release of APRSdroid. Instead, I used a community fork maintained by NA7Q, which includes additional features and improvements.

This version adds several useful capabilities, including:

Direct support for DigiRig-style interfaces
Improved hardware compatibility
Additional configuration options
Support for OpenStreetMap (OSM) tiles

✅ Perfect Match: Since the USB2Rig behaves like a DigiRig-style interface, this modified version of APRSdroid worked very well for my setup and simplified configuration.

If you are experimenting with USB soundcard interfaces or DigiRig-compatible hardware, this version of APRSdroid may be worth trying.

⚙️ Configuring APRSdroid

Setting up APRSdroid with the USB2Rig was straightforward.

Configuration Steps

1 Connect the USB2Rig to the phone using a USB OTG adapter.
2 Connect the radio cable between the interface and the transceiver.
3 Open APRSdroid.
4 Select AFSK via USB sound card as the connection type.
5 Adjust audio levels if necessary.

Once configured, APRSdroid handled the APRS modulation and demodulation using the USB audio interface.

APRSdroid Configuration Screenshot

APRSdroid configuration settings for USB2Rig

📡 On-Air Testing

After configuring everything, I transmitted a few APRS beacons.

🎯 Test Results

✓ APRS packets decoded reliably
✓ Audio levels were stable
✓ Transmissions were clean
✓ No noticeable latency issues

From a functional standpoint, the interface behaved exactly like a DigiRig-style device.

Example APRS Decodes

Successful APRS packet decodes from on-air testing

⚖️ USB2Rig vs DigiRig Mobile

Feature	DigiRig Mobile	USB2Rig
Original Design	✓ Yes	✗ No (clone-style)
USB Sound Card	✓ Yes	✓ Yes
CAT/PTT Control	✓ Yes	✓ Yes
Compatible Cables	DigiRig cables	DigiRig-compatible
Android Compatibility	✓ Yes	✓ Yes
APRSdroid Support	✓ Yes	✓ Yes
Size/Form Factor	Compact	Very similar
Price	Higher	Typically cheaper

📝 Field Notes

Practical Observations

🔌

Android detected the interface immediately as a USB audio device.

📦

No additional drivers were required.

🎚️

Audio levels were fairly forgiving, but transmit audio needed minor adjustment in APRSdroid.

⏱️

The interface remained stable during long APRS beacon intervals.

🔋

Power consumption was low enough that it worked well from a phone battery.

🎒

The setup is compact enough to fit in a small pouch, making it suitable for portable APRS operation.

🎚️ Audio Level Tuning Tips

🎛️ Critical: Proper audio levels are important when using soundcard-based packet systems.

Transmit Audio Issues

❌ Too High

Packets may be distorted
Nearby stations may not decode your signal

❌ Too Low

Digipeaters may fail to decode your packets

Recommended Approach

1 Start with the APRSdroid audio level around 50%.
2 Transmit a beacon.
3 Monitor your signal with another receiver or SDR.
4 Increase or decrease the level until the AFSK tones sound clean.

🎯 Target: The characteristic APRS tones should be clear and consistent, without clipping.

🗺️ Portable APRS Use Cases

A setup like this is surprisingly versatile. Some potential uses include:

📍

Portable APRS Beacons

🏔️

APRS During Hiking

🎒

Field Operations

🔄

Temporary Digipeater

💬

`A65KJ-7>APDR13,WIDE1-1,WIDE2-1::A61BN-10 :USB2Rig APRS test via Android{01`

APRS Messaging (No Internet Required)

  Since everything runs from a phone and a small interface, it makes a very compact digital station.

📸 Station Setup

Complete portable APRS station in action

🎒 What's in the Kit

A small portable setup like this typically consists of:

📱 Android phone running APRSdroid
🔌 USB2Rig interface
📻 Handheld radio
🔗 Short radio interface cable
🔋 USB OTG adapter

Everything can fit into a small pouch, making it easy to deploy during portable operations or experiments.

🏁 Final Thoughts

After testing the USB2Rig with APRS and examining the hardware inside, it appears to be a functional and reasonably well-designed digital radio interface. The device combines a USB audio codec, a USB-to-serial converter, and supporting circuitry behind a small USB hub, allowing it to present itself to the computer as a USB speaker, microphone, and serial port simultaneously.

From a practical standpoint, the interface worked well with APRS using APRSdroid, and the setup process was straightforward once the audio levels were adjusted properly. The hardware design includes the essential components expected in this type of interface, such as audio isolation, serial level conversion, and configurable control signaling.

While the design clearly mirrors the concept of the DigiRig Mobile, the USB2Rig appears to implement the same basic architecture using lower-cost components such as the CH340 serial interface. In terms of functionality, however, it still provides the key features needed for digital operation, including audio input/output and serial-based PTT or CAT control.

⚠️ Documentation Gap: One notable drawback is the lack of official documentation. Important configuration details—such as serial mode selection, solder jumpers, and pinouts—are mostly discovered by examining the PCB or through community discussions. For operators comfortable with a bit of experimentation this may not be a major issue, but beginners might find it confusing.

Overall, the USB2Rig proved to be a useful and compact interface for digital radio experiments, particularly for portable setups using a smartphone or small computer. With the right cables and configuration, it can support a wide range of applications including APRS, packet radio, and other soundcard-based digital modes.