Push-to-Talk

Control radio keying and channel access

Push-to-talk (PTT) configuration tells graywolf how to key your radio for transmission. Each radio channel has its own PTT configuration. If a channel has no output device or PTT method set to none, it operates in receive-only mode.

PTT configuration showing a serial RTS setup for the VHF APRS channel

Detect Devices

Click Detect Devices at the top of the page and graywolf scans your hardware: USB audio adapters with CM108 PTT, /dev/gpiochip* nodes on Linux, USB-serial ports, and so on. Two lists come back:

• Recommended for PTT — the best match for your hardware (e.g. an AIOC or Digirig adapter showing as cm108 on /dev/hidraw1). Click it to pre-fill the form.
• Other Detected Devices — everything else worth showing, with type and path. Pick one only if you know it’s right for your adapter. Some entries carry hints — for example, an AIOC’s serial port is annotated Use /dev/hidraw1 for PTT on this adapter; this serial port is for data — so you don’t accidentally pick the wrong one.

Detection is the fastest way to set up PTT correctly. Use Add PTT manually if your hardware doesn’t enumerate the way you expected.

PTT Methods

PTT Settings

VOX PTT

VOX (Voice-Operated Switch) keys the radio from the transmit audio itself, with no control wire. Select the vox method when your radio's VOX is enabled and you have no serial, GPIO, CM108, or CAT keying line available. No device path is required.

A VOX circuit takes a moment to close its transmit relay once it hears audio. Without a head start, the beginning of the packet — the part the receiver needs to lock onto — goes out before the radio is actually transmitting and gets clipped. To prevent that, graywolf plays a steady 500 ms lead-in tone at the channel's mark frequency immediately before every packet, giving the VOX time to engage so the full frame makes it onto the air.

Digirig Lite Tone PTT

The Digirig Lite has no serial or HID control line for PTT. Instead, its onboard detector keys the radio whenever it hears an audio tone on the right channel, while the AFSK packet audio travels on the left channel. Select the digirig_tone method to use it. No device path is required, but the channel's audio output must be routed to the Left channel of the stereo Digirig Lite device.

For every packet, graywolf emits a steady keying tone at the channel's mark frequency on the right channel for the full transmission, and prepends a 500 ms silent lead-in on the left (AFSK) channel so the tone starts slightly before the packet. That head start lets the Digirig's detector and the radio's transmit switch engage before any data goes out, so the start of the frame is not clipped. The keying tone rides its own channel and never mixes with the packet audio.

On Android

The Android app offers this as “Digirig Lite (tone PTT)” in the channel's PTT method picker. No device needs to be selected: the app auto-routes audio to the connected USB sound device and drives the keying tone itself. Unlike the desktop, there is no output-channel selector — the mapping is fixed to the Digirig Lite wiring, with the AFSK packet on the left channel and the keying tone on the right. The same 500 ms silent lead-in applies so the radio keys before the packet starts.

Serial PTT

Serial RTS or DTR is the most common PTT method. The modem asserts the chosen pin when transmitting and de-asserts it when done. Most USB sound card interfaces (Digirig, SignaLink USB) use this approach.

# Find your serial device
ls /dev/ttyUSB* /dev/ttyACM*

GPIO PTT (Raspberry Pi)

GPIO PTT is Linux-only. It targets single-board computers with exposed GPIO headers — Raspberry Pi, BeagleBone, and similar — where you can wire a GPIO line directly to your radio’s PTT input. On non-Linux hosts the gpio method is hidden from the method dropdown and the backend returns gpio ptt is only supported on Linux if forced via API.

Graywolf uses the chardev v2 API (/dev/gpiochipN). The legacy sysfs interface (/sys/class/gpio) is deprecated and not supported.

In the PTT dialog, select gpio, then pick the gpiochip device from the auto-detected list (typically /dev/gpiochip0 on a Pi). The UI then calls /api/ptt/gpio-lines and populates a line selector with the lines reported by the chip. Each entry shows the offset and the kernel’s line name (for example, Line 14 — TXD1). Lines already claimed by another kernel driver — SPI, I2C, UART, and so on — are annotated [in use: <consumer>] in the label and are rejected by the form before submit with a message like Line 2 is claimed by i2c-bcm2835. Pick another line.

When GPIO PTT is active, the consumer label graywolf-ptt shows up in gpioinfo output so you can confirm which process is holding the line. Graywolf drives the line low before releasing it, so the PTT input is left inactive when graywolf stops or the channel is reconfigured.

If you run graywolf on a Linux host where no gpiochip devices are available — for example, inside a container without a bind mount for /dev/gpiochip*, or as a user without read/write access — the GPIO method stays visible in the dropdown but the line selector reports No GPIO chips detected. Check that you have access to /dev/gpiochip* (typically the ‘gpio’ group on Raspberry Pi OS).

sudo usermod -aG gpio $USER

CM108 PTT (USB HID GPIO)

CM108-family USB audio codecs (C-Media CM108, CM108AH, CM108B, CM109, CM119) expose a HID interface alongside audio that can toggle GPIO pins for PTT. This is the method used by Digirig, AIOC, and many homebrew adapters that combine a USB sound card with PTT on a single device.

Select the cm108 method and choose the detected hidraw device. The GPIO pin selector is annotated for you: pin 3 (the default) is the most common, pin 2 is unavailable on CM108AH/B, and pins 5–8 only exist on CM109/CM119.

sudo usermod -aG plugdev $USER
sudo udevadm control --reload-rules
sudo udevadm trigger

CSMA Channel Access

CSMA (Carrier Sense Multiple Access) prevents graywolf from transmitting while another station is on the air. The TX governor checks the DCD (Data Carrier Detect) state before keying.

The p-persistence algorithm works as follows: when the channel is clear (DCD inactive), graywolf generates a random number 0–255. If it’s less than persist, it transmits immediately. Otherwise, it waits one slot_time_ms and tries again.

TX Rate Limiting

The TX governor enforces per-channel rate limits to prevent excessive transmissions. Configure these through the TX timing API: