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USB Audio Interfaces

Setting up USB audio interfaces is much easier with ASL3.

  • The USB audio interface "tune" settings have been moved into their respective configuration files, simpleusb.conf and usbradio.conf. The separate tune files (e.g. simple-tune-usb1999.conf) no longer exist
  • The device string is automatically found when the USB setting devstr = is empty
  • rxchannel=SimpleUSB/USB1999 has been changed to rxchannel=SimpleUSB/1999. Same for rxchannel=Radio/1999 for consistency with other rxchannel= settings
  • A new asl-find-sound utility can be used to help identify the compatible device strings for attached interfaces
  • We now support the ability to specify vendor and product identifiers (VID and PID) for non-natively supported audio interface chips

The asl-menu and Asterisk CLI USB config commands handle these changes.

USB Interface IC Support

Nearly all USB Interface products that are used with ASL nodes use C-Media CM108 and CM119 USB interface ICs (or their variants). These ICs provide the following features:

  • Two 16-bit DAC audio outputs, capable of driving line level or headphone outputs
  • One 16-bit ADC mic/audio input with switchable mic preamp
  • Internal hardware mixers allowing ADC input and DAC output levels to be adjusted within a range of approximately 40dB, making it easy to optimally match input and output audio levels within the ASL tune utility settings, as well as a mixer allowing input audio to be mixed into output audio with zero latency
  • GPIO lines that support a PTT output, a Status LED, a Clip LED, COS and CTCSS inputs, and one or more additional general-purpose I/O (GPIO) lines that can be controlled by ASL in response to events or DTMF commands
  • Support for an attached EEPROM IC to store user configuration and manufacturer information.

The CM119 provides an additional 4 GPIO lines (GPIOs 5-8) versus the CM108. The original CM108 and CM119 ICs were replaced in the early 2010's with the CM108AH and CM119A. These ICs are still used in many USB Radio Interfaces (URIs). Newer versions of the CM1xx ICs, the CM108B and CM119B, are now available and have some minor differences in gain settings and other specifications, but are fully compatible with the earlier IC versions and with ASL.

Supported C-Media Chips

ASL3 natively supports C-Media chips with the vendor identifer (VID) 0x0d8c with the following list of product identifiers (PID) .

Chip Product Identifier (PID)
CM-108 0x000c, 0x000d, 0x000e, 0x000f
CM-108B 0x0012
CM-108AH 0x013c
CM-119 0x0008
CM-119A 0x013a
CM-119B 0x0013
N1KDO 0x6a00

There are more variants of the C-Media chips that can be used with ASL3. If your sound card is not natively supported, you can configure ASL3 to recognize your card's vendor id (VID) and product id (PID). To determine the vendor id and product id of your sound card, use the lsusb command at the Linux CLI prompt.

You will see something similar to the following:

Bus 001 Device 003: ID 0d8c:013b C-Media Electronics, Inc. USB PnP Sound Device

The vendor identifier (VID) is 0d8c with a product identifier (PID) of 013b. This chip does not appear in the list of natively supported chips. You can enter this non-natively supported chip in res_usbradio.conf.

NOTE: You can enter multiple VID:PID pairs by separating the pairs with a comma.

Edit /etc/asterisk/res_usbradio.conf with your favorite editor. You will see the following:

;usb_devices = 1209:7388    ;comma delimited list of usb
                            ;descriptors to allow.
                            ;format vvvv:pppp in hexadecimal
                            ;vvvv=vendor id, pppp=product id
                            ;
                            ;1209:7388 = AIOC (all in one cable)

NOTE: The All-In-One-Cable (AIOC) emulates the C-Media CM-108 chip. The sample above shows the VID:PID for the AIOC. If you will be using the AIOC, just remove the ; before usb_devices.

Replace ;usb_devices = 1209:7388 with usb_devices = vvvv:dddd for your specific C-Media chip. Save your changes and restart Asterisk. The added VID:PID will now be available to the SimpleUSB and USBRadio channel drivers.

NOTE: The asl-find-sound utility will include the VID:PID pairs entered in res_usbradio.conf.

Setting Audio Levels

When setting up a new node, or if any changes have been made such as use of a different radio, microphone, other audio hardware, or gain settings, checking and setting audio levels is very important.

If you have a radio attached to your node, you are stongly encouraged to tune your audio settings (and this your transmit deviation) using a communications service monitor. Failure to do so can easily result in you modulating distorted audio, as well as potentially causing adjacent channel interference due to excessive deviation or splatter.

Radio equipment such as HTs or mobile radios, and many electronics products such as VOIP phones often restrict end-user audio adjustment capabilities to ensure consistent audio levels. AllStarLink nodes and USB interfaces however, generally allow greater flexibility and thus extra attention is required to be sure your audio interface gains are properly matched to the external audio input and output levels. Failure to calibrate your audio levels could mean your node is much quieter or louder than it should be when linked to other nodes, which could be disruptive to other users and systems. It can also mean that the signal from your RF transmitter may cause interference with other radio systems, which carries serious consequences from spectrum regulators.

Most nodes use the SimpleUSB channel driver (chan_simpleusb). Nodes that require more advanced features such as DSP squelch detection, CTCSS tone encode/decode, or additional audio filtering options may use the USBRadio channel driver (chan_usbradio). These channel drivers both have tune utilities for setting audio levels and other functions. Once a node has been configured in the asl-menu Node Settings, you will end up in the appropriate tune application. The following options should then be used to check and optimize audio level settings:

NOTE: Be sure you are not connected to any other nodes when doing any of the following tests and adjustments.

  • Set Rx Voice Level (using display) provides a level meter. Talk at a normal and steady volume into the microphone and be sure that the average level does not go past the "3KHz" point, and that peak levels do not go significantly past the "5KHz" point. Note that for proper mic technique you should talk no closer than 2 to 3 inches (5 - 10 cm) from the microphone, and it should be held at an angle to minimize pops/plosives. Once you see where the audio levels are, hit the Enter key and you'll then be prompted to change the setting. If for example the current setting is 500 and your levels were too high, try entering 400 for a new value and then repeat the test. Continue to iteratively adjust the settings until the average audio levels stay around the "3KHz" mark. For a more precise calibration with radio nodes a 1KHz test tone can be input into another radio at a level that results in 3KHz FM deviation, then calibrate the node to the 3KHz mark, however this can be more complicated to set up and may not account for other differences such as different mics or mic gain settings.
  • Set Transmit A Level, Set Transmit B Level (or Set Transmit Voice Level for USBRadio) allow the node audio output levels to be set. For nodes connected to a radio transceiver the level should be set such that the transmitted FM deviation is no higher than 3KHz, and the audio does not sound distorted or overly loud. When listening on another radio it can also be helpful to confirm the audio levels from your node sound about the same as levels on FM repeaters in your area.
  • Toggle RX Boost enables a preamp in the CM1xx for use with very low mic-level input signals. This should be disabled in almost all cases.
  • View Rx Audio Statistics shows precise audio level information. If when running this option and talking at a normal to loud volume into your mic, you see Pk levels of any higher than -3.0 dBFS, Avg Pwr levels of higher than -12 dBFS, or any ClipCnt value greater than 0, you should reduce your rxmix setting (using the Set Rx Voice Level menu option), and then repeat this step. "dBFS" is a logarithmic unit of measurement, where 0dBFS equals the Full-Scale (largest possible) value of a digital signal. Digital signals cannot exceed 0dBFS. If an Analog-to-Digital Converter (ADC) is over driven, clipping, distortion and greatly reduced audio quality results.
  • Toggle Echo Mode will record each of your transmissions and echo ("parrot") back to you. After setting your levels with the above menu options first, echo tests are the final step to confirm your audio is clear and intelligible. If anything does not sound right or you have any questions be sure to visit the AllStarLink Community for additional help. When done with Echo Mode testing don't forget to disable it. DTMF commands can be enabled to make it easy to do echo testing any time. To enable parrot mode DTMF commands uncomment the 3 lines in /etc/asterisk/rpt.conf that start with 921 =, 922 =, and 923 =. Then, restart Asterisk or reboot the node. Entering *921 on a DTMF keypad will then enable parrot mode and *922 will disable it.
  • Print Current Parameter Values shows the current audio level settings.
  • Write (Save) Current Parameter Values saves any changed settings to disk. Be sure to execute this option before exiting the menu if you have changed any settings and want those to be the new defaults.

The tune utilities can also be run from the command line without needing to use asl-menu. For SimpleUSB run sudo /usr/sbin/simpleusb-tune-menu or for USBRadio run sudo /usr/sbin/radio-tune-menu.

EEPROM Operation

The SimpleUSB and USBRadio channel drivers allow users to store configuration information in an EEPROM IC attached to the C-Media CM1xx USB Interface IC. The CM119A can have manufacturer information in the same area that stores the user configuration. The CM119B does not have manufacturer data in the area that stores user configuration. The manufacturer data cannot be overwritten. The user configuration data has been moved higher in memory to prevent overwriting the manufacturer data. If you use the EEPROM to store configuration data, you'll need to save it to the EEPROM after upgrading. Use susb tune save or radio tune save in the Asterisk CLI.