splread - Read Sound Level from GM1356 meters

This tool is intended to read, and print to stdout, the current sound level from a USB-connected GM1356 sound level meter.

The GM1356 is not a precision instrument, but it's handy if you want to measure and monitor sound levels over a long period of time.

Requirements

You will need to have installed the following:

• libhidapi

For Ubuntu Linux, you can install libhidapi as follows:

apt-get install libhidapi-dev

You can simply build the tool by invoking make in the source directory. The output file, called splread is invoked with the appropriate command line arguments.

For information on invoking the binary, run splread with the -h command line argument.

I want to run this automatically!

You can install the included systemd units as a user. There are two required units - one for a FIFO that splread will feed its messages to, and one for the splread service itself. Edit them both to ensure the paths to the FIFO, the EnvironmentFile and the service ExecStart make sense for your machine.

Ediut the contents of the environment file to match your use case. There is a sample splread.env file included in the source code distribution.

Once you've done that, copy or link the files to ~/.config/systemd/user/ and invoke:

• systemctl --user daemon-reload
• systemctl --user start splread.service;

You should be good to go. Any status or error messages from splread will go to the journal for later consumption.

I keep having to run this as root!!1

Copy the file 99-gm1356.rules to /etc/udev/rules.d then go through your distribution's dance to reload udev rules. For many it will simply be:

udevadm control -R

then either unplug/replug the device or invoke

udevadm trigger

if you're too lazy to replug.

Details

I don't understand the details of the protocol super well, but I can say the following:

1. The protocol is USB-HID carried over a full-speed USB link
2. Each HID report (input and output) is 8 bytes long

There are two types of messages I've seen: CONFIGURE and MEASURE. A CONFIGURE message seems to only involve the first two bytes of the report:

1. 0x56 - indicates the CONFIGURE command
2. 1 byte of flags where:
   • 0x40 indicates we're in fast mode (bit not set == in slow mode)
   • 0x10 indicates we're measuring dBC rather than dBA (bit not set == dBA)
   • bits 0-3 are one of the enumerated supported ranges:
     • 0 == 30-130 dB
     • 1 == 30-80 dB
     • 2 == 50-100 dB
     • 3 == 60-110 dB
     • 4 == 80-130 dB
     • All others are unknown/unsupported

It seems this gets a response where the first byte is 0xc4 consistently. You should poll until you get this response, because HID messages can and will get lost in transit.

The MEASURE/TRIGGER message is as follows:

1. 0xb3 - indicates the MEASURE/TRIGGER command It seems that all other bytes are "don't care" values.

The response is as follows:

1. 2 bytes representing the sound level, in 100ths of a decibel (i.e. divide by 100 to get dB value)
2. 1 byte of flags where:
   • 0x40 indicates we're in fast mode (bit not set == in slow mode)
   • 0x20 indicates we're in hold max mode (bit not set == normal mode)
   • 0x10 indicates we're measuring dBC rather than dBA (bit not set == dBA)
   • bits 0-3 are one of the enumerated supported ranges:
     • 0 == 30-130 dB
     • 1 == 30-80 dB
     • 2 == 50-100 dB
     • 3 == 60-110 dB
     • 4 == 80-130 dB
     • All others are unknown/unsupported

Still working out what the remaining 5 bytes actually represent.