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Plugging in the device

Table of contents :

Presentation

This document describes the study of the communication between the computer and the device when the device gets plugged into the computer. It also explains the study results.

USB frame capture workflow

In order to capture the discussion between the computer and the device, I first plugged the device into the host machine, but I let it unplugged into the Windows VM (unticked checkbox in the VirtualBox's Devices -> USB -> Pioneer DJM-850 menu).

Then, I started to capture usb frames on the host using wireshark, and I ticked the Devices -> USB -> Pioneer DJM-850 checkbox of the VM in order to connect the device to the VM. The Setting Utility program showed up on the VM, and I stopped the wireshark capture a few seconds later.

Discussion details

As we can see in the capture file, the Setting Utility starts to poll the device to gather data for the Mixer input tab.

Additionally, we can see a discussion similar to the behavior of the Mixer output tab, where the Setting Utility tells the device what are the selected options for USB output.

You can use this wireshark filter to see these requests :

usb.transfer_type == URB_CONTROL && usb.endpoint_address.direction == OUT

USB output options frames

These 5 requests are the USB output options settings : one per USB X/Y output audio source, and one for the USB output level. The device also responds to these requests by aknowledging the options configuration.

As the Setting Utility is able to send the latest configured options for USB output when it launches, it means that it is also able to store these options on the computer, in order to send them to the device for the next launch.

Configuration storage

By digging into the Windows WM, I saw that the USB output options were stored into the Windows registry at the Computer\HKEY_CURRENT_USER\SOFTWARE\Pioneer\DJM-850 key, alongside to other settings :

Setting Utility configuration storage

Here are the config details :

Parameter name Description
AsioConfigLaunch Unknown parameter.
Level Stores the USB output level option.
Output1 Stores the USB 1/2 output audio source option.
Output2 Stores the USB 3/4 output audio source option.
Output3 Stores the USB 5/6 output audio source option.
Output4 Stores the USB 7/8 output audio source option.
PosX Stores the position of the Setting Utility window on the X axis.
PosY Stores the position of the Setting Utility window on the Y axis.
Tab Stores the latest viewed tab of the Setting Utility (0, 1, 2, 3).

Note that the stored values here for the USB output options differ from the values sent to the device. I don't know why the values differ, but this is an implementation detail of the way the Setting Utility stores the config, and what really matters here is that we are now aware that this config is stored.

Implementation guide

The clone of the Setting Utility should also send the latest used USB output configuration to the device when it launches and the device is plugged in.

In order to do so, the clone should save the configured USB output options when it gets closed. We can think to a simple text file (e.g. yaml) to store this config, as it would stick to the Unix philosophy (all is file), and would also be human readable. The config file could directly store the values to send to the device. The clone would make sure that the values are valid, otherwise it would use default values (i.e. None option for USB output audio source, and -19dB option for USB output audio level). The default values should also be used in case where the config file is not present. The config file should be stored in the user's home directory, i.e. ~/.DJM-850/config.yml.