Home Entertainment Linux MP3 Player

Imagine lying back on your sofa in the living room with your remote control unit. You press the channel selection button, and a synthesized voice speaks out, “Alternative”. You keep pressing the channel-up button and the voice speaks out different music categories: “Children's”, “Classical” and others. You chose the “Classical” category and press ENTER on your remote control, and the voice starts listing the albums in the same manner. You select “John Williams, Spanish Guitar Music”, and the near CD-quality guitar music starts playing.

No, the music is not playing off some CD changer, and the synthesized voice is actually interactive, responding to your remote controller actions. The music stored is your complete CD collection, plus those old tapes (somehow still around), digitized and compressed as MP3 files. The whole show is controlled and played by a Linux server hidden in your closet, connected by an audio cable to your home entertainment amplifier.

As soon as I realized the scenario described above is already within our reach, I decided to code the missing pieces and somehow glue it all together.

The remote control unit is the central part of my design. The controller I used, called “MouseREMOTE”, is part of the “BigPicture” package that lets you transmit audio/video signals from your computer remotely to your TV. Being a “control freak”, I purchased the whole package some time ago and set up all the house lights to be remotely controlled. The remote mouse is especially useful, as it performs like any other controller for audio/video components (it can also control X-10-based home automation devices). In addition to every other button commonly found on a universal remote, it has a rubber mouse pad on the face and two buttons on the back of the unit. The controller sends RF signals when its buttons are pressed to the receiver unit which, when its buttons are pressed, has a regular computer mouse pass-through, so you can still use your regular serial or PS/2 mouse. The remote-mouse software packets will be inserted in the stream of packets sent by your regular computer mouse. Unfortunately, the MouseRemote comes with only the MS Windows software to assign actions to different keys. I installed a high-quality audio cable from the Linux sound card line-out connector in the server room down to the living room amplifier unit's RCA input.

That was all I had to do on the hardware side. For software support, I had to modify the mouse-server program to accept the codes sent by the MouseRemote unit and pipe them to an MP3 player program, which will perform different functions depending on the remote-mouse selections.

Table 1

I determined the MouseRemote specification by reading codes using the modified gpm program. MouseRemote is detected as a bare-mouse type, and it returns packets of three bytes per event. Table 1 shows the codes returned for the events. As you can see, almost all of the keys return some code, giving this remote unit tremendous potential versatility.

There are few peculiarities: mouse button codes (left/right buttons located at the bottom of the controller unit) are transmitted three times in a row. Only one button can be pressed at a time; the other one is ignored while the first one is being pressed. You can press and hold one mouse button and press any key. If you hold down mouse buttons, the codes will not start repeating; only mouse pad (mouse move) codes repeat with no apparent delay. All other keys are repeating with a short delay of approximately 1/20 of a second.

The mouse movement pad detects three levels of pressure that can be used by the mouse driver software to accelerate movement. These levels set bits 0, 1 and 2 of the second and third byte in the mouse packet. Those bytes are used to determine the difference in movement between two consecutive packets, so the firmer the pressure, the faster the movement appears.

I decided to modify mouse server gpm (version 1.13). The key function added is x10codes in the file gpm.c (see Listing 1). The function argument data is an array of three bytes making up the current mouse packet. The detection of the MouseRemote device is not deterministic. I rely on the property that all its buttons return codes in the specific range; that is, the range of the possible, but not probable, combination of codes returned by the normal mouse movement. In particular, the first code is always in the range of 44 to 47, with the third code always being 0x3F. A regular mouse would have to be moved quickly at just the right speed in order to get the same code out. In the unlikely case it happens, the second level of safety is the existence of a named pipe, /dev/x10. If the pipe does not exist, the packet will be passed on for normal processing. I used that named pipe to connect to a reader of remote-mouse codes.

Listing 1