--- a/djm-fix.cpp	Mon Dec 21 16:44:39 2020 +0100
+++ b/djm-fix.cpp	Mon Jan 04 00:15:56 2021 +0100
@@ -32,9 +32,53 @@
 	std::cerr << "interrupt()" << std::endl; // TODO: do not mess STDIO
 }
 
+/**
+ * The support for Pioneer DJ DJM-250MK2 (an external USB sound card / mixer) was added to the Linux (kernel) by these patches:
+ *
+ *  - https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/sound/usb?id=73d8c94084341e2895169a0462dbc18167f01683 (playback)
+ *  - https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/sound/usb?id=14335d8b9e1a2bf006f9d969a103f9731cabb210 (recording)
+ *  - https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/sound/usb?id=cdc01a1558dedcee3daee7e1802d0349a07edb87 (mixer setup)
+ *
+ * These patches are enough for playback and for recording from post CH faders.
+ *
+ * However this mixer is somehow incapacitated and if we want to record the raw signal from the PHONO or LINE channels,
+ * we only get silence. This feature is important for DVS (Digital Vinyl Systems) setups where
+ * the timecode signal from special control vinyls flows from mixer to the computer
+ * where it is interpreted in a software like MIXXX and used for controlling the playback of files on our computer.
+ * The signal (usually music) from these files flows back to the mixer and then to speakers and headphones.
+ *
+ * To make this work and enjoy all the features of the device we have bought, we need to tell the mixer that we
+ * want the signal instead of silence on given channels. And this is the purpose of the djm-fix utility and
+ * it is done by sending some magic packet to the mixer.
+ *
+ * Implementation of this magic in the AlsaBridge.cpp file is based on publicly available documentation
+ * that can be found at <https://mixb.me/CDJHidProtocol/hid-analysis/handshake.html>.
+ * This page pointed me to the proper hash function (according to the constants, it is bit uncommon but publicly known Fowler–Noll–Vo hash function, FNV)
+ * and some magic bits. I wrote this standalone C++ program that talks with the mixer over MIDI SysEx messages and does the magic.
+ *
+ * When this program is started, it finds the mixer and makes it fully working.
+ * It needs to be running all the time, otherwise we will get silence on the PHONO/LINE channels again.
+ *
+ * Install dependencies:
+ *   apt install mercurial make pkg-config g++ libasound2-dev    # in Debian or Ubuntu (it will be similar in other distributions)
+ *
+ * Download djm-fix:
+ *   hg clone https://hg.frantovo.cz/midi/djm-fix/               # primary source
+ *   hg clone https://hg.globalcode.info/midi/djm-fix/           # or we can use this mirror
+ *
+ * Compile:
+ *   make                                                        # we can skip this step, it will be compiled on the first run
+ *
+ * Run:
+ *   make run
+ *
+ * Stop:
+ *   press Ctrl+C
+ */
+
 int main(int argc, char**argv) {
 	std::string deviceName = argc == 2 ? argv[1] : "hw:1"; // FIXME: parse CLI options + automatic device search
-	
+
 	signal(SIGINT, interrupt);
 	std::unique_ptr<djmfix::DJMFix> djmFix(djmfix::create());
 	std::unique_ptr<djmfix::alsa::AlsaBridge> alsaBridge(djmfix::alsa::create(djmFix.get(), deviceName));