Audio. From http://en.wikipedia.org/wiki/Word_clock :
Stand-alone synchronizers: http://www.bhphotovideo.com/c/buy/WordClock-Syncronization/ci/14835/N/4294550050 , http://www.sweetwater.com/c953--Synchronizers .
Stand-alone synchronizers: http://www.bhphotovideo.com/c/buy/WordClock-Syncronization/ci/14835/N/4294550050 , http://www.sweetwater.com/c953--Synchronizers .