A trade name of the Ampex Corporation used to describe the adding of live sound to a spare track to synchronize with recordings on a multitrack tape or film by the temporary use of other tracks of the record head for replay. Sel-Sync is an abbreviation of selective synchronization.
"Just a short note to thank you for the wonderful job you did transferring my old cassette of Japanese koto music to CD. I haven't be able to find a CD version of this tape in Japan and had stopped playing it to avoid wearing it out. Within days of sending it to you I received back an excellent CD of this beloved tape at a very low cost. Thanks ..."