Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!mcgill-vision!snorkelwacker!bu.edu!rpi!zaphod.mps.ohio-state.edu!sdd.hp.com!hplabs!otter.hpl.hp.com!otter!sgm From: sgm@otter.hpl.hp.com (Steve Methley) Newsgroups: sci.electronics Subject: Re: Digital Out (Philips CD620) Message-ID: <1770018@otter.hpl.hp.com> Date: 12 Sep 90 13:52:48 GMT References: <704@hexagon.se> Organization: Hewlett-Packard Laboratories, Bristol, UK. Lines: 39 esmith@goofy.apple.com (Eric Smith) writes in : > The reason the data is sometimes inverted is that the digital audio > interconnect standard specifically states that the polarity of the > signal is undefined, and is allowed to change (even on one piece of > equipment). Since the code is self-clocking (Biphase Mark), the > polarity doesn't matter at all. > In biphase mark encoding, there is a transition on the line between > each pair of bit cells. There is an additional transition in the > center of the bit cell for 1 bits. This is equivalent to frequency > modulation. A little confusion here, I think (correct me if I'm wrong): Data is composed of information bits and when a coding scheme eg Biphase Mark is applied _symbols_ are the result, which are transmitted along the line. In Biphase Mark, what doesn't matter is the polarity of the _symbols_. Data 1's are data 1's whether the _code_ is upside down or not. (If the polarity of the data truly didn't matter, then we couldn't transmit information at all). What sometimes happens is that the data is inverted before coding - so that the symbols with the transition in the center of the bit cell actually desribe zeros not ones. ie we transmit data(bar) not data. We have to know which this is before further processing; the code cannot help us. Some CD players, a minority, transmit data(bar), but don't make a point of saying so. This was really the point of my original posting. > To receive the code, you use a PLL to recover the clock It's surprising how often this simple approach is missed in commercial audio equipment. I could name two audio products which go to excessive lengths with non-linear elements for clock recovery, simply because they don't realise that Biphase Mark belongs to the set of comms codes which possess a discrete clock component for random data. A simple filter is all that's required. The other thing about Biphase Mark is that it has no dc component, another typical comms code property. Cheers, Steve. (my own opinions)