🚀 go-pugleaf

esmith@goofy.apple.com (Eric Smith) writes in
<ESMITH.90Sep11181129@goofy.apple.com>:

> 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)