Date: Wed, 3 Nov 93 17:30:48 PST

There's been some discussion on the net about how the DSD does
auto-detection of the sampling frequency.  Rob has done an
extensive analysis of the various options and it is attached.

It is still my STRONG belief that there is no need for manual sample
rate control on the DSD.  In fact, it was popular demand which forced
me to work with others to create the auto-frequency elements of the
DSD design.  (My first prototype was manual sample rate control.)

However, in the interest of keeping everyone happy, here's the
information.  If it ends up that you make one of the changes described
below out of necessity (i.e. not just because you want some more neato
switches on your DSD!) please let me know.

Again, I strongly recommend building the kit as described and testing
it first before considering pursuing any of the below options.  Also,
please be nice to Rob, just like the rest of us, he's often got other
fish to fry...

Bob

-----------------------------------

The standard DSD automatically extracts the sample rate bits from the
Channel Status Block (subcode)of the incoming digital audio stream.  It
then re-encodes these bits in the Channel Status Block of the outgoing
digital audio stream.  These bits are also displayed on a pair of
LEDs.

Recently, discussion on DAT-Heads has led some DSD kit builders to
desire the ability to modify the sample rate bits.  I am still very
skeptical about there being any real need for this.  I also believe
that the standard DSD design completely meets all of its objectives,
in a very robust way.  Nevertheless, I am presenting some options for
adding manual sample rate bit control to the DSD.  Interested parties
should select an option and then let me know, and I will provide design
info as necessary.

-----

I have come up with two options that allow the auto sample rate to be
overriden.  Both options require no physical board changes.  They only
involve changing the PAL program.  The PAL supplied in the DSD kit is
electrically eraseable and reprogrammable.

Standard DSD PAL Program:

      Switch    Definition 
      ------    -------------
        S1      Bypass
        S2      Select In #3
        S3      Select In #2
        S4      Select In #1
        S5      Select In #0

Option 1:  Auto-frequency-detect is completely defeated.
Frequency must always be manually encoded with switches S2 and S3.
The priority encoding scheme used to select inputs is replaced by a
binary encoding scheme.  All four inputs are still functional but are
selected by two switches instead of four.  

      Switch    Definition 
      ------    -------------
        S1      Bypass
        S2      C24 } sample rate
        S3      C25 } subcode bits
        S4      } binary encoded
        S5      } input select
        
	    S5  S4  Input
	    --- --- -----
	    off off 3
	    off on  1
	    on  off 0
	    on  on  2

Option 2:  The Bypass feature is eliminated.  When S1 (which used to be
the Bypass switch) is ON, the DSD behaves as in Option 1, above.  When
S1 is OFF, the DSD behaves like the standard model.

Here are two other options that do involve physical changes:

Option 3:  The PAL is reprogrammed such that the FC1 and FC0 outputs of
the PAL are always tri-stated (disabled).  This is effectively like
bending the PAL pin up.  You can then connect switches and pull-up
resistors to the FC1 and FC0 inputs on the CS8402 to manually control
the sample rate subcode bits.  The switches pull the inputs to ground.
The pull-up resistors pull the inputs up to Vcc.

Option 4:  This is like Option 3 except that the PAL is reprogrammed
such that the Bypass feature is eliminated.  When switch S1 is ON, the
FC1 and FC0 outputs of the PAL are tri-stated.

Option 5:  The PAL is unchanged.  You cut the PCB traces for FC1 and
FC0 and insert switches between the PAL and the CS8402, in addition to
the switches and pull-up resistors described in Option 3.  The
advantages of this approach are: (1) the indicator LEDs will continue
to display the subcode bits from the received audi stream; (2) you
still get full Bypass and Input select functionality; you can choose
when to override the automatic frequency selection.

The disadvantage of Option 4 or Option 5 is the need to make physical
board changes.

If there is serious interest in Option 4 or Option 5, I will  mark up a
schematic and send it out.  You will need to figure out the physical
aspect yourself.  

If there is serious interest in Options 1-4, I will make the
appropriate PAL programs (Abel source and JEDED files) and put them on
the FTP site.  You will need to reprogram your own PAL.

Note that there are many many different variations on Options 1-4.
It's just a bunch of equations.  You can do whatever you want, but
there are no extra pins on the PAL, so if you want to add a switch, you
have to take away a switch.

Rob