[ibis-macro] IBIS-AMI Correlation and BIRD Update - comments

  • From: "Muranyi, Arpad" <Arpad_Muranyi@xxxxxxxxxx>
  • To: "IBIS-ATM" <ibis-macro@xxxxxxxxxxxxx>
  • Date: Tue, 1 Apr 2008 21:59:33 -0700

Hello IBIS-AMI experts,

I want to preface this message with a "warning" to eliminate the
possibilities of going off in a tangent of personal remarks, and
ending up hurting each other's feelings.  My intensions are NOT
to end up with a blood bath, but the seriousness of the issues I
want to raise could very easily take us there if we don't handle
the topic in a professional way.

First, I would like to comment on the presentation we saw today
in the IBIS-ATM meeting.

1)  I am fine up to pg. 9., but I have a little problem on
pg. 9.  This is really a small thing, but can be confusing
considering the big picture.  Based on pg. 8, I gather that
the meaning of the arrow pointing to the black box from above
is "this is what's inside the box".  On pg. 9, however, the
same notation seems to mean "this is the input to the black
box".  (I am saying this based on the example Tx model).  As
I said, this is a small detail, but the reason I mention this
is because this leads me to something bigger later.

2)  On pg. 10 I am missing a statement that would clarify that:

   h_teg(t) = h_cr(t) * h_tei(t)

(where "_" stands for subscript and "*" stands for convolve),
or, on the right side of the bottom half I would have used the
same expression that is found on the left side of the bottom
half of pg. 11 to better clarity.  Again, I say this based on
what I see in the example Tx model.  (You actually show that
equivalence on pg. 12, but pg. 12 is still misleading somewhat
because it gives me the impression that all of that is inside
GetWave, when in reality that top arrow is an input to GetWave).

In terms of the drawing, I am missing an arrow indicating that
the output of the Init box is fed into the GetWave box.  I would
have drawn a similar arrow that you have on pg. 11, except
pointing to the GetWave box instead of the expression on the
left of the bottom half.

3)  I admit that these are nitpicky comments and I can understand
that Todd's busy schedule may have played a major role for missing
such minor details.

4)  However, as I am studying the example Tx model's source code,
I feel compelled to bring up a serious concern I have regarding
the coding style which may have an effect on this BIRD (and the
specification).  You may say, who am I to complain about coding
style when I made such a fool of myself (somewhat deliberately)
in my last DesignCon presentation, pretending that I don't know a
thing about C programming...  Please be patient and try to hear
me out despite of that.

Here is my understanding of the structure of the example Tx model:

- the impulse response is passed to Init via a pointer variable
  along with several other parameters, including the filter tap
- the code convolves the impulse response with the tap coefficients
  and returns the results "in place", i.e. in the same memory 
  location where the impulse response came in
- in preparation for the convolution in GetWave, an integration
  is performed on the equalized impulse response to obtain a step
  response.  One could argue that this code would really belong
  in the GetWave function, but I can see it being here too, since
  it is related to the impulse response in some ways.
- NOTE: this step response is NOT returned in any of the function
  arguments to the caller.  It is just left in memory for GetWave
  assuming that no garbage collection is happening until we are
  done with GetWave.

- the stimulus waveform is passed to GetWave via a pointer variable
  along with some other parameters
- the output of Init that was left in memory is used AS THE SECOND
  INPUT to the convolution algorithm.  This second input is not
  passed into the function as a function argument "normally" as
  the other inputs are.
- additional code takes care of the block by block execution of this
  function using the same technique of leaving things in memory to
  pass left overs from a previous run as input for the next run.
- the result is returned "in place", i.e. in the same memory 
  location where the stimulus waveform came in.

What bothers me the most about this example Tx model is that not all
of the input and output arguments are passed through the function call
interface.  Stuff is going in and out of the functions BEHIND the
SCENES through the backdoors!  Don't take me wrong, I think this is
a wonderful and clever engineering marvel for situations when there
is no other way to achieve things, hats off to whoever developed it.
But we are defining a new specification, we have all the freedom
to do it right, yet we are already setting the stage for doing
things in a kludge way.

As far as I am concerned, each function should have all of their
inputs and outputs go through the function arguments (and returns).
I don't think this would have to result in memory penalty (due to
duplication of data when calling or exiting the functions) if
pointers are used appropriately.  Even for the block by block
repetitive execution of GetWave, I could see mechanisms for
passing and returning the "left overs" around the boundaries
between the calls.  Something similar to "carry out" and "borrow"
could be implemented on the function calls for that, but I could
see even better ways of doing that without having to write any
code in the GetWave function itself (to reduce the burden of the
model maker).

5)  Now, we could argue that this is just a coding style problem,
we could fix it by writing a better example Tx model.  Unfortunately
not so.  The reason being that Section 10 of the BIRD describes
each function with a precise list and description for each argument.
There are no provisions there to pass two inputs to GetWave as it
may be necessary if we use the SiSoft interpretation of how data
flows.  There are no provisions there to do a "carry out" and
"borrow" for running the GetWave multiple times for block by block
execution either.

But even more, pg. 18 of today's presentation proposes a new
parameter associated with GetWave: "Use_Init_Output".  How would
the caller of GetWave pass in the output of Init without an
additional function argument for the second input?  This is only
possible through the backdoor technique I described above!

6)  I think we should have a spec with a function interface which
provides all of the necessary inputs and outputs, so that model
makers would not need to resort to computer science back door
trickery in order to achieve the fundamental goal of this

A properly designed function interface would also make the use of
other languages easier, because the function interfaces are much
more similar between the languages than the backdoor capabilities.

I firmly believe that correcting these issues would make the life
of the model maker much easier.  People usually understand function
calls much more readily than back door tricks which rely on memory
management features of a specific language.  These types of things
are invented by experienced programmers, not electronic engineers...

In summary:

I would like to take this opportunity to clean up a little bit as

a)  Each function should have all of its arguments on the function

b)  Each function should be an independent function on its own, i.e. one
function should not depend on memory allocations in the other, other
using pointer variables in the argument.

c)  The functions should not rely on stuff left in memory, i.e. no
back door data exchange should be allowed between functions (unless
someone is a hacker, just kidding).

d)  The caller of the functions should take care of passing arguments
around from one function's output to another function's input.

e)  The caller of the GetWave function should take care of breaking
up larger data blocks into smaller pieces and executing GetWave
repetitively without relying on any code related to this in the
GetWave function itself.

There may be more (or less), but I hope you all get the point.

I hope this will not result in a bunch of virtual rotten eggs and
tomatoes thrown at me...


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