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'[EE]: MORE PIC Sound: Black vs Axtell BTC method..'
2004\07\07@203941 by James Newton

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source= http://www.piclist.com/piclist/2004/06/29/070856a.txt?

Michael Rigby-Jones  says:
> Irrespective, your suggestion was to add another pin and
> resistor to enable the shallower or steeper slopes to be
> obtained.  My suggestion was simply that by using differently
> weighted resistors, you can achieve even more variants.

My question is: What ratio should the two resisters have to maximize the
range of possible values and provide the most linear result? I've day
dreamed about this before and never really came up with anything worth
posting. Like most analog electronics, it just makes my brain hurt, but then
there are lots of things that do that!

pin connection
1 2 and value
- - -------
0 0 GND 1/1/R1+1/R2
0 Z GND R1
0 1 DIV R2/R1
Z 0 GND R2
Z Z FLOAT
Z 1 VCC R2
1 0 DIV R1/R2
1 Z VCC R1
1 1 VCC 1/1/R1+1/R2

So if you were connected to a cap, the voltage on the cap should go to:
   RAIL @ RATE
  ---- -------
0 0 GND 1/1/R1+1/R2
Z 0 GND R2
0 Z GND R1
Z Z  FLOAT
1 Z VCC R1
Z 1 VCC R2
1 1 VCC 1/1/R1+1/R2

For a total of 6 useful states.

I don't know what to say about the two "DIV" or voltage divider modes... I
guess they would tend to return the charge to a point above or below the
midpoint of GND and VCC depending?

Now, the interesting thing about this is that I think that is exactly what
Bob got in his analysis with the two resistors of the SAME value. No? But he
used a different set of 6 states!

I think the only advantage of the resistors being different values is that
it makes the two extra DIV states, but I don't see what good they are.

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2004\07\07@225903 by Bob Axtell

face picon face
Thanks for commenting on my suggestions. I thought nobody saw my posting
at all, yous is the first comment.

When I first saw the Black information in the PIClist archives awhile
back, I brushed it off as another PWB DAC scheme. Later, a client paid
me to research everything known about squeezing up audio and
regenerating sound with minimal cost and minimal memory. Being paid, I
studied the Black piece more closely- and decided that there was some
brilliance in his work there.

In NO way is Black's method just another DAC. What he is generating is
slopes. A '1' causes his BTC output to rise at an angle, and left to its
devices (no other input) the BTC output will rise to the TOP rail and
stay there. A '0' causes his BTC output to fall at an angle, and left to
its devices (no other input) the BTC output will fall to the BOT rail
and stay there. He adjusts the rise or fall so that during the interval,
the rate of change is 1/8. So to recreate a sound, his program carefully
generated a series of 1's and 0's that moved the BTC data in such a way
that the output resembled the original sound path. The concept is truly
awesome, and holds the possibility of a genuine advance.

But when I tinkered with it a little, I THINK that Black overlooked some
points:

1. The PIC is not capable of super high intervals, and the slope needed
to regenerate some sounds had to be sharper (1/3 perhaps) at some points
to recreate high frequency sounds without too much distortion.
It looks like the basic scheme is better suited for very high-speed
intervals, perhaps a UBicom chip. But,the _demand_ is for the scheme to
work at an 8Khz to 12Khz interval: i.e. PIC speeds.

2. There are times when the slope should be halted in its rise or fall.

This can be accomplished by tristating the PIC pin, but now 1-bit per
interval becomes 2 bits per interval.

3. There needs to be a way to force the signal to a nominal center
before the regeneration begins, and again at the end. I think this
problem causes some of the distortion seen.

To these ends, and many others, I have decided to create a BTC
breadboard that will allow me to explore the concept thoroughly.
There are MANY variables possible here.

The digital pot is very interesting, because I can change the R values
between intervals, which would probably eliminate a second pin, as if R
goes to R/2, the slope steepens considerably.

---

I was looking for folks to help me pay for the minimum of 5 pcs of the
PCB. I can get 5pcs done for about $140 USD, so each one is 140/5 or
about $30USD + shipping. I will lay it out.

If anybody is interested, email me and I'll send you the schematic PDF I
am considering. Its a work in progress...

--Bob



James Newton wrote:

{Quote hidden}

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2004\07\08@105741 by hael Rigby-Jones

picon face
{Quote hidden}

Having read James post, I understand what you are are getting at.  However,
I still think this is simply a semantics problem.  Firstly, it is
unequivocally a form of DAC.  It converts a digital value to an analog one.
In this case the DAC is a delta sigma converter, which was invented by
Philips in the 60's (have a look at
http://www.beis.de/Elektronik/DeltaSigma/DeltaSigma.html )

James is quite correct that with equal value resistors, two of the states
are different to the rest in that they output a voltage which is not either
Vcc or ground, and in the context of charging a capacitor this obviously
makes a difference to the end point voltage.

However, using trinary weighted resitors and you have a classic DAC.
Integrate the output of this and you can generate arbitrary slopes.  The
essential difference is that the orignal design has a current output so a
capacitor can be charged directly.  With the trinary weighted resistors the
DAC has a voltage output, so an active integrator would need to be used.

Whilst Romans implementation may be novel, at the end of the day it's simply
a low bit rate, low resolution digital audio system, and suffers from the
same problems as any such system, i.e. limited bandwidth and high harmonic
distortion.  The sample rate still has to be significantly higher than the
maximum frequency of interest, as with any system

As Roman himself states "it is basic encoding methodology, just done in a
clever way to make it very easy and fast on a PIC."

Regards

Mike

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2004\07\08@151104 by Bob Axtell

face picon face
Michael Rigby-Jones wrote:

<snipped so that I don't overburden the Piclist Server>
{Quote hidden}

I am still gonna spend some time on it. I'll let you know what happens,
Mike.

--Bob

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2004\07\09@045220 by hael Rigby-Jones

picon face
>-----Original Message-----
>From: Bob Axtell [@spam@engineerKILLspamspamCOTSE.NET]
>Sent: 08 July 2004 20:11
>To: KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU
>Subject: Re: [EE]: MORE PIC Sound: Black vs Axtell BTC method..
>
>
>I am still gonna spend some time on it. I'll let you know what
>happens, Mike.
>
>--Bob

That would be great, irrespective of the details it's still sounds like a
usefull method of producing speech at least.

Regards

Mike

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