From: julian@bridge.welly.gen.nz (Julian Wright)
Subject: Re: Arc sound - impedance matching
Date: 14 Mar 92 11:47:06 GMT
Reply-To: wright_j@kosmos.wcc.govt.nz (Julian Wright)

sgccreg@citec.oz.au (Rob Geraghty) writes:

The Problem:
> I have my Arc connected to the auxilliary inputs on the stereo.

The Symptom:
> The high end of the Arc sound seems to get lost in the translation

The Cause:
Low Pass filter present.

The Solution:
Tap sound output before it reaches the Low Pass filter.

The Method:

- Open your 300, 400(/1), 500, or 5000 series machine up, and find a 5x2 pin
  connector somewhere in the video circuitry. There's only one, and on the
  310's I have seen it is near the back-right corner of the circuit board.
  On the 5000 it is around the back-center of the board I think (it's the
  only such connector that I can see on the board). It looks something like:

    9 . .
      . .
      . .
      . .
    1 . .

- All the pins on the right (even numbered ones) are ground. Pin 1 is LEFT
  sound output and pin 9 is RIGHT sound output. I have no proof but guess
  that pins 3, 5, and 7 are raw R, G, and B video signals respectively.

- Attach a 3.5" socket to a short length of ribbon cable and clamp a 5x2 IDC
  plug on the other end. (Much better than soldering wires directly to the
  pins). I use pin 8 for ground but in theory any even numbered one will do)
  I assume you know the layout of a 3.5" socket 'cos I can't for the life of
  me remember it at the moment! It's in the User Guide in one of the
  appendices anyway...

- Dangle the socket out the back through one of the cracks (if there are
  any...) or better still, drill a new hole in the bottom back panel right
  next to the current headphone socket hole and mount the new 3.5" socket in
  it. You now have TWO output sockets, one filtered, one unfiltered.

- Attach your stereo to the new socket, which works equally well as a
  headphone socket or a line out.

I find that without the filter STMs sound much better, but low grade samples sound a lot more scratchy. This is understandable, as STMs are played at
29KHz, while low grade samples are less than 20KHz, usually 13 or 10KHz.

Apparently the filter is there to make the sound from the internal speaker
sound reasonable. Shame it also affects the standard external connector,
otherwise all this hacking around wouldn't be necessary.

This "upgrade" is also possible on the A3000, but you have to hook into a
couple of resister legs (I don't know which ones sorry) as the 3000 doesn't
have the 5x2 connector on the circuit board.

    Cheers, Julian.


From: s047@SAND.SICS.BU.OZ.AU (Jeremy Lee)
Date: 20 Mar 92 05:21:09 GMT


	I just checked the schematics for my A310, and there are
a number of outputs on that Aux. Audio connector inside the machine:
Filtered Left
Filtered Right
Aux In (Yes, *in*)
Unfiltered Left
Unfiltered Right
	In ALL CASES these lines are Buffered. There is therefore NO RISK
to the vidc (within reason, of course) from connecting external
equipment to this connector. That was it's purpose! The guys at acorn
are not stupid. (At least, not the hardware designers ;-)
	The present Audio out signal is taken straight from the Filtered
lines.
	The signal moves around the system like this:
	The left and right channels are independantly put through an
Op-Amp to buffer the signal and remove digital noise (With small smoothing
capacitors). The unfiltered output is taken from here. The seperate
channels then pass onto a second Op-Amp with a transitor output stage
to do what looks like basic amplification and impedance matching for
output to the headphones, presumably it does a little more filtering
here because the Filtered outputs are taken from here.
	At this stage, the outputs from the two stereo channels are
combined, and mixed with the Aux In line, send to another Op-Amp, and
this output is used to drive the internal speaker.
	You can safely take output from any point on this connector, and
moreover, you can also safely feed an audio signal into the internal
speaker!!
	Learn something new every day.

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*    . Jeremy Lee s047@sand.sics.bu.oz.au    Student of Everything    *
*   /|            "Where the naked spotless intelect is without       *
*  /_|              center or circumference. Look to the light,       *
* /  |rchimedes      Leland, look to the light" - Dale Cooper         *
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