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'[OT] ISD sound chip questions'
1998\04\06@235345 by Mike Keitz

picon face
On Tue, 7 Apr 1998 13:28:14 +1200 Russell McMahon <>
>>Does anyone have a circuit for electricaly coupling the
>sound output from an
>>ISD sound chip to a small, one transister FM transmitter. I
>have tried, but
>>had to settle for acoustical coupling.
>I use USD chips extensively. Electrical coupling should be
>extremely simple. Coupling from 1 speaker output pin to the
>transmitter input with a capacitor (0.1uF probably OK)
>should be enough.

The one-transistor FM transmitters with the modulation coupled to the
base of the transistor usually need only a small modulation signal.  Too
much, and the transmitter's frequency will go outside the range of the
receiver.  The raw output from the ISD chip is likely too much.  In order
to keep control the amount of modulation, a voltage divider should be

ISD chip --|  (one of the speaker pins)
  10K pot \------||---- Transmitter

The coupling capacitor from the pot's wiper is probably already on the
transmitter.  Just remove the microphone and any biasing resistor for it
and connect the ISD chip and potentiometer instead.  Connect the
transmitter ground to the ISD ground as well.  Once a level has been
found that produces acceptable results, the potentiometer and 100K
resistor can be replaced with 2 fixed resistors.  If the modulation can't
be made strong enough with the pot turned all the way up, replace the
100K resistor with a smaller one.

The speaker outputs are differential (one
>is high when the other is low) and both are always above
>ground so you don't need to use both to get a single ground
>referenced output.

Unfortuantely, on the ISD1000 (I don't know about the others), when the
chip is not playing, both outputs are forced to ground.  Using one for a
single-ended output produces a "thump" when the chip starts and stops
playing.  A high quality system will probably need to use a differential
amplifier or transformer to always take the difference between the two
speaker pins.  Since both are forced to ground at the same time, no
"thump" will come from the differential converter.

>>Also, any ways to copying the recordings from one ISD chip
>to another?
>The only way that i am aware of is at the analog level - 1
>chip is set to play back and the other records. This is not
>ideal but you do not have access to the inner workings of
>the ic.

This is generally the process.  Of course, don't use the microphone
amplifier and AGC on the destination chip.  Put a gain adjustment between
the two chips and adjust so both reordings are the same level.  You
should use the external clock feature to clock both chips at the same
rate.  The recordings should then end up at exactly the same address,
regardless of variations in the internal clock generators.  If preserving
the signal frequency content is more important, use the internal clock
generators.  The clock rate can't be increased to speed up the process
because the analog lowpass filters in the chips are not driven by the
clock, so they would excessively filter a faster-sampled signal.  ISD
mentions they sell a box to replicate a chip's content either singly or
in gang.  I didn't see any details about the "programmer"'s availability
or price.

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1998\04\07@085809 by wwl

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>Unfortuantely, on the ISD1000 (I don't know about the others), when the
>chip is not playing, both outputs are forced to ground.  Using one for a
>single-ended output produces a "thump" when the chip starts and stops
This is true of all the chips.
{Quote hidden}

There is however a subtle issue when copying chip to chip which may
be a problem in some applications. When you stop recording, record
continues to the end of the segment (50ms). There is also a delay
between starting playback and the playback actually commencing. What
this effectively means is that if you copy from chip to chip, the
copied message will tend to be one segment longer than the original.

I've found that you can do chip to chip copying at least 4 or 5 times
between chips with no noticeable drop in quality.
For the analogue interface, you need to use a differential amplifier
to avoid the abovementioned clicks at the start/end.
The cct below gives the correct level for chip-chip copy on ISD14xx

SP- --[10K]---[ -     |     |              1uF
             [ 1/2lm358 >--------[22K]----[]|--->AIN
SP+ --[10K}---[ +                       |
           |       |                  1K
          10K      |                   |
           |      GND                 GND
Note that the lm358 is assumed to be running from at least 9v, so the
5v reference is far enough from either rail for sufficient swing.
For a 5V-only system, you should be able to connect the + input of the
opamp to the junction of two 20K resistors, one to +5v, one to 0V
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