This is my first attempt at a web site.  So things,  
will no doubt be, dynamic for a while. Change is
the only constant.
Audio :
  • Digital / Analog
    Audio Test &
  • Prototype
    Evaluation /
  • Test
  • Data Processing
    / Report

Audio Precision
System One & Two
Equipment :
Audio Precision
System 2322,
HP 54602b 4 ch
digital scope +math,
HP E3631a
supply, GPIB
A good article concerning Audio Taper :

"Notes on Audio Attenuators" at:
An Index into several Word docs containing
basic information and links relating to Audio.

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 These are audio interface switch boxes that I designed to be used with Audio Precision Test Systems I & !!.

The center area:
Allowed selecting of 1 of 8 test inputs to each of the two equipment inputs.  

Left side :
The sync. and gen. refs. are brought  up front  The gen. outs toggle with an external source like a sound card or FM tuner.

Right side:
Provides equipment  bypassing, and for sending the equipment monitor outputs or the bypass to a scope .  There is an external loop for
additional measurement filters.  All passive, just switching.
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This is a 6 channel buffer that I made
for test purposes.  Each channel had
selectable :
1-of-2 single ended inputs
One instrumentation type 40dB +
differential input.  

This is followed by a +/- dB gain stage
that drives a pair of line level outputs.
One of the outputs can be inverted to
provide a push-pull type output.  

There is also a bypass to allow setting
a reference level input.

It runs off an external +/- 9v 200 ma
supply.  So it can be used as a
channel, floating differential line
Private Sub MORE_TO_COME(Tym as Int)
Dim now as Int

While now < tym
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Audio Precision System 2322 Dual Domain
Audio Analyzer
HP 54602b 4ch Digital Storage Scope &
54657A Memory / Measurement Module
HP 3561a Dynamic Signal Analyzer
Spectrum Analyzer 125uHz to 100kHz
HP E3632A Triple Power Supply.
HP 400FL  RMS Volt Meter
The volt meter every other volt meter
wants to be.
                 Background :
This was my bench setup at Dolby.
Basically incorporated :  2 PCs, 2 APs, a 2ch A/D&D/A
fs 48kHz, analog constant power matrix encoder, also
implemented with AP in Digital Domainand 2 bit
stream swtiches as well as an analog signal switch box.

This provided a fairly flexible test environment.
More Audio ?

Go =>
More Audio
!! New Flash !!
40 year old Heathkit Scope,  still works.
And I still have it.

I took the photograph of the Scope below on 25 JUL 2008.
assembeled it, (a HeathKit), in 1969 almost 40 years ago.
My first piece of equipment.
I was in 9th grade.
I worked on it all night.
It was the first time I ever stayed up all night.
OK, here we go.  One more time.

"Takes a licking and keeps on..." !! ? Rats....,

This is a "Sinclair" not a "Timex Sinclair".

Oh well,  never mind.
I never did have their watch...
Takes a licking and keeps..?.. - !!.., never mind.
That reminds me.  ? .....
The HP-400FL has a linear dB scale.
Calibrated for 0 dBr = 0
dBv,  (little "v").
Across 600 Ohms,  0 dBr  = 0
dBu,  (1mW/600).

Solve for E as
0 dBv : E^2/R = P
E^2/600 = 0.001 : E^2 = 0.6 : E =
0 dBv is about = 0.77459666924148337703585307995648... volts
0 dBv is typically considered as 775 mV.

More about dB on my E_Notes-01 page.
The HP-400FL AC Voltmeter.
The '55 Cheve of Audio Volt Meters.
This is the Dolby standard AC voltmeter.
    The Dolby CAT-98A Noise Weighting Filter.  Similar to the CCIR-468 weighting filter.  Ray shifted
the unity gain point to 2kHz and specified RMS volts and a average responding meter.

    The "ARM", Average Responding Meter,  referees to the ballistics of the meter movement,  not the
voltage reading.  Note the small text at the bottom center of the 400FL meter face below.
The HP-400FL combined with the CAT-98A was company
standard noise measurement mechanism.
More about the CCIR-2k filter,  
schematic and AP plot,  can be
found near the bottom of my
"E_Notes_01" page.
                                                                                           It was in the Fuzz.

   Some would argue that using a scope is not necessary with an AP.  I disagree,  a simple analog scope can quickly answer measurement questions that the AP on it's own,  (without an
experienced user), would not find.

  An example of Noise Interference.
  A product test engineer and technician brought a car casseiver to my bench because they were having trouble measuring the performance of the noise reduction processor.  They had
spent several days trying to figure out why the processor appeared not to be working.  I connected it to my AP,  powered the deck up,  looked at the scope monitoring my AP and the
answer was obvious to me right off.  There was
fuzz on the scope.  

  The quiescent, no signal condition,  of Dolby NR processors is full encode or decode.  Full encode or decode means that the variable shelf of the processor is,  "all the way in", (for
sliding bands) or at "maximum boost", for "flat bands",   providing maximum boost or cut of the audio signal.  Control voltages developed by the detector circuit force the band out and
reduce the amount of processor action.   

   I explained that the
fuzz was a high frequency oscillation probably in the 100k - 200k Hz region.  The HF signal was being seen by the detector of the NR decoder as a, "relatively",  
high level,  high frequency audio signal similar to main audio program material.  This type of signal typically does not need NR processing.  The processor was behaving normally.  The
HF signal dominated the test signals and  was simply forcing the sliding band to park at it's outside limit preventing any NR action.

   I said that the HF oscillations are typically due to insufficient resistive decoupling of the test lead between the NR IC and the test equipment.   The OpAmp inside the IC, that is driving
the pin being monitored is being turned into a Phase Shift Oscillator by the capacitive loading of the test lead.  The frequency will depend on the "slew rate" of the OpAmp and the
capacitance of the test lead.  Typically a 100 Ohm series resistor at the IC pin will prevent the oscillation,  Sony chips usually have a higher "slew rate" and may require as much as 200

   Car audio products can be rather densely packed relative to products for the home market.  The engineer and technician said they had looked and there was not enough room in the
deck to install resistors at the IC and re-assemble the deck for testing.    I asked if they would leave the deck with me for a bit and I would get back with them after lunch.  I installed the
resistors and showed them that the HF oscillations were gone.  I said that as a quick check,  B-type at 2k Hz & -25dBr should give 7.0 dB of compressor action.  I showed them the 7 dB
of decode action and said it was probably OK to continue testing the product.

   I did find a parasitic oscillation from capacitive loading in a product sample and suggested that they also add a small resistance near the pin.  Two other sources of HF signals that can
fool a NR processor are tape record bias and FM pilot tone.

   Basically the AP System 2 is limited to less than 80k Hz, (except for the Jitter meter).  Without a scope,  they were blind to the source of their problem.  I used an old Tek T922 15MHz
scope to monitor my AP.   There were several T922s collecting dust in the wear house,  having been replaced by more current scopes.  I suggested they make use of one and I think
they did.   

 A lot can be learned from a single glance at an analog O-scope or spectral display.    

   My bench setup at Dolby,  typically contained an HP-3561a Dynamic Signal Analyzer, (pictured above).  The Lab purchased it for me,  so that I could test, Dolby HX Pro,  a dynamic
bias servo for tape recorders.  I needed to do time captures of record bias signal envelopes so that I could automate the calculation of time constants.  When not being specifically used,  
I just had it continuously updating the display with a test point spectrum.  A discontinuity of the display that is enough to catch the eye,  usually indicates that something needs looking
into.  That analyzer cost the Lab, about $13k in around 1986.   At that point in time there were not many options available to digitize a 100+ kHz signal,  it also provided an IEEE-488 port
that I already used with Tektronix and Panasonic ATE.   It is a simple hardware based signal processor, circa 1983.  From the human perspective it is relatively fast,  I can't think of any
other piece of audio test equipment that does FFTs so fast.  

     I never considered that I could own a 3561a until I was researching on some basic test equipment for a client.  It just happened to pop up in a listing.  I searched on HP3561a in
Goggle and the first few hits were high,  at least $5k.  But when I saw two 3561a with certification for sale at $1,500 each,  I bought one.   For the price it provides a lot of power and in my
mind,  rounds out my selection of ATE, and audio test equipment in general.   I can cover analog signals from virtually DC to several GHz and digital up to fs 48kHz at 24-bit deep.  
Again,  the ability to quickly and easily look at the signal environment can significantly reduce test time.  If provided with the proper antenna assembly this analyzer can measure down to
125uHz, ( 8,000 seconds per cycle), VLF and ULF radio signals as well as those possibly related to earth quake phenomenon.

   When I purchased the 3561,  for grins I looked to see if they had any HP-400FL AC voltmeters.   The "FL" was important because that means the main scale is in linear dB and us  
more suited to audio than linear volts.  They wanted $75,  I wanted it mainly because it is a classic.  

     The only serious test value would be for measuring noise for which a mechanical movement provides the eye with more spectral information than a segmented display can.   

     Because of the large movement it can be seen from a distance.  A sound pressure meter can be easily connected to the 400FL and placed central to a listening environment.  This
makes balancing a sound system with a noise sequencer easy and with minimum obstruction of the sound  field.  Normalizing the audio system in the listening room to the device being
tested,  was the first step in any listening test.  For matrix decoders relative level is more important than absolute level,  for noise reduction,  absolute level is critical.  For my test
purposes,  I made this
6 channel A/B switch,  (left middle of this page),  primarily for listening to multi channel decoders.  It could A/B two decoders and two program sources.  I typically
would match levels to within 0.1 dB,  more than 0.3 dB differences needed an explanation.
Above, a simple DPDT switch allowing me to send both of the AP analog and digital signal monitors
to a single scope.  Older scopes that were taken out of service worked well to monitor the AP.
Dolby CAT-98A  CCIR-2K  Weighting Filter.
     While looking for the image of a T922 as seen to the left,  the
first price that I saw for this scope was $1,195.00  !! WOW.  

     Then I found it on EBay for $75,  then ranging from about $10
to $175.  Not a lot of features.

     My main analog O-Scope is the 100 MHz Tektronix 2235
shown above.  More complex triggering and delayed time base.
Massive 16k of additional, wire wrapped, RAM.

It lasted me until I tried to calculate Pi.  

That is when Ray offered to finance PCs for
employees who wanted them at about half the
going commercial interest rate.
Tektronix T922 Scope.
      I thought that maybe a very simplified topology of a Sliding Band NR Compander may help to
understand the "Noise Interference" problem, that I mention in the text on the left.

      I just put the above drawing together a few minutes ago.  It's been a while so I may decide that I need
to change somethings.  (For the final word on all this see the link below.)  

      But the basic idea is there.  
A variable shelf RC filter using a FET as the variable resistance element.  
The FET is controlled by a filtered copy of the input signal.  
The output of the variable EQ network is summed with the input signal to produce the encoded output.
A decoder is implemented by placing an encoder in the feedback loop of an OpAmp.
A Very Basic "Sliding Band" NR Process
The Dolby Online Technical Library     
 For the final word on Dolby Audio technology,  follow this link to the Dolby Technical Library, (
http://www.dolby.com/resources/tech_library/index.cfm ).  Everything you could ever want to know
about Dolby Audio IP written by the people who invented it.  Lots of good general information relating
to both consumer and professional audio standards as well.
     This B&K VCO and Level Recorder was our standard until replaced by the
Audio Precision Equipment.   When we decided to sell them off,  I purchased the
units I had used at my bench.  Both for about $630,  with the 25dB pot as shown
as well as a 50dB and linear pot.  And just as important about 30 rolls of chart
paper.  A single roll of the size shown at the left cost us about $35 each.

     The VCO shown below is a bit more useful as it can be controlled by a
programmable power supply or a DAC.    I used the DC power supply of my Tek
5000 ATE to drive the B&K 2010 Super Heterodyne Tracking Filter so I could read
it with the Tek 5000 AC volt meter.

The cannons shown,  are used for the 1812dB dynamic range test.  For
which the speakers shown here were used.   Actually the first atomic bomb
only pushed about 240 dBSPL,  with 190 dBSPL being lethal.  If I remember
correctly,   0 dBSPL,  is 1 Joule at 1 meter.  A Joule being a watt second.  A Joule
is also the energy contained in 1/100 of a droplet of beer.  Who knew ?

 Or would you believe that my Step Father liked to photograph and make
scale models of old cannons ?  He cast brass into the basic shape of the cannon
he wanted to model,  then turned it in a metal lathe he also constructed the

     He had a Masters in Industrial Arts and taught Machine Shop, Auto Mechanics
and Printing.  He worked as both a Chemical Analysis and Mechanical Draftsman
for Bechtel.  He got his Bachelor's at Texas A&M and paid for it by running his own
photo gallery including all the lab work.  

     Along with the more common home shop tools,  we had the metal lathe a blast
furnace,  oxygen/acetaline,  drills,  taps and dies, rulers and micrometers.  We also
had a dark room with gallery camera for resizing images.   I used the camera to
make transparencies for photo etching PCBs for things I wanted to build.   I used
Ferric Chloride to etch the copper so my hands were constantly stained that brown
orange color.  

     For one of his classes,  my Step Father constructed a plasma beam torch  in
our basement.  I owned an electric powered, 12"x18" Chandler & Price Platen
Press, (similar to the one shown on the right ) and he had several offset presses.

     His major hobby was electronics and our basement was filled with piles of
boxes of surplus electronics hardware produced in Silicon Valley.  I simply had in
my basement, piles of electronic components that most kids my age,  who were
interested in electronics could only read about.  It was a simple opportunity for
open loop learning.  All the parts and answers at home.  In about 11th grade I
made flip flops and gates on PCBs,  that I also made,  and constructed a digital  
adding/subtracting machine.   

     My first in class electronics courses were in the Air Force.  I kept a 94%
average while in the Basic Electronics and then radar schools.  

      Promotions in the AF are primarily governed by two point scores.  One set of
points is the result of testing on general military and AF knowledge.  The other
score is based on testing of job specific knowledge.  I my case that was, "Ground
Based Automatic Target Tracking Radars and Associated Analog Ballistics
Computer Systems, AFSC 303X3.   I took a lot of additional correspondence
courses in electronics through the Air Training Command.  The first time that I
tested for Staff Sergent,  I had the highest combined test score Air Force wide for
all those testing for the rank of Staff.   

      It all started back in about 9th grade with a pile of electronic junk in the
basement and a HeathKit Scope.