This is my first attempt at a web site. So things,
will no doubt be, dynamic for a while. Change is
the only constant.
|"Potz.exe" help file.
| Calculated difference between two tone bursts.
On the left, difference between the two tone bursts shown in the bottom half, is plotted as dBr/time along with a limits window on the top half of
the graph. The measurements were taken with an Audio Precision System One..
| These screens are used to qualify spectral
components of measurements made with my Audio
Precision System Two 2322.
Measured data is displayed with a background color
code indicating unexpected results.
Expected results can be displayed and the Error
information is placed in two text boxes and can be
pasted into a Word document.
| Above is my first attempt at using Visual Basic.
In this case the input is a power spectral density plot of a noise sequencer output. The program allowed placing up to 4 data tied and 2 free index
points on the screen and data. Vectors are created between selected index point and the resulting information is displayed. There are various features
for storing and using reference data.
|Predict Tape Noise Reduction Effect.
Above is the process flow diagram of a program I wrote to analyze the effect of internal processor and tape
machine noise and predict the amount of noise reduction effect produced by a noise reduction processor
without the typical record or playback circuitry.
[)(] LISTENING TESTS
Dolby Noise Reduction Processors
The diagram above is of an analog signal interface for performing the critical listening tests for noise reduction processors or ASICs.
I didn't finish this box. I got as far as the BNC connectors and signal switching. I used external buffers for level normalization.
The reference for all of the consumer Dolby NRs, were the professional NR units manufactured and sold by Dolby Labs.
The professional processors were designed to work best with signals in the 0dBv to +6dBv range, around 1 to 3 volts RMS.
Consumer processors were originally designed to operate correctly with a reference signal level of between 580mV to 775mV.
The reference level called Dolby Level for mains powered NR processors evolved down to 300mV. This allow for 20dB of headroom from a 5 volt DC
power supply. The RMS of 5 volts being about 3.5 volts and 20 dB below that is 350mV.
For NR processors used in portable battery powered cassette players, the Dolby level went down to as low as 25mV. For a Dolby B-Type NR
decoder processor, the point where the system noise starts to influence the position of the Sliding Band is about -45dBr of Dolby Level, for C-Type NR
that level drops down to more like -63dBr.
To be a little more correct, the technical definition of Dolby Level for compact cassette tape at 4.75 cm/s is, 400 Hz at 200 nW/M^2 ANSI, "high
efficiency head method".
When the noise contribution of "biased blank tape", (tape that has been erased without ever being recorded on.), is considered in the noise reduction
calculation. To obtain 10dB of noise reduction the tape noise needs to be around -53dBr of Dolby Level with a playback EQ amplifier noise of about
-56dBr. (More precise numbers can be seen on the display of a noise reduction calculator that I wrote to model the noise reduction process on a
cassette tape recorder that I used for evaluating ASICs.)
The big idea was that I needed to interface a 2 volt RMS device with a 25 mV RMS device while not exceeding combined noise floor and headroom
limits of both processors while listening to normal program material contained on standard Compact Disk. This is Critical Listening, something that is
learned, and in the case of NR processors, is not pass/fail but a judgement call on how well the DUT matches the standard. (link to online technical
library at Dolby Laboratories) => standard processor. A lot of good information there. It is a balancing act between noise floors and headroom limits
to obtain the best quality signal path for the test material.
But I was talking about CAD here, so..
| NR EFFECT PREDICTION
The image above is a link to a diagram of this program contained on my VB 6.2 page.
The single numbers above the columns of floats, correspond with the locations marked
"(DB#)", on the diagram. They are noise source arguments to the calculation.
To approve a NR processor IC for manufacturer it has to be capable of producing a
minimum amount of NR Effect in a cassette deck.
I needed to predict how a particular processor fit into the standard numbers for the class of
product that it was being developed for.
The Power Calculator.
Numeric Key Pad.
Target Value Search Tool.