Spectrum Analyzer (High Performance Forensics Version)

[Craig Maier]

Using the High Resolution Mode:


Set the FFT Size to 4,096 (or greater). If you then click on 0.67 Hz, the system will re-scale the horizontal (X) axis of the spectral graph to indicate 1,380 Hz full scale rather than 20 kHz when you run the Spectrum Analyzer. You can resolve frequency increments as small as 0.02 Hz as indicated by the settings shown on the resolution chart. To augment this capability, use the “Range” control setting set to 20 dB and the “Offset” control to zoom in on the signals of interest. This combination of features are particularly useful when trying to determine if a forensics recording is a "dub" of the original by looking for two discrete "hum" frequencies on the spectrum. For more information on tape authentication, please refer to the “How do I?” section of this user’s manual.

Important Note:
When using the Spectrum Analyzer in High Resolution mode, it will take a long time for the system to integrate. A message will appear at the bottom of the display during this interval stating “Sampling, Please Wait”. Refer to the resolution chart for latency details.
Measure the Spectrum of Either L or R Channel(s) (Tutorial)


The Spectrum Analyzer displays the algebraic sum of both channels in normal operation. If you want to obtain the spectrum of only one channel, use the following procedure:

1. Bring up the Spectrum Analyzer.
2. Set its parameters appropriate for the measurements that you wish to make.
3. Bring up the File Conversions feature found under the Filter menu.
4. Choose Stereo to Left only or Stereo to Right only, depending on the channel of interest.
5. Click on Preview, and only the chosen channel will be presented to the Spectrum Analyzer.

Measuring the %THD of an Electronic component (Tutorial)


The Spectrum Analyzer contains a special feature which allows you to measure the % Total Harmonic Distortion of a piece of electronic equipment, whether it be a CD player or an audio amplifier. It does this with a high degree of accuracy. This measurement will require, however, the use of a professional grade, high performance sound card and/or CD player in order to achieve accurate results. To achieve accurate results, it is necessary that the signal sample rate be at least 44.1 KHz in order to account for all of the Harmonic Distortion products within the audio spectrum. There are two procedures that you can choose from. The first procedure is simple to perform, but less accurate than the second procedure which is more complicated.

Spectrum Analyzer settings for %THD Measurements:

Display Mode: Fast
FFT Size: 16,384
Frequency Resolution: 2.69 Hz
Range: 150 dB
Mode: Bar or Line (your choice)
Window Type: Kaiser-Bessel
Show Peak: Check the checkbox
Show THD: Check the checkbox

Note 1: The Distortion Analyzer feature is accessed via its Options Menu.

Note 2: If you are using the High Precision Spectrum Analyzer found in the Forensics version of the software, set the Start Frequency at 10 Hz and the Stop Frequency at 22,050 Hz.

• Procedure Home (Simple Method):

1. Create a 10 minute (600 Seconds) Stereo, 16 bit, 44.1 kHz sampled 1000 Hz* Sine waveform .wav file by using the Diamond Cut “Make Waves” feature found under the Edit Menu.
2. Name the file, and make sure you know the path on which it was stored.
3. Use your ROM burner software to make a CD of this .wav file.
4. Connect the output of the CD player to the component under test’s input
5. Connect the output of the device under test (DUT) to your sound card’s line input.‡
6. Launch DCArt10 or DC Forensics10
7. From the View menu, enable the Spectrum Analyzer.
8. In the Spectrum Analyzer, enable the THD meter feature.
9. While operating in “Live” mode, play the CD.
10. You should see a dominant spectral spike at 1 kHz.
11. The Distortion Analyzer will calculate the %THD and display it in the THD meter window.
12. Done

• Procedure Forum (High Accuracy Method):

1. Perform steps 1-3 from the Procedure Home (Simple Method).
2. Connect the output of the CD player to the line input of your sound card.

1. Launch DCArt10/DC Forensics10.
2. From the View menu, enable the Spectrum Analyzer.
3. In the Spectrum Analyzer, enable the THD meter feature.
4. While operating in “Live” mode, play the CD.
5. You should see a dominant spectral spike at 1 kHz.
6. The Distortion Analyzer will calculate the CD and sound card %THD and display it in the THD meter window. Write this value down and call it %THD(I) (instrumentation).
7. Disconnect the CD player from the line input of the sound card and then plug it into the electronic device under test’s (DUT) line input.
8. Connect the output of the Device Under Test (DUT) to your sound cards line input.‡
9. In the Spectrum Analyzer, observe the THD meter feature and write down the %THD indicated. Write this value down and call it %THD(T) (total).
10. Calculate the DUT distortion using the following equation:

%THD(DUT) = (((%THD(T))^2) – ((%THD(I))^2))^1/2

* Note: Lower Frequency .wav files can be used.

‡Note: If a power amplifier is being tested, appropriate resistive loading must be connected to its output, along with an appropriate attenuator between the amplifier output and the sound card input. DO NOT connect the output of a high-output audio power amplifier directly to the input of your sound card. Direct connection of the output of an audio power amplifier to a sound card input is a hazard that could result in injury and could result in the destruction of your hardware.
Spectrum Analyzer – High Precision

(DC Forensics Version Only)

A High Precision Spectrum Analyzer is provided in the DC Forensics version of the Diamond Cut version 10 product family (which is sometimes referred to as an FFT analyzer in the Forensics field). It uses techniques including higher FFT sizes, chirp Z-Transforms, decimation methodology, a widened array of window types, waveform storage, and a new graphical display interface in order to achieve its higher level of precision compared to its standard precision counterpart. This high precision spectrum analyzer is useful for making acoustical measurements, authenticating forensics audio recordings by analyzing for potential dubs, verifying that forensics recordings have not been edited, assuring compliance with OSHA Noise Standards (both broadband as well as pure tones), performing mechanical vibration testing, measuring feedback control loop performance, and many other applications.


The High Precision Spectrum Analyzer
The High Precision spectrum analyzer is described here, highlighting the features which are different compared to that of the Standard Precision version found in DCArt10. It is highly recommended that you read the section that precedes this section before learning about the High Precision Analyzer, since many features are shared by both. Here are the salient differences provided by the High Precision Spectrum Analyzer over the Standard Analyzer:

• • • Added Zoom capability with a Start and Stop setting coupled with Frequency Axis Scrolling Capability
    • Added a Memory feature to store and recall spectrums, both Left + Right and individual Left and Right Spectrums
    • Added a Vertical Marker Measurement System
    • Added Hi-Resolution mode so that all of the FFT bins are applied to the frequency span selected by the user. For example, if you have an FFT size of 4096, and a 1 Octave span, then all of the FFT bins would be spread over just that single octave presenting the user with a very high frequency resolution display.*
    • Added Kaiser 10, Kaiser 20, and Welsh Window types
    • Increase FFT Size to 132K maximum
    • Added 1.0 Octave and 1/3 Octave Analog Filter based modes for noise and acoustical measurement applications – similar to an analog based Real Time Analyzer (RTA).

*Note: The selectivity of the High Precision Spectrum Analyzer is still limited by the basic FFT size and the Window choice.
The first difference that you may note between the DCArt10 and the DC Forensics High Precision Spectrum Analyzer is the extensive list of functions provided under the drop-down menu on the left side of the system labeled “Options”. Clicking on this menu provides you with the following options from which to choose. Option Menu (Forensics Version)

• • Show Peak Checkbox – Provides for the automatic measurement and display of the Peak Amplitude Signal level in dB and its Frequency. The two numeric values are shown in Green in the upper right hand corner of the Spectrum Analyzer. Its format is: Frequency in Hz, Level in dB. (Format is Hz, -dB.)
  • Dual Waveform Display Checkbox – When this checkbox is not ticked off, the system displays the summed average of both the Left and Right channels. When it is checked, it provides you with the ability to discretely display the Left and Right channels. The Right channel information will be displayed with a Red trace while the Left channel is displayed in Green.
  • Bar Graph Mode Checkbox – When this checkbox is unchecked, the system displays each data point with a line drawn in between. When it is checked, the data is represented in a bar graph format.
  • Standard FFT Checkbox – This applies the number of bins (FFT Size / 2) to be distributed over the entire audio spectrum.
  • Hi Precision FFT Checkbox – This mode allows the number of frequency bins to be applied strictly to the range (or span) of frequencies selected by the user. This mode effectively improves the Frequency resolution of the system when the range of frequencies selected is less than the entire audio spectrum.
  • 10 Band Octave Analyzer Checkbox – This changes the mode of operation from an FFT Based system to an IIR based simulation of an Analog, Octave frequency weighted system (like an RTA).
  • 30 Band Octave Analyzer Checkbox – This changes the mode of operation to an IIR based simulation of a 1/3^rd Octave frequency weighted system like an RTA).
  • Smooth Spectrum – This smooths the graphical display based on a set of frequency domain calculations using adjacent sides of a bin within an FFT.

Note 1: The two IIR based Analyzers are useful for measuring relative noise levels against local Environmental laws, especially pure tones. For more information about pure tones, please refer to the Glossary of Terms section of this User’s Manual.
Note 2: The 10 Band Analyzer breaks the audio spectrum down into the following center frequency buckets:
31 Hz, 62 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz
8 kHz, 16 kHz
Note 3: The 30 Band Analyzer breaks the audio spectrum down into the following center frequency buckets:

25 Hz, 31 Hz, 40 Hz, 50 Hz, 62 Hz, 80 Hz, 100 Hz, 125 Hz, 160 Hz, 200 Hz, 250 Hz, 320 Hz, 400 Hz, 500 Hz, 640 Hz, 800 Hz, 1 kHz,
1.3 kHz, 1.6 kHz, 2 kHz, 2.5 kHz, 3.1 kHz, 4 kHz, 5 kHz,
6.2 kHz, 8 kHz, 10 kHz, 13 kHz, 16 kHz, 20 kHz
Display Mode

• Fast: Shows the spectrum in almost real time with a rapid update rate.

• Slow: Shows the spectrum with slower / averaging (and leaky) ballistics.

• Averaging (On/Off): This allows the system to provide you with the average signal spectrum rather than in real time. It will integrate for as long as the file is being previewed, played, or loop-played. Its ability to respond to changes becomes slower as the integration interval lengthens. In other words, the longer that the file is played in this mode the slower its response will be and the more averaged will be its resultant display.

Amp Range (Amplitude Range)


This feature controls the Vertical Axis full scale range. You can select between the following:

• 150 dB
• 100 dB
• 50 dB
• 20 dB

Note: Lower values of dB produce greater degrees of amplitude resolution.

FFT Size


The FFT Size determines the basic frequency resolution of the system with the number of frequency bins equal to the FFT size / 2. Higher FFT sizes produce higher selectivity or frequency resolution with the tradeoff of longer integration time. You can choose between the following FFT sizes:

64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 37768, 65536, 131072

Note: Frequency resolution in terms of absolute Hertz value at a given fft size will be degraded as file sample rate is elevated. For example, a file having a certain value of frequency resolution at 4096 ffts and 44.1 kHz will have a frequency resolution twice as wide with 88.2 kHz sampled files with all other factors being equal. In other words, bins will become twice as wide at 88.2 kHz compared to 44.1 kHz with all other things being set equally.
Start and Stop Frequency Values


The Start Frequency and Stop Frequency Data Entry fields allow you to establish the horizontal axis frequency range displayed on the Spectrum Analyzer. The Start Frequency sets the left-most graphical setting while the Stop Frequency sets the right-most setting. The frequency set-ability of this system is 1 Hz for each of the two parameters. The maximum range of adjustability for Start and Stop Frequency runs from 1 Hz to half the file sample rate*. So, by way of example, using a 44.1 kHz file, you can set a Start Frequency as low as 1 Hz and a Stop Frequency as high as 22,050 Hz (which covers more than the entire audio spectrum of 20 Hz to 20,000 Hz). On the other hand, you can set a frequency range as small as 1 Hz. For example, you can set a Start Frequency of 59 Hz and a Stop Frequency of 60 Hz. It is important to note that sample theorem comes into play and the highest frequency that you can actually plot with the Spectrum Analyzer shall be the .wav file sample rate / 2.

*Note: As an example, a 44.1 kHz sampled rate file will allow for a maximum Stop Frequency of 22,050 Hz. Similarly a 96 kHz sampled file will allow for a maximum Stop Frequency of 48,000 Hz. Frequency Axis Scroll Bar


At the bottom of the Spectrum Analyzer display, you will find a scroll bar. This allows you to move horizontally along the frequency axis in the spectrum display. Use your left mouse button coupled with a drag motion, either in the left or in the right direction with your Scroll Bar.
Window Type


You can choose between the following Window types:

Bessel, Blackman, Hanning, Hamming, Kaiser 10, Kaiser 20, Kaiser-Bessel, Rectangular, Triangular, Welsh

Descriptions of the characteristics of many of these Window types can be found in the section describing the Standard Precision (DCArt10) Spectrum Analyzer.
Mem (Memory) Button


This button activates the trace storage and trace recall features of the High Precision Spectrum Analyzer. You can store a large number of Spectrum Analyzer traces in the directory of your choice. The file extension is .spt for these data. You can only display one set of recalled data at any given time along with the present display data. This feature is very useful for comparing spectral data to a reference set of data and presenting it on the same graphical display. Clicking on it (Mem) brings up the following options:

• Save Trace: Browse to the Directory of your choice.
• Save Left Trace: Same as above, but applies to the Left Trace only.
• Save Right Trace: Same as above, but applies to the right Trace only.
• Load Trace: Browse to the directory containing the trace to be recalled and click on it and it will appear in the Spectrum Analyzer Display.
• Remove Trace: Eliminates a Memory location based displayed signal trace. Clicking on this button clears the Spectrum Analyzer Display Window of the previously recalled spectral data.

Close Button


Clicking on this button closes down the Spectrum Analyzer Display Window. Hold / Run Button


Clicking on this button alternately freezes (holds) the display or runs the Analyzer in a toggle manner of operation.
Point, Click and Measure


To measure the Amplitude and Frequency of any given spectral line or signal, just point your mouse at the signal of interest and left – mouse click. The two numeric values are then displayed in Yellow in the upper left hand corner of the Spectrum Analyzer. Its format is: Frequency in Hz, Level in dB (Hz, dB).
Settings for High Accuracy Measurement of %THD with the Precision Spectrum Analyzer


Display Mode: Fast
FFT Size: 65,536
Start Frequency: 20 Hz
Stop Frequency: 20,000 Hz
Frequency Resolution: 0.67 Hz
Resolution Mode: Check “Standard FFT” in the Options Menu
Range: 150 dB
Display Mode: Bar or Line, your choice as found in the Options Menu
Window Type: Kaiser-Bessel
Show Peak: Check this checkbox in the Options Menu
Show THD: Check this checkbox in the Options Menu
Using the Spectrum Analyzer in Real Time Mode

1. 1. Bring up the Precision Spectrum Analyzer and set it up as desired.
   2. Bring up the Multifilter and remove all of the filters from its lineup or click on the “Bypass” checkbox.
   3. Click on “Live Preview”
   4. The Spectrum Analyzer will then display the frequency domain content of your signal in real time as presented to the input of your soundcard after a relatively short latency time period.