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Intro to the six basic audio measurements - Part 1

Posted: 04 Dec 2007 ?? ?Print Version ?Bookmark and Share

Keywords:FFT analysis? basic audio measurements? DUT? DSP?

By David Mathew
Audio Precision

Much audio testing is concerned with a small number of performance benchmarks, which we call the Big Six measurements: Level, frequency response, THD+N, Phase, Crosstalk and SNR.

We'll start out by looking at what testing is required for different audio devices, and we'll choose a device to test and a signal path. Then we'll discuss basic setup considerations. Then we'll look at each of the six measurements in detail and make each of the measurements using analyzer reading meters and one frequency sweep.

For simplicity, we'll make all the measurements in the analog domain, and we'll test a common audio device. The concepts and approaches shown here can be extended to the digital domain, to cross-domain measurements and to other types of audio equipment.

While these basic approaches will often be all that is required, understanding their principals will provide an excellent framework for working with faster and more informative techniques such as FFT analysis, multitone testing and the continuous sweep (log chirp) method used in the APx Series of analyzers.

DUT and signal path
The equipment you want to test may be a receiver for a home theater, an audio power amplifier or a DVD player, or one of hundreds of other devices that require audio testing.

DUTs ! When discussing a measurement, we refer to the equipment to be measured as the DUT.

Signal paths, connections and more ! Different DUTs may require different signal paths. Let's look at the signal paths associated with the three types of devices mentioned above. For example:

? A home theater receiver has many inputs and outputs, and you must choose which you are going to test. Both the inputs and the outputs may be analog or digital.

? An audio power amplifier has both inputs and outputs. The inputs may be analog or digital, and the output is invariably analog.

? A stand-alone DVD player has no audio inputs, only outputs. The audio outputs may be carried as analog audio or as digital audio.

For the examples in this technote, we will use a home theater receiver as a DUT. This receiver has many inputs and outputs, and we have chosen to test the path from the CD Left and Right analog inputs to the Left and Right power amplifier outputs.

In most cases, DUTs with different signal paths will be tested using very similar techniques, simply reconnecting cables or using the digital domain generator or analyzer. Playback-only devices (such as a stand-alone DVD player) require discs or other media with pre-recorded test signals and external sweep or external source measurement techniques.

Connecting the DUT to the analyzer ! Most professional, industrial and broadcast audio devices use balanced analog inputs and outputs; consumer analog equipment is typically unbalanced. Whether or not your DUT has balanced or unbalanced inputs or outputs will determine your selection of generator and analyzer connections and the type of cables you must use.

Our home theater receiver has unbalanced inputs, but its power amplifier outputs are balanced. This is not always the case, but power amplifier outputs are often balanced, even in consumer devices.

Using terminating loads ! Certain DUTs must have their outputs terminated in a specific load impedance to perform as designed or to match specified measurement conditions. An obvious example is the power amplifier, which in use must deliver its output voltage at the current drawn by its load (the loudspeaker). For amplifier measurement, the loudspeaker is typically replaced by a power resistor of the specified resistance, usually 8Ω.

Choosing a measurement level ! Most audio equipment has a nominal operating level within a few decibels of 1Vrms. Specialized equipment such as microphone preamplifiers or high-power amplifiers are designed to operate well below or above this range, and certain tests may require very low or very high levels.

Our home theater receiver has the typical 1V nominal input levels, and power amplifier stages that can deliver up to 100Vrms at each speaker output.

Choosing DUT gain and effects settings ! Some DUTs have no settings at all, just input and output connections. Others may have gain controls, equalization or bass and treble controls, even surround and reverberation effects accomplished using internal DSP. These settings will affect the measurements you make, so you must be careful to set them properly for testing (usually by disabling them).

The gain or volume control is typically set to a nominal operating level, and other effects and controls are set to their OFF or NEUTRAL positions. Other gain settings may be specified or necessary for certain tests, and in rare cases there may be a reason to set other controls or effects to ON. See the section Gain considerations for level measurements below for more information.

We will connect our DUT as shown here:

The equipment you want to test may be a receiver for a home theater, an audio power amplifier or a DVD player, or one of hundreds of other devices that require audio testing.

Setup considerations
So, let's set up our home theater receiver. We will use this setup (with one minor change for input/output phase) for making all the measurements.

Connecting the DUT in the measurement signal path ! We have chosen to use the CD inputs in the measurement signal path for our receiver. These are unbalanced RCA jacks, and we're assuming they have a typical nominal operating level of 1 Vrms. We will connect these to our instrument generator unbalanced analog outputs.

We will measure our DUT at its power amplifier outputs (speaker connections), using the left and right channels. These are available as spring clips for connecting speaker cable. For our test we will ignore the surround and subwoofer channels. We will connect the left and right outputs to a pair of 8-Ω power resistors as a terminating load, and will also connect parallel lines from the resistors to our instrument analyzer balanced analog inputs.

Setting the DUT controls ! For this part, refer to the following:

  1. Turn the DUT ON.

  2. Turn the DUT Volume control to minimum.

  3. Select CD. (Our signal path choice).

  4. Set DSP to OFF. (In our DUT, other choices were Stadium, Theater, Club, etc. These all add processing that would adversely affect measurement.)

  5. Set SURROUND MODE to STEREO. (In our DUT, this bypasses surround processing for a stereo input).

  6. Set BASS and TREBLE to 0.

Setting up the analyzer control software ! Turn on the analyzer, and launch the AP2700 or ATS control software.

  1. If you have already been working in the control software, open a New Test (this step ensures that all the analyzer default settings are in place).

  2. Double-click the title bar of the Analog Generator panel to open the larger panel display.

  3. Set Configuration to unbalanced.

The Big Six measurements
This section provides step-by-step instructions to make the Big Six measurements using an Audio Precision analyzer and our home theater receiver DUT.

The instructions are written assuming the use of a SYS-2722 audio analyzer. Where instructions for the ATS-2 differ, notes are added in [brackets].

Level
About Level measurements ! Each DUT may have a number of level measurements that are of interest. You must choose which level you are seeking. Target levels include

  • an input level that produces a given output level, such as 1V, or 1W, or unity gain (see below for a discussion of DUT gain);

  • an input level that produces a certain output distortion, such as 1 percent THD+N;

  • a level that provides good noise performance with comfortable headroom, often called the operating level;

  • an input or output level specified in a testing document.

Any of these levels may be used as a reference level on which we can base further measurements. Frequency response measurements, for example, are expressed relative to the level of a mid-band frequency; THD+N measurements are made at specified levels, which should be reported in the results.

Gain considerations for level measurements ! The ratio of a DUT's output voltage level to its input voltage level is the voltage gain of the DUT. For example, in a DUT with a gain of 2, an applied input of 2V will produce an output of 4V. A gain of 1, where the output voltage equals the input voltage, is called unity gain.

Some DUTs offer no gain adjustments, and are said to have fixed gain. The gain may be fixed at unity, or at some other value.

Measuring level in variable gain DUTs ! A DUT with a volume control or other setting that affects gain is a variable gain device. When setting and measuring level, it is essential to consider whether or not the DUT gain is variable (not only volume controls, but tone controls and other settings can change gain), and, if it is, how to set the DUT controls for the desired test results.

Making Level measurements ! We will make level measurements using three different methods that produce results that are commonly used in audio specifications.

With a 1Vrms applied signal, we will set the volume control to the position that produces 1Vrms at the speaker outputs (actually, across the 8Ω terminating load resistors). Then we will readjust the volume control to produce 1W in the load, and then we will drive the amplifier into distortion to find the level in watts at 1 percent THD+N.

Hint: making very small level adjustments ! Our DUT has a stepped volume control, with steps of a few decibels. Very fine adjustment is not possible. To find the precise level where the DUT clips, we use a Bar Graph control in the AP software to make small adjustments in Generator level.

Here's how: first find the highest distortion less than 1 percent, using the DUT volume control. Then set up a Bar Graph to control the generator level, using very small steps.

Create a bar graph by right-clicking in the Amplitude setting field. Then set the Increment to 1mV in the Bar Graph Setup dialog. Slide the bar control with the mouse, or click in the bar to engage the keyboard arrow keys. Click the arrow keys to adjust the level in these small steps until you find the highest THD+N reading you can get that is less than 1 percent.

Initial setup ! Start with the DUT and control software setup instructions listed in Section 2.

Adjust DUT for Unity gain ! To do this, do the following:

? Turn the Generator outputs ON. With the default New Test settings, this will output a 1kHz sine wave at a level of 1Vrms.

? Observe the Level meters on the Analog Analyzer [ATS-2: Analyzer: Audio Analyzer], and slowly increase the DUT volume until you have a reading of about 1 V. Since our input is 1 Vrms, this volume setting produces unity gain.

Adjust DUT for 1W ! This is simple. First, on the Analog Analyzer [ATS-2: Analyzer: Audio Analyzer], drop down the units menu for each of the Level meters, and select the watts units. Next, observe the Level meters on the Analog Analyzer [ATS-2: Analyzer: Audio Analyzer], and slowly increase the DUT volume until you have a reading of about 1 W.

Adjust DUT for 1 percent THD+N ! Double-click the title bar of the Analog Analyzer [ATS2: Analyzer: Audio Analyzer] to enlarge the panel, drop down the Function Reading [ATS-2: Measurement Function] menu and select THD+N Ratio. After this, observe the Function Meter and slowly increase the DUT volume until you have a reading of about 1 percent. You may find that the distortion jumps suddenly from some ratio just below 1 percent to a very high ratio of distortion. This is caused by the onset of amplifier clipping. Find the volume that produces the highest distortion that is below 1 percent (see Figure 2 below).

The equipment you want to test may be a receiver for a home theater, an audio power amplifier or a DVD player, or one of hundreds of other devices that require audio testing.

For our DUT, this output level was about 97W (about 28Vrms in 8!). This level is often called Maximum Output Level, or MOL.

CAUTION: At this level, the amplifier will be producing its greatest undistorted output. Depending upon the design of the DUT and its output rating, this condition may stress the amplifier, and may heat the amplifier heat sinks and the terminating load resistors. Be sure that your load resistors are designed to safely handle the rated output of your DUT, and that they are well ventilated. Leaving the amplifier at its maximum output may damage the amplifier, the load resistors and may be a fire hazard. Generally, you should make maximum output levels tests brief, and be sure you turn the generator OFF and / or the volume control down as soon as you have your measurement.

Reference testing level ! Once you have found a useful level using one of the above methods, you can set it as a reference in the 2700 or ATS-2 memory. To set the current Analyzer input levels as a references, select Edit > Set Analyzer dBr Ref or press F4 in the control software. Separate references are set for analyzer input channels A and B, called dBrA and dBrB. To set the current Generator output level as a reference, select Edit > Set Generator dBr Ref or press F3.

Since the DUT gain setting affects level, you should note the DUT volume setting as a reference as well.

About the author
David Mathew
is technical publications manager at Audio Precision. He is an Emmy-winning sound mixer who can still align a 3M 24-track in his sleep. He enjoys playing electric bass badly.




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