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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
- Signal-to-Noise Ratio
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.
Device Under Test (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 Device Under Test, or 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 re-connecting 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 8 Ω.
Choosing a measurement level
Most audio equipment has a nominal operating level within a few decibels of 1 Vrms. 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 1 volt nominal input levels, and power amplifier stages that can deliver up to 100 Vrms 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 digital signal processing (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:
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.
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