When reduced to its fundamentals, audio measurement is concerned with a select number of performance benchmarks:
• Frequency Response
• THD+N (Total Harmonic Distortion plus Noise)
• SNR (Signal-to-Noise Ratio)
In this article, we’ll look at what testing is required for different audio devices, choose a device to test and a signal path, discuss basic setup considerations, and look at how to proceed with the first couple of tests in detail, using your analyzer's meters and one frequency sweep. In the second part of this article, we'll examine the final four tests.
For simplicity, the examples given here will assume you're making all the measurements in the analog domain using a typical audio analyzer, and testing 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.
Device Under Test & Signal Path
The equipment you want to test may be a receiver for a home theater, an audio power amplifier, a DVD player, or one of hundreds of other devices that require audio testing, such as ADCs, DACs, semiconductor devices, mobile phone and telecommunications equipment, etc. When discussing a measurement, we refer to the equipment to be measured as the Device Under Test, or DUT.
Signal Paths & Connections
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. Inputs and outputs may be either analog or digital.
• An audio power amplifier has both inputs and outputs. The inputs may be analog or digital, with the output usually 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 article, we will use a home theater receiver as the DUT. This receiver has many inputs and outputs, but by way of providing a test example, let's assume 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 the use of 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 systems.