Stereo receivers, which dominated the home audio market prior to the advent of affordable surround sound, were relatively simple devices–a tuner, a few analog inputs, and a 2-channel amplifier. Hence, for the test engineer equipped with a good audio analyzer, conducting tests on a typical receiver was a relatively straightforward task.
With the recent explosion in popularity of digital media applications, modern home theatre receivers have developed to the point where the term receiver is really a misnomer. A modern receiver is actually a complex switching device with a myriad of audio and video inputs and outputs, both analog and digital. In addition to the traditional analog audio inputs and outputs, typical receivers now support switching of digital audio from various streams, and both analog and digital video. Furthermore, with increased adoption of surround sound systems in the consumer market (e.g., Dolby or dts), digital audio streams are typically encoded, and output channel counts have increased substantially from stereo to 2.1, 5.1, and 7.1. Even car audio receivers have increased channel counts as manufacturers adopt techniques like bi-amping and tri-amping to meet consumer demands. Add HDMI with its encoded digital audio to the mix, and the prospect of conducting audio tests becomes daunting.
In addition, class D amplifiers may contain elevated noise in the region above 20 kHz. Normally, a 20 kHz low-pass filter is used to limit noise measurements to the audible frequency range. However, the 3 to 6 pole low pass filter used in most audio analyzers may not be sharp enough to eliminate all of this extra noise, so the measurement accuracy may be impacted. A better filter to use is a low-pass filter conforming to the AES-17 standard. Originally intended for measuring D/A converters, this sharp low-pass filter is also ideal for measuring class D amplifiers. For Audio Precision's 2700 Series, the optional S-AES17 filter is available. All of the built-in digital filters present in APx analyzers already provide a sharp cut-off, so this issue is no longer a concern.
Fortunately, Audio Precision offers a complete line of audio analyzers with a powerful feature set to simplify and speed up even the most demanding audio tests.
- For multiple channel test applications, Audio Precision offers a variety of solutions. The new APx series offers 8 or 16 input channels (APx585/APx586) with 8 output channels. For higher channel counts, users with Audio Precision's ATS-2 or 2700 series 2-channel audio analyzers, the SWR-2755 Switchers are available to enable testing of multiple input or output channels in switched mode. These switchers are in a 12 x 2 configuration, and can be cascaded to achieve a maximum of 192 channels. The switchers can also be used with the APx series 8 or 16 channel analyzers, if required.
- The new High Definition Multimedia Interface (HDMI), which carries audio signals along with HD video, presents even greater challenges in audio testing. HDMI streams have up to 8 channels of audio in a variety of Dolby or dts encoded formats (both lossy and lossless formats) with sampling rates up to 192 kHz. In addition, 2-way metadata is bundled within the stream, allowing output devices to know how many channels an input device can handle. The Audio Precision APx585 Multichannel Audio Analyzer with HDMI is the only audio test solution for HDMI enabled devices and is ideally suited for the application. The analyzer even comes with a disc containing a variety of audio test signals in HD formats for testing Blu-Ray disc players.
- Switch-mode audio power amplifiers (e.g., Class D or Class T) are becoming increasingly popular, and they are beginning to displace more traditional linear amplifier designs in home entertainment systems. Measuring the performance of switch-mode amplifiers presents some new and unique challenges to the audio test engineer. These amplifiers inherently generate signal components that are much higher in frequency than the audio band. In addition, the nature of these ultrasonic components can cause nonlinear behavior in the input stages of high quality audio test and measurement equipment. The consequence of this is that unless effective measures are taken to prevent this nonlinear behavior, a test will yield absolutely worthless and inaccurate results. Fortunately, Audio Precision offers a solution to this problem in the form of the AUX-0025 and AUX-0100 filter accessories.
- For production line audio testing where speed is essential, Audio Precision has developed a High Speed Tester (HST) application. This application makes use of the powerful multitone features built into Audio Precision analyzers to quickly and accurately provide test results. With this technique, in a little as 1 to 2 seconds, engineers can test a device for DC offset, frequency response, phase response, THD spectrum, noise spectrum, and crosstalk.
- For receivers equipped with digital I/O (e.g., S/PDIF or TOSLINK), Audio Precision's family of dual domain audio analyzers facilitate testing in either the analog or digital domains or both (cross-domain).
Tips for Optimum Testing
- Features like spatial processing or equalization that are available in most receivers will significantly affect test results. These features should be disabled before testing.
- Reversing the polarity in a surround sound system can degrade the surround experience, so be sure to test the phase response of each channel.
- Typically, multi-channel receivers can't drive all channels simultaneously with any significant load before they go into protection mode. This needs to be taken into account in designing a test.