Pre-Analyzer & AES17 Filters
Ensuring precision in special measurement applications
Why a Pre-Analyzer Filter?
Audio Precision offers several pre-analyzer filters for special measurement purposes. In conventional audio analyzers, signal filtering is implemented late in the measurement path, following several gain stages. This approach provides repeatable and accurate results for devices whose noise spectrum is somewhat constant or diminishes slightly with frequency, typical of a wide range of audio devices. Audio Precision’s analog analyzers and DSP-implemented analyzers follow this model.
However, the output signal from some devices can exhibit high levels of high-frequency out-of-band energy. This is characteristic of digital-to-analog converters (DACs) and switch-mode or Class D amplifier outputs. This high-level out-of-band energy can overload analyzer input stages and produce inaccurate measurements.
A pre-analyzer filter is a low-pass filter that is designed to reduce this out-of-band energy to manageable levels before it is applied to sensitive analyzer input circuits. Audio Precision offers several pre-analyzer filters optimized for different measurements.
The Audio Engineering Society publishes its recommendations for the measurement of digital audio signals in the document AES17. Recognizing the problem in measuring signals in the presence of out-of-band noise, AES17 recommends the use of a pre-analyzer filter when measuring DAC outputs. Our S-AES17 low-pass pre-analyzer filter satisfies the recommendation in AES17.
OPT-2020 is a low-pass pre-analyzer filter similar to the S-AES17. While S-AES17 is a two-stage design with a gentle pre-analyzer filter and a second brick-wall filter later in the path, the OPT-2020 accomplishes all its filtering in one stage. Although the overall performance of the S-AES17 is superior, some engineers prefer the OPT-2020’s sharper cutoff in the first stage and the availability of an open secondary filter slot for their applications.
The AUX-0025 is a low-pass pre-analyzer filter specifically designed to reduce the out-of-band energy in the output signals of switch-mode amplifiers, protecting the analyzer input from signals that can produce slew-rate limiting. Custom inductors and careful engineering create a very low-distortion filter with negligible insertion loss. See the AUX-0025 / 0040 / 0100 page for more details.
The AUX-0040 is a two-channel passive filter, with a 20 Hz to 40 kHz passband. It is designed to be used with two-channel analyzers, including the APx555, APx52x Series and 2700 Series test systems. See the AUX-0025 / 0040 / 0100 page for more details.
The AUX-0100 is the same filter design as the AUX-0025, but providing 8 channels of filtering in a convenient 2 U chassis for multichannel applications. See the AUX-0025 / 0040 / 0100 page for more details.