Created on 2009-01-23 16:00:00
Selecting Appropriate AES-17 Pre-analyzer Filters for the 2700 Series Analyzers
Question
I'm going to be measuring some DACs with my 2700 Series analyzer. Which AES-17 filter should I use?
Resolution
AES-17 pre-analyzer filters are low-pass filters with a steep brick-wall cut-off just above the measurement range. They conform to AES (Audio Engineering Society) Standard 17 for measuring DACs, digital amplifiers, and other digital devices with strong out-of-band content. The filters remove this out-of-band noise, which gives more accurate measurements and allows the analyzer to properly set input ranges to yield best resolution.
Note that only the 2700 Series analyzers accept optional AES-17 filters. The APx Series and ATS-2 don't need them—their standard low-pass filters are implemented digitally and already have a brick-wall roll-off that meets the AES-17 specifications.
For the 2700 Series, in almost all cases we recommend using the S-AES17 filter. It has excellent performance, with virtually no impact on noise or distortion. The S-AES17 is a two-stage filter, with a gentle roll-off in the first stage before the analyzer's measurement auto-ranging circuit, and a brick-wall roll-off in the second stage after the circuit.
The gentle roll-off in the first stage protects the auto-ranging circuit from high frequency out-of-band content that might interfere with its operation. It may be invoked independently by selecting 20 kHz SPCL or 40 kHz SPCL in the Bandwidth (BW) low-pass select box on the Analog Analyzer panel.
The brick-wall roll-off in the second stage removes essentially all remaining out-of-band content, so that the measurements accurately reflect only the audio band. It may be invoked independently by selecting 20 kHz (40 kHz) Brick-wall in the Filter (Fltr) select box. The steep brick-wall stage of the S-AES17 filter has been placed after the auto-ranging stage to achieve the best performance. Note that selecting both halves of the filter together—20 kHz (40 kHz) SPCL in the BW select box, and 20 kHz (40 kHz) Brick-wall in the Fltr select box—is equivalent to selecting the entire 20 kHz AES17 filter in the BW select box.
Filter response 10 kHz - 40 kHz
Graph Key:
Yellow: 20 kHz SPCL, Magenta: Built-in 22 kHz low-pass, Cyan: 20 kHz SPCL + 20 kHz brick-wall (same as 20 kHz AES17)
The single-stage OPT-2020 filter has exactly the same roll-off curve as the S-AES17, but its THD+N performance is not quite as good. Additionally, the OPT-2020 also has further degraded performance at high input amplitudes, as found in power amplifier testing. Nevertheless, there are some cases where the OPT-2020 may be a better choice. Since its steep brick-wall filter comes before the analyzer's measurement auto-ranging circuit, it does a better job protecting that stage from overload caused by high amounts of interfering signals just above the cut-off frequency. This can occur in the case of converters using low sample rates, since the sigma delta process causes the noise to start at double the sample rate in use. These low sample rate applications are often in telephony or audiology where very low residual THD+N performance is not of concern. The other advantage of the OPT-2020 is that it does not occupy one of the regular filter slots, so it can be combined with other filters, such as the A-weighting filter.
Switching (Class D) power amplifiers present a special case. Because their speaker outputs often contain high frequency switching noise at much higher voltage levels than line level devices, slew-induced distortion can be caused on the audio analyzer's input stages. This holds true for all audio analyzer models and brands. To eliminate this problem, passive filtering must be done before the signal enters the analyzer. AP makes the AUX-0025, AUX-0040 and AUX-0100 filters for this purpose. As the primary function of the external passive filter is to prevent input overload, the steeper brick-wall S-AES17 filter is still necessary to eliminate all out-of-band noise that might influence measurements.