Low-pass filter meets digital testing requirements

The new Audio Precision S2-AES17LP filter, now available exclusively in the UK from TTi, meets the requirements for digital audio testing laid down by the Audio Engineering Society.

The new Audio Precision S2-AES17LP filter, now available exclusively in the UK from TTi (Thurlby Thandar Instruments), meets the requirements for digital audio testing laid down by the Audio Engineering Society.

The AES has defined the techniques for quality measurements of digital audio equipment (systems that contain digital/analogue convertors).

The AES17-1998 standard specifies, in section 4.2.1, the use of a standard low-pass filter that has a sharp roll-off above the audio upper band (20 kHz).

The filter is specified to have a stopband attenuation of 60 dB or better above 24 kHz, an aggressive slope.

The filter must be inserted early in the measurement path in order to remove the out-of-band noise before the measurement notch filter and its subsequent gain.

This will ensure that the +N part of the THD+N parameter contains only the in-band noise and distortion.

Without the filter, the automatic gain ranging that normally follows the THD+N notch filter can behave incorrectly and the resulting measurement will be in error.

The S2-AES17LP filter meets these requirements, and is now available for use with Audio Precision's System Two and System Two Cascade systems.

The passband response is ñ0.10 dB from 10 Hz to 20 kHz, with stopband attenuation greater than 60 dB from 24 kHz to 200 kHz.

Residual THD+N (1 kHz) is less than 0.0004% (-108 dB) + 1.0 æV.

Unlike other optional filters from Audio Precision, the new device is a larger module inserted in a different location in the measurement circuit, just after the input conditioning amplifier and prior to any measurement circuits.

Conventional band-limiting and noise weighting filters cannot solve the problem because they are located too late in the measurement chain and the damage is already done.

The S2-AES17LP costs ?300 (plus VAT).

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