Specifications

2.21

Applications Manual

Revision 1.11Prism Sound dScope Series III

3.2.6.3 Signal modulation noise

Applies to:

EUTs with analogue or digital inputs and analogue or digital outputs.

Aim:

To measure signal modulation components produced by gain modulation in the

EUT

Method

A sinusoidal stimulus is applied at –5

dBFS

with a frequency of 0.4999 times the

upper band-

edge frequency

. An FFT of the 1000Hz frequency band above half the upper band-edge frequency is

displayed for each channel, so that any modulation products can be identified (the AES17 standard is

restricted to products within a 500Hz frequency band above the upper band-edge frequency). The Y-

scales and cursors are calibrated in dB, relative to the stimulus tone.

This FFT method is more useful than the 1/3 octave filter method described in the AES17 standard,

since it allows individual components to be identified and measured if required.

3.2.6.4 Low-level noise modulation

Applies to:

EUTs with analogue or digital inputs and analogue or digital outputs.

Aim:

To measure variations in the noise floor of the

EUT

produced by the presence of low-level LF

signals. These are often the result of incorrect dithering.

Method

In preparation for the test, the idle-channel noise of the EUT over a 22Hz..22kHz bandwidth is first

measured. Then, a 41Hz sinusoidal stimulus is applied at to the EUT at an amplitude such as to

produce an output amplitude of –40

dBFS

. A bank of 21 FFT Detectors are set up, each one notching

out the 41Hz tone from the EUT output and measuring the residual in a different 1/3 octave band

between 200Hz and 20kHz. The measured amplitude of each Detector is recorded for the initial

–40dBFS output amplitude.

The amplitude of the stimulus is then reduced in 10dB steps, until it is below the idle-channel noise

amplitude. At each step, the amplitude readings of all the FFT Detectors are recorded.

A single result is presented for each channel, that being the maximum difference in measured

amplitude in any single Detector from its initial recorded result for a –40dBFS output amplitude. In

addition, a graphical output is provided showing the worst case deviation in each of the 21 1/3 octave

bands.