Created on 2013-02-01 00:17:00
Audio amplifier circuits are required to operate over vast dynamic ranges: 80, 100 or even 120 dB or more. Distortion and noise at the extremes are givens.
The need for ranging
The input circuits of an audio meter or analyzer employ switchable attenuation or gain to accommodate the extremes of dynamic range for accurate, high-resolution measurement.
When an input signal is low, the analyzer gain should be set to amplify that signal to an optimum level for measurement. When an input signal is high, the analyzer gain should be attenuated to prevent overload and to bring the signal down into the optimum measurement range.
Most meters and analyzers provide a way for you to manually set ranging to accommodate the input signal level. In Audio Precision instruments, the ranging choices are shown in a dialog that allows you to select the maximum level you expect.
For the APx585 and 586, six ranges are available for both unbalanced and balanced inputs:
· 0 Vrms to 320 mVrms
· 320 mVrms to 1 Vrms
· 1 Vrms to 3.2 Vrms
· 3.2 Vrms to 10 Vrms
· 10 Vrms to 32 Vrms
· 32 Vrms to 100 Vrms
Choose the lowest range whose upper limit is greater than the expected maximum input signal.
Autoranging employs a circuit that measures the input voltage and automatically selects the correct range for the signal. As the signal level varies, the range will be switched up or down to keep the signal in the best range for measurement.
Autoranging works well for steady signals or signals whose levels change slowly. Most test stimulus signals are consistent, making autoranging a good choice for most testing.
When autoranging isn’t suitable
However, signals that are very dynamic (such as music, voice or noise signals) may cause the autoranging circuits to switch too often for your test purposes. Signals with very fast pulses or spikes change too quickly for the autoranging circuit to react and may cause incorrect measurements due to ranging errors. PESQ and other perceptual audio measurements use voice samples as test signals, and may benefit from the use of fixed ranging.
In these cases, evaluate the signal range carefully, or estimate the range if you must, and manually set the analyzer ranging to accommodate the input signal.
Also, autoranging takes time. When the signal levels are known and a sequence would require many range changes, setting ranging to Fixed will provide faster testing.
Track first channel range
For convenience, in APx500 ranging for channels above 1 are set to follow the channel 1 range setting. This behavior can be defeated by unchecking the “Track first channel range” checkbox.
Autoranging for Continuous Sweep and Frequency Response measurements
Acoustic Response, Continuous Sweep and Frequency Response measurements use a modified autoranging process to find the correct input range.
For these measurements, the sweep acquisition begins in a particular range called the range floor, and moves up from there if necessary. By default, the range floor is the lowest range (0 Vrms to 320 mVrms), but you can specify a higher range by entering the value in the Minimum Range field in the Advanced Settings dialog.
If the initial range is correct for the signal, the acquisition is made and the data are processed and displayed. If this range is too low, data from the out-of-range sweep are discarded. The input range is moved up and the sweep is repeated. This process may result in several sweep attempts before the correct range is determined; however, since the sweeps are very fast (typically one second) the total acquisition time is short.
If your sweep is a step in an automated sequence that you would like to run as fast as possible, you can optimize the speed of the ranging process by setting the range floor to the correct range for the measurement.