Introduction to the Six Basic Audio Measurements - Part 2

About Phase measurements
In audio engineering, phase measurements are used to describe the positive or negative time offset in a cycle of a periodic waveform (such as a sine wave), measured from a reference waveform. The reference is usually the same signal at a different point in the system, or a related signal in a different channel in the system. This choice of references defines the two most common phase measurements: device input/output phase, and interchannel phase.

Phase shift varies with frequency, and it is not uncommon to make phase measurements at several frequencies or to plot the phase response of a frequency sweep. Phase is expressed in degrees.

Making an Interchannel Phase measurement
Once again, we have to decide on a level. Phase measurements are not particularly level-sensitive, as long as we are above the noise and below distortion. We will make our test at 1 Vrms, with the DUT set for unity gain. These steps assume a DUT like our home theater receiver.

Initial Setup
Start with the DUT and control software setup instructions in Section 2.

Adjust DUT for Unity gain

Phase between channels often varies with frequency. Experiment by changing the Generator frequency to 100 Hz, and then to 10 kHz. The results varied slightly in our DUT: +0.02 degrees at 100 Hz, and +0.51 degrees at 10 kHz. To view a complete phase response, a sweep measurement plotting interchannel phase versus frequency is often made.

Making an Input/Output Phase measurement
The other common phase measurement compares the phase of the signal at the input of the DUT to the phase of the same signal at its output. A simple way to make this measurement for the Left channel in our DUT is to select GenMon as the Channel A analyzer input and then connect the left channel DUT output to the Channel B analyzer input.

Notice that we now show a phase difference of +179.17 degrees at 1 kHz (see figure below).

Results at 100 Hz and 10 kHz are similar (+187.50 and +163.73 degrees, respectively). This indicates that the output of our DUT is out of polarity with its input (180 degrees out-of-phase at all frequencies) with some additional phase shift at the frequency extremes.

Crosstalk

About Crosstalk measurements
In audio systems of more than one channel, it is undesirable for the signal in one channel to appear at a reduced level in the output of another channel. This signal leakage across channels is called crosstalk, and in practical devices it is very difficult to eliminate.

Crosstalk is expressed as the ratio of the undesired signal in the unstimulated channel to the signal in the stimulated channel. Crosstalk is largely the result of capacitive coupling between channel conductors in the device, and usually exhibits a rising characteristic with frequency.

Making a Crosstalk measurement

Initial Setup
Start with the DUT and control software setup instructions in Section 2.

Crosstalk A into B

Crosstalk B into A

Other Crosstalk techniques
The method just described provides a quick, single-number result for crosstalk, and is often the method of choice. However, a crosstalk versus frequency sweep will show how a DUT performs across its operating bandwidth.

Signal-to-Noise Ratio (SNR)

About SNR measurements
How much noise is too much? That all depends on how loud your signal is.

Signal-to-noise ratio (or SNR) is a measure of this difference, providing (like THD+N) a one-number mark of device performance. The signal is usually set to the nominal operating level or to the maximum operating level of the DUT. When SNR is made using the MOL, the result can also be called the dynamic range, since it describes the two extremes of level possible in the DUT. (Dynamic range in digital devices has a somewhat different meaning). SNR is usually stated in decibels, often shown as negative.

Using traditional methods, SNR requires two measurements and a bit of arithmetic. First you measure the signal level, then turn off the generator (and often, terminate the DUT inputs in a low impedance as well, to fully reduce the noise in the device). Then the noise level (often called the noise floor) is measured, using filters to restrict the measurement bandwidth. The ratio between the two is the SNR.

Making SNR measurements
Since SNR is the relationship between two measurements, first we measure the value at a specified signal level. For convenience we will store the value as analyzer input dBrA and dBrB references.

Then we measure the noise in the channel, using the dBr references as the units reference. This result is the SNR.

Initial Setup
Start with the DUT and control software setup instructions in Section 2.

Measuring and calculating SNR

In the two BW filter fields, set the high pass and low pass filter selections to define the measurement bandwidth. SNR measurements should be made in a limited, defined bandwidth, typically about 20 Hz to 20 kHz. This measurement bandwidth must be stated with the distortion result. We will use the built-in Audio Precision filters at 22 Hz and 22 kHz. [ATS-2: 22 Hz and 20 kHz LP.] For noise measurements, weighting filters are often used instead of bandwidth-limiting filters.