9. Digital Interface Tests
For error-free transmission of
digital audio data, the physical digital signal must be investigated in
addition to the digitally coded audio signal. For this purpose a physical
digital signal with analog parameters such as pulse amplitude and clock frequency
is used.
To be able to perform digital interface tests UPD has to be fitted with Options
UPD-B2 (AES/EBU Interface) and UPD-B22 (Jitter and Interface Test), and UPL
with Options UPL-B2 (Digital Audio Interfaces) and UPL-B22 (Jitter and
Interface Test).
For these tests the measuring instrument has to be configured as follows:
o In the Src Mode line of the digital generator panel the user determines the tasks for which the built-in generators are to be employed. These signal sources can generate audio data and/or operate as jitter modulator and superimpose a balanced common-mode signal onto the audio data stream.
o
The application of the built-in
measurement functions is specified in the Meas Mode line of the digital analyzer.
The measurements functions allow not only the audio contents to be analyzed but
can also be used for jitter and digital phase measurements or for determining
other physical signal parameters such as pulse amplitude or common-mode
signals.
9.1. Common-Mode Interference, Digital Pulse Amplitude and Sampling Frequency
Setups:
o
DCOM_DD.SAC
In the following example the
generator is used to superimpose a sinewave common-mode signal of 100 mV onto
the balanced audio line. Signal shape, level and sweep functions can be set.
This setup allows the degree of immunity of the DUT to such type of interference
to be checked.
The analyzer measures the incoming common-mode components. Level measurement,
FFT and waveform function can be selected, the setup shows the rms value of the
common-mode signal.
To find the digital input levels to
which a DUT synchronizes correctly, it is possible to set the pulse amplitude
of the digital data stream in the generator panel. The switchable cable
simulator simulates the lowpass behaviour of a 100m cable by smoothing the
signal edges.
With this setup the analyzer indicates the pulse amplitude as a peak value.
Cable attenuation can be determined by referring the measured value to the
generator level.
Digital input stages must also be able to handle clock frequency offsets. To
investigate the capture range the user can vary the clock frequency in the
generator, the analyzer displaying this frequency.
Setups:
o
JITAM_DD.SAC
In this example the generator is used as jitter source. A sinewave
jitter of 100 Hz with an amplitude of 0.1 UI has been selected. If necessary,
the jitter signal can also undergo sweeping.
The analyzer indicates the jitter amplitude in all common units.
Setups:
o
JITSP_DD.SAC
Setup of
generator as described above. FFT analysis is performed. The jitter spectrum reveals the cause of the
jitter if, for example, a frequency used in the switching power supply is
included in the jitter spectrum.
Setups:
o
JITWA_DD.SAC
With this setup the generator
produces a noise-like jitter signal, the waveform function of the analyzer shows
this signal in the time domain.
9.5. Jitter Susceptibility
! At present, this measurement function is only
available in Audio Analyzer UPL !
Setup:
o
JITSU_DA.SAC
o
JITSU_DD.SAC
Definitions
and test conditions:
The measurement of jitter
susceptibility is at present being incorporated into AES 17. It describes the effects
of jitter at the digital audio or reference input on the quality of the audio
contents. The digital input receives a sinewave signal of 1/4 of the sampling
frequency at a level of -3 dBFS, this signal being at the same time subject to
jitter. The jitter frequency is swept from 80 Hz to 20 kHz at a jitter level of
40 ns. The audio output signal is examined by graphically displaying the THD+N
value versus the jitter frequency.
Notes
on measurements:
The measurement of jitter
susceptibility was not possible with Audio Analyzers UPD and UPL up to now. UPL with firmware version 1.01 and Option
UPL-B1 (Low Distortion Generator) can now perform this measurement which involves
the simultaneous generation of an audio signal and of jitter. UPL-B1 provides
the second generator required for this purpose.
9.6. Phase between Audio Data Signal and Reference
Signal
! This
measurement function is available only in Audio Analyzer UPL !
Setups:
o
DPHA_DD.SAC
Definitions
and test conditions:
If a digital audio equipment is
synchronized using an external clock, the audio frames and the reference clock
must be within a specific time range. As regards the phase shift between
synchronization input and audio output of digital components, AES 3 specifies a
limit of 1/4 of the frame length (=32 UI).
Notes
on measurements:
Audio Analyzers UPD and UPL feature comprehensive synchronization
facilities. In the generator section the phase shift between audio frames and
reference clock can be selected between -64 UI and +64 UI to determine the
synchronization range of the DUT.
The JITTER/PHAS menu in the Meas Mode line of the analyzer allows
the phase between reference input and audio input of the analyzer to be
measured, which corresponds to the phase shift between synchronization input
and audio output of the DUT assuming appropriate cabling.