Cirrus Logic AN331 - Manual (Page 5)

AN331
5
4. Demonstrating the Technique
Assembling a test system to demonstrate this technique is a relatively simple matter using standard Cirrus Logic
evaluation boards and the Audio Precision System 2. The block diagram in Figure 4 shows a test set-up which in-
cludes the CDB5381 and CRD43530, the evaluation boards for the CS5381 A/D and the CS495313 audio DSP. The
Audio Precision System 2 is the source of the analog signals as well as the analysis tool used to generate perfor-
mance data and plots. The digital interconnections between the evaluation boards and the Audio Precision System 2
are the standard S/PDIF (IEC-60958) interface. The evaluation was performed at a 48 kHz sample rate but the per-
formance improvement is valid at all sample rates.
Figure 4. Test System Block Diagram
The A-weighted THD+N versus amplitude plots shown in Figure 5 clearly show the 3 dB performance improvement
that can be achieved with the channel summation technique. These plots were produced with the Audio Precision
generating equal amplitude and in-phase signals to both inputs of the CDB5381. The equation of e
o
= A/2 + B/2 was
performed in the DSP.
4.1 The Subtraction Approach
The analog inversion with digital subtraction technique is also easily implemented with the test system. The only
differences are that the analog outputs of the Audio Precision are configured to generate an inverted signal to one
of the A/D inputs and the DSP is configured implement the subtraction, e
o
= A/2 - B/2. This configuration was tested
and there was no appreciable difference between the subtraction and the addition techniques with the CS5381 in
this test set-up. To a large degree, this is a result of the differential architecture of the CS5381. Implementations
using different A/D architectures or systems designs have been shown to benefit from the subtraction technique and
produce improved results.
Right Channel A/D
Left Channel A/D
CRD49530
Cirrus CS495313
DSP
Analog Inputs
SPDIF Interface
Summed-Channel Output
Audio
Precision
System 2
Audio
Analyzer
SPDIF Interface
CDB5381
Evaluation Board