Sound

Digital Delay For Application in Surround Sound

4.0 Results

4.1 ELECTRICAL TESTING

All measurements were conducted using MLSSA (Maximum Length Sequence System Analyser), unless otherwise stated. A bandwidth of 25kHz was selected, as well as a Chebyshev anti-aliasing filter.

4.1.1 Frequency Response

The frequency response was averaged over ten readings, see FIG 4.0. At 20.047kHz the magnitude is 4dB down.

FIG 4.0 Frequency Response

4.1.2 Signal to Noise Ratio

The signal to noise ratio, S/N, was measured using a real time analyser and confirmed using a Lindos LA1 Distortion Analyser.
Firstly the S/N was investigated without any off-sets configured to the ADC or DAC, however it was discovered that this gave a poor result. Subsequently off-set trim-pots were added and the measurements carried out again.

TABLE 4.0 Signal to Noise Ratio (Linear weighted)

4.1.3 Phase Characteristic and Delay Time Measurement

The overall phase response of the processor was determined for a delay setting of zero.
From further phase measurements taken at 1, 2, 3, 4 and 5 ms the accuracy of the delay time could be calculated.
Each measurement was averaged over ten readings.

FIG 4.1 Overall Phase Response - Delay = 0 ms

FIG 4.2 Phase Response - Delay = 1 ms

FIG 4.3 Phase Response - Delay = 2 ms

FIG 4.4 Phase Response - Delay = 3 ms

FIG 4.5 Phase Response - Delay = 4 ms

FIG 4.6 Phase Response - Delay = 5 ms

From analysis of the phase figures, the delay can be shown to be working correctly.

TABLE 4.1 shows changes in phase at one frequency (515.9198 Hz) for incrementations of 1 ms.

 

Delay (ms)	Change in Phase (Degrees)
0 to 1	-182.667
1 to 2	-186.78
2 to 3	-186.391
3 to 4	-186.83
4 to 5	-185.361

TABLE 4.1 Change in Phase at 515.9198 Hz for Varying Delay Times

The actual phase difference for a signal of frequency with period t subjected to a time delay -T, can be found from the following equation:

Therefore for a frequency of 515.9198 Hz, period t = 1.938 ms subjected to a 1 ms time delay, the phase difference would be -185.73°.

The average phase change from TABLE 4.1 is -185.606°.

If more readings were taken for different time delays then the values should correlate even better.

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