Appendix II
LE3000S
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Appendix II: Testing Of Asynchronous Sample Rate Converters
The theoretically ideal sample rate converter is a device that converts the data transfer rate
without changing the content of the material.
Listening tests:
Many devices are judged by their particular sonic quality. A recording engineer may prefer, for
example, a small amount of distortion to add some "color" to the sound. The same engineer,
testing a number of processing devices separately, may choose his building blocks to achieve
a certain characteristic sound. Let us assume that the desired characteristic is based on some
small amount of second order distortion which imparts a characteristically “warm” sound. A
problem may arise when processing the sound through more than one unit. The "desired
distortion" may be compounded beyond the desired level (in our example, the second order
distortion may increase each time the sound is processed to an unacceptable end result).
Sample rate converters may serve to reduce excessive clock jitter. This improvement can take
place for any sampling-rate ratio (including 1:1). Jittery incoming data introduces signal
dependent noise and distortions (increasing with signal amplitude and frequency). Such jitter
reduction yields noticeable sonic improvement, thus complicating the objectivity of listening
tests. Some manufacturers' comparison tests inappropriately compare a high jitter input to a
low jitter output. Further confusion is due to the fact that digital domain FFT tests do not
adequately show the effects of input jitter.
Listening tests should be based on comparing the audio of a directly applied signal against the
converted version of the same material. The greater the difference, the less ideal the
converter.
The sample rate converter should receive a low jitter data source, and drive a reference grade
D/A converter, a high quality power amplifier and top grade speakers, all matched to 0.1 dB.
"Blind" listening comparisons (A/B/X tests) by recording professionals yield the best unbiased
results.
Measuring performance:
The most commonly used measurements are based on a standard FFT, THD plus Noise
testing (in the digital domain) and phase linearity. Interpreting measurements performed on
asynchronous sample rate converters is less straightforward. The asynchronous sample rate
converter can not control the input and output rates (these rates are forced by the driving
source and required destination devices). The converter is required to reconstruct the data
content while accommodating receiver and transmitter clock rates.
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