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Music servers and other computer based digital audio technologies.

RE: USB cable errors - NOT

"How would hard data errors and the lack of retries manifest themselves audibly in the audio playback chain?"

There are two possibilities:

1. process the erroneous data. What you get is what you hear.
2. ignore the erroneous data and attempt to guess what it was. What you hear depends on the algorithm.

In the case of SPDIF there is no further error detection, so it's case 1. You will hear clicks, amplitude depending on which bits in the PCM word get corrupted.

In the case of CDA (Red Book coding) there is a strategy to correct errors and to guess at single sample uncorrectable errors by interpolation, otherwise to mute. Most single errors are successfully interpolated without clicks. There are test tools that CD mastering engineers run to evaluate the quality of the raw bits on a disk to see what margins for error correction remain.

With USB audio, the DAC would implements some kind of strategy. I suspect this includes discarding any packets that have CRC errors. Here error detection is done on a packet basis which contains roughly 1 millisecond of audio. One can simply duplicate the missing sample or one can drop it and slip the tempo one millisecond, etc... If this doesn't happen too often it won't be noticeable on most music to untrained listeners. (The details will depend on the particular USB protocol the DAC uses, which may be proprietary.) The aural effects are likely to be similar to buffer underruns due to computer system overload. One strong reason for discarding packets that fail the CRC concerns the USB frame format. Bit stuffing is used to delimit the data packets, which means that a single bit in error can turn into a different number of bits in the output, resulting in loss of PCM word framing. If PCM framing is lost the results can be a horrible (if not dangerous) burst of noise.


Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar


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