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In Reply to: RE: subtractive dither posted by The Sound Guy on August 19, 2009 at 12:31:21:
Subtractive dither requires that the dither signal be created at the time of encoding and be made available at the time of decoding. It has to be a random signal (or a pseudo-random signal). If a random signal is used then it has to be included on the medium (or elsewhere) and a separate signal is needed for each stereo channel. Storing the required dither to compress a 24 bit signal down to 16 bits would add an extra 8 bits, for no net gain. So a pseudo-random signal is needed. These can be of arbitrary complexity, as much as a cryptographic algorithm that couldn't be broken by espionage agencies, or something much simpler. However, this algorithm must be agreed up, i.e. standardized, otherwise subtractive dither won't work. Given that SP controlled the standards for RBCD and didn't do this then we don't have it. In addition, there is the necessity to synchronize the dither on encoding and decoding. These all involve details of encoding, but this can be done, indeed the low level bits stolen by HDCD can be used to accomplish this.
Subtractive dither was first disclosed in the early 1970's by Larry Roberts, in the video context as part of an MIT Master's thesis. It was the original dither. What got implemented by SP the better part of a decade later was a bastardized form.
In addition to the requirement of standardization there may be patents involved.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
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Follow Ups
- RE: subtractive dither - Tony Lauck 13:55:45 08/19/09 (0)
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