Wednesday, February 18, 2015

"Information" is not just my imagination

When I say that PCM systems preserve information better than Delta Sigma systems, I mean it.  I'm not only talking about my subjective experience (and I don't have a strong belief in subjective experiences, I believe we're easily fooled and fooled all the time).  I'm talking about something which I strongly believe could be measured, in principle.

One type of measurement would be to create a modulation transmission system using a digital system.  Take two 56k modems, connect one to the input of the digital sampler, and the other to the delta sigma output.  See the maximum rate of information that can be transferred.

Now a 56k modem would not be a good test as it doesn't transmit as much information as a 20kHz bandwidth system could. It would be only testing the middle frequencies available, in which delta sigma systems might be superior to some PCM systems (say, DSD vs 44.1kHz PCM).  So imagine an even better modem, a 20-20kHz modem, that perhaps transmits data at 1Mbps, I don't actually know what it could potentially do.  But such a modulation transmission system would, I believe, show PCM systems to be superior.

Meanwhile, even a 56k modem could show that low bit rate MP3 loses lots of information.  Obviously a modem stream riding on MP3 can't transmit a higher bitrate than the MP3 itself.  We are audibly fooled because of limitations in our ability to hear, but the modem isn't fooled.

With an even higher bandwidth transmission test, say 20-50kHz, high resolution PCM would be far superior to 1x DSD.  While DSD nominally has frequency response to 50kHz, it's very noisy response.

In a way, you could see the issue simply in the rising noise above 10kHz.  But static noise does not perfectly correspond to information loss because there is also dynamic noise, modulation noise, and THAT is the big problem with delta sigma systems.

Just looking at the decoder, a delta sigma or sigma delta DAC seems to have excellent amplitude characteristics.  It can be made with higher apparent linearity than a PCM DAC.  But it achieves this by smearing the time response.  You have a feedback system that takes a variable amount of time to stabilize the amplitude level, or a feedforward system based on the addition of known quantities--either way information is smeared.  True PCM systems put out the best approximation to an analog voltage level at predetermined times, times that don't vary in a way correlated with the amplitude level.  The latter preserves actual information better because any kind of correlation smears information.

I'm pretty much flying by my intuition in describing the whole problem, and I could be wrong here, but it makes sense to me.  Meanwhile, there's also a problem in that it's not clear humans can perceive the higher rate of information.  My argument remains that it may not make a difference in a single listening session, but over the course of a lifetime of listening there will be more different listening experiences because of greater information available.  Such differences would be nearly impossible, effectively impossible to produce DBT proof for.

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