Xref: utzoo sci.electronics:18308 comp.dsp:1370 Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!uunet!motcid!duerr From: duerr@motcid.UUCP (Michael L. Duerr) Newsgroups: sci.electronics,comp.dsp Subject: Re: A question about the Nyquist theorm Message-ID: <4692@apricot30.UUCP> Date: 8 Mar 91 21:26:23 GMT References: Organization: Motorola Inc., Cellular Infrastructure Div., Arlington Heights, IL Lines: 32 From article , by wilf@sce.carleton.ca (Wilf Leblanc): > When I bought my CD player, it said on the front panel 'Dual D/A > converters'. For fun, I asked the salesperson what that meant. > The reply was rather funny, and of course completely inaccurate. > > What does this really mean ? (I figured maybe two distinct D/A's rather > than 1 D/A and two sample and holds ??). Yes, it means that. Or, more likely, one dual-channel D/A. There are a couple of reasons why. Sample and holds have errors known as pedestal - a voltage step that occurs when they transition between state - and droop, where the signal decays as it is held. The would be irrelevant to sound quality, except that they are nonlinear and thus introduce distortion. Yes, D/A's have distortion too, but adding more only degrades things. Also, a S/H will have some feedthrough. Thus, when it is holding and the D/A is producing a value for the second channel, some of the second channel will feed through. While the ammount is slight, remember that the output of the FIR into the DAC may be 22 bits. That represents 132 dB of dynamic range. At 1 volt levels, -120 dB would be 10 mV. Depending on the noise level that may be burried, but the it is amazing what the human ear can integrate up out of white noise. Thus, channel isolation problems are potentially lessened by using dual D/A's. Of course, its easier to use a dual D/A than a single one plus two more S/H chips, even if the isolation between D/A sides is not an issue or the DAC itself has bad isolation. This is probably the biggest reason for Dual D/A - less chips, less board space, less $. Sound quality improvements will be undiscernable to most listeners, who probably buy more based on ( real and perceived ) features.