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Re: Erroneous Stuff...

Hi Todd,

I'm just about to head out, but I thought I'd reply real quick. I haven't had a chance to read over your arguments in detail, but just reading your linked message, I think you might have missed something. (or, rather that Doug is right about the time-smear issue but he has missed a very important stipulation about post-filters)

You're right - if you use a brickwall-type analog post filter with an upsampler, you don't get any benefit in the area of phase distortion. However, the over/upsampler allows you to use a gentler low pass filter; that is the textbook reason for why oversampling was developed in the mid-80's. Doug was not clear on the point of "in theory you can use a whatever-the-hell-analog-post-filter you want" and perhaps the issue is made complicated by the fact that it is not always clear whether that run-of-the-mill over/upsampling DAC chip you just pulled off the shelf employs a brick-wall filter or gentle-slope filter.

My expertise is in non-oversampling chipsets, so I am not familiar with all the "advanced" chipsets on the market. But it would be completely idiotic if all available converters require you to use the built-in analog filter, especially in the high-precision units intended for high-end audio - the most advanced designs really should employ external discrete analog post-filters built with specialty parts. I'm sure not every over/upsampling die nowadays requires you to use the built-in filter - though who knows, with all these do-it-all monolithic chipsets coming out it seems this might be the way things are going.

Best,

-Chris

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