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In Reply to: Then it would cease to be a Benchmark DAC1 but...... posted by rfbrw on May 8, 2005 at 12:13:39:
The AD1853 includes a digital filter with 8x oversampling so you wouldn't use it if you wanted to change the digital filters, but it isn't designed for a specific input rate like 4Fs. It's compatible with sample rates from 32K to 192K. The DAC1 precedes it with an upsampling digital filter set to 110 KHz which is a little below 3 Fs. But you're right in that the changes required would be too extensive and he'd be better starting with a design that was closer to his own philosophy.
Follow Ups:
Unlike some of its rivals the 8Fs filter in the AD1853 cannot be bypassed but thats a moot issue. The point is at 44K1, the output of the DF1704 is 352K8 or 8Fs. The max input sample rate of the AD1853 is 192Khz approximately 4Fs, assuming Fs is 44K1. Interfacing the two would require a decimating digital filter if done correctly or the discarding of every other sample if not.
I'm not sure why you talk about interfacing the two digital filters since if one was to use a DF1704 they would discard the AD1853 and use separate DACs. Probably go with PCM DACs instead of hybrids. Sure, Todd's not really a technical guy and probably didn't fully appreciate what would be involved when he tossed out that idea, but no one would try to interface the DF1704 to the AD1853. The data format isn't even compatible and the whole idea in the first place was to get rid of some of the digital filtering by removing the upsampler, not increase it. Of course, you could replace the AD1853 with the somewhat comparable TI PCM1792 which does have the slow rolloff feature he was after and maintains a similar topology to the 1853. Then he would just have to pull the oscillator and the ASRC. But like I said, might as well start with something closer to that goal :)
The issue was about modifying the Benchmark DAC1 not designing from scratch. At some point it ceases to be a DAC1 and I think it is pretty safe to say removing the AD1853 takes one beyond that point.
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