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Comment by dist-epoch | Hacker News Reader

I saw a talk recently where an experienced engineer was arguing that for some modern applications, it's better in both cost and quality to convert the analog signal to digital, filter it in the digital domain, then convert it back to analog (analog signal -> ADC -> digital filter -> DAC -> analog signal).

I don't remember the exact domain, possibly very high frequencies.

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sure, you do that whenever you can, which is pretty much anytime you don't have very high frequencies

in order to get to where you don't have very high frequencies, you need analog filtering on the input and the output, and you need adcs and dacs suitable for the application. you might also need attenuation, amplification, impedance transformation, and/or mixing on the input and output. all this stuff is analog and requires this kind of analysis to understand

but whenever you can do stuff in the digital domain, you do, because you can easily reduce the error introduced by digital computation as small as you want, and you can redesign your signal processing after your space probe has passed neptune so a soldering iron would be inconvenient. and you can fit arbitrarily complex filtering and other signal processing into an arbitrarily small package, near enough; what would be another precision capacitor in an analog circuit is just another 16-bit coefficient loaded from your terabyte microsd card in a dsp setup, if you're processing a slow enough signal

One reason you can get away with this sometimes is that ADCs themselves act like a low-pass RC filter. The ADC input is itself a capacitor that gets charged to the voltage that needs to be measured, and it has an appreciable resistance (which can be supplemented with an additional input resistor). Sometimes that's all the low-pass filtering you need to prevent aliasing, and the rest can often be done in the digital domain.

But if the signal you're measuring is very weak (or poorly matched to the ADC range), you probably need some kind of amplifier for it anyway, and in that case you may as well make a filter out of it at the same time to maximize the signal-to-noise ratio.

This is the default solution for anything even slightly complicated. At very high frequencies(Rf), then you may be forced to use passive filters. The range where it makes sense to use active op-amp filters has shrunk considerably.

You get ease of prototyping,modification, and excellent performance. In these systems there may be still analog filter components(anti-aliasing,reconstruction), but with digital components so cheap,good, and fast this "mostly digital" technique is almost always the best option.

The most common type of amplifier at audio frequencies usually works the same way, these days. ADC -> DAC. It fully decouples (signal-wise) the input and output (no possibility of distortion). And component tolerance is less important. It's also more power efficient, potentially exceeding 90%, since power loss only occurs when the transistors generating the output switch on/off, and at audio frequencies transistors switch almost instantly.

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