analog filters vs software filter/editing

[Eujene]

Can you give examples where analog filters can do things that can't be done by any software filter? By software filters are meant after the analog signals got into the ADC (Analog to Digital Converters).

I mean, back in the 1970s. They used sharp audio band filters or LCR band filters and then used spark/arc plotters to plot the envelope amplitude output from each of those filters. Can software filters like from REW able to do these too? If not. Please give examples something that can't be captured by software filtering and editing and need 1970s analog technology.

Remember some used vacuum tubes or 1950s technology for audio.

 

[fpitas]

I think you'll find that any filter you can write an equation for can be duplicated in DSP.

 

[solderdude]

DSP is more versatile, lower noise but requires digital sources or ADC + DAC.
Analog is usually more noisy, limited in bands etc but is easy to use with analog sources. Also it is less prone to clipping.

 

[fpitas]

DSP is more versatile, lower noise but requires digital sources or ADC + DAC.
Analog is usually more noisy, limited in bands etc but is easy to use with analog sources. Also it is less prone to clipping.

Why are you thinking analog is more immune to clipping?

 

[solderdude]

An all analog is. DSP is not.
Analog can clip of course but usually has a lot of headroom above its nominal input level.

 

[fpitas]

An all analog is. DSP is not.

I'd guess an op-amp filter is just as likely to clip as DSP.

 

[solderdude]

Yes but how many analog opamp EQ circuits clip at 0dB input levels ?
Of course when it clips at 10V for instance you could call that 0dB but usually the nom. input voltage would be between 0.5V and 2V or so.

 

[Sokel]

I'd guess an op-amp filter is just as likely to clip as DSP.

I think of it just like an analog pre amp that you can apply positive gain easily,a digital one will go as far as 0db.
I'm I wrong?

 

[fpitas]

I think of it just like an analog pre amp that you can apply positive gain easily,a digital one will go as far as 0db.
I'm I wrong?

That's how I think of it. Usually gain of each stage is adjusted along the path to optimize performance.

 

[fpitas]

It's much easier to give examples where DSP does things not practical with analog. Like ultra-high PEQ (72 max for my DSP) adjustable in 1Hz increments.

 

[MaxwellsEq]

Can you give examples where analog filters can do things that can't be done by any software filter? By software filters are meant after the analog signals got into the ADC (Analog to Digital Converters).

I don't think there are any things that analog filters can do that can't be done by a DSP processor. If, however, you consider digital sampling and encoding a terrible thing, then analog filters have the benefit of allowing you to avoid ADCs and DACs.

There are downsides to analog filters. To keep component count to a economically viable level, analog filters usually implement one filter method, rather than the multiple methods available to DSP solutions. They also often are unable to implement very complex filter architectures. In practice this means noise handling and phase handling are not as effective as DSP implementations.

 

[antcollinet]

You can't use a digital filter for anti aliasing on the input to your ADC.

 

[Cbdb2]

You can't filter out RFI with an audio dsp.

 

[DVDdoug]

Most DSP is done in floating point, which for all practical purposes will never clip.

BUT your DAC (and ADC if there is one) is hard limited to 0dBFS.* And since most digital audio is normalized ("maximized" for 0dB peaks) or nearly normalized, boosting of any frequencies can potentially clip the DAC.

For that reason many (most?) software EQs have a "preamp" adjustment which is most-often used as an attenuator rather than an amplifier.

You can't use a digital filter for anti aliasing on the input to your ADC.

TRUE!

You can't filter out RFI with an audio dsp.

Since the signal is limited to half the sample rate, any digitization MUST perfectly filter-out any RF (unless you have RF sample rates). But you do need that anti-aliasing filter, or the RF will fold-back into the audio range.



* There are now some floating point ADCs that can go over 0dB. I'm not sure if there are any floating-point DACs (yet). They won't have the same dynamic range as floating point in the digital domain but I wouldn't be surprised if they can go to +10 or +20dB before clipping.

 

[antcollinet]

You can't filter out RFI with an audio dsp.

Well there won't be any RFI in the AF digital signal, since it is band limited to half the sample rate. So nothing to filter out.

On the other hand if you had super RF sampling rates that had captured RFI you could filter out the RFI using a digital filter.

 

[Cbdb2]

Well there won't be any RFI in the AF digital signal, since it is band limited to half the sample rate.

Yes by an analog filter. So its still an answer to the OPs question.

 

[antcollinet]

Yes by an analog filter. So its still an answer to the OPs question.

Except it isn't, as pointed out by my post above. If RF sampling were used, then digital filters could filter out captured RFI

Technically RF starts at 20kHz - so it is actually already done. For example a 192kHz sample rate can capture RF frequencies between 20kHz and 96kHz (we would call this ultrasonics - but it lies in the RF Band). This can be filtered by digital filters.

 

[MaxwellsEq]

Radio Frequencies start at 3Hz to 30Hz for the ELF ITU band. Everything below 30kHz is pretty much used for low data rate broadcasts underground or underwater. But it is a good idea not to confuse the waves in air we associate with sound versus EM waves

 

[Cbdb2]

Except it isn't, as pointed out by my post above. If RF sampling were used, then digital filters could filter out captured RFI

Technically RF starts at 20kHz - so it is actually already done. For example a 192kHz sample rate can capture RF frequencies between 20kHz and 96kHz (we would call this ultrasonics - but it lies in the RF Band). This can be filtered by digital filters.

So change RFI to greater than 100mhz. I think everyone else understood what I meant. How do digital filters work without an analog filter first?

 

[MaxwellsEq]

So change RFI to greater than 100mhz. I think everyone else understood what I meant. How do digital filters work without an analog filter first?

A couple of people have pointed out, there needs to be a filter ahead of any analogue-to-digital converter, and since we are in the analogue domain, this needs to be analogue.

Also, if you are working exclusively in the analogue domain, e.g. microphones, an analogue mixer, analogue monitor speakers, then all filters will be analogue. Under those circumstances, it's a good idea to use an analogue filter to remove RF at the start.

But once a signal has been converted to digital, you can mix it with any amount of RF, as long as data is not corrupted. In the digital/data domain analogue concepts such as RF don't really mean anything, unless they are so severe as to corrupt the data. In these circumstances, there is no need for a filter.