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Topic: DACS with filters (Read 9447 times) previous topic - next topic
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DACS with filters

I wanted to get input from members here on the purpose behind the filters of DACS. I've noticed a number of DACs have filters one can choose, that change the sound quality.

Are the filters like an EQ? Is there a technical purpose behind them? If there are any DAC designers, please share why people include filters on DACs. I thought a DAC was just a digital to analog converter, but if a DAC has filters to choose from, then it becomes something else?

DACS with filters

Reply #1 –
In a DAC, a reconstruction filter is used to prevent imaging (some call it aliasing).
Contrary to popular belief, using a much steeper filter than was used to bandlimit the signal in the first place will not add ringing in the time domain.

Other filter options are:
a) non-linear phase: causes potentially audible phase shift, imho does not make sense with 44.1 kHz

b) slow roll-off: reduces the ringing in the time domain by starting attenuation earlier (attenuating more of the signal), but also pushes the ringing frequency down. If the cutoff frequency is too low you may actually hear the roll-off.
These filters may not reach the stopband (full attenuation) at half the sampling rate. In this case they will only partly attenuate images above half the sampling rate.
"I hear it when I see it."

DACS with filters

Reply #2 –
Quote from: Rich B on
I wanted to get input from members here on the purpose behind the filters of DACS.


One common good technical name for the brick wall filters at the outputs of DACs is "Reconstruction filter". They finish the job of reconstructing a smooth analog waveform. They are as much a part of a DAC as wheels are parts of cars.

Quote
I've noticed a number of DACs have filters one can choose, that change the sound quality.


Those would be boutique items for collectors and dilettantes.

Quote
Are the filters like an EQ?


They are not supposed to be like an Eq. They are there to produce a product that is sonically transparent, that is one that does not audibly change sound quality.

Quote
Is there a technical purpose behind them?


Asked and answered.

Quote
If there are any DAC designers, please share why people include filters on DACs.


Asked and answered.

Quote
I thought a DAC was just a digital to analog converter,


it is, and a reconstruction filter is a component of any proper DAC.

Quote
but if a DAC has filters to choose from, then it becomes something else?


Well yes and that question has been asked and answered as well.

If a DAC has filters to choose from, then it becomes a toy.

DACS with filters

Reply #3 –
Small nitpick Arnold. Why do you quote a sentence of mine, answer it, and then answer another quote by saying "asked and answered"? You tend to quote comments unnecessarily. Of course you answered my comment, but you then quote a question that I asked that you already answered in the same post. You could have rather chosen not to quote me several times over rather than quoting every single question ... that you already answered. Seems like a lot of work quoting comments for no reason other than to tell me that you've answered me.

DACS with filters

Reply #4 –
The reason I asked the question is that I've noticed some DACs available that have multiple digital filters. So in my mind, a DAC that offered that flexibility would just be an EQ box, otherwise what's the point of offering multiple digital filters?

DACS with filters

Reply #5 –
Quote from: Rich B on
The reason I asked the question is that I've noticed some DACs available that have multiple digital filters. So in my mind, a DAC that offered that flexibility would just be an EQ box, otherwise what's the point of offering multiple digital filters?


"More choice is better" for some buyers, even if the choices don't make sense.

DACS with filters

Reply #6 –
Quote from: Rich B on
The reason I asked the question is that I've noticed some DACs available that have multiple digital filters. So in my mind, a DAC that offered that flexibility would just be an EQ box, otherwise what's the point of offering multiple digital filters?


Foolish people with more money than sense.

DACS with filters

Reply #7 –
Quote from: Rich B on
The reason I asked the question is that I've noticed some DACs available that have multiple digital filters. So in my mind, a DAC that offered that flexibility would just be an EQ box, otherwise what's the point of offering multiple digital filters?

I don't see how you logically conclude that this makes it an EQ box.

A DAC needs some kind of filtering in order to work as expected. The design of this filter is a matter that can be discussed, as there are some trade-offs. By offering the user a set of such filters, the user is given some flexibility in over-riding the prioritizations usually set in stone by DAC designers.

Oh, and I don't think that the user (rationally) needs this flexibility any more than he needs the option of replacing the condencers in his amplifier with "audiophile grade" ones.

-k

DACS with filters

Reply #8 –
Quote from: Rich B on
The reason I asked the question is that I've noticed some DACs available that have multiple digital filters. So in my mind, a DAC that offered that flexibility would just be an EQ box, otherwise what's the point of offering multiple digital filters?


Does a filter enable you to alter all the frequency bands or even bass, mid and treble? If not it's not an EQ is it?

DACS with filters

Reply #9 –
Quote from: probedb on
Does a filter enable you to alter all the frequency bands or even bass, mid and treble? If not it's not an EQ is it?


A filter designed for that purpose, yes, of course, but the reconstruction filter of a DAC is a low pass filter, typically designed to only kick in above 18-20 kHz. There are a lot of variations of reconstruction filters, varying in steepness and other parameters, and some are better than others - but none of them are used to alter bass or mid frequencies (OK, I am sure there must be some mad hat "audiophile" DAC out there where that is not the case).

DACS with filters

Reply #10 –
This may give an idea what these filters do or not from a measuring standpoint. Again our member Archimago spent some time doing colorfull things:
http://archimago.blogspot.de/2013/06/measu...ilters-and.html

I have a network player that had an option added at one point to use a slow filter. I was expecting a miracle but only found it killed sound for me. Others found several superlatives for its imorovement.
Measuring the change showed a clear early drop of highs altogether. I am leaning forward to the wording Arnold uses.
If a DAC offers several filters it makes more a toy as a DAC especially when the audible band is clearly bent. The effects of the better ringing behaviour is moot imho.
Is troll-adiposity coming from feederism?
With 24bit music you can listen to silence much louder!

DACS with filters

Reply #11 –
Less (pre)ringing would make (more) sense if the ringing was in the audible range.

Btw, you can to some extent "bypass" the DAC filter by resampling before D/A conversion.
Let's assume your DAC filter reaches the stopband at 0.6*Fs, so roughly 26 kHz for CD audio, 58 kHz for 96 kHz audio ...
If you resample CD audio in your player to 96 kHz, you can completely suppress images >22 kHz to 74 kHz.

Also, if the roll-off started at 0.45*Fs, which is 20 kHz for CD, it would start at 43 kHz for 96 kHz. Similarly, if the DAC filter produced a large phase shift at 20 kHz it would get a lot smaller.
"I hear it when I see it."

DACS with filters

Reply #12 –
Oh and let me add: given the flexibility of the SoX resampler (there's also a foobar2000 plugin), you can do tests yourself.

You can configure steepness, phase, and if you want to allow imaging.
"I hear it when I see it."

DACS with filters

Reply #13 –
Quote from: Rich B on
The reason I asked the question is that I've noticed some DACs available that have multiple digital filters.


I've noticed that if I drive south to Canada a few miles from where I live,  I can pay $4.32 US$ for the same gas that costs about $3.60 in a nearby gas station.  The point is that all sorts of so-called opportunities are available that don't make a lot of sense.

Quote
So in my mind, a DAC that offered that flexibility would just be an EQ box, otherwise what's the point of offering multiple digital filters?


This and other posts I've read lately make me suspect that you may have conflated every component that by accident or intention has a non-flat frequency response with an equalizer. Every component with a non-flat frequency response is not  useful as an equalizer.

Let's start out with the meaning of a common word, Equalize.  To Equalize means to fully correct something so that it meets a certain standard. To be a useful equalizer, a device with a nonflat response needs to be able to introduce frequency response alternations that can compensate for audio systems that are nonflat response in common ways.

Here are the available response curves for a typical CD player with multiple filters:





Here is your challenge: find real world audio systems that will have in some sense ideal frequency response as a result of the application of those curves.

IME they will have a rate of incidence that is on the same order as hen's teeth!

Therefore, this CD player is not generally useful as an equalizer.

An example of a useful equalizer might be a typical parametric equalizer with 5 or more independent sections, and the typical range of frequencies, bandwidths, and ranges of amplitude adjustments.


DACS with filters

Reply #14 –
Quote from: Arnold B. Krueger on
Quote from: Rich B on
The reason I asked the question is that I've noticed some DACs available that have multiple digital filters.


I've noticed that if I drive south to Canada a few miles from where I live,  I can pay $4.32 US$ for the same gas that costs about $3.60 in a nearby gas station.  The point is that all sorts of so-called opportunities are available that don't make a lot of sense.

Quote
So in my mind, a DAC that offered that flexibility would just be an EQ box, otherwise what's the point of offering multiple digital filters?


This and other posts I've read lately make me suspect that you may have conflated every component that by accident or intention has a non-flat frequency response with an equalizer. Every component with a non-flat frequency response is not  useful as an equalizer.

Let's start out with the meaning of a common word, Equalize.  To Equalize means to fully correct something so that it meets a certain standard. To be a useful equalizer, a device with a nonflat response needs to be able to introduce frequency response alternations that can compensate for audio systems that are nonflat response in common ways.

Here are the available response curves for a typical CD player with multiple filters:





Here is your challenge: find real world audio systems that will have in some sense ideal frequency response as a result of the application of those curves.

IME they will have a rate of incidence that is on the same order as hen's teeth!

Therefore, this CD player is not generally useful as an equalizer.

An example of a useful equalizer might be a typical parametric equalizer with 5 or more independent sections, and the typical range of frequencies, bandwidths, and ranges of amplitude adjustments.


In the Richard Clark thread you guys were saying that tubed amps often behaved like non-adjustable, fixed EQ. So ... if one of these digital filters alters the frequency response then why wouldn't it be the same?

DACS with filters

Reply #15 –
In it's simplest form, the low pass filter can be looked at as a fixed crossover. Its job is to steeply attenuate above a predesignated frequency point. In exploiting that property, it can be used to reconstruct the high frequencies.

The important point is that the knee occurs on the fringe of the upper audible range, so its effects as an equalizer (in the traditional sense) are minimal. If it does not alter the audio frequency band enough to be detectable, then I guess we could look at it as not equalizing.

It goes without saying that it's undesirable for the filter to alter the signal and that such behavior should not exist in any idealized design.

DACS with filters

Reply #16 –
Quote from: julf on
A filter designed for that purpose, yes, of course, but the reconstruction filter of a DAC is a low pass filter, typically designed to only kick in above 18-20 kHz.


Cheers, that's what I was getting at

DACS with filters

Reply #17 –
Quote from: Rich B on
In the Richard Clark thread you guys were saying that tubed amps often behaved like non-adjustable, fixed EQ.


That was a joking criticism, and  not the most technically profound statement.

A non-adjustable eq is like a car without a steering wheel.

If I said that all GM cars drove like they had no  steering wheels, would that be a statement of profound technical truth or a joking criticism?

Having real world experience with proper equalizers which do have steering wheels may be required to get the joke.





DACS with filters

Reply #18 –
Quote from: xnor on
Less (pre)ringing would make (more) sense if the ringing was in the audible range.
Yes. Where DACs do have significantly different filters available, if those filters were acting in the audible range (say ~5kHz rather than ~20kHz), the difference between them could/would be audible.

As it is, the differences are usually in the frequency range where most people can hear absolutely nothing (their ears are stone dead to those frequencies), and a few people can hear something if the sound pressure is extremely high. No one hears these differences themselves. I've read some theories as to how people might be aware of them, but these theories don't stack up IMO.

Non-ideal properties of other equipment may cause inaudible frequencies to become audible. This is easily demonstrated with test signals. It's hard or impossible to find this effect with real music. I'm sure most reports of hearing differences between these different DAC filters are placebo, but it's reasonable to play with this stuff IMO.

It's not really an equaliser. Very few of these filter options do anything significant within the audible region. Some of them roll off the upper frequencies, but it's a small effect, and inaudible to older listeners.

EDIT: I may be basing my opinions on out of date information. It seems that more recent DACs with selectable filters have some rather "different" options. To hark back to another thread, I'd call some of these options broken.

Cheers,
David.

DACS with filters

Reply #19 –
Some people use things as equalisers. This includes cables and imperfect kit. They are changing (at least in their heads) the tonal balance to suit their taste, so I suppose one could call that using an equaliser.

Those people are called audiophiles. Not all audiophiles do this stuff: some are quite sane!

Using a chisel to open a paint can doesn't make it a screwdriver.
The most important audio cables are the ones in the brain

DACS with filters

Reply #20 –
It's all already in the recording anyway. If they used a slow roll-off minimum phase filter with aliasing, then on the recording you may have:
a) rolled-off highs
b) large amounts of phase shift in the audible range
c) a little post-ringing potentially in the audible range
d) aliasing (frequencies above Fs/2 folding back potentially into the audible range)

If they used a "normal" filter, then you get:
a) flat highs to maybe even above 21 kHz
b) no phase shift
c) ringing outside the audible range
d) no aliasing or at least outside the audible range
"I hear it when I see it."

DACS with filters

Reply #21 –
Quote from: Thad E Ginathom on
Some people use things as equalisers. This includes cables and imperfect kit. They are changing (at least in their heads) the tonal balance to suit their taste, so I suppose one could call that using an equaliser.

Those people are called audiophiles. Not all audiophiles do this stuff: some are quite sane!

Using a chisel to open a paint can doesn't make it a screwdriver.



and using a screwdriver to open one doesn't make it a...