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Soundcards without reconstruction filters?, Split from Topic ID: 81672
DVDdoug
post Jun 16 2010, 21:05
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QUOTE
and if you google "analog vs digital audio" this image has most hits (among other funnies):
Why is that funny???? Most audio editors represent the waveform as a series of stair-steps. And, the soundcard output from 2 of the 3 computers I use at work (where I have an oscilloscope) is completely unfiltered and looks exactly like the stair-stepped version!

It sounds fine to me... I can't hear the ultrasonic harmonics and I'm sure the speakers are mechanically smoothing/filtering the waveform, and it's probably filtered by the speaker's amplifier too.
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Arnold B. Kruege...
post Jun 16 2010, 21:31
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QUOTE (DVDdoug @ Jun 16 2010, 16:05) *
QUOTE
and if you google "analog vs digital audio" this image has most hits (among other funnies):
Why is that funny???? Most audio editors represent the waveform as a series of stair-steps. And, the soundcard output from 2 of the 3 computers I use at work (where I have an oscilloscope) is completely unfiltered and looks exactly like the stair-stepped version!


What are those sound cards?
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DVDdoug
post Jun 16 2010, 22:58
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QUOTE (Arnold B. Krueger @ Jun 16 2010, 13:31) *
QUOTE (DVDdoug @ Jun 16 2010, 16:05) *
QUOTE
and if you google "analog vs digital audio" this image has most hits (among other funnies):
Why is that funny???? Most audio editors represent the waveform as a series of stair-steps. And, the soundcard output from 2 of the 3 computers I use at work (where I have an oscilloscope) is completely unfiltered and looks exactly like the stair-stepped version!


What are those sound cards?
I only know what Windows Device Manager tells me:
C-Media
"CM18738/C3DX PCI Audio Device"
(This one is a PCI soundcard)

Realtek
"Realtek AC'97 Audio"
(This one is a soundchip on the motherboard)

This post has been edited by DVDdoug: Jun 16 2010, 23:02
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Arnold B. Kruege...
post Jun 17 2010, 13:11
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QUOTE (DVDdoug @ Jun 16 2010, 17:58) *
QUOTE (Arnold B. Krueger @ Jun 16 2010, 13:31) *
QUOTE (DVDdoug @ Jun 16 2010, 16:05) *
QUOTE
and if you google "analog vs digital audio" this image has most hits (among other funnies):
Why is that funny???? Most audio editors represent the waveform as a series of stair-steps. And, the soundcard output from 2 of the 3 computers I use at work (where I have an oscilloscope) is completely unfiltered and looks exactly like the stair-stepped version!


What are those sound cards?
I only know what Windows Device Manager tells me:
C-Media
"CM18738/C3DX PCI Audio Device"
(This one is a PCI soundcard)

Realtek
"Realtek AC'97 Audio"
(This one is a soundchip on the motherboard)



The CMI 8738 is pretty pretty old and very substandard, even when it was new. Its basic converters have only about 8 bits woth of actual resolution, and it is believable that they lacked the output filters required for a clean output. You can obtain superior replacement sound cards for less than $20.

"Realtek AC'97 Audio" is a title that has related to a number of different chips over the years. More modern Realtek chips usually have something in their name about "Hi Definition" and also have proper output filtering, if memory serves.

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2E7AH
post Jun 17 2010, 13:24
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first link from post #10 (of the original discussion) shows oscilloscope output from Realtek AC97

This post has been edited by greynol: Jun 17 2010, 21:23
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DVDdoug
post Jun 17 2010, 19:18
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Sorry for continuing this, I do think we're getting off-topic... The point of my post was NOT to say that "squared-off" waveforms are normal... These are cheap-crappy soundcards/soundchips... My intent was to say that's it not unreasonable to explain digital how digital audio works by showing a quantized (digitized) waveform. (I'd probably do something similar with zoomed-in pixels to explain digital imaging.) It would be more precise to graph the digital data as a series of unconnected dots, but it's more common to see the samples connected by horizontal and vertical lines, and this "zeroth order hold" is exactly what you get from an unfiltered DAC. Of course, that raw-unfiltered signal/waveform should NEVER come out of the soundcard!!!!


QUOTE (Arnold B. Krueger @ Jun 17 2010, 05:11) *
The CMI 8738 is pretty pretty old and very substandard, even when it was new. Its basic converters have only about 8 bits woth of actual resolution, and it is believable that they lacked the output filters required for a clean output. You can obtain superior replacement sound cards for less than $20
WOW! Only 8 bits! ohmy.gif That wasn't obvious from looking at the waveforms. The gross "stair-stepping" is clearly due to the limited number of samples-per-cycle, not the amplitude resolution. (I could see ~2 steps per 20kHz cycle, and IIRC I could see 48 steps on a 1kHz cycle.) No need to upgrade... there are NO SPEAKERS connected to that computer! biggrin.gif

QUOTE (2E7AH @ Jun 17 2010, 05:24) *
first link from post #10 shows oscilloscope output from Realtek AC97
It could be a different "Realtek AC97" chip, or a perfectly-good DAC chip used without a filter.
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2E7AH
post Jun 17 2010, 20:24
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are you saying that your Realtek AC97 chip (DAC) doesn't have reconstruction filter?
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DVDdoug
post Jun 17 2010, 23:00
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QUOTE (2E7AH @ Jun 17 2010, 12:24) *
are you saying that your Realtek AC97 chip (DAC) doesn't have reconstruction filter?
Right! The waveforms are surprisingly rectangular... I don't even see any "accidental" smoothing/filtering. The waveforms look exactly like what I see in GoldWave. If you'd like to see some 'scope images, PM me with your email and I'll dig 'em up.

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saratoga
post Jun 17 2010, 23:16
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Post the scope images. It sounds like you're not measuring correctly (perhaps saturating the scope?).
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2E7AH
post Jun 18 2010, 00:06
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same discussion about your sound cards and even blocky waveform diagrams were discussed more then year ago with more or less same posts by you Doug and Arnold here http://www.hydrogenaudio.org/forums/index....showtopic=69533 biggrin.gif

hopefully someone posted link to The Sampling Theorem that apparently we all know, so one more reminder doesn't seem like bad idea for anyone catching this: http://www.dspguide.com/ch3/2.htm

have a look at some sheets here: http://www.realtek.com.tw/products/product...el=5&Conn=4 - even the lowest chip ALC101, seems to have filter of course:



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DVDdoug
post Jun 18 2010, 04:48
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Here's a 15kHz "sine wave" sampled at 48kHz:



Pretty shocking eh? crying.gif

P.S.
Besides the "blockiness", notice how the frequency/period is wrong. The period is always an exact multiple of of the 48kHz sample rate. (Different waveform "captures" of this signal will have different frequencies/periods, but the average frequency is 15kHz.)

This post has been edited by DVDdoug: Jun 18 2010, 05:10
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WernerO
post Jun 18 2010, 07:51
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This is absurd.

How was the 15kHz source generated?

(Edit: this doesn't matter. But it still is absurd.)


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greynol
post Jun 18 2010, 08:30
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It isn't absurd at all. Put that signal through a reconstruction filter and you will get at 15kHz sine wave.


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WernerO
post Jun 18 2010, 09:53
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I meant that the notion of a soundcard without reconstruction is absurd.

But apparently this is the case with the OP's example.

The datasheet of the CMI**** device isn't very helpful here.
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Cavaille
post Jun 18 2010, 12:00
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This is very unexpected. Remember how some of us talked about the drawing of several programmes like Sound Forge or iZotope? To me it appeared that Sound Forge displays waveforms without taking reconstruction filters into consideration. I did a small, non-representative test with the ASUS Xonar Essence ST (also CMI-based). The results were perfect waveforms.

But this is very unfortunate. Do you use the newest driver? I donīt know too much about the technique but could it be that an "improved" driver could switch on the reconstruction filter? I mean, we are not talking about normal D/A converters here, arenīt we? If I understood it correctly they are basically programmable DSP - so it should be possible in my mind to activate it by improving the driver.

Or am I totally wrong here?


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Arnold B. Kruege...
post Jun 18 2010, 12:50
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QUOTE (WernerO @ Jun 18 2010, 04:53) *
I meant that the notion of a soundcard without reconstruction is absurd.

But apparently this is the case with the OP's example.


Yes, facts are facts.

Since virtually every real world audio system has a system response that includes a high frequency roll-off, there is always a reconstruction filter of sorts, usually unintentionally implemented in the final electroacoustical transducer. Failing that, there is always the inherent roll-off of the human ear.

Needless to say these unintentional reconstruction filters, which have varying and inconsistent corner frequencies, and usually only a 12 dB/octave roll-off, aren't all that great technically speaking.

If you check out the spectrum of a partially-reconstructed digital signal, it will have quite a bit of content at the sampling frequency and odd harmonics of the sampling frequency. There may be a strong image of the original analog signal, starting at the sampling frequency and with all of the frequencies mirror-imaged and working down. However, musical recordings usually don't have strong content above 15 KHz, so the energy in the image is not very strong below about 25 KHz. If the image(s) and harmonics of the sampling frequency don't cause a lof ot IM, they won't have a lot of audible effects.

A digital interface that lacks reconstruction fitlers may be *useful* for emphasizing the audibility of differneces between good SS power amps, and SET power amps, for example. The audible effect might be perceived as "greater air" and "wider sound staging" by some.
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Arnold B. Kruege...
post Jun 18 2010, 13:05
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QUOTE (Cavaille @ Jun 18 2010, 07:00) *
This is very unexpected. Remember how some of us talked about the drawing of several programmes like Sound Forge or iZotope? To me it appeared that Sound Forge displays waveforms without taking reconstruction filters into consideration. I did a small, non-representative test with the ASUS Xonar Essence ST (also CMI-based). The results were perfect waveforms.

But this is very unfortunate. Do you use the newest driver? I donīt know too much about the technique but could it be that an "improved" driver could switch on the reconstruction filter? I mean, we are not talking about normal D/A converters here, arenīt we? If I understood it correctly they are basically programmable DSP - so it should be possible in my mind to activate it by improving the driver.

Or am I totally wrong here?



You are right in that a DAC that lacks a reconstruction filter is not a normal DAC. It is by definition, incomplete.

I wouldn't mix DSPs into the discussion, because they are inherently separate. There are DAC chips that have on-chip DSPs, and there are DSPs that include on-chip DACs, but they are always separate functions.

Audio interfaces are usually implemented one of 3 ways:

(1) One chip with all audio interface functions on it - now the rule for low and medium range audio interfaces. The required functions are a PCI or USB interface, a DSP, ADCs, DACs, and possibly digital line driver and receiver chips if there is a digital interface. Then there are the analog buffers. This is what you find in your typical on-board audio interface, such as the Realtek and CMI chips.

(2) One chip that does the PCI or USB interface, another chip with the DSP functions, and additional separate chips for the DAC, ADC and possible line receivers and drivers. The analog buffers are usually implemented as separate chips as well. This is what you find in most higher end products. Sometimes the DSP functions are implemented not as a DSP but rather as random logic, often using a PLA (programmable logic array).

(3) One chip that includes everything but the ADC, DAC and analog buffers. There are then separate chips that do the rest. The Via Envy series of chips used by M-Audio in their Delta series audio interfaces (and others) are popular example of this.

Of course there are many ways to mix or match these approaches.

One-chip approaches put a premium on efficient use of silicon chip real estate, so minimal implementations of less-critical functions like reconstruction filters are possible. The worst audio interfaces in a PC are usually found in CD player headphone jack drivers.

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Arnold B. Kruege...
post Jun 18 2010, 13:07
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QUOTE (greynol @ Jun 18 2010, 03:30) *
It isn't absurd at all. Put that signal through a reconstruction filter and you will get at 15kHz sine wave.



A wide range of low pass filters will do the job.

The chip specs mentioned in a previous post by 2e7ah show how sloppy these filters are in low-cost interfaces. The pass band does not even go up to 20 Hz, and the stop band is not speced to start until 28.8 KHz. This leaves an approximate 9 KHz band of "everyman's land" which mixes the high end of the desired audio signal with a frequency-reversed image of the same audio signal.

The good news is that hearing, cheap loudpseakers, and audio content that normally does not have a lot above 12-15 KHz partially mitigates the mess.

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Martel
post Jun 18 2010, 13:16
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Could you run the pictured waveform through some sort of spectrum analyzer (on the 'scope)? I doubt there would be much additional/unwanted content in the audible range apart from the 15kHz. So how would this sound different from the properly reconstructed wave?


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pdq
post Jun 18 2010, 13:20
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DVDoug, if you get a chance could you repeat the experiment with a low-amplitude sine wave to see if the DAC has less than 16-bit resolution as well? Or perhaps even better, some low-amplitude noise could show the individual quantization levels.
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pdq
post Jun 18 2010, 13:22
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QUOTE (Martel @ Jun 18 2010, 08:16) *
Could you run the pictured waveform through some sort of spectrum analyzer (on the 'scope)? I doubt there would be much additional/unwanted content in the audible range apart from the 15kHz. So how would this sound different from the properly reconstructed wave?

Wouldn't it be interesting if "audiophiles" were unable to ABX this horrible-looking waveform from an analog-source original? laugh.gif
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WernerO
post Jun 18 2010, 14:52
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Actually this type of soundcard could be marketed to adepts of non-oversampling,
of course at a seriously oversampled price.

The CMI**** datasheet mentions pins named XLFILT and such, and also
user-configurable interpolation for the DAC, but no more detail.
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DVDdoug
post Jun 18 2010, 17:51
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QUOTE (Martel @ Jun 18 2010, 05:16) *
Could you run the pictured waveform through some sort of spectrum analyzer (on the 'scope)? I doubt there would be much additional/unwanted content in the audible range apart from the 15kHz.
Sorry, I don't have access to a spectrum analyzer. sad.gif (Almost everything we do where I work is digital.)

QUOTE
So how would this sound different from the properly reconstructed wave?
I don't hear anything wrong! I haven't done any "critical listening" or ABX tests (I don't have an ABX box anyway). This is "work computer" with cheap speakers that that I occasionally use for background music. Maybe somebody can hear this, but I'm not making any audibility claims one way or another... But it is a FACT that most of the components that create this waveshape are ultrasonic (above human hearing). Take a look at Arnold's post above.

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DVDdoug
post Jun 18 2010, 18:15
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QUOTE
DVDoug, if you get a chance could you repeat the experiment with a low-amplitude sine wave to see if the DAC has less than 16-bit resolution as well? Or perhaps even better, some low-amplitude noise could show the individual quantization levels
Maybe sometime... I have to come into work on a weekend to "play with" the equipment.

You can see noise in the waveform, so I would guess that low-level signals are mostly noise.

IIRC, the waveforms were captured from the CMI soundcard. The Realtek waveform shapes were almost identical, so I only captured & saved waveforms from one system. But I really don't remember if one system showed more noise than the other. (Arnold mentioned that the CMI cards were know "to have only 8 bits worth of actual resolution." I believe it!)

P.S.
As I menetioned before, there are no speakers connected to the system with the CMI soundcard and I'm not using it. The system with the Realtek chip has speakers and I do use it for occasional "background" listening.

This post has been edited by DVDdoug: Jun 18 2010, 18:47
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saratoga
post Jun 18 2010, 18:42
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Clearly I am wrong to doubt there are DACs with filters. But still I wonder if it was designed this way or was meant to work correctly and then failed (bad cap maybe?).
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