CHANGSTAR: Audiophile Headphone Reviews and Early 90s Style BBS
Lobby => Amp and DAC Measurements => Topic started by: firev1 on October 28, 2014, 04:51:56 PM
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Did some measurements recently to check out the implementations of various reconstruction filters to take a look at how they affect the analog output. Fortunately I have dacs on hand that have some variability in their filters to take a look at this.
Some subjective discussion on various filter types and especially Han's thoughts about the various filters of a Gamma 2 he tried are here.http://www.changstar.com/index.php/topic,1535.msg40729.html#msg40729 (http://www.changstar.com/index.php/topic,1535.msg40729.html#msg40729)
Personally I have little to say about filter sounds except that I find the Ipod4g Touch's audio performance to be more than reasonable for my uses. The products themselves are too far apart in how they are built and their analog performance to provide any useful subjective opinions on filtering.
Keeping in mind the 0404's ADC limitations with rising noisefloor with increased bandwidth, I thought the measurements I did with the DACs on hand were useful presentations of their filter performance.
Methodology used can be found in JA's article: http://www.stereophile.com/content/arcam-fmj-d33-da-processor-follow-april-2013 (http://www.stereophile.com/content/arcam-fmj-d33-da-processor-follow-april-2013)
Test Regime:
DAC sample rate: 44.1khz ADC sample rate: 192khz
A spectrum of -3dbFS white noise and a spectrum of a full scale 19.1khz test tone was combined to form a graph. 19.1khz being very close to the upper limit of the 44.1Fs bandwidth, resulted in an aliasing product at 44.1-19.1=25khz.
White noise is played to simply observe the rolloff characteristics of the DAC's filter.
First up is the Ipod4g Touch which uses minimum phase filters in a sharp rolloff, suppression of the aliasing product is not complete but reasonable I reckon. The Ipod touch might be using some inband noise shaping to provide better measurements within the audio range. With the white noise, some rippling can be seen in the stopband, this is more of a filter design decision(high order but lower rejection) and not a limitation of the analog circuitry.
(http://i.imgur.com/TvSP165.png)
Next the Audio GD NFB2 which uses a linear phase slow roll-off filter. Slow roll-off means little rejection of aliasing products. AGD having no feedback, harmonic products of the 19.1khz tone and the 25khz aliasing product subside into the noise floor as frequency goes up.
(http://i.imgur.com/VZRVQc3.png)
Last is off course the ODAC with its traditional linear phase brickwall. Excellent aliasing rejection of ~-118db. Unlike the AGD harmonics go all the way past the measurement bandwidth.
(http://i.imgur.com/LWxBBE9.png)
Forgot to put in the wideband noise measurement for the 0404 as a baseline reference.
(http://i.imgur.com/q4hcNuF.png)
Disclaimer: Maybe you can hear it, maybe you can't? LOL YMMV, not Science, BS.
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Nice work!
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Interesting. The ODAC seems to have a fairly low noise floor and a pretty strong brickwall there (almost Elliptic like). Not sure where those distortion products (N*19.1kHz) are coming from but seem to be fairly common.
Looking at the Audio GD NFB2 website, it seems one may be able to configure the filter response depending on some jumper settings:
http://www.audio-gd.com/Pro/dac/NFB-2/NFB2EN.htm
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Mine is the very first NFB2 though so its only factory configurable sadly. I too have no idea where the N*19.1 distortion products are coming from but I hope with more measurements in the future its not the ADC section of the 0404 since I'm unable to do a loopback of this particular measurement. The products between the Ipod4g and the ODAC seems to differ enough to make me think that the 0404 is sufficient for the measurement though.
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Though part of it could be interaction between the DAC and measurement equipment, I don't necessarily think it your 0404. Consider AJ's uber-AP dScope-pooper size-matters rig shows this as well for pretty much all DACs to varying degrees.
Maybe the DAC opamps do this... Dunno.
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Maybe, could be anything lol, added in baseline ADC measurement for reference. I use the high Z 1Mohm input for 2V DACs(ODAC, NFB2) whenever possible. For the low power devices though, I use the 1.5kohm input though(XLR to RCA adapters).