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Old 22nd March 2014, 10:05 PM   #2291
miero is offline miero  Czech Republic
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Notice: The I2S source is BeagleBone Black with internal low quality oscillator and the Linux with sound driver I've wrote -- http://bbb.ieero.com/ ... so there might be problem with my driver also.

I've been doing more research on the roll off noise issue with ES9018 described above.

Recent findings:
- the white noise in SoX->BBB->I2S->Buffalo is louder by 6dB than ARTA->PC->SPDIF->Buffalo
- when playing via I2S with volume reduced by 6dB the roll off noise is actually better than with SPDIF
- white noise with volume reduced by less than 6dB has roll off issues

$ play -r 48000 -c 1 -b 24 -n synth white

The stats of "full volume" white noise generated by SoX:
DC offset 0.000745
Min level -0.999998
Max level 0.999999
Pk lev dB -0.00
RMS lev dB -4.77
RMS Pk dB -4.58
RMS Tr dB -4.98
Crest factor 1.73
Flat factor 0.00

play -r 48000 -c 1 -b 24 -n synth white vol -6dB

The stats of "-6dB" white noise generated by SoX:
DC offset -0.000290
Min level -0.501187
Max level 0.501187
Pk lev dB -6.00
RMS lev dB -10.77
RMS Pk dB -10.55
RMS Tr dB -10.99
Crest factor 1.73
Flat factor 0.00

From the stats I think that full volume white noise should not cause the issues. Or am I wrong and the volume must be really reduced?
Attached Images
File Type: png white_noise_0dB_i2s.png (33.5 KB, 308 views)
File Type: png white_noise_-3dB_i2s.png (32.9 KB, 306 views)
File Type: png white_noise_-6dB_i2s.png (32.7 KB, 282 views)
File Type: png white_noise_0dB_spdif.png (32.1 KB, 280 views)
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Old 23rd March 2014, 09:02 AM   #2292
miero is offline miero  Czech Republic
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The same situation happens with SPDIF input.

Measurement setup (Linux):
SoX -> Int. soundcard -> SPDIF -> Buffalo 3 -> Preamp -> Juli@ -> ARTA

But now, the SPDIF white noise -6dB is correctly filtered. The measurement in previous post was done on different PC with Windows.

Is the white noise with peaks at -0dB a valid test signal?
Attached Images
File Type: png spdif_white_-6dB_peak.png (39.2 KB, 252 views)
File Type: png spdif_white_-0dB_peak.png (39.5 KB, 21 views)
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Old 23rd March 2014, 10:21 AM   #2293
miero is offline miero  Czech Republic
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I don't know the theory behind, but roll-off filter seems to fail when oversampling operation in the DAC makes the audio clipped.

Two more graphs of square wave frequency spectrum.
Attached Images
File Type: png spdif_48k_square_-1.9dB.png (40.5 KB, 31 views)
File Type: png spdif_48k_square_-0.0dB.png (41.4 KB, 28 views)
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Old 23rd March 2014, 10:56 AM   #2294
peufeu is online now peufeu  France
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DAC oversampling is not supposed to clip the output, it should have a safety margin, unless the DAC's filter coefficients are a bit too hot...

Can you post a scope trace of what happens during this clipping ?
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Old 23rd March 2014, 09:08 PM   #2295
PMA is offline PMA  Europe
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It may happen easily.
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File Type: png sq_minusnulapetdB.PNG (54.4 KB, 61 views)
File Type: png sq_minusnulapetdB_overs.PNG (33.4 KB, 60 views)
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Old 23rd March 2014, 09:14 PM   #2296
miero is offline miero  Czech Republic
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The most extreme case - 1kHz square wave at full volume -0dB ... 48kHz / I2S

It clips much and the roll-off filter does cut at 24kHz.
Attached Images
File Type: png square_1k_-0dB_48k_i2s_SoX_wave.png (32.7 KB, 56 views)
File Type: png square_1k_-0dB_48k_i2s_SoX_spectrum.png (28.3 KB, 52 views)
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Old 24th March 2014, 11:29 PM   #2297
Bunpei is offline Bunpei  Japan
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Hi, meiro!

As far as I know, you have never shows details about your Buffalo DAC and its settings.
I'm sorry if my understanding is incorrect.

Do you set "SPDIF auto detect = OFF" when you make experiments through I2S?

Bunpei
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Old 25th March 2014, 06:23 AM   #2298
miero is offline miero  Czech Republic
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You are right, here is my B3 settings & wiring:
- SPDIF autodetect OFF
- Oversampling & reclocking ON
- Fast FIR roll-off
- DPLL Lowest, 128x multiplier (BBB has low-quality onboard clock)
- Quantizer 6, True differential, outputs joined
- "Buffalo II compatible input wiring configuration" for both PCM & DSD stereo input
- Input remapping ON

Nevertheless, I've been testing the most extreme cases when data gets clipped during oversampling. In the normal conditions it works perfect. Sorry for confusion if it was not clear.
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Old 25th March 2014, 07:14 AM   #2299
peufeu is online now peufeu  France
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OK, so the DAC doesn't have enough headroom built-in... at least it clips cleanly and doesn't wrap around like some buggy ones do !

Since it is just a setting (not a hardware bug) it will go away if you reduce the digital volume a little.

The spectrums you get are normal, if the oversampled signal clips, it will generate out of band noise.
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Old 25th March 2014, 07:27 AM   #2300
PMA is offline PMA  Europe
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Quote:
Originally Posted by peufeu View Post
OK, so the DAC doesn't have enough headroom built-in...
Almost none have. Oversampling DACs cannot handle artificially generated signals like 0dBFS square in principle (oversampling + interpolation principle).
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