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Old 27th February 2015, 11:29 PM   #231
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Quote:
Originally Posted by derekr View Post
Paul, did you ever go back and alter the NOS file posted?
I did, but haven't listened to it as yet.

This is basically a "full bypass" in that all filters except the 2 for cd de-emphasis are set to a single coefficient of 1.000 and a gain of 1.

cheers
Paul
Attached Files
File Type: zip 1021filtNOS_fullbypass.skr.zip (559 Bytes, 46 views)
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Old 28th February 2015, 12:38 AM   #232
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Quote:
Originally Posted by robertrowett View Post
I am a little confused about FIR2, is it also bypassed/upsampling by the NOS file or is it still active? What is it doing if it is not bypassed?
It wasn't bypassed in the previous version. FIR2 is set well above audio - both are flat out to 100kHz and is -80dB by either 220/240kHz.

I've attached a few screenshots of the first 60 seconds of Miles Davis's "So What".

First is a straight rip from CD:
Click the image to open in full size.

Second is up sampled x 8 with no FIR filtering applied:
Click the image to open in full size.

Third is a basic linear phase FIR filter. You can see images of the sub-2kHz audio mirroring. These images are around -80 to -90dBFS.
Click the image to open in full size.
Attached Images
File Type: png SoWhat.png (644.5 KB, 1039 views)
File Type: png SoW_nofir.png (619.9 KB, 949 views)
File Type: png SoW_remez441.png (205.0 KB, 654 views)
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Old 28th February 2015, 12:45 AM   #233
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continuing...

Next one is upsampling x8 of the upsampled x8 SoWhat, with no filtering applied:
Click the image to open in full size.

And as above with only FIR2 filter applied
Click the image to open in full size.

The analog filter at the DAC output has a corner frequency of 212kHz so this is not what you see at the output terminals. Even so it should illustrate with the 2 tiers of FIRs are doing in conjunction with up/over sampling.
Attached Images
File Type: png SoW_nofir2.png (617.8 KB, 673 views)
File Type: png SoW_fir2.png (308.0 KB, 655 views)

Last edited by spzzzzkt; 28th February 2015 at 01:00 AM.
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Old 28th February 2015, 01:00 AM   #234
derekr is offline derekr  Barbados
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Quote:
Originally Posted by spzzzzkt View Post
I did, but haven't listened to it as yet.

This is basically a "full bypass" in that all filters except the 2 for cd de-emphasis are set to a single coefficient of 1.000 and a gain of 1.

cheers
Paul
Thanks for providing it. Shall listen to it. I am going through the available filters one by one. Fun stuff.
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Old 28th February 2015, 08:47 AM   #235
TNT is offline TNT  Sweden
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spzzzzkt, are these measuremnts on the analog outputs or analysis/simulation of the digital domain?

//
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Old 28th February 2015, 09:12 AM   #236
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Quote:
Originally Posted by TNT View Post
spzzzzkt, are these measuremnts on the analog outputs or analysis/simulation of the digital domain?

//
Digital domain.

see Filter brewing for the Soekris R2R

Stage 1:
Code:
sox /path/to/input.wav -b 24 -r 352.8k /path/to/upsampled_output.wav upsample 8 \
[fir /path/to/fir1_coefficents.txt vol 8]
Stage 2:
Code:
sox /path/to/upsampled_output.wav -r 2822.4k /path/to/final_output.wav upsample 8 \
[fir /path/to/fir2_coefficents.txt vol 8]
Purpose is to visualise the cascading effect of FIR1 and FIR2 not to analyse the DAC output, as the question posed was "what is the purpose of the FIR2 filter?"

Last edited by spzzzzkt; 28th February 2015 at 09:14 AM.
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Old 28th February 2015, 09:13 AM   #237
TNT is offline TNT  Sweden
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OK - thanks!

//
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Old 28th February 2015, 03:25 PM   #238
derekr is offline derekr  Barbados
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Quote:
Originally Posted by spzzzzkt View Post
I did, but haven't listened to it as yet.

This is basically a "full bypass" in that all filters except the 2 for cd de-emphasis are set to a single coefficient of 1.000 and a gain of 1.

cheers
Paul
Paul, I've loaded this twice and both times the volume is very low - at V00 it's just audible.
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Old 28th February 2015, 03:35 PM   #239
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Thanks Paul, I'll give the full bypass a listen this afternoon.
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Old 28th February 2015, 09:29 PM   #240
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Quote:
Originally Posted by derekr View Post
Paul, I've loaded this twice and both times the volume is very low - at V00 it's just audible.
I guess you must be running SE or unbuffered.

Yes it is quiet, but you did ask to listen to NOS for both filters with gain multipliers set to avoid adding distortion

Upsampling x8 in the DAM is done by inserting 7 zero values between each pair of samples. This means that the energy of the original audio is reduced to 1/8th of the original, or -18dB.
Because NOS retains the 7 mirror images the total energy in the data is the same as the original audio but you can only hear the energy within the audio band. Adding gain to the x8 up sampled and unfiltered audio data will cause data to exceed 0dBFS - in other words it will clip.

There is a bit of wiggle room because Søren has said that the processing done in the FPGA has 2-4 bits headroom. As we don't know which applies under what circumstances we have to assume the lower figure is going to be relatively safe.

1 bit allows double the amplitude of a signal or 6dB gain, 2 bits allows x4 the amplitude or 12dB gain, and 3 bit allow x8 amplitude or 18dB gain.

You possibly see from this how the multiplier is used in the DAM filter configuration, and why the x8 oversampled settings which reduce energy in the filtered file to 1/8th the original require the data to be multiplied by 8.

Anyway back to the NOS.

This is the filter config for the too quiet "double bypass NOS"

Code:
dam1021,44100,8,1,1,1
Input FIR, 44.1 Khz Samplerate, Bypass
1.000

dam1021,48000,8,1,1,1
Input FIR, 48 Khz Samplerate, Bypass
1.000

dam1021,88200,4,1,1,1
Input FIR, 88.2 Khz Samplerate, Bypass
1.000

dam1021,96000,4,1,1,1
Input FIR, 96 Khz Samplerate, Bypass
1.000

dam1021,176400,2,1,1,1
Input FIR, 176.4 Khz Samplerate, Bypass
1.000

dam1021,192000,2,1,1,1
Input FIR, 192 Khz Samplerate, Bypass
1.000

dam1021,352800,1,1,1,1
Input FIR, 352.8 Khz Samplerate, Bypass
1.000

dam1021,384000,1,1,1,1
Input FIR, 384 Khz Samplerate, Bypass
1.000

dam1021,384000,8,2,1,1
Final FIR, 384 Khz Samplerate, Bypass
1.000

dam1021,352800,8,2,1,1
Final FIR, 352 Khz Samplerate, Bypass
1.000

dam1021,352800,8,30,5,1
Deemphasis IIR, 352.8 Khz Samplerate 50/15 uS
0.3533735087400593
-0.6245393735573087
0.2759469933909753
1.8617085892222387
-0.8664897177959644

dam1021,384000,8,30,5,1
Deemphasis IIR, 384 Khz Samplerate 50/15 uS
0.3520148312325706
-0.6284427828676498
0.2804855763857754
1.8726010243259992
-0.8766586490766952
I suspect that to avoid issues at higher bit rates the gain will have to be distributed between the fir1 and fir2. I will have to clarify the range of headroom with Søren...

The mods below add 6dB gain at FIR1 and 6dB at FIR2.

You'll have to test this for yourself and there is no guarantee it will not clip.

Code:
dam1021,44100,8,1,1,2
Input FIR, 44.1 Khz Samplerate, Bypass
1.000

dam1021,48000,8,1,1,2
Input FIR, 48 Khz Samplerate, Bypass
1.000
Code:
dam1021,384000,8,2,1,2
Final FIR, 384 Khz Samplerate, Bypass
1.000

dam1021,352800,8,2,1,2
Final FIR, 352 Khz Samplerate, Bypass
1.000
cheers
Paul
Attached Files
File Type: zip 1021filtNOS_fullbypassv2.skr.zip (543 Bytes, 24 views)
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