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Old 3rd May 2008, 09:22 PM   #1
Netlist is offline Netlist  Belgium
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Default Analogue vs. Digital RIAA

Iíve been experimenting with digital RIAA equalization over the last couple of days and after many trial and error and having read a lot about the subject, I brew my own setup which is a combination of available equipment and software, and a mix of knowledge I gathered together on this forum along with bits and parts of what I found on the web.
(Thanks to everyone for sharing so much valuable info all over this mighty cyberspace)

The basic idea was to minimize the analogue signal path as much as possible and do everything in the digital domain.
I know a little about analogue and very little about digital which will probably reflect in this post but nevertheless, I achieved Ďsomethingí.

Having a MC cartridge that likes a 47k input resistor on the preamp, it was not a good idea to plug it directly into the M-audio Firewire 410 instrument input which has something like 3k input impedance although that would have been the shortest possible path.

So, I took my beloved Ono clone, consisting of the MC and the MM part and I separated both sections. The reason I kept the MC part was
1) I needed the 47k input impedance and
2) I could use the gain of that stage because, even if the gain of the M-audio instrument input is fairly high, it couldnít provide enough output with a low enough S/N ratio.

I recorded this non RIAA-equalized signal with Audition and applied an fft filter with the inverse RIAA parameters.
Before that, I removed the ticks and pops and afterwards, I amplified the signal by something like 4dB and finally compared the result with the same recording I previously made with the full fledged Ono.
I have read somewhere that all this is not a very good idea and one of the reasons would be that the low frequencies would be captured with a too low resolution.
Because the RIAA curve boosts the bass at the lowest regions by about 20dB, one needs to record at least 20dB below 0dBfs without the analogue RIAA equalizer.
I donít get the bass-and-resolution part very well so I would be pleased if someone could explain a bit on the subject.
In any case, comparing both recordings, I have the impression that the new (to me) technique sounds better. Visually (Waveform display), I can see more dynamics but Iím aware that my eyes could trick my ears.
Iíll upload a small extract of both files somewhere and would be happy if some of you could have a listen and let me know what is most pleasing to the ear.

/Hugo
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Old 3rd May 2008, 10:02 PM   #2
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Link to the file:
http://rapidshare.com/files/112338404/Extract.flac.html
The moderate large (13.376 kB) file is encoded with flac and thereís a 2 sec. silence gap between both pieces. Enjoy.

/Hugo
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Old 3rd May 2008, 10:46 PM   #3
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Actually it's 13 megabytes, too big for my dial-up, but I'm very interested in the results. From a pure technical standpoint you shouldn't be able to equal the results from a good analog RIAA section. OTOH, if you record the flat signal with enough bits, say 24, and if you can keep the s/n ratio high enough, I'd think you could get a good enough result to make it hard to tell the difference. I've found you have to be very careful of things that sound better during a short listening trial; often they have some subtle flaw that shows up under long term listening. There's also the issue of how well the two RIAA curves match; this is both harder to do and more critical than one might think. You could easily be hearing a mismatch between the hardware and software equalization.
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Old 4th May 2008, 07:35 AM   #4
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The recording was indeed 24bit and I'm well aware of the many culprits of critical listening. I'll burn the two versions on different CD's, mix them up and try to find out which one sounds best.
I know the Ono has a very accurate RIAA response, and I checked my diy version to make sure I build it right.
So, if the fft filter is as good as the Ono, the only difference should be the greater resolution due to a smaller signal path.
I have uploaded a VBR MP3Pro file which is only 2.2Mb but notice that much of the fine details are lost:
http://rapidshare.com/files/11241074....flac.mp3.html

/Hugo
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Old 4th May 2008, 08:02 AM   #5
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I've listened to the Flac-version, and frankly, i can't hear any big differences.
Admitted, i'm listening via my computer, but with a fairly good amp, and a pair of reasonable speakers.

You are making me curious about your fft. I've tried this trick before with Cool dit pro, ( now called Audition ), but wasn't satisfied.

Are you interested in sharing your fft ?

Best regards
Ebbe
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Old 4th May 2008, 08:50 AM   #6
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Of course and I'm trying to refine it ATM.
When recording with AA and perhaps also Cooledit, the fft range varies with the sample rate.
If you record with a 44.1 sample rate the range is 21.39Hz up to 22.050kHz.
I've inputted about thirty frequency points on the scale with the corresponding RIAA numbers and I'm currently comparing a wide range of frequencies by generating sine waves with a fixed frequency and let the fft do the job. Then, with the amplitude stats, I calculate the difference between the original and the RIAA'ed amplitude. The fft size is 8192 points, which should be pretty accurate. I'll post the RIAA curve if you can't find it here on the forum.

/Hugo
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Old 4th May 2008, 09:03 AM   #7
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The "Why doing RIAA digitally is a bad idea" argument is all to do with dynamic range and easily explained. Draw yourself a graph of dynamic range (in dBs) against log of frequency, then add the RIAA curve at the top with the 50Hz peak just touching 0dBFS (dB Full Scale). Now draw a straight horizontal line at -110dB to correspond to the noise floor of a good practical ADC. Now look at the distance between the RIAA curve and the ADC noise floor at various frequencies; this is your new dynamic range. Finally, consider what happens with variable recording levels, cartridge sensitivities and clicks and bangs. Now you know why people don't usually like to do RIAA digitally.

Still, it's an interesting experiment...
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Old 4th May 2008, 09:55 AM   #8
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Hmm..have to ponder a bit on this one.
Let's say I generate a 50Hz sine wave at -17.03dB (that's what is needed to make the RIAA eq boost it to 0dBFS). Then I'm stuck...
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Old 4th May 2008, 06:55 PM   #9
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Fwiw i didn't like the sound of either segment too much. Still, would prefer the first one for low level resolution and ambience. The second sounds a bit warmer and distortion may well be higher.
Listened through a pair of Leak TL12.1 which i just finished restoring. My normal amp will likely make the difference even more noticeable.
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Old 4th May 2008, 08:16 PM   #10
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Thanks for taking the time.
I will not yet reveal who is who as to give others a chance to listen unbiased.

I'm slowly getting a grip on the dynamic range issue.
I statistically compare both files and the digitally eq'd one has a used dynamic range of 32.95dB, while the analogue RIAA has 38.65dB.

/Hugo
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