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Thread: Audible Jitter/amirm vs Ethan Winer

  1. #111
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    Lovely charts Don. Great way to visualize the data across the range.

  2. #112
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    Lightbulb

    Quote Originally Posted by DonH50 View Post
    And, now you know why we musicians just ignore the ape with the stick up front...
    Q: What's the difference between an orchestra and a bull?
    A: A bull has the horns in front and the assh0le in the back.



    BTW, I have the utmost respect for conductors. Well, the good ones anyway...

    --Ethan

  3. #113
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    Lightbulb

    Quote Originally Posted by amirm View Post
    here is a less silly demonstration by stereophile magazine ... Real jitter will look worse than above
    I know you're kidding, or exaggerating to make the point, but photo "jitter" in realistic amounts would not be visible. As proof, I edited an old photo of me playing the cello and "jittered" it by shifting the image a worst-case -80 dB, which is 10,000 to 1, or 0.0355 pixels for the width of the attached photo. It looks just like the original version, but maybe you can see the jitter?

    --Ethan

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  4. #114
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    Quote Originally Posted by Ethan Winer View Post
    As proof, I edited an old photo of me playing the cello and "jittered" it by shifting the image a worst-case -80 dB, which is 10,000 to 1, or 0.0355 pixels for the width of the attached photo.
    Shifting is not jitter. It is a static delay. Give me the original picture though and I will try to demonstrate something else .

    t looks just like the original version, but maybe you can see the jitter?

    --Ethan
    I think you looked younger in the original picture! Seriously, how do you expect me to answer this with just seeing the modified image???

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  5. #115
    Member Sponsor [Technical Expert] DonH50's Avatar
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    Hey Amir -- Truncation Correction

    Hi Amir,

    I was working on some other stuff and remembered that the truncation plots you presented earlier in this thread had me bothered. They claim to show the results of truncating a 24-bit signal to 16 bits with resulting very high distortion. I responded that truncation was not the same as sampling to 16 bits and that truncation could cause distortion, but that I also felt something else was going on.

    Truncation error occurs at the lsb level of the new (truncated) signal, 16 bits in that example. I ran a few tests and verified that, while truncation does slightly increase the noise floor, and very slightly increases distortion, it is a few dB at most. Nothing like shown in that plot. I think the signal was clipped in the truncation process, based upon the odd harmonic sequence, but there's other stuff going on as the harmonic amplitudes are a little high. Looks sort of like some loop (modulator) issues, and/or maybe perhaps the (noise) filters were not tweaked? Just truncating a 24-bit signal to 16 bits does not cause the output to be much worse than sampling at 16 bits, and in fact in most cases the difference is insignificant. I'm a little confused, nothing new!

    FWIWFM - Don
    Don Herman
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  6. #116
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    Hi Don. I have been meaning to comment also in our differing views regarding quantization noise. I think some of that has to do with our perspective. I always look at the issues there post A/D conversion. There, we use the term quantization noise when we go from one resolution, e.g. 24-bit, to another, e.g. 16-bit. Similarly in video that is captured at 10-bits from telecine equipment and then provided in 8-bit more for consumption on Blu-ray/DVD. That conversion in absence of dither, produces quantization noise. Same thing happens when we encode audio in lossy manner when in frequency domain, we reduce the bit resolution.

    As to your question here, I have not checked the work of the article I linked to. But his results are completely consistent with countless other simulations and practical measurements of the same. I am not home and am typing this on a slow link. But in a quick search online, here is another case of 24 to 16 bit conversion but this time, using Pro Tools (audio workstation software): http://www.themasteringhouse.com/dither/dither.html

    First the oirginal 500 Hz signal:


    Now with it converted to 16-bit without dither:



    Note the peaks at the odd harmonics of 1.5 KHz, 2.5 Khz and so on (500 Hz times 3, 5, etc)

    Now with the same signal with dither applied to it prior to conversion to 16 bits. But this time, he uses "noise shaping" in Pro Tools to push the noise into ulatrasonic region so that it is not audible:


  7. #117
    [WBF Founding Member] Gregadd's Avatar
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    "...Where does he get those toys?"

    Amir Every time I see one of your graphs I think of Jack Nicholson playing the Joker in Batman I. After Batman uses some device to foil the Joker, he remarks:"Where does he get those toys."

  8. #118
    Member Sponsor [Technical Expert] DonH50's Avatar
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    OK Amir, we're getting somewhere! I think we are in agreement, with the caveat that quantization noise, while adding discrete spikes, is not the same as nonlinearity to me. But, look at those first two spectral plots on page 11 of this thread, and compare them to what you just posted. The first plot on page 11 has a noise floor below 90 dBFS or so, with a few distortion spurs sticking above (nonlinearities in the converter, or maybe the signal is slightly overdriving the ADC). Now, after truncation to 16 bits (next plot), you have spikes up around -40 dBFS -- yikes! That does not fit any conventional Nyquist sampling theory I know; I would have expected a few dB rise above a 16-bit noise floor.

    Now look at the plots you just posted. The 16-bit plot has many more spurs, as expected, but their level is still below -100 dBFS or so. Now, that should yield around 16-bit SNR (98'ish dB) and fit the world I know. That's why I feel there is more than just simple truncation going on in your previous plots -- the spurs are just too high to be only truncation.

    One hooker in all this is that it looks to me (from the shape of the noise floor) that you are plotting a delta-sigma converter, not a regular Nyquist (flash, SAR, etc.) converter. Due to the way the modulator loop works and relationship to the filtering going on, dither can have a much larger impact on the noise (spur) floor, but there's more going on in your last plot on this page. You stated he uses noise shaping to push the noise to the ultrasonic region -- that's what a delta-sigma modulator does! This is more than just added dither in-band; the modulator's noise response (transfer function) actually pushes the quantization noise way up in frequency, where it is easily filtered.

    I know you know this, but let me digress for others who might not...

    If I build a perfect 16-bit ADC sampling at 44.1 kS/s, put in a full-scale tone somewhere in the audio band, run the FFT, and calculate the SNR, I'll get about 98 dB. If I double the sampling rate (oversampling by two), the noise is now spread over about 40 kHz instead of 20 kHz. The total quantization noise (energy) is the same, but spread over twice the bandwidth. Now, if I filter out the upper half I don't need (20 to 40 kHz) and use only what's left in the audio band that I care about, I've gotten rid of half the noise, and the SNR goes up by 3 dB (1/2 bit). Double again, gain another 3 dB, and so on. We gain 0.5 bits (3 dB) for each doubling in the oversampling ratio (OSR).

    The magic of a delta-sigma converter (ADC or DAC) is that the signal transfer function is not changed much (it still passes the signal through), but the noise transfer function is shaped so that noise gets pushed to the upper end of the Nyquist band. The total noise is the same, but the frequency response is much different; at low frequencies it is very low, and at high frequencies it is very high. Now, when we oversample and filter out the unused upper frequencies, we are getting rid of a larger portion of the noise, and end up with much higher SNR in the signal band. In fact, for an ideal M-order modulator and perfect noise filters, we achieve (M+0.5) bits (or, (M+0.5)*6 dB) for each doubling in OSR. Where our simple Nyquist converter gains a lowly 3 dB when we double the sampling rate, a 3rd-order (relatively low today) delta-sigma modulator gains 21 dB for that same doubling -- and delta-sigma modulators operate with much, much higher OSRs so the effect is large. In the real world, naturally, other implementation issues limit the SNR, not these theoretical limits...

    So, I think we are on the same page, finally. I just needed to know what was really going on, and you needed me to clarify my position based on what I saw and how I related to it. Please let me know if I still glitched somewhere...

    Whew! - Don
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  9. #119
    Addicted to Best! marty's Avatar
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    First, congratulations to Amir and Ethan for a very informative "debate". Both sides presented their arguments well. My reading and observations nets out on the side of Amir. Ethan's key argument that jitter, if it is 100dB below the signal, must by definition be inaudible, reminds me of the old argument that an amplifier's performance at 40KHz must be irrelevant since we can only hear up to ~20KHz. We now know that not to be the case as the harmonics of signals at 40KHz may be heard in the audible range. The main reason I think jitter is likely audible is the recent work of some vehement anti-jitterholics such as Ed Meitner, whose recent effort , the XDS1, greatly impressed me.
    http://www.whatsbestforum.com/showth...0031#post20031
    Perhaps the reasons I was so impressed have to do with more than jitter reduction, but I have to defer to Meitner when he states his belief this is in large part the case. In any case, thanks to Amir and Ethan for their invaluable education. I remember the days there used to be great articles in Audio Magazine on technical stuff like this, but those days are long gone. "What's Best" is a more than satisfactory replacement because of the inherent strengths of blogging in real time!

  10. #120
    [WBF Founding Member] Ron Party's Avatar
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    Quote Originally Posted by marty View Post
    We now know that not to be the case as the harmonics of signals at 40KHz may be heard in the audible range.
    I must have missed this one. How do we know that?
    Peace.

    Ron Party

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