Digital Bass Management and Clipping Distortion

Discussion in 'Audio Hardware' started by multiformous, Jul 16, 2017.

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  1. multiformous

    multiformous Active Member Thread Starter

    Location:
    Newfoundland
    Here, I've prepared a very specific example of how audible clipping distortion may arise from using a typical bass management system in an A/V Receiver or audio player. I recorded the song "Yoru Ga Mita Keshiki" from the album Landscape In Portrait by Marihiko Hara. The file I played is the FLAC ripped from the CD. There is no clipping in the original. I chose this because it's the album I was listening to when I first heard this distortion and sought to find out what exactly was causing it. Long story short, using the "Pure Audio" setting on my receiver cleaned the distortion immediately and I reasoned it was due to the filtering involved in the bass management system.

    Instead of recording from the pre-outs on my reciever, however, I've opted to record directly from the analog output of my Oppo UDP-203 with and without the player's bass management system enabled. The audible results are the same as using the receiver's bass management system: The filters cause the signal to be clipped before it reaches the analog output stage of the player (feeding the full range digital signal to the receiver with bass management enabled results in distortion in the main channels when amplified). You can hear it for yourself:

    Full Range clip
    Bass Managed clip

    And for a surgical look, here are some waveform/spectral analysis screengrabs:

    Full range (waveform)
    Bass Managed (waveform)
    Full Range (spectral)
    Bass Managed (spectral)
    Full Range (spectral zoom)
    Bass Managed (spectral zoom)
    Full Range (waveform zoom)
    Bass Managed (waveform zoom)

    At bottom, this is due to a lack of sufficient headroom in a digital recording for the kinds of processing typically employed by an audio processor or A/V receiver. The hardware or bass managements systems aren't implemented in error, per se. But this is something this is technically happening all the time when bass management is enabled; it just is more or less audible depending on the program material. If music is mastered near the digital maximum, then it doesn't take much to push it over the edge when a filter is applied. Just look what happens if I simulate the bass management system by using a simple high pass filter on this Dirty Projectors song:

    Original waveform (note zoomed out dB scale on the right)
    High pass filter settings
    High pass filtered waveform

    That's an awful lot of stuff "above" 0dB, isn't it? Now imagine this filtered version hitting the DAC and then the amplification stage... Not pretty. The "easy" solution here is to create headroom, obviously. However this isn't always easy with digital sources depending on the player. If you're playing from a computer or something, just turn down the volume (in Spotify, foobar, Windows, etc.) before it hits the receiver's processor. Or turn down the level of the analog output of a player (like the Oppo) before it hits the receiver's ADC and bass management system. That adds an additional conversion, but them's the breaks.
     
    Last edited: Jul 16, 2017
  2. back2vinyl

    back2vinyl Forum Resident

    Location:
    London, UK
    If this is "technically happening all the time when bass management is enabled," what audio enthusiast would ever use an A/V receiver with bass management enabled for playing music? And yet they do. I would like to think it's something that only happens in certain circumstances and I would be interested to know what those circumstances are.

    Interesting post and thanks very much for going to so much trouble to share.
     
  3. pdxway

    pdxway Forum Resident

    Location:
    Oregon, USA
    Hmm, going from either Oppo's HDMI or Roku's HDMI to my Pioneer SC-95, playing bunch of piano, pops, electronica, rocks from CD, Blu-ray, wav files, or YouTube videos, I have yet to encounter such issue.

    I crossover at 100hz to my sub via the Pioneer.

    Your observation sounds like a preamp is output at max. For example, when I turn my external DAC to max volume to amp weak signals, the speakers would have similar kind of distortion.
     
    Last edited: Jul 17, 2017
  4. multiformous

    multiformous Active Member Thread Starter

    Location:
    Newfoundland
    When the bass management filters are enabled, they are doing weird things to the signal. I think it's some combination of phase shift and the Gibb's Effect. I don't know why or how, but the filtering adds some gain or 'overshoot' to the original signal the causes amplitude changes. If the original signal is near 0dB, this overshoot can exceed the digital maximum. Bass management is a digital process, and if there isn't enough headroom for the processing then the signals are clipped. This is a simple, inescapable fact. These filtering effects are happening, but will not always cause distortion (especially if the original file has enough headroom, or if the effects are minimal), and may not necessarily cause audible distortion. But in a digital bass management system, this is happening. (And yes, I've tested multiple receivers and players, etc. Turn the filters on, you get distortion. Go full range or Direct or Pure Audio, distortion is gone. Chris Kyriakakis of Audyssey confirmed it. And yes, I've noticed distortion on other recordings before, but when I went searching for the distortion later in headphones, I could never find it.)

    Perhaps it is better illustrated this way. Below are the original waveform for "Yoru Ga Mita Keshiki" (FLAC, ripped from CD) and the high pass filtered version (80Hz, 12dB slope) as processed by Audacity. This simulates what's going on in the digital bass management system. If you're not familiar with the software, those red lines in the second image alert us to clipped signals. Sure enough those are precisely the spots that distort when I play this file in a bass managed setting. (Try playing the song yourself from Spotify using the link. Turn on your bass management, and listen for the distortion -- be sure the player volume is at 100. Then try turning off bass management, or adjust the volume in the Spotify player window until enough headroom is achieved to defeat the distortion.)

    [​IMG]

    [​IMG]
     
  5. pdxway

    pdxway Forum Resident

    Location:
    Oregon, USA
    I just did a test with your Full Range clip.

    Clip in USB drive -> Oppo 203 -> HDMI to Pioneer SC-95 receiver.

    Pioneer does the bass management. Crossover is 100 Hz. Sub is 6 dB hot.

    Play back measured with SPL meter at listening position, C weighted fast, peak at 90 dB. Did not observe distortion.

    Note that it is at -10 playback level of my receiver. I still have about 16 dB to go before reaching max volume of my receiver.

    Would you please do the same and see if there is distortion?
     
  6. Mike-48

    Mike-48 A shadow of my former self

    Location:
    Portland, Oregon
    When digital is involved, you have to manage the gain path so there are no overlevels. The original track seems quite compressed, with many peaks at 0 dB. So if the bass management is raising levels anywhere, you will have clipping. (Well, I suppose you know that already.) Is there no adjustment in your system that lets you turn down the incoming digital level by, say, 3dB to 6 dB before processing? There should be!
     
  7. pdxway

    pdxway Forum Resident

    Location:
    Oregon, USA
    BTW, I regularly test bass tones recorded at 1db below maximum possible volume for CD in my system. Let's say that my system is built for playing very loud. May be that was why I didn't hear any clipping using your Full Range Clip.
     
  8. multiformous

    multiformous Active Member Thread Starter

    Location:
    Newfoundland
    No, there is no such setting and this would be the ideal solution -- a setting that would attenuate incoming digital signals before it is processed for bass management. But turning down the volume in foobar works (when playing digital audio via optical/HDMI directly from my computer to my receiver), which is what I do now if I want to listen in a bass managed configuration. A -6dB setting would probably be enough based on this filtered waveform, though it would also be nice if recordings themselves had more headroom... but we all know that's not gonna happen.

    The Full Range Clip I provided in the original post is a recording of the analog output of the Oppo UDP-203. It is absolutely nothing to do with the levels of my playback system or how loud my receiver is. Indeed, the distortion is just as audible at low levels as it is high levels. The distortion is introduced before the amplification stage; the signal is clipped when it undergoes digital to analog conversion.

    You probably won't hear the distortion if you try the Full Range Clip I provided because that clip doesn't reach -0dB like the original untouched recording. I provided that clip to show the effect of the bass management filter. It has nothing to do with playback levels, and everything to do with the levels of the recording itself and how the bass management filters affect those signals before they undergo digital to analog conversion. I already provided a link to particular song I used in this example on Spotify in the original post, but if you'd like an untouched clip of the original FLAC recording for testing purposes, see below. Listen to this clip in a bass managed setup and in a Direct / Pure Audio mode on your receiver. You don't need to turn it up. Just play the file without making any volume adjustments before it gets to your receiver (so play it via USB or Network or something).

    09 - Yoru Ga MIta Keshiki (untouched clip)
     
  9. pdxway

    pdxway Forum Resident

    Location:
    Oregon, USA
    I check all three files just now.

    Full Range - no distortion
    Bass manage clip - very obvious, can hear distortion on both left and right
    Untouched - I see that it is recorded at a higher volume level than other two songs. I can hear slight distortion on the left if I turn up louder at -6 of my Pioneer. It is much better than the bass manage clip. If I use the same -10 dB volume like yesterday (since it is recorded higher, thus measured at 95db peak from my seat), there is no hint of distortion. I do agree that bass management is making the song easier to distort, but not at a volume level I am comfortable with anyway.
     
  10. multiformous

    multiformous Active Member Thread Starter

    Location:
    Newfoundland
    The original Full Range Clip from the first post of this thread is the song being recorded from the analog output of my Oppo player. It's a recording of a recording. There is headroom in that digital capture of the analog conversion because that's how I recorded it. So that file will not distort when bass managed because there is sufficient headroom. That original Full Range Clip was for demonstration purposes as a "control" for the Bass Managed Clip recording (this file also has headroom because it is my own capture of the analog output, but the distortion is in the recording because of the bass management -- it is independent of receiver or output levels). You should not use these first two files to test your setup; they are just for illustration of the problem.

    The file to use for your own tests is the Untouched Clip; or just hunt the album down on your digital music provider of your choice (CD, download, streaming). That clip is just a section of the untouched digital file from CD -- not being played back or captured through any setup or processed in any way.

    Changing the playback volume may make the distortion during bass management more or less noticeable. However, it is happening regardless because the distortion is introduced before the signal is amplified to your speakers. Listening through your receiver's headphone jack should be clean, though, since bass management is bypassed regardless of your settings.
     
  11. pdxway

    pdxway Forum Resident

    Location:
    Oregon, USA
    Yes, I did test the untouched one you provided, song #9. It was the third song I tested and mentioned in my previous post. Did you read all the way through my post???

    Anyway, since my system with bass management works just fine with the Untouched song at loud volume, I am not going to worry about bass management in my system.
     
  12. multiformous

    multiformous Active Member Thread Starter

    Location:
    Newfoundland
    I apologize if my desire to be absolutely clear about the information and resources I've provided is perceived as having not a read post entirely. I assure you, I have -- which is why I felt the need to emphasize those points about the clips for other readers.
     
  13. pdxway

    pdxway Forum Resident

    Location:
    Oregon, USA
    No problem.

    If "Bass managed clip" is what you hear from your Oppo and Onkyo at all sound level when bass management is enabled, then I would say it is pretty bad.

    But, since my Pioneer sounds very clean and only have slight distortion when the peak is near 100db with the "Untouched" clip, all I can say is not all companies implements bass management the same way.

    Oppo and Onkyo not so good. Pioneer better. :D
     
  14. multiformous

    multiformous Active Member Thread Starter

    Location:
    Newfoundland
    When I play the music on a Pioneer SC-1228-K with bass management enabled, the distortion is the same. Sounds clean in Direct mode. The same goes for my Onkyo TX-SR706, and on another more recent Onkyo I've tried. It happened on whatever system this person used. It was fairly easy to determine that it wasn't just my particular hardware that was the issue, but rather the underlying system of bass management itself.

    The bass management systems may not all be the exactly the same (different slopes or whatever), but the effect of the filters are inescapable. Whether or not it is always audible is another story. The point is the effect of the filters in a bass management system aren't always benign when there is a lack of digital headroom. Sometimes we can hear the problem, sometimes we can't. Doesn't mean it's not happening.
     
    Last edited: Jul 18, 2017
  15. multiformous

    multiformous Active Member Thread Starter

    Location:
    Newfoundland
    Finally, let me illustrate all this another way. What follows are the amplitude statistics of the song "Yoru Ga Mita Keshiki" as ripped from CD, and the same file high pass filtered at 80Hz with a 12dB slope. Notice the changes in peak amplitude:

    [​IMG]

    But that's just some nice, sparse piano music. What about a loud pop/electronic song like "Keep Your Name" by Dirty Projectors? The effect is much larger:

    [​IMG]

    I don't know exactly why the filters cause these changes to the peak amplitudes, but there it is. On a related note, I suspect these filtering effects (high/low pass filtering, elliptical filters to progressively sum bass to mono) are also why DR scores of vinyl rips tend to be so much higher than the digital releases.
     
  16. pdxway

    pdxway Forum Resident

    Location:
    Oregon, USA
    I don't know. May be the ESS Sabre DAC of Pioneer SC-95 has better digital headroom?
    ES9016 Ultra DAC has up to 128dB dynamic range and -110dB total harmonic distortion.
     
    Last edited: Jul 18, 2017
  17. pdxway

    pdxway Forum Resident

    Location:
    Oregon, USA
    One thing I see is original is 16 bit format and the high pass filtered is in 32 bit format. Do you have anything with high pass filtered in 16 bit?
     
  18. multiformous

    multiformous Active Member Thread Starter

    Location:
    Newfoundland
    That's just how Adobe Audition reads the file after applying the filter. I show those stats to illustrate the waveform changes. Playing the file after filtering results in the level meter being driven into clipping. The amplitude "above 0dB" is just theoretically there, and has to be clipped when played or saved. If I save it after running the filter, it's still 16-bit but then everything is clipped at 0dB:

    [​IMG]

    Here's what this 16-bit file post-filtering looks like in Audacity.

    [​IMG]
     
    Last edited: Jul 18, 2017
  19. multiformous

    multiformous Active Member Thread Starter

    Location:
    Newfoundland
    Just to demonstrate how I create headroom by turning the volume down in my player (foobar on my PC, for example) and the effect on the amplitude statistics. First, I turn the original file down by 6dB in Audition, and apply the high pass filter. Basically, the results are the same: sufficient headroom for processing before it hits the DAC. -6dB is plenty for this music; perhaps just barely enough for some other recordings.

    [​IMG]
    [​IMG]
     
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  20. pdxway

    pdxway Forum Resident

    Location:
    Oregon, USA
    So, is 0db the absolute maximum signal before clipping? Is this a 16 bit limit?
    I wonder if the 16bit was transcoded to 32bit (I do select upsampling option of 32 bit in my Pioneer) before bass management is applied in my Pioneer.
    Since 32 bit can handle higher dynamic range and the DAC can also handle it, the extra increased dB caused by bass management is not causing serious clipping.

    May be that is why I don't hear clipping (light buzz from left side) until very loud, like close to 100db at sitting position?

    Did you try upsampling to 32bit in your receiver?
     
    Last edited: Jul 18, 2017
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  21. multiformous

    multiformous Active Member Thread Starter

    Location:
    Newfoundland
    0dBFS is the limit regardless of bit depth. What changes between 16, 24, and 32 bit is the level of the noise floor, or the signal-to-noise ratio. Higher bit depth means lower noise floor, but 0dB is still 0dB. This is why the headroom needs to be in the recording that is being processed. This headroom refers to the level of the signal below 0dB. If audio is mastered to -0.3dB like the song that I've been using as an example, it doesn't matter if the noise floor is -96dB for 16-bit or -144dB for 24-bit. It matters that the original -0.3dB peaks exceed the digital maximum when the filter is applied. The digital audio workstation (Adobe Audition amplitude statistics screenshots) is able to "see" that information above 0dB after processing, but that information cannot be rendered when converted to analog.

    This is why I "create headroom" by lowering the volume of the player before it is processed by the receiver. Lowering the volume by 6dB is equivalent to 1-bit of resolution; which in effect raises the digital noise floor. Not that I'll ever hear that noise anyway... but still. It's something to consider. This is why artists like to record at higher bit depths because it leaves so much room for processing and they never have to worry about elevating the noise floor to audible levels. That all changes once the audio is mastered to near the maximum level... we listeners at home have that headroom space taken away by the mastering engineer, which is where we can run into trouble when we do our own processing (bass management included).
     
  22. RiCat

    RiCat Forum Resident

    Location:
    CT, USA
    Clearly bass mis-management. I could find nothing on the web that tells you what that feature does to the signal. Sad to say. Why use it? What about your system requires that feature to be used?
     
  23. back2vinyl

    back2vinyl Forum Resident

    Location:
    London, UK
    Everyone knows digital processing will create digital overs if there's no headroom. That's why it's essential to leave headroom if there's to be any digital processing. I'm really struggling to believe that no one up until this day has ever realised that bass management will create overs (and audible distortion) if there's no headroom in the file being played.

    I would have thought that any good receiver would have something built in to prevent this. I don't have a receiver so I can't test.

    Maybe try asking a manufacturer?
     
  24. EddieVanHalen

    EddieVanHalen Forum Resident

    I have a Pioneer SC LX-760European model also sold under a different reference in the US on the Elite range and the first thing I thought is exactely what you are saying. 32 bit upsampling seems not to do any difference on sound quality until it's necessary, I found that playing The Film Music Of Jerry Goldsmith bot in multichannel 5.0 (no LFE channel) and stereo, disengaging 32 bit upsampling seems to render some audible distosion on this recording which has a wide dinamic range and deep bass on some tracks, engaging 32 bit upsampling made no distosion to be heard.
     
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  25. pdxway

    pdxway Forum Resident

    Location:
    Oregon, USA
    I searched online for 32 bit floating point processing and from one grlearslutz article, someone said this:
    32 bit floating point has the advantage that range can be 'float' across the amplitude spectrum well below or above 0 dBFS.
     
    Last edited: Jul 18, 2017
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