Signal 'clipping' with a BFD 1124p?

Discussion in 'Archived Threads 2001-2004' started by Chris Demaree, Dec 20, 2001.

  1. Chris Demaree

    Chris Demaree Stunt Coordinator

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    I just started using a BFD 1124p. After getting it partially set up (3 filters in use) I remembered a post about watching for signs of a 'clipped' signal in the BFD. If I remember correctly, to check for this, just play a loud/low scene in a movie (THX intro to TPM and 'comming mother' from 'The Haunting'), then watch for any yellow or red LED's on the signal strength meter for the channel you are concerned with. I am really happy with the BFD, it is already making a very audible difference [​IMG], but I am very much 'in the dark' about what could 'help' the BFD clip a signal vs. what would make it less likely.
    Does using a 'wider' amount of bandwidth, per filter, increase the chance of clipping vs. using more filters with a smaller amount of bandwidth?
    Does cutting the gain by a HUGE amount (-25db), on a single filter, increase the chance of clipping or only if you add 'gain'?
    Is clipping simply a function of input signal strength before processing, after processing?
    Currently, both scenes listed above are still in the 'green' on the BFD, at the loudest I normally watch movies. I just don't want to hurt my SVS!
    Equipment:
    SVS 20-39pc
    Outlaw Audio 1050
    BFD 1124p
    Paradigm Titans (front), CC170 (center) and Energy take2's (rear)
    Sony kv36-fv26
    Toshiba sd2200
    AVIA
    Radio Shack analog SPL meter
    :b
     
  2. Brian Fellmeth

    Brian Fellmeth Supporting Actor

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    I just spent my first session working with the BFD. That thing is amazing. Unfortunately, I have the older 20 bit version so clipping is more of an issue for me- you've got 4 more bits (16 fold) of headroom.

    Anyway, clipping occurs when the POST filtered signal saturates the ADC (analog-digital converter) so that it goes beyond the largest digital number it used.

    The yellow LED is the warning- signal not yet clipped until the red lit up. If you don't even see yellow playing as loud as you will play, you're in good shape. The filters themselves are not set to protect against clipping. If your lapping against the limits, the solution is to cut the overall stregnth of the line level input signal and make a compensatory increase on the sub amp's gain.

    Also, a little dose of clipping will not fry your SVS. The BFD is designed to "soft clip" by not allowing the clipped signal to wrap around.
     
  3. MarkDesMarais

    MarkDesMarais Stunt Coordinator

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    Right on, but one little nit- unless there is something funny going on, going from 20 to 24 bit will not give you more headroom- it isn't a capacity measure, it is an accuracy measure.

    24 bit will let you measure your input value more precisely- 16 million possible values- vs 20 bit- 1 million possible values. But if the input (or caculated output) is too large, you get clipping regardless of how finely sliced the signal is.

    Clipping can occur on the input if the signal saturates the A/D, or on the output if the D/A capacity is exceeded. Safeties can be put in in both places to smooth things out.

    Markd
     
  4. Brian Fellmeth

    Brian Fellmeth Supporting Actor

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    Mark,

    I assumed that the extra bits were used to extend the range rather than slice it finer. You're probably correct.
     
  5. brucek

    brucek Second Unit

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    Chris,

    When setting up the input levels to the BFD you have to remember it isn't a normal fixed line level situation. Consider the output of a CD player for instance. It is a line level that has a varying output depending only on the music being played, but it still has a fixed maximum line level output that isn't passed through a volume control. This line level would be very easy to match to the input of the BFD because it's predictable.

    This unfortunately isn't the way you're using the BFD. You are feeding it a line level signal that is affected by your preamps volume control. See the problem? I can play a DVD at a very low level or a very high level and the BFD will be fed different maximum inputs.

    One of the problems with ADC's is that they tend to become fairly non-linear and rather ineffective at low input levels. They operate best if they are fed a signal which has a maximum that is just below their maximum input level. This ensures that the maximum signal does not clip and the quietest signal gets properly evaluated and assigned the appropriate low order bits to correctly represent its actual level when converted back to analog by the DAC. This is where quantization error (noise) exists, at these low levels.

    Your goal then, is to decide what the probable maximum listening level will be for your volume control of your receiver or preamp while playing a bass heavy disk like U-571 and then set the "subwoofer output" level of your receiver to where you see the red LED's turning on the BFD.

    The BFD does not clip when the red LED turns on, but it does clip with a signal slightly greater than that. I've done quite a few bench tests on mine and I found the red LED turns on at about 1.5volts peak and the output begins to clip at about 1.6volts peak (+4dBv). This is the case when you use the operating level sensitivity switch set at -10dBv (which is the only setting you should use).

    Yeah, I know the book says the max input level is +2dBV (1.26volts peak) but my measurements show this is when the yellow LED turns on and you still have quite a bit of headroom right up to +4dBv before clipping occurs.

    Once you have this subwoofer output level set, then you know the maximum sent to the BFD will be a maximum signal that it can tolerate and you'll know normal levels will be in the linear region. Now you can set the output level of your subwoofer with its own volume control to whatever level you enjoy...

    Note that the BFD IN/OUT switch will enable and disable your filters with its light "ON" and "OFF" respectively. In these two conditions the LED indicators are showing the BFD's output level.

    But, if you push and hold the IN/OUT switch and it begins flashing, you bypass the filter section completely and the LED indicators are showing input level. This is the condition to set it up in that I am discussing above.

    brucek
     
  6. Chris Demaree

    Chris Demaree Stunt Coordinator

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    thanks everyone, that clears things up [​IMG] After calibrating the sub's total volume to match my other speakers, I have yet to see any yellow on the level meters with the most demanding scenes I could think of (THX intro on TPM, The Haunting (DTS) ..and soon U-571). My frequency response is still a bit ragged in a couple spots, but it doesn't measure more than a few dB's above/below other frequencies. Now I will just enjoy a few movies and tweak the response a bit further in a month or so [​IMG]
     

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