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Gain structure of summed inputs on the routing page (for mono subwoofers) 1 month 3 weeks ago #56711

  • jaistanley
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I appear to have answered my own question following a lot of head scratching today; but can someone confirm how to sum inputs from L&R into a mono sub channel?

I ask because yesterday I was setting the gain on my amps, and first checked that I was getting an un-clipped wave-form from the dsp. I was seeing clipping on the output of the CDSP when two subwoofer channels were summed into one.. Unless I reduced the gain in the routing panel.  I have since concluded that I was clipping the input stage of my cheap USB oscilliscope and frystratingly now have to remove three of my amplifiers to get to the gain adjuster of my subwoofer amp (that's on me!).

But to make sure:

When sending a 0dB tone to the L&R digital inputs (ensuring ALL x-over, PEQ, gains etc etc are zero'd) I should see clean 0dB (full scale) on the summed output channel?  As in a 0dB waveform plus a 0dB waveform should sum to a 0dB waveform?

Thanks in advance for my now seemingly stupid question!

Kind regards,

Jai Stanley

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Gain structure of summed inputs on the routing page (for mono subwoofers) 1 month 3 weeks ago #56713

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Nope.  If you sum two channels that are 0dbFS you could achieve +6dbFS. Depending upon how correlated the two channels were.
If they were identical signals then you would have +6dbFS and you could see some clipping in the DSP internals.  In that case you would reduce the attenuator of each input channel to -6db, and that would eliminate the possibility of clipping from the summing configuration.  (You might still have the possibility of clipping from other means.)

Dave.
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Last edit: by dreite. Reason: update

Gain structure of summed inputs on the routing page (for mono subwoofers) 1 month 3 weeks ago #56718

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Oh for god's sake. I THOUGHT that was how you did it, and how I set the gain of the amp. Now I'm second guessing how I actually did it (I spent ages getting the setup ready and flew through the setup so now can't remember). The only trouble is, to get to the gain knob of the amp I might have to remove all the amps above it, and it's at the bottom of the 'rackmount' style amp holder thing I made in my car (space is very limited).

One thing is for sure: with the gains set on all other channels, the subwoofer channel is WAY too low. I was expecting this to an extent: subwoofers are generally a lot less sensitive, but it's so low I'm sure I've made a mistake.

Reading car audio forums, when you mention using a scope to set gains, you basically get people rolling their eyes and saying it doesn't work. I'm really not sure why: surely setting your amps so you can't clip them is insurance against distortion/voice coil heating? 

Thank you for your reply. I'll go back when we've done the family outing to a safari park, and have a tweak.

Kind regards,

Jai

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Gain structure of summed inputs on the routing page (for mono subwoofers) 1 month 3 weeks ago #56721

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It has been over a decade since I have done anything with car audio but back in the mid-2000s it was relatively common to set gains using a scope. That being said it was also relatively common to use a 10 dB gain overlap per Autosound 2000 (Richard Clark / David Navone) recommendations -> www.google.com/url?sa=t&rct=j&q=&esrc=s&...6U3ihRhyP0Tr_e81U-Ea.

Setting your gains so you never clip is probably not the greatest idea as it limits what you can do from a volume perspective with recordings mastered at a lower level. I like about a 6 dB difference between max output voltage and amplifier input sensitivity, YMMV. 

Michael
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Gain structure of summed inputs on the routing page (for mono subwoofers) 1 month 3 weeks ago #56736

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Thank you very much for that link. I read it with interest, and also clarifies a few things/eases my mind about my choice to use the scope.

I was thinking about it yesterday (not had a chance to go back to the car system yet). I set the gains with a 0dB signal at 40hz to the sub: then switched back to my previous DSP config/preset briefly and tested it; resulting in a very low audible output from the sub, but the tweeters/mids were fine (although slightly hotter, the amp gains were all set to the same halfway point previously, with the only one being significantly reduced beign the sub gain so this makes sense).  My config had the most EQ work done on the subwoofer output following my initial nearfield measurement of each driver. Being a small sealed box not designed specifically for this driver: there was a significant hump in the response which I flattened out with quite a large EQ cut, hence futher reducing the gain of the sub a lot over much of the pass-band.

I think what I will do is re-set the gains with some gain overlap as described in that link. Maybe first see if the volume of the system output on the higher frequency drivers is enough.  I would like to be in a position where I never send a clipped signal to the drivers (fully active system) that are most delicate and prone to damage when a clipped signal is sent to them; IE tweeters, 2" mids.  Then increase the gain on the sub/possibly midwoofers to match their output; accepting that at 0dB it will be possible to introduce a tiny bit of clipping, but on drivers that are beefy enough/have high enough thermal capacity to deal with it.

Also: I wonder if it would be prudent to measure the peak to peak voltage output of the amp at max power/unclipped output; then introduce my EQ'd DSP config and pink noise and check the voltage of the subwoofer amp is below this level?  Maybe that is a bridge too far considering what I just wrote above.

Thank you again for your input.

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Gain structure of summed inputs on the routing page (for mono subwoofers) 1 month 3 weeks ago #56739

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This thread makes me realise this is a potential issue I'd not considered, as I do send a summed signal to my sub. I've always considered the digital head-room required for applied EQ but not this. I can't say I've ever experienced an unexpected issue though.

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Gain structure of summed inputs on the routing page (for mono subwoofers) 1 month 3 weeks ago #56749

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Much depends upon where the system volume control is.  If it's upstream of this summing junction, it's probably not an issue.

Dave.

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Gain structure of summed inputs on the routing page (for mono subwoofers) 1 month 3 weeks ago #56756

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Much depends upon where the system volume control is.  If it's upstream of this summing junction, it's probably not an issue.

In my case volume control is via the SHD itself for music, as it's all streamed from Qobuz via Volumio. 

(For movie using I use the SHD at fixed gain.)

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Gain structure of summed inputs on the routing page (for mono subwoofers) 1 month 3 weeks ago #56759

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So I did some more setup and head scratching.

I spent some time looking at the output of the dsp with a scope as I tinkered, to double check thinking and make sure everything was clear in my head.

The two inputs (L&R digital) are summed into the sub channel at -6dB and this does give a 0dB output at the output level meter for the sub channel, and indeed give a clean waveform on the scope. With this checked, I then un-bypassed the EQ/Filter/crossovers I use for the subwoofer channel and did a sweep from 15 to 100 hz manually in 0.5Hz steps.  Each time checking the output from the DSP on the scope and a the output level meter (dsp software).  I found that with my filters, the output level never rose above -6dB - explaingin why my subwoofer level was so low!  When doing my filtering I only used cuts and the occasional boost after cut to flatten a little - none of which took me above the -6dB level.

With this being the case: I then removed all filters and set the output to  'clean' (filter bypassed) at -6dB volume.  Then I used the scope over the dummy load of the amp putput and set the amp gain to just under clipping.

The filters were then re-engaged and the output volume on that channel set back to 0dB.

The result of this is a strong, clean and correct level subwoofer output I can tweak per song/road conditions with the remote control at the front of the car (comes with the CDSP and set to subwoofer volume function).

Also: with all my gains set with no gain-overlap; 100% volume on the head unit is PLENTY loud, but not so excessive that it is turned down a lot. I reason this is a good thing as using the volume on the Android head unit is probably just cutting bits from the 16bit internal digital signal and reducing dynamic range a lot.

I intend to use the remote and do volume control with the CDSP but having the subwoofer volume control is more useful to me at present.  I will order myself an OLED remote control soon, as then I can switch between volume/sub volume on the remote itself without the need for the laptop.  Then I can leave my head unit set to 100% and leave the volume control to the CDSP (which I ASSUME uses a MUCH higher internal bit-depth for volume control than the 16bit android heat unit?).

Massive thanks for all the help.

Kind regrads,

Jai

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Gain structure of summed inputs on the routing page (for mono subwoofers) 1 month 3 weeks ago #56764

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Glad to hear you got everything worked out. I was going to warn the opposite, that if you had boosts programmed in the DSP cutting 6 dB might not be enough, but cuts can also result in a lower level than you anticipate. It is always a good idea to have an understanding of where you sit from an overall level perspective after all processing is applied.

I would personally be surprised if the android HU volume control was 16 bit. Which model are you using? But yes volume control in the CDSP (are you using an 8x12 or 6x8?) is more of a known entity.

Michael

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