Basis - several questions

Moderators: Cameron Shoffner, Christian Cook, Kirk Fyvie, Martin Barbour, Fred Thomke, Gary Evans_QSC

Mon Jul 14, 2008 1:46 am

  • Hello,

    I've got some observations and questions about how to use 922az properly:

    -what to do with input sensitivity? I have set to 1.5V, that gives desired output level for most desks 0 dB output. But then I don't have any headroom, any +dB on mixer gives input clip on the processor. When I set to higher input voltage I don't get proper output level. Where it's safe to add some level- I mean is there a place after DAC where is some VCA output or something?

    -processor reboots eveytime I disconnect LAN cable. I've set control over CN as I use one cable to run audio and control. Sometimes I use analog audio and connect PC only to see impedance or change input delays. When I disconnect the cable after changes are done processor reboots (shows on the screen) but audio is not discontinued. Sometimes changes are not saved as well

    -snapshots. I have some snapshots I want to create for particular event. What I do- I take some existing snapshot, do some changes then mark all controls and save as snapshot. Maybe I should do it other way, because after unplugging cable new snapshot is not stored but changes are applied to previous snapshot. Everything is done into Live mode.

    -QSCreator - is there a way to change properties for bunch of controls? I mean I could copy all controls what I need for 1 processor, and then I could set them up in desired layout, afterwards I'd like to copy all these to control processor 2, 3 and 4. I.e. I'm trying to set up one window where I can see all 4 processor outputs and inputs. What would be fastest way?

    kind regards, Einars
    ddff
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Mon Jul 14, 2008 10:53 am

  • Hello Einars,

    A reference point is needed to begin. Since you indicate 0db, I will base my reply with 0db meaning 0dbu; you are referring to input voltage. The 1.5v input sensitivity setting in the Basis922az correlates to about +5.7 dbu which is very close to twice to reference voltage so there is headroom available. Are you using 0db on the console as a maximum/fixed point of reference or a starting point? Also, are you using the Basis’s DataPort connections with a DataPort equipped QSC amplifier?

    When using a direct connection into a Basis unit, the recommended communication port is the QSControl port. A Basis unit uses CobraNet traffic to sync its CN port, when you disconnect the CN port, the Basis unit realizes CN traffic is not present any longer and then syncs to its internal sync clock. When a Basis resynchronizes, the Basis resets internally.

    QSC has created How To tutorials for recommended QSControl.net programming. There is a session on snapshot creation. I would suggest downloading that session. I can quickly suggest that you only mark the parameters you wish to change with respect to the snapshot you are presently creating. “Marking allâ€
    Brad Sambrano
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    QSC Audio Products, LLC
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    Brad Sambrano
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Tue Aug 05, 2008 12:58 am

  • Thank you for reply - now it's clear about control port and creator. The creator takes quite a while to build one panel with controls from 4 processors, so I was seeking for shorter way :)

    The input gain situation is like that- I've set sensitivity to 1.5V, this gives full amplitude of power amp connected via dataport when mixing console shows 0 dB on the meter (this is +4 dBu in voltage). So far so good, but there are FOH engineers who like to mix with +10dB on the mixer, then processor shows clip in whole signal chain and this is logical. So my question was if I set sensitivity to 3V or 9V will I get this extra gain somewhere after DSP and D/A to have full power amp output at +4dBu on input and rest of the signal just would enable output limiter without driving in clip whole unit. Great thing would be to see block diagram for this Basis unit, this probably would answer my question in seconds. Like, for example, dbx driverack 4800 has nice diagram and I can see that gains input and output are built on simple opamps with VCA steering, so they're placed outside digital path of signal.

    The second problem is not all the mixing consoles are having same output level at 0 dB on the meter, but this is something we can't change, so it's better to prepare a processor for all these situations.

    Einars
    ddff
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Tue Aug 05, 2008 12:00 pm

  • Hi Einars,

    With respect to the QSControl.net units that have DataPort outputs (and analog local inputs), a Basis922az in this instance, the only analog domain level control is the input sensitivity setting: 1.5Vrms, 3Vrms, 9Vrms and 18Vrms.

    The detachable header (Euro) output versions (and analog local inputs) have input sensitivity plus output level control in the analog domain. The output analog domain level control is a trim though; there isn’t gain, only attenuation.
    Brad Sambrano
    Systems Support Engineer
    QSC Audio Products, LLC
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    Brad Sambrano
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Tue Nov 04, 2008 7:44 am

  • OK, I've set input sensitivity to 9V, this is safe setting and I haven't seen anybody getting into digital clip so far. However I've lost output level. If I try to add it in last chain of signal (limiter) I can get clip there. But there is another +12 dB available in output section where amp impedances can be seen. Is this point already after DAC and has some analog VCA steered gain? At least here I can't see any dBFS measurement.

    I have a question about limiter as well - when limiter treshold occurs it starts to reduce a level with every extra dB on the input I got minus dB on the output. Like a dbx 160, where you can set 1:-1 ratio. Is this normal operation mode? For now I use power limiters as they take off about -2 dB constantly after treshold.

    thanks in advance, Einars
    ddff
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Thu Nov 06, 2008 11:35 am

  • Hello Einars,

    The output section you refer to “where (the) amp impedances can be seen” is the channel strip page or the amplifier screen. The information shown is in the analog domain, the info shows the input and output signal flow through the amplifier attached to a DataPort. Unless you are engaging the Auto Power Limiter or have the amplifier’s clip limiter function engaged, no action on the signal is taken other than signal level control.

    Are you using the Peak Limiter object or the Compressor object for a limiter? The Peak Limiter isn’t adjustable with respect to the knee ratio but the Compressor is.
    Brad Sambrano
    Systems Support Engineer
    QSC Audio Products, LLC
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    Brad Sambrano
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Fri Nov 14, 2008 2:21 am

  • OK, that makes sense. So in the output section I can go with +12 dB without worry that I'll drive DAC mad. On the amps I have enabled clip limiters. For voltage limiting I use a limiter function, threshold I calculated by measuring Basis output on dataport and then did power to level calculation 20*LOG10((SQRT(Speaker_power * Speaker_Impedance)/0.775 . This works fine. However, I didn't find much information how to calculate attack/release times for variable bands. I've set 10/100ms and that works, however there should be some theory how to set it depending on coil size, freq. band or whatever.

    Then I found those Auto Power limiters - looks like unit can measure output voltage, actual impedance and then calculate power that is given to speaker. Attack times are quite long, so this makes me believe these limiters are intended for long term signal protection - maybe some material with small crest factor, like sine. Attack and Release parameters are clear, but what is Avg time? Is that kind of averaging buffer? Is there some theory out there how to set these limiters? Tried following - 18 Sound speaker 18ND9300, rated AES power 1000W (pink noise, crest factor 6, 2 hours), set 500W with attack 0.5 sec, release 2 sec. and turned on sine 80 Hz. Of course, I fried the coil. Then I got some idea from Bassmaxx people (we use their bins), quote follows:

    > My suggestion for having worked in the real world with many 18' woofer
    > is that it will never be able to really handle anything really
    > continuous over 200/250 W, so I would limit the peak power to 1000 W @
    > 8 Ohm, that means about 128 V on the peak limiter with an attack of
    > about 10ms and a release of about 80/100ms, while I would set the
    > power limiter to 200/250 W with an attack of about 500/600 ms and a release of about 2000/2400 ms.

    This makes sense, but where these 200W come from - should I stand with pyrometer and wait for critical temperature? How QSC are calculating these powers? Is there some chart where I can predict long term power capacity for high density (low crest factor) signals? I see some really big figures when selecting speakers from preset list. Are they tested and working properly?

    thanks, Einars
    ddff
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Fri Nov 14, 2008 4:24 pm

  • Hi Einars,

    Take a look in the Venue Manager help file under “Auto Power Limiter Panel”. The help page describes its functionality. In short, the auto power limiter feature is a thermal failure prevention.

    Although you can add a custom or non-QSC loudspeaker to the list, I wouldn’t tinker with it with respect to using the auto power limiter for non-QSC loudspeakers. Please use the limiter(s) in the DSP that is more in use with the data given by Bassmaxx.
    Brad Sambrano
    Systems Support Engineer
    QSC Audio Products, LLC
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    Brad Sambrano
    QSC Audio Products
     
    Posts: 198
    Joined: Thu Mar 06, 2003 2:01 am
    Location: Costa Mesa, CA



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