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Old 14th Sep 2017, 4:32 pm   #1
Biggles
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Default Equalisers and their differences.

Not quite sure which section to put this one, but I was just interested to find out what the difference is between a parametric equaliser, and a graphic equaliser, or are they basically the same beast with a different name? Thanks.
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Old 14th Sep 2017, 5:28 pm   #2
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Default Re: Equalisers and their differences.

Simply put, graphic EQ comprises of a series of fixed frequency bands the gain of which is controlled by the faders. A parametric EQ has at least one sweepable frequency control, and one 'Q' control, enabling the bandwidth of the selected centre frequency to be controlled and one gain control. Pro and semi-pro mixers usually have parametrics with at least two sweepable bands plus a high and low shelving EQ, graphics tend be outboard.
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Old 14th Sep 2017, 5:29 pm   #3
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Default Re: Equalisers and their differences.

Ah, equalisers!

This is something I have studied in depth!

A parametric equaliser allows you to specify, say, a frequency response 'bump' in relatively few settings, e.g. centre frequency of bump, width ('Q'), and height of bump. A parametric equaliser might provide a number of such programmable bumps.

A graphic equaliser lets you program the frequency response verbosely - numerous frequency controls, most of which won't differ much from their neighbour.

There's a second dichotomy: Minimum phase equalisers and Linear phase equalisers. The former represents the best you can do with analogue equalisation. A minimum phase equaliser can get the magnitude of the response right but may leave phase shifts that cannot be corrected. If you want the phase right too then you need a Linear phase eq - a digital monster that imposes a processing delay (latency) that makes it useless for live work.

To make matters even more complicated, a minimum phase eq, though a traditionally analogue device, is almost always implemented digitally now.

Hope this helps.
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Old 14th Sep 2017, 5:30 pm   #4
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Default Re: Equalisers and their differences.

You just beat me to it, ITAM805!
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Old 14th Sep 2017, 5:51 pm   #5
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Default Re: Equalisers and their differences.

I got as far as "There's a second dichotomy..." before my head started spinning.
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Old 14th Sep 2017, 7:28 pm   #6
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Default Re: Equalisers and their differences.

Cheers. That's something new to me. I hadn't really considered the technicalities before. My interest was raised by seeing an advert in one of the radio magazines for a parametric equaliser unit, presumably to add to a receiver to improve the audio by tuning out heterodynes, whistles or what have you. It all makes sense now.
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Old 14th Sep 2017, 8:09 pm   #7
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Default Re: Equalisers and their differences.

Parametric equalisers and graphic equalisers are really tools for making relatively blunt adjustments to the frequency response of a system to change it's sound or character, however you want to phrase it. Think of them as more comprehensive tone controls.

A parametric equaliser splits the audible range into relatively few bands and allows you to not only cut or boost those bands, you can also move the boundaries between them around.

A graphical equaliser splits the audible range into a large number of bands. You can't change where the boundaries are. You can only boost or cut each one, usually with an individual slider. But there are many bands and the fixed boundaries are therefore less of a limitation. These machines are arranged so that the positions of all the slider knobs form a graph of the frequency response you have set it to give... hence the name.

For removing heterodyne whistles in communications receivers it is usual to employ a very sharp notch filter. The aim is to really attenuate the annoying whistle while removing the minimum of wanted signal around it. These notches tend to be very sharp and their frequency is adjustable. The receiver by my elbow goes so far as finding a whistle and tuning its notch filter onto it automatically! Usual graphical and parametric equalisers don't go this narrow, and usually don't go as deep.

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Old 14th Sep 2017, 10:03 pm   #8
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Default Re: Equalisers and their differences.

Put simply:

Parametric = concise, usually because knowledge of the underlying process allows a taylored eq with relatively few adjustable parameters.

Non-parametric = graphic = verbose, usually because the underlying process is complex or unpredictable. Many adjustable parameters gives maximum flexibility at the cost of a more laborious setting up time. Also, lots of sliders sells

There is a notion of a 'perfect sound' which has been coloured, often by room acoustics. We seek to create an electronic inverse of this colouring process to recover the original ('dry'?) sound. Where this process has a stable inverse, a simple analogue minimum phase equaliser can get that wanted sound back.

Sometimes though, usually as a result of multi-path effects, the process does not have a stable inverse. A minimum phase eq can still get the spectrum gain right, but there will be residual 'all pass' elements that cause the phase to turn in loops. Often, this can be tolerated. If these phase rotations cannot be tolerated then a linear phase eq must be used, in which case the phase can be unwound using a digital filter. There's a price though - such processing imposes a delay, the duration of which depends on the lowest frequency at which the phase starts to take off.

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Last edited by Karen O; 14th Sep 2017 at 10:05 pm. Reason: simple typo
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Old 14th Sep 2017, 10:22 pm   #9
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Default Re: Equalisers and their differences.

Just to add a minor complication on the linguistics front.

A graphic equaliser was so named because in it's most typical physical form, the controls appear to plot a frequency response graph. This made perfect sense when the analogue graphic eq. typically had a row of sliders whereas the parametric typically had clusters of rotary controls.

Now we have touch screen digital sound desks with either no physical knobs or just a few multipurpose assignable ones, all types of eq. are now represented in graphical form on the screen.
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Old 14th Sep 2017, 10:25 pm   #10
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Default Re: Equalisers and their differences.

Thanks David and Karen. I would imagine the theory and mathematics are quite complicated when you get into it. I suppose the "modern" way of doing it all is by way of DSP. Personally I haven't really got round to any of that yet, relying on my ears to do the filtering. I have read about those DSP speakers made by BHI? but never had the chance to try one out to see if they make much difference.
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Old 15th Sep 2017, 3:34 am   #11
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Default Re: Equalisers and their differences.

Quote:
I would imagine the theory and mathematics are quite complicated when you get into it
I used to think that but the underlying principles are really quite simple. Text books always use obscure language (to my mind) that makes understanding difficult. It's a bit like medicine and its use of Latin

Digital filters are wonderful - they can implement virtually any magnitude and phase profile you might want. But they can be over-used. Recall the 'colouring process' I mentioned in my previous email? Well, if a good old analogue eq can match that process then that analogue eq will restore phase as well as magnitude. A linear phase eq would do nothing to the phase and leave that aspect of the colouration un-restored.
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Old 15th Sep 2017, 7:54 am   #12
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Default Re: Equalisers and their differences.

The failing point of equalisers is that they apply their 'corrections' to all the components of the sound in the chosen frequency bounds.

It's often the case that recordings have a mixture of several reflection paths as well as the main direct path. Which one do you want to 'equalise'! They can be set to give a sort of general correction for the level of the sum of the paths, but it's a rough fix. The longer paths create time delays, which means a slope of phase versus frequency. The different paths have different delays, hence different slopes all needed at the same time.

The art of audio equalisation at recording time is to arrange things so that the minimum is needed.

At replay time, you get the same issues... multiple paths in the room from speakers to lug 'oles. And different paths for everyone listening.

With DSP you can create models of the delay paths and replicate their content and get a reasonably good cancellation of the unwanted effects. But it would only get it right for one listener in a room, or when recording for one instrument in an orchestra.

So the preference amongst recording engineers is to shove an individual mike up every single instrument and to feed them all into an awesome mixing desk the size of a small sports field. When these channels are panned and combined to make a stereo whole, the different time delays from each instrument to the listener are gone. Each channel has a parametric equaliser, but that doesn't do the delay. You get a clean sounding recording as the output, but a lot of the realism has gone. Have a listen to some of the old 50s and 60s Decca classical recordings, made in a carefully chosen hall and with quite simple mikeing, and you'll find them rather good.

But these are tools aimed at entertainment audio. The OP mentioned tone removal in radio reception and that can be done surprisingly well but it needs specialised notch filters, far narrower and far deeper that would be wanted in equalising music. They are also far simpler!

David
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Old 15th Sep 2017, 4:50 pm   #13
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Default Re: Equalisers and their differences.

I would agree with you on the live music debate. Live music wins hands down, long before it meets anything electronic. I have mentioned in a different thread how surprised I was when I went to my first classical music gig a few months ago. Now my usual thing in the past was to see various rock bands in the City Hall in Newcastle, and they all had a particular sound, but the whole classical thing was a bit of an eye opener with how pure and unspoiled the music sounded. Probably would have been even better if my ears were forty years younger. So I suppose every electronic circuit adds it's own bit of character, even when designed not to.
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Old 15th Sep 2017, 5:10 pm   #14
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Default Re: Equalisers and their differences.

Quote:
how surprised I was when I went to my first classical music gig
THey can be fairly (one way of spelling it) loud too.
 
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