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Will a DSP be effective when 1 input into the DSP will eventually be outputed to 20 speakers in mono?

Started by kam_lime 4 years ago22 replieslatest reply 4 years ago120 views

The way my system works is that my receiver sends one (analog) input into my amp, then my amp will send that input to 20 channels (for 20 speakers). I would want to put a DSP between my receiver and amp, so that the dsp will have 1 input and 1 output, but that output will be eventually sent to 20 in-ceiling speakers in a very large ballroom, all playing in mono. 

However, will any dsp be effective in this situation? From the point of view of the dsp, the 20 speakers are really one giant speaker. But in reality they are 20 speakers. I'm not sure how this affects performance of a dsp (I'm a newbie). 

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Reply by jbrowerMay 17, 2018

Kam-

Many years ago I used to help guys with sound systems in concert halls, churches, etc.

One thing always of concern:  equalizing sound arrival at certain points in the audience (typically a line down the middle of the seating, if that's possible).  To do this, output to different speakers may need slightly different delays (depending on shape of your ballroom and where you can place speakers), and to do that you need a sampling rate high enough to support your desired delay granularity.

So I think you might want N outputs, not sure if 20, but more than 1.

As other answers have indicated, at some point you may want other processing in addition to delay.  That's where flexibility of a DSP becomes helpful -- add what you need when you need it, fix what's not working, just by reprogramming.

-Jeff

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Reply by kam_limeMay 17, 2018

Hi Jeff,


Unfortunately, I won't be able to have more than 1 output. 


There are 5 rows, with 4 speakers in each row. None of the speakers are down the middle (naturally, as there are even number of speakers in each row). If the speakers were completely spread out evenly, then there would be 6 rows of speakers, with 4 speakers in each row, but it was unnecessary to have speakers along the last row.


This ballroom is 75' x 40' x 14'.

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Reply by kam_limeMay 18, 2018

Do you think there will be sound arrival issues with this room that is 75' x 40' x 14'?


Also, if it's worth mentioning, I have passive crossover built-in between the tweeter and woofer.

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Reply by kam_limeMay 28, 2018

Hi Jeff,


I'm sorry to bother you again, but if I had 5 left speakers, 5 middle speakers, and 5 right speakers, and I had 3 inputs to the dsp (left, middle, and right), do you think that would be good enough to deal with time delays or no?


I'm thinking of using the minidsp ddrc-88a, but I'm open to suggestions from people more knowledgeable than me.

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Reply by jbrowerMay 28, 2018

Kam-

But you only have one output, correct ?  The typical approach is one output per speaker, get the system up and running, then take careful seating measurements and adjust your DSP delay line and output level for each speaker so that sound arrival is mostly simultaneous and uniform at desired seating locations.

If your seating arrangement is a rectangle, and you line the speakers down the middle (or two rows down each side) with spacing that sufficiently limits overlapping coverage, then maybe one output can give decent results.  But that puts the onus on placement and precise adjustment of output level for each speaker, and other "hardwired" adjustments.  Sorta defeats the purpose of a flexible digital signal processor.

What is to be avoided in large public venues are phase differences in arrival times at desired seating locations.  Sound arriving at different times will be perceived as echo or reverb.  There may be some cases -- like concert halls -- where impulse response (equalization) is a big issue, which leads to concerns about construction material, geometry of the walls and ceiling, movable curtains and canopies, etc.  (Search on architectural acoustics, they use terms like "afterglow", "warmth", "richness", etc, which are basically referring to manipulation of the impulse response tail and its duration).

But you have a ballroom, so at first, my suggestion is try to keep it simple and worry about echo.  Your ballroom seems on the small side and not very wide, so maybe just 3-4 speakers down the middle would work.  If that's possible.  You could mount side speakers also, and keep their output level lower.

-Jeff

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Reply by kam_limeMay 28, 2018

Hi Jeff,


I'm sorry, I'm confused. If by output you mean volume output, then the amp allows me to individually adjust volume for each speaker, so all 15 speakers can have a different volume output. It's just that the dsp can only recognize 3 speakers (left, middle, and right).


Also, I should mention that this isn't for live performances. If there was a live performance which utilized the small stage, then they would bring their own loudspeakers. This is just for groups that want to play music (either low volume or loud "party" volume) via online music streaming, cd player, and aux line-in.

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Reply by jbrowerMay 28, 2018

Kam-

> the dsp can only recognize 3 speakers (left, middle, and right)

Does that mean you have three (3) outputs ?

> the amp allows me to individually adjust volume for each speaker

Well that's better than hardwired, but still not as flexible as letting your DSP program do it.

-Jeff

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Reply by kam_limeMay 28, 2018

Hi Jeff,

First off, I just want to say thank you so much for taking the time to help inform a novice like me!


In the post above that I wrote, I meant there would be 3 outputs.

After reading your post above and looking up these things, I realize how important having one output per speaker for indoors is. 


How about: the ballroom has 9 speakers: 3 on the left, 3 down the middle, and 3 on the right. Each speaker will have its own output so the dsp can do its job fully. 


Do you know any good dsp that can handle 4 zones (1 zone for the ballroom and 3 zones for the 3 outside areas) with 9 speakers in the first zone, and one speaker in the other 3 zones (so from the perspective of the dsp, it is handling 12 speakers, with 9 in one zone)?


Also, for the ballroom, I was looking at these speakers, which have sensitivity 88 dB. The amp pushes out 65 watts RMS to each speaker, each speaker being 8 ohms: 


https://www.htd.com/Level-THREE-Bookshelf-Speakers...


Do you think 9 speakers, each 88 dB getting 65 watts RMS, and will be DSPed (which I think will reduce the output volume a bit?) will be enough for big parties? I know this is a very subjective question, I just trust your judgement. Although much more expensive, if it's necessary I can look at different speakers or a different amp.


Again, I can't tell you how much I appreciate your help!


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Reply by jbrowerMay 30, 2018

Kam-

With three outputs, and the relatively small size of your ballroom, I think you would be in reasonable shape.  As long as cost is not prohibitive, there is no reason not to install 9 speakers in a grid, even if it turns out that the middle speakers can provide coverage and the outer ones should have reduced output level, no harm done.


If you can actually implement 9 independent outputs, that would be an ideal configuration.  Again it may turn out that you don't fully utilize that capability, but if you're doing this professionally then it might serve as a model / testbed for future installations.

As far as amps and speakers, I haven't looked at state-of-the-art specs for some time.  It seems you're up on the curve in that area anyway.  For the DSP I would suggest something from TI, maybe one of their Keystone II devices with one ARM core and one c66x core.  That's actually overkill (you don't need anywhere near that much horsepower for delay line and output level handling) but it keeps you in TI's sweet spot in terms of tech support and community support on their e2e forum.

You also need a multi-output D/A converter device, maybe something like:

  http://www.ti.com/product/DAC8568

This one is 16-bits which is fine for a public venue.  I didn't check to see if there is an EVM board with this device, but if so that's what you want in order to get your design running and tested.

-Jeff

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Reply by DaniloDaraMay 17, 2018

This is not clear, Kam.
"Effective" in doing what?
I could guess that you are talking about equalization.
I mean - frequency response equalization and phase composition.
Do you?

The dsp will play no magics.
It will just do what you tech him to do.

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Reply by kam_limeMay 17, 2018

Yeah, a decent "flat" response

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Reply by SlartibartfastMay 17, 2018

What makes you think it wouldn't be?   If you provide some detail about your application and your concerns you might get better answers.

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Reply by probbieMay 17, 2018

It depends what you want the DSP to do to the resulting sound. If you are simply applying some Equalisation, then the *change* in EQ will be the same if there is one speaker or 1000. If however you are seeking to achieve a 'flat' response everywhere then this is either very difficult or impossible; requiring each speaker to be fed from a different FIR filter so that they acoustically sum to flat in as many locations as possible. Such expert systems are available.

So first, you need to quantify what outcome you require..

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Reply by kam_limeMay 17, 2018

I'm sorry, I did a poor job of explaining my full situation.

I have an HTD whole-house system, with 4 zones (1 ballroom with 20 speakers [5 rows of 4 speakers], 3 outside zones, each with 8 speakers). All the speakers are full-range speakers, all wired standard (1 channel per speaker, no parallel, exc.).

This place is used mostly for events like weddings.

I was hoping that a dsp could help make a decent "flat" response, even though for each zone, only one pair of analog RCAs would be going into the dsp and only one pair of RCAs would be coming out of the dsp for that zone (the amp takes care of sending that info to each speaker in the zone). 


I was looking at the minidsp ddrc-88a because my understanding is that it allows you to send 4 analog RCA pairs and you can have 1 configuration for 4 separate zones potentially all playing simultaneously (or the more standard 4 different configuration for 1 zone). Also, the ddrc-88a/dirac live has an "auditorium" setting. 


The reason why I was concerned whether or not any dsp could help make a decently flat response is because usually, the dsp can manipulate each speaker in a room independently so the total sum leads to a good flat response. But in my case (I can't afford changing my set-up), the dsp will only have "1 degree of freedom" to help make a flat response for the entire room (or outside "room"). 


Unfortunately, I can't even change the situation to have 2 pairs of analog RCAs per zone (even if I bought a ddrc-88a) because I'm rarely at this location and for operating purposes, that would be more difficult for the people there to work.


I was hoping I could spend a good amount of time initially setting up a dsp, but then basically never touch it again, since I will rarely ever be at the location and the people there will not be able to handle it. 


If it's relevant: there are no dedicated subwoofers. The ceiling speakers are rated at 40 Hz at -3dB and the outside speakers are rated at 60 Hz at -3dB. Most of the time, all 4 zones will be played simultaneously, with each speaker playing in mono.


Would there be any solution to this situation or should I just not get a dsp at all? 

Please let me know if there is other relevant information that I forget to mention.

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Reply by kam_limeMay 17, 2018

If it matters, the ballroom is 75' x 40' x 14'. If the speakers were completely spread out evenly, then there would be 6 rows of speakers, with 4 speakers in each row, but it was unnecessary to have speakers along the last row. So there is just 5 rows of speakers, with 4 speakers in each row.

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Reply by kam_limeMay 17, 2018

The speakers will be used for music. Occasionally, there will be a microphone input for speeches, but this situation doesn't have to be optimized. Regardless of the number of zones on, they will all be playing the same music source.

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Reply by probbieMay 18, 2018

So it sounds like you can have a DSP channel dedicated to each zone, which will allow you to EQ each zone independently, which has to be better than having no DSP at all. You might also find the DSP useful for applying some high-pass filtering to eliminate very low frequencies which your ceiling speakers will not be able to reproduce and so would otherwise just waste amplifier headroom.

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Reply by kam_limeMay 18, 2018

High pass filter is a good idea. 

The following sentence may really show how much of a novice I am with dsps, but the one thing I absolutely don't want is some high-pitched screetching sound coming from the speakers, whether it's a result of the speakers, room characteristics, or if even the dsp tried to make some corrections high into the kHz and it didn't have the intended effect. What do you think I should do to prevent such a unwanted screetch from happening?

Should I also have a low pass filter at 20 kHz? Or even lower? I like "warmer" music as opposed to "bright", especially for events.


Also, if it's worth mentioning, in my set-up there is a passive crossover built-in to the tweeter and woofer.

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Reply by probbieMay 19, 2018

I don't think any of the punters will notice a LPF at 20kHz. If you desire a less bright sound, then may I suggest a more gentle shelving filter which attenuates frequencies over, say 5kHz but just a few dB, say in the region of 6dB. Alternatively, if the system seems to have a prominent frequency due to drivers or acoustics, then you could use a parametric 'Bell' EQ to attenuate the offending area.

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Reply by kam_limeMay 19, 2018

Sounds good, thanks!

The speaker company for the ceiling speakers (tweeter and woofer with passive crossover) I ordered says the frequency response is 40 Hz - 22 kHz at -3dB. For the high pass filter, should I have it at 40 Hz, or maybe since companies stretch the truth, maybe 45 Hz? 


Also, based off my set-up and the fact that I have 4 zones so need a dsp to handle 4 analog pairs separately, is there a dsp you recommend for me? My budget is up to about $1500 US.

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Reply by DaniloDaraMay 18, 2018

It seems to me that you are either underestimating the complexity of your specifications, or overestimating what Digital Signal Processing can do for you.

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Reply by kam_limeMay 18, 2018

Right. I wasn't expecting great results. I wanted to see if I could obtain solid results. I realize that what qualifies as "solid" is very subjective, but I wanted to hear from people much more knowledgable than me.