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mr_ro_co 15th January 2004 07:15 PM

need for simple all pass digital delay
 
I'm building a three way, active speaker that uses a horn for the high frequencies (TAD 2001 on an 13" OD round horn). It's a deep horn, about 10 inches. This introduces a considerable amount of delay (about 600 microseconds or so) that will introduce a lot of excess phase between the output of the horn and the two other direct radiator drivers that are directly below it (an 8" and 15").

My need is to add simple, all pass delay to the amp that will be driving these horns.

To all the gurus on this particular forum, what reasonably priced pro delay units, capable of at least 16 bit 44 KHz resolution, are out there that can add this much delay, preferably in 10 microsecond increments or finer?

I'd also be interested in building my own, if someone can direct me to the chip(s) that this would require and the industry application notes that would show me how to build the circuit.

Thanks in advance...

Steve

Devil_H@ck 15th January 2004 08:04 PM

There's a circuit on Mr. Linkwitz's site that does exactly this. Just a sec, I'll search for it.

Here are some images from Linkwitz's site (if this kind of image linking is not allowed please edit my post):

Image 1:
http://www.linkwitzlab.com/images/graphics/allpass.gif

Image 2:
http://www.linkwitzlab.com/images/graphics/allpass2.gif

Link: http://www.linkwitzlab.com/filters.htm#4 (if the #4 thing doesn't work, search for delay on that page)

mr_ro_co 16th January 2004 03:17 PM

Thanks for the assisitance...

From what I can tell, these analog circuits merely delay low frequencies below a certain fo that is chosen based the crossover region frequency. This is useful for delaying tweeter ouput through a crossover region to preserve the filter slope's phase shift and get correct summation between the low and high pass filters.

I'm not sure if sufficient delay could be added, starting at one octave or so above my crossover frequency to my midbass and woofer channels using ganged up stages of this all pass network to get the amount of delay I need. I'll have to play around with the RC values and see what's possible.

Thanks...

Steve

Ulas 17th January 2004 08:44 PM

You could use an analog bucket brigade. They are shift registers that use a sample-and-hold register at each stage instead of a latch and are commonly used for audio effects, such as, chorusing. As you can imagine, after traversing 1024 S/H registers, the output has a little more distortion then the input. In fact, anything you do to electronically delay the signal is going to mess it up to some degree. Why not just reposition the horn relative to the other speakers to reduce the delay.

awboy 19th January 2004 04:57 PM

need for simple all pass digital delay ?
 
What is the digital source?
You are speaking of a 3way active loudspeaker so I suppose you have three digitally filtered low level signals going to the 3 amplifiers.

If it is spdif you could (simply?) use an TTL shift register of the appropriate length in the tweeter digital source.
If it is music via a soundcard then there is software that can delay channels.

mr_ro_co 20th January 2004 02:39 PM

Ulas,

Thanks for the suggestion. How much distortion are we talking here?

I cannot reposition the horn. I can't make that cosmetic compromise because the horn is not a separate unit from the rest of the speaker. The 12" OD horn, 8" midbass, and 15" woofer are all flush mounted in a nice cabinet. That is, the compression driver is concealed inside the cabinet.

awboy,

The source is not digital. It is simply an analog preamp driving amps that have passive inline RC filters for simple 6dB/Oct filter slopes. So the preamp output would have to to through a A/D stage, delay stage, and then D/A stage for the feeds that go to the woofer and midbass amps.

I've played around with units like Brook Siren Systems loudspeaker processor boxes, and I know of other very sophisticated pro-audio processors. They just cost a lot of money, and all I want to do is add an exact amount of delay across the entire audio band.

Thanks,

Steve

Ulas 20th January 2004 11:04 PM

As I recall, the typical distortion is around 1%. I suggest you do a Google search, download the data sheets, and determine for yourself it these parts are suitable.

Another possibility is to use a 96k/24-bit ADC/DAC with a few MC14557 shift-registers in between.


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