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Old 11th September 2004, 05:22 PM   #1
lulu is offline lulu  Chile
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Default digital reverb or delay

hey guys i am looking for a good digital reverb design. i am completely clueless about it, i guess it has to pass the following stages: adc -> some "storage" option -> some chip that reads the memory a certain number of times -> dac

i hint this might be a little harder than your average diy project, maybe, but i am looking foward to trying good designs. if you could give me a hint i would truly appreciate it.
thanx

Luciano
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Old 13th September 2004, 11:56 PM   #2
ingrast is offline ingrast  Uruguay
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I was looking for the same kind of product about a month ago.
Some years back there were analog bucket brigade chips for this use but they have apparently gone out of production.

An alternative limited only to delay effects should be A/D followed by FIFO memory and D/A, all of this coordinated by a local clock and some glue logic.

I choose to follow the DSP route which provides also much more processing possibilites like filtering, tremolo, equalization, flanger etc.
It implies higher costs and a substantial learning curve but I felt worth it. This project will be adressed towards year's end / next year.

Rodolfo
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Old 14th September 2004, 12:39 AM   #3
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There are Reverberation Amplifiers on ebay all the time that go cheap and they often use those analog bucket brigade shift registers. I have a nice one scored at a yard sale (Sansui or maybe Pioneer) It has a way out time tunnel display on the front panel. I also have a Radio Shaft mixing console with this form of echo built in and a separate Radio Shaft echo box also using bucket brigade. These could maybe be coaxed from my paws with offers of cash.
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Old 14th September 2004, 04:21 PM   #4
lulu is offline lulu  Chile
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Quote:
Originally posted by ingrast
I was looking for the same kind of product about a month ago.
Some years back there were analog bucket brigade chips for this use but they have apparently gone out of production.

An alternative limited only to delay effects should be A/D followed by FIFO memory and D/A, all of this coordinated by a local clock and some glue logic.

I choose to follow the DSP route which provides also much more processing possibilites like filtering, tremolo, equalization, flanger etc.
It implies higher costs and a substantial learning curve but I felt worth it. This project will be adressed towards year's end / next year.

Rodolfo

can you tell me more about the dsp?? i have found one, the tas3103 but it seems a bit complex... can you tell me how you use it?? or which one do you use??

and how you solder it, since it has those little legs??

thanx in advance
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Old 15th September 2004, 09:11 PM   #5
ingrast is offline ingrast  Uruguay
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Default DSP for reverb

Luciano:

Getting into DSP is not the casual type of project. DSP works by processing digitized signals on the fly with algorithms in turn derived from a mathematical formalization of signal theory.

If you have a maths and physics background, I can point you reference material to check in this respect, and to application specific literature and producs.

To start actual work, you should select a manufacturer and product family that better suits your needs, and purchase a developement kit which usually bundles a card with the DSP chip and support circuitry, a cable for communicating the card and your computer, and software both for download / debug and for program developement. Prototype kits usually cost several hundred dollars depending on brand and model.

Rodolfo
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Old 16th September 2004, 09:59 PM   #6
ingrast is offline ingrast  Uruguay
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Default digital reverb

Luciano:

Stirred by your enquiry, I checked some parts and found a rather decent slapback/reverb, even basic chorus can be readily built from a few tens of dollars worth of components.

I requested samples and will be back in a few days with a more definite idea.

Rodolfo
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Old 17th September 2004, 12:48 AM   #7
OliverD is offline OliverD  Germany
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Basically this can be achieved without a DSP chip, using cheap standard components.

You implement a ring buffer which consists of an address counter and a static RAM. Then add a parallel ADC and DAC, with associated analog signal conditioning. The databus of ADC, DAC and RAM would be connected. A clock and decimal counter provides the timing for the steps of operation:

1. Read a sample from the ring buffer and transfer it to the DAC
2. Read a sample from the ADC and transfer it to the RAM
3. Advance the address counter
4. back to step 1

Feedback (for multiple echos) and mixing the original with the delayed signal would be done in the analog domain.

The delay time can be adjusted by resetting the address counter at adjustable positions or by changing sampling frequency (tricky).

A long time ago I built such a beast based on 8bit Ferranti DACs and ADCs. Must have been one of my first projects. It was based on an article in an electronics magazine which went out of business in the 80s.
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Old 17th September 2004, 04:22 AM   #8
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have a look at the PT2399 chip - big with the guitar effects boys
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Old 17th September 2004, 07:56 PM   #9
ingrast is offline ingrast  Uruguay
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OliverD:

You are quite right with the proposed configuration. I should suggest a FIFO type memory for the delay element, for they are made with that purpose in mind to simplify circuitry. You don’t need for example address generator and latching of input / output data since FIFOs are dual ported.

What I have in mind nonetheless is something a little more involved, short of a more accurate realization that mostly makes sense with a DSP chip instead of specially built hardware.

A more realistic reverberation consists of an initial delayed echo, followed by several decaying closely spaced secondaries. More sophisticated implementations involve pre-delays and various mixings of secondaries at different time intervals.

A basic yet workable hardware realization can be accomplished for example with a first delay stage – tunable from 10 to 40 ms corresponding to what is known as “slap back”, and a second stage tunable from 3 to 10 ms with feedback to accomplish the decaying secondaries train.

I have ordered a set of samples including 74V225 1Kx18 FIFO (U$ 8.- ea.) for the second stage, 74V245 4Kx18 FIFO for the first stage (U$ 8.50 ea.), PCM1802 96 KHz / 24 bit A/D converter (U$ 3.35 ea.) and DSD1791 192 KHz / 24 bit D/A converter (U$ 3.15 ea.). Add to this some TL074 low noise opamps for antialiasing filters, post-conversion filters and you are mostly done.

An inexpensive and interesting addition should be to enable not only variable clocking speed (to vary delay) but also to modulate it with a low frequency to achieve a basic chorus effect and similars. The design should end up with lots of switches and knobs, but musicians use to love to have lots of things to tweak with. (interestingly, it is fairly easier to provide lots of switches and knobs in a purely hardware approach than a software based one !)

I will pursue this development in the next weeks (as soon as I receive the samples and design and build a prototype PCB).

If there is interest in this subject, I will post more details (tentative schematics etc.) and will of course welcome any suggestions you deem worth considering.

PT2399

I did some search on this part and found neither Digi-Key nor Newark carry it. It is probably the type of discontinued or not widely available part I should not try in a new design. A reference to it mentions it is rather noisy too.

Rodolfo
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Old 17th September 2004, 10:49 PM   #10
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Yes I assume it (pt2399) is obsolete or obscure

I mention it because it just seems so simple.
( I have no exerience with it )

It and it's obscure BBD chips seem to be available here
http://www.smallbearelec.com/Ordering/ICsDelay.htm

I eagerly await a more substantial design- as yours appears to be


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