If there was ever a good idea on controlling a SDRAM with a microcontroller, this is it! The diagram says it all....
As mentioned, the PIC16F887 does the "grunt" work of controlling the SDRAM, and the Arduino does all the fancy stuff. To clarify, the PIC chip controls the addressing, loading, and transfer of data to and from the SDRAM, ADC, Latches, and DAC. Not shown it also will control De-glitching of the DAC output if needed.
Apart from receiving "orders" from the Arduino, this is all the PIC will do.

  The Arduino will:
-RX Commands from the Main Control (ATMEGA) and run sequences.
-Keep track of raw addressing pointers, or retrieve address data from PIC
-Send Address data to PIC
-Perhaps control 4 SASH (Sample And Sorta Hold) circuits to control audio

  These SDRAM's are similar to the old DRAM's in that they have equiv. CAS and RAS address lines, but the sequence of multiple CLK pulses, WE, CAS, RAS, & CS make for a fairly complex series of events. Throw in the latch LE, de-glitch, ADC's start acquisition, and CS, things become somewhat complex. To further explain the Row Refresh I intend on using, which I'm sure will work fine, here's a table..

  As my method/application only needs 1 byte at a time, and re-loading the RAS address, (which will also set up the SDRAM for a refresh), the Sample PIC will step through ROW's (4096) then increment COL's. Each time the RAS is stepped into, the refresh cycle will happen, after proper clocks.

Presently, I am trying to design with a board stacking in mind, to keep it compact. So once that is figured out, work can begin!

 I found a PIC18F4220 in my box of PIC microcontrollers, and wow it can go fast! I can double the clock frequency internally, which makes 40 MHz! This made a great improvement and allowed for simultaneous Record/Play, which is really needed for layering.

  As can be seen above, there are 3 main boards, but these sandwich another board that I found on EBay, the SDRAM breakout. Because of double headers it was a bit cumbersome, but the boards came out good enough that it wasn't an issue.

The most recent schematic I have is the one to the right, (click to see much larger!) which is pretty accurate except the LPF has been replaced by a TDA7718 audio processor. It's basically a 3 band EQ designed for car audio, but has 6 "speaker" outputs and is controlled via I2C like all TDA's.

 For a while I thought this would defeat the purpose of the MCP41010 digital Pot, but as it turned out it was needed as well. There was also no need to "deglitch" the DAC output.
The TDA chip provides the delicate balance of feedback required much better than SASH circuits did.

  In the schematic above, you can see the diodes feeding the arduino's A0 analog input. This is for Peak Slicing, one of the 3 types of slicers. It has a drop-off of about 100mS (depending on where the peak level is set) and worked much better than I thought. It not only slices at a voltage crossing, but recognizes a drop, then rise as another slice. So if there's a cymbal  trailing and a hat occurs, the hat will be registered as a new slice. Nice!

 The circuit has of course been changed. An Op-AMp has been added to give 5:1 gain before feeding into the cap and diodes.

  I've dubbed the sampler/looper the "Borg Cube" as with 6 layers, or levels, and it's seeming complexity, it was the logical name. ( If you're a trekkie like me! )
So what if there's a few headers on it!  The photo below shows the audio section with the new Mod, the TDA board, which was one of the 2 extras from the main mixer section I had left over.

   Also, if you look at the pics closely, you can see an Arduino Pro-Mini in there somewhere (behind the crystal in the pic bottom right) It's soldered right on instead of mounted on a header. I just put a 1K resistor on the FTDI programmer interface's TxD line so I would'nt fry the tiny FTDI chip with MIDI data!

A 9 Volt zener limits the voltage to the TDA which can only accept 10 V maximum. The voltage comes from the 12 Volts mains.

 The analog and digital grounds have been joined together and the noise reciprocated by dropping to virtually nothing.

The small board above the ADC was an add-on super low noise Op-Amp to remove any stray noise between the audio board and the ADC input. It is recommended in the datasheet.
I used the LM833, which is very low noise ( 4.5nV/√Hz ), but hasn't the big price tag.

 The board below, left, is the DAC out board, which uses a PCM 53JP 16 bit DAC. These DACs were purchased from China on Ebay, fairly cheap, but I suspect the design is decades old. It has all of the earmarks of the 80's, i.e. reversed MSB/LSB (Q0 is MSB), big power draw, case style, no input gate or latch, a "de-glitching" circuit in the datasheet.

 It works will, but is a power pig. The SIAB now has 2 of these, which draw a total of  140mA. That's quite a bit, and they warm up to prove the point! Oh well.

The other 2 chips are 74HCT573 8 bit latches. This load the data from the SDRAM's 16 bit data bus at the appropriate time. The HCT types seem to work well will 3.3V logic inputs.
The white box is a multi-turn trim pot that centers the ADC voltage at 1/2 of 4.096V. Because it is capacitively coupled, the input to the ADC may drift off and a sudden record cycle after sitting quiet can cause a "pop" at the start, which can be annoying if looping or echoing!

 The photo, lower right, shows a side view of how the boards all fit together. Count them, 6 layers in all. Pretty cool!

Using The Sampler:

 Each part of the SIAB project has been incredibly fun to build, then use, but the Sampler has been the best so far. I'm not familiar with hardware sampler/looper's in general, so am not sure if the features I've added are standard or bizarre or what, but if you're building a sampler, this may give you some ideas.  Here's the modes it has:

                        1)         Note Sampler
  The Note Sampler is basically like the old "Emulator" in that keys can be programmed with a sound, which can then be played with other keys. Refer to the keyboard diag. above.

                        2)         Layer-Loop Tempo 
  The Layer-Loop Tempo is one of the main features of the Sampler.  This Mode allows controlled looping in layers. That is a tempo-synced loop can be initiated from a foot switch, or via MIDI, then "record paused" to continue looping until the next part is added.
At that point, another tap anywhere in the loop will commence a new recording at the loop point set by the 2nd tap on the footswitch.

                        3)         Layer-Loop Free
  Sometimes, just a vocal or guitar loop is needed without a tempo, or more importantly as a part of a band, where tapping is king!  A tempo signal doesn't need to be, or have been, present. This means no preparation and the only job at hand being tap accuracy. Other than that, the operation is as with the Layer-Loop Tempo.

IIntroduction for all Slicers:  The Slicers allow a recorded sample to be divided in 3 different ways, then assigned to notes in sequence (Notes 72-104, see above diagram) to be played in a new way! The slices don't loop individually but follow through as the original recording. This is more intuitive as the original recording may contain a sound immediately after that wouldn't sound good by itself.   There can be a maximum of 32 slices.
*  The Played Notes of the sliced sample are velocity sensitive only if the velocity is below half (64) at which point the level of sound falls toward zero. It can never be 0 though or will be interpreted as a note off. Playing slices by hand usually yields velocities of over 64 so this isn't an issue, but in a sequencer, there is some control over sample levels.

                        4)         Tempo Slicer
  To record, simply press the Record or Footswitch to begin, then again to end. If there is a tempo active, (MIDI or internal blue light flashing) the record will "wait" for the next bar. This is 99% the way it will be used, so it's automatic. The sample will be sliced according to the Tempo Sync Setting (1/32,1/24,1/16,1/12,1/8,1/4 etc)

                        5)         Peak Slicer
  The Peak slicer is the trickiest of the three slicers.  Record procedure is the  same as all slicers, but slices are created  from audio levels rising above the level of Peak (CC#82)
This Mode can be great for acquiring slices of drums or other percussive sounds, but can work well with voice, and even guitar if the notes are "plucky". For DJ'ing, from canned music, it can add an element of randomness which is really fun to play with.

                        6)         Divide Slicer
  The Divide Slicer does what it's called, Divides the Recording into slices. The number of slices can be selected using the Sync setting's alternate values (32,24,16,8,6,4,3,2,1)
This must be set before the record is commenced, even though the slicing happens after the Record stop tap. This mode is very useful for sampling/slicing canned music by pressing the footswitch on the down beat, then on another downbeat for a predictable slice size and position.

                        7)         Echo Looper
 The Echo Looper is the most "hands-off" Mode. Apart from requiring a start from the foot switch (Record), and extremely long period, it resembles a standard echo.
This echo requires a tempo to be present as echoing this long would be useless otherwise. You'd never know when the wrap around will be. As with all modes, this mode can be automated or manually controlled. The echo time can be very short (<1/4 note or a few mS if set that way)  or very long, up to 184 seconds. The echo stop command includes a nice fade out.
 A tempo isn't required but if a song was playing since this mode selected, that tempo will be used for start synchronization. (Good for trailing echoes after a song stop)

                        8)         Echo Free
 The Echo Free is similar to the Echo Looper, but can be started any time, whether MIDI is playing or not, and, of course, will not be on-sync. The echo time in fact is controlled by:
Granulizer Size (CC#85) -        This is the course tune, 1 mS to .5 seconds
Granulizer Speed (CC#86) -     This is the fine tune, mostly for 1,2,3 mS setting on course.
 The tighter settings can create a cool flange effect, or a ringing effect if the EQ is balanced properly. This mode is a good way to test EQ dynamics affect on longer echoes or for layering as it will show the changes quickly. If, for example, you wanted the "older" layers to sound tinnier and distant, set the EQ here then move to the Layer Mode.

                        9)         Foot Hold
  The foot hold is very simple compared to other Modes. It is always recording, so when the footswitch is pressed, it'll hold the last recorded space that the granulizer Slice is set to.
This is great for holding a vocal note, or guitar note. releasing the footswitch causes the level to fade down, pressing again halts the fade and holds at new level.

                        10)       Extras Option
  Modes 10-16 have been left open, but included in the selector for future mode ideas that may become obvious.

FX Modifiers:

                        1)         Pitch Notes
  These notes will shift the sample rate of the sample presently playing or recording. The diagram above shows the range and notes (+/- 1 octave)

                        2)         Pitch Shifter & Pitch Wheel
  The Pitch Wheel (on most keyboards) will overtake the Pitch Notes or Dashboard Panel Pitch setting

                        3)         Stutter
  Sampler Effects Old school! The stutter control is really simple. You set the Adj.(CC104) to a value and this value is AND'ed to the address values (on play side only) to the SDRAM which creates a stutter or buzz effect. The stutter setting determines what binary value is AND'ed.

                        4)         Reverser
  Reverses the presently playing sample for time the note is held. Great for seguaying into a different part of a song. I used during an Echo Mode, Echo is cancelled.

                        5)         Granulizer
  A standard granualizer with controls for progression (forward/reverse) speed and grain size. When activated, starts looping a small section of the sample playing, but slowly moving so the loops change giving a "slo-mo" effect. LFO can act on this creating some excellent FX.

                        6)         LFO
  The LFO controls the sample rate with various envelopes from wave shapes and stepping. The envelopes are sine, triangle, ramp up, ramp down, PWM up, PWM down, Random (stepped with limits), and Audio Level (the envelope is actually the level of audio input)

  The pitch Shifter (notes) will work with most of the above, but should only be used with note sampler and Slices during playback for "non-space alien effects" operation lol!

  The LFO will work with most of the modes above, but will also record while LFO varies the sample rate. The LFO has several envelopes, some specialized for sampling.

       This is a really basic look at the Modes and FX. For a more in-depth look, download the user manual (it's in .rtf but MS Office stuff will open it like a .doc)
Here it is in the  long-awaited PDF format

This didn't affect the next layer recording.

  It has been a month or so and I have discovered that samplers are in demand in the modular synth world, and there is (at least some) interest in my design.
So, I am considering modifying the Sampler above into a CV/Trigger controllable sampler module. The hardest part of this for me is not dreaming up functions and features, but how to fit it all into a modular panel of reasonable size, and make it look good! Euro seems the best way to go with the 3.5mm (as opposed to 1/4") jacks, and more compact form factor.

 The next decision will be whether or not it should be a DIY kit. As shown above, some of the TSOP stuff is basically outside of the soldering abilities of the average hobbyist. So those chips would need to be either already on the boards or on break-out boards up to DIP size.

 It's all up in the air right now, but it would be an interesting challenge to take up once the SIAB project is complete. Until then, if you're a modular freak or Muff Wiggler, these samps I've dug up may describe a small section of the sampler's capabilities.