Sunday, December 11, 2011

Audio Delay

I've always wanted to build an audio delay box, something that would take an audio input (mono or stereo) and delay it by some variable amount (user determined). Why? Because when I've watched the Saturday afternoon game and the baseball playoffs for my home team (SF Giants), I realize just how lousy those national league announcers are. But, playing an out of sync radio broadcast with the TV broadcast is the stuff of nightmare. The radio is always ahead of the TV broadcast because the radio is transmitted locally and the TV broadcast goes around and around, up and down, cable, satellite, whatever.

So, I need an audio delay device. The Arduino family seems like a great place to start building such a device. Just before starting to build this project, I looked around to see if anyone had built one of these. There are plenty of options for products between $60 and $200 that delay between 1 second and 1 minute. I found one product that could delay up to 20 minutes.

I only need a delay of max 5 seconds, but I'm an engineer, so as big as I can make it will make me happy. Plus, I need to build it for less than $60 in parts, or I might as well buy it. Actually, that's not exactly true, any cost over $60 pays for my enjoyment in building the project, but you get what I mean.

If I use the Arduino (or the Mega) that's half or all of my budget, so this engineering process should get interesting.

I have a few ideas for designs and will make follow up posts detailing them with some pros and cons. In short, here's what I've thought through so far:

  1. ADC -> MEM -> DAC + CPLD/FPGA + Arduino
  2. ADC -> Flash Mem -> DAC + CPLD/FPGA/Arduino
  3. ADC -> MEM -> DAC + Arduino Mega (which has lots of address pins)
  4. Delta-Sigma Modulator + Delay Queue (implemented w/ Arduino + majolsurf mem expansion)
For MEM, I was thinking of an old DDR2 board.

Basically, the designs come down to some combination of the following elements:
  • An Analog to Digital Converter that outputs one or two channel (mono v. stereo) and 1 bit or N bit (generally, 16 bits). However, even the N-bit ADCs really stack up 1 bit outputs and then release them as N bits, but on a serial line! Therefore, the N-bit ADCs are simply bursty one bit ADCs. None of them have 8 or 16 parallel output, so their output needs to be thrown into a shift register if to be used in parallel.
  • A Digital to Analog Converter (1 bit or N bit). If data held in parallel, then it needs to be placed in an SR and sent to DAC, serially. N-bit DACs don't accept input in parallel.
  • Memory
    • Flash memory accepts input as a bit stream and it should be in page chunks, that is, written or read sequentially. Flash memory really isn't meant to be Random Access, or least NAND based flash isn't.
    • DDR2 has some interesting requirements (a section of low addressed memory that is off limits for storage) and 200 PINs to drive or read. The Arduino Mega could run a DDR2 board.
  • Data Transfer Protocols
    • I2C or SPI or Parallel or NIH (Not Invented Here or Roll My Own).
    • The challenge of the first two are lack of familiarity and/or speed limitations.
    • The challenge of the second is a heck of a lot of wiring - ugh!
    • The challenge of the third is inventing a stable (serial) protocol better than I2C or SPI.
  • Delay Control - one or two knobs that allow the user to dial in a delay (coarse/fine grained delay). It would be great if the delay could be from sub microsecond up to limit of the system. Also, if the system delay maximum is large, the coarse grained control should be non-linear in response (smaller steps closer to zero).
  • Display Delay - it would be nice if the user could see how much of a delay the system was using.
    • Controller/Control Signaling - somehow, all of these elements need to communicate with each other and do the delay work.
    In the end, the system converts analog voltage levels into frequency bit streams that get delayed then returned back into a voltage signal. Everything else is just implementation.

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