Exploring audio DACs in simulation – Part 3 – 2021-04-03

Having set up a simplistic testbench in Verilator, I want to be able to use it to evaluate several different DACs. The ones I will be testing are:

• The simple 1st-order Sigma Delta I included in my last post.
• A 2nd order DAC found on Github: https://github.com/hamsternz/second_order_sigma_delta_DAC/blob/master/second_order_dac.v – however, I’ve adapted this to halve the input volume by discarding the lowest bit, since second- (and higher) order 1-bit sigma delta DACs are unstable at the extremities of their input range – they require some headroom. Since the Amiga application only requires 15-bit input this is not a problem.
• A 3rd-order DAC found in Mark Watson’s Atari 800 repository. I’m not sure where it originally came from or what its copyright status is, since it’s not listed in Mark’s otherwise comprehensive list of which license applies to which files. The 3rd order DAC already attenuates the signal to maintain stability so I didn’t need to modify this one, except as detailed below.
• Finally, the simplest form of my hybrid DAC.