Now that I've had a few weeks with the Okto dac8 pro thought it would be good share my experience integrating a few DSPs with the Okto in Pure AES mode as it has NOT been completely straight forward.
My primary application is DIY two channel active speakers. The main DSP I have been using is a miniSHARC + DIGI-FP + 4x AES out, see write up on the miniDSP forum at the link below for more info.
https://www.minidsp.com/forum/hardware-support/18550-minisharc-8-channel-digital-output
When I first received the Okto I was doing some basic testing with the miniSHARC+Okto with a MOTU M4 to make sure everything was working as expected. On loopbacks with no filters applied I noticed differing phase response on many of the outputs. At 20 kHz the phase offset was 75 or 150 deg depending on the channel. These offsets exactly correspond to a 1-2 sample delay at a 96 kHz sample rate so I figured some channels were being delayed by 1-2 samples by the Okto. After doing some more investigation it seems like channels 1 and 2 are always perfectly synced but will always be some amount out of sync relative to other channels. Typically this is 1 sample ahead of all other channels but the behavior is inconsistent. I've tried running the miniSHARC at both 48 kHz and 96 kHz and have tried other DSPs such the SHD Studio and the nanoDIGI and the Okto has the same behavior with all of them. From doing a bit of googling it seems most likely these delays are happening in the XMOS as I came across many similar complaints in the XMOS developers forum.
As I mentioned above the delays are inconsistent and I have found that they may be changed by the following actions:
-Power cycling DSP
-Power cycling Okto
-Unplugging / re-plugging Okto AES input 1/2
-Switching between Pure AES, Pure USB and USB/AES input modes on Okto
The following actions do NOT trigger a change in delay:
-Soft powering down Okto with remote (I believe this is more of standby mode as Okto remains visible to a computer)
-Changing DSP configuration
-Changing DSP inputs
As I leave the DSP on at all times and only use Pure AES mode I have found that I can compensate for the Okto delays in the DSP and they do not change as long as I do not do any of the actions mentioned above that change delays.
It is important to remember that in applications other than active speakers where you need to worry about phase alignment between channels these delays will be completely inconsequential. Even in active speakers these delays are likely inaudible. Nevertheless it is a bit disappointing to experience this issue with a device that advertises top-tier objective performance.
I've attached some screenshots that show a 1 kHz from channels 1-2 in perfect alignment, a 1 kHz tone from channels 2-3 showing a 1 sample offset at 96 kHz and then a 1 kHz tone from channels 2-3 with a 0.01 ms delay on channel 3 to compensate for the 1 sample delay. I've also attached some phase response screenshots for 48 and 96 kHz sample rates showing the phase response with a 1 sample delay.
I have also been doing a bit of experimenting with a nanoDIGI as it is a low cost option with 8 channels of digital output. Unfortunately it only has coaxial SPDIF outputs so it will not interface directly with the Okto's AES inputs. At first I tried using 6 ft long RCA to XLR cables from monoprice. This worked well for up to 4 channels of output from the nanoDIGI but if more channels of output are used there is significant noise / crackling. I also tried using 1.5 ft RCA to BNC cables to Neutrik NADITBNC-M transformers to 3 ft AES cables. In this configuration I could use up to 6 channels of output with no issue but if I tried to use all 8 channels I would get noise / crackling. I did not have any shorter cables to try and I wonder if I could use all 8 channels with shorter cables. In any event for applications that require 6 channels of output the nanoDIGI is a great low cost option for the Okto.
Michael