James Richmond has direct experience of the best current high end interfaces providing high channel counts. In this article he shares his thoughts on DADlink, a new protocol which has latency so near to zero that it left him very impressed indeed!

For some time now I’ve been using an Avid MTRX audio interface as my main Pro Tools interface and monitor controller. Regular readers of the blog may also have seen the addition of a DAD AX64 to the studio too, which supplanted the MTRX as the Pro Mon Monitor Controller, primarily because of the massive increase in routing possibilities, 256 channels over Thunderbolt 3 and SPQ. The original MTRX remained as my Pro Tools interface, and I connected the AX64 and MTRX over MADI or Dante.

Avid announced the MTRX II a few months ago and it is now here and I am pleased to report it is (almost) everything that the AX64 is with the addition of Digilink ports for Pro Tools compatibility. 

I’ve been using the MTRX II for the last couple of weeks and it is fantastic but I don’t want to talk about the MTRX II in this article. I want to talk about multi-unit synchronisation and Digital Audio Denmark’s (the OEM manufacturer of the MTRX, MTRX II, MTRX Studio) new protocol, DADlink.

Both the MTRX II and AX64 have a maximum analogue IO capability of 64 channels, via the 8 channel input or output cards. I like the DAD AX/MTRX family products because they are scalable. If you want 8 inputs and no outputs (say, if you have monitors with a digital input) then that can be accommodated. If you need huge channel capability then that can also be done. You buy what you need you also therefore pay for what you need and not for things that you do not. 

The issue is what you do when you want more than 64 channels of analogue conversion? Dante and MADI expansion onboard allows you to affordably expand IO. When using hardware inserts with Pro Tools HDX that include Dante or MADI transport to the master interface additional latency needs to be calculated. This additional latency isn’t a huge problem, but it does need to be considered. 

In my case the instance of wishing to serve the AX64’s analogue IO up to the MTRX II’s Digilink ports I would need to add the converter latency to the MADI latency. When using hardware inserts in Pro Tools this means calculating each hardware inserts IO based on the total latency and if you have some IO internal to the MTRX, some on MADI, some on Dante and some on AES then you have different values that need to be inputted into Pro Tools’ Hardware Insert Delay.

Fig 1. Shows the latency values table from the MTRX II manual.

DAD have an ace up their sleeve with these second generation devices that has not been hugely publicised and that is this newly created DADlink protocol.

The June 2023 software update release notes contain the following text:

"DADLink is a new proprietary interface by DAD. It uses the MADI Mini Module equipped with optical SFP modules to transfer up to 128 channels @ 48 kHz, 64 channels @ 96 kHz or 32 channels @ 192 kHz per link. The link rate is 312.5 MHz. The links are "zero" latency (approx.1 micro second) between the connected units meaning that all inputs and outputs are phase aligned provided the Digi delay is set to the same value in the two or more units connected via DADLink. For proper alignment of inputs and outputs it is recommended that either DADLink or WCLK is used as the sync source between the unit set as Clock leader and the unit(s) set as follower.”

DADlink is actually pretty straightforward. Both devices (in my case the MTRX II and the AX64) need to have MADI daughter cards installed with SFP’s capable of Gigabit speeds installed and an optical cable to connect the two units. These ports are assigned to ‘DADlink’ in DADman, the slave device (the AX64) clocking is set to follow DADlink and… that’s it.

Compared to using MADI the advantages of DADink is double the channel count (128 vs 64) plus near zero latency, at 1 microsecond.

This required testing.

Fig 2. shows the same audio signal multed to 3 analogue inputs. Channel 1 is the MTRX II, channels 2 is the AX64 using DADlink and channel 3 is a Ferrofish Pulse 16 DX CV connected over MADI.

Fig 3. zoomed in shows that the results are that channels 1 and 2 are perfectly time aligned, but channel 3 (MADI) is 3 samples behind.

Let’s look at the fairly simple mathematics of the problem.

To calculate the number of samples of latency you simply multiply the delay in milliseconds by the sample rate, which in my case is 48kHz.

1 microsecond is 0.001 milliseconds.

0.001 X 48 = 0.048 samples.

DAD have reduced the time it takes to transport audio from one device to another below one sample. That is simply astonishing.

The pre-requisites for DADlink are two compatible DAD/Avid interfaces which at the time of writing are the Avid MTRX II, DAD AX64, AX Center, Core 256 with the MADI daughterboard installed in both units. Compatible SFP’s are required and that wasn’t as simple as I first thought it would be.

Avid sell compatible SFP’s but they are £300 in the UK. I had been using a much cheaper (but I assume unsupported) alternative product when using the MADI which is the Broadcom HFBR-57E0LZ. 

This 100Base-FX SFP is not compatible with DADlink which requires 1000Base-FX or faster. I happened to have a pair of Ubiquiti UF-MM-10G SFP’s to hand, which are probably overkill for the task but they also only cost me £33 per unit. They work perfectly well and don’t cost as much as a mini-break to Paris.

The Avid MTRX II and the DAD AX64 are both stellar audio interfaces. In my opinion Digital Audio Denmark make the most flexible, high channel count audio interfaces currently available. DADlink further expands their usability for those that need high channel count and sample accuracy across multiple units by effectively eliminating latency between units.