MADI distorted audio, synchronizing the Gemini system to an external 48kHz word clock?

Summary of the defect:

After a Gemini installation in July 2019, a shortcoming in the MADI card design and functionality was uncovered. The defect reported was the audio on the MADI source had a small but noticeable amount of distortion when listening to a MADI channel on a panel headphone.

Reason for the defect:

The Gemini system is based on 48kHz samples. This is derived from a local 24.576MHz. So the mixing engine, expansion TDM stream and sampling on the analogue boards (AEB’s) are all performed using the same clock source.  In the configuration editor the “TDM” clock option for MADI is this clock.

The MADI stream received carries samples from another source. These samples are 48kHz, but not the same 48kHz as Gemini. The MADI expansion card receives the stream and recovers the clock.  The recovered clock is only used as an option to transmit the out-going MADI stream. 

The other two options for the outgoing MADI clocking are to use

  • external word clock; or
  • the TDM clock (Gemini system clock).

Note this is the outgoing clocking of the MADI. The Gemini system is still performing the mixing at the local system clock rate. There will be sample rate mismatch issues because the Gemini system is not locked to the MADI stream.

The accumulation of the sample rate mismatch causes the distortion heard within the speech on an AEB output.

The Solution:

To overcome the sample rate mismatch issues both Gemini and the MADI source could use the same reference clock. For example, feed a word clock to both the MADI kit and Gemini.

The 03 version of the Gemini matrix has the ability to synchronise its local clock to the recovered HSL clock. With some addition hardware modifications, it is possible to modify a system to accept 48kHz word clock on the fault loops D-type. Thus, the word clock source can be used to lock the Gemini.

The following documents how to modify and configure the system so that Gemini can synchronize to an external 48kHz word clock such as that generated by the MentorRG/XL systems.

As well as supporting the configuration of this mode, the MADI interface card firmware will need to incorporate a new Gemini FPGA build.

***Note, this modification only applies to the revision 4 Gemini baseboard which is in the Gemini -03 system build ***


Hardware modification details

The fault loops D-type connector (PCB ref. J5) has spare pins that can be used for the external word clock input. Pin 5 shall be used for the ground connection and pin 9 shall be used for the single-ended word clock input. The new pin-out is shown below.

Figure 1 – Pin-out for Fault loops port.

The Gemini baseboard needs to incorporate a modification to receive the external word clock on the fault loops port and route to the appropriate circuitry.

IMPORTANT: The modification supports a TTL-compatible 48kHz word clock to be used as the external source. The absolute maximum voltage level is 7Vp-p.


The following details how to modify the revision 4 baseboard to get word clock in to the Gemini.

  • Using 7/0.2 awg equipment wire (twisted black and red), on the underside of the baseboard solder the red wire to pin 9 of J5 and the black wire to pin 5 of J5.
  • On the underside of the baseboard, cut track to Pad 1 of J51 as shown below.
  • Solder the twisted pair black wire to pad 2 and the twisted pair red wire to pad 1 as shown below.

Figure 2-Wire Attachment to J51 on underside

  • Remove R519 and U106.
  • Cut track on topside of baseboard between pins 1 and 3 of U106. (see diagram below)
  • Refit U106.
  • Solder kynar wire link from U106 pin 4 to pad of R519 (closest to FPGA device). (see diagram below)
  • On the underside of the baseboard carefully solder kynar wire to pad 15 of J51 (same pad as twisted pair red connection.
  • Pass the kynar wire through via of W13 to top side of the board and solder the kynar wire to pin 3 of U106. (see diagram below)

Figure 3 – Modification to component side around U106

  • It might be useful to consider changing the hardcoded variant version of the platform so that the firmware can identify that the feature is available. This can be done by removing the 0R resistor on J46 so that the platform variant is reported as “001” (instead of “000”).

This completes the hardware modification.

Firmware modification details

FPGA build version ‘D’ (V6) or greater shall be required to accompany this modification. Please note version ‘D’ FPGA build can be used without the modification in place.

Word clock adaptor.

For professional digital audio equipment, it is common for word clock to be transported via 75 Ohm BNC terminated coaxial cables. MentorRG/XL presents word clock on a BNC connector and has a 75 Ohm output impedance.  This presents an 8Vp-p signal level when un-terminated. A 75 Ohm termination resistor is required to match the cable characteristics which will reduce the signal level to 4Vp-p (i.e. TTL compatible).

In order to connect word clock to the Gemini system a BNC to D-type connector adaptor shall be required. If the source is Mentor RG/XL then a 75 Ohm termination resistor must be incorporated into the adaptor to correctly terminate the signal and ensure the signal level is TTL compatible. For a source that is TTL compatible then the 75 Ohm resistor must be omitted.

Figure 4 – Adapter for BNC connection


Configuration for locking the Gemini system to word clock

To enable the word clock source to lock the Gemini the following check box should be enabled via the Gemini browser > Admin > Settings > System Settings page. A reboot is required for the setting to take effect.

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