How do I connect the M Scaler to DAVE, and why does dual BNC only work with the output set to white?

Practical answer. Connect both of the M Scaler's dual BNC outputs to one BNC pair on DAVE (BNC 1 and 2, or BNC 3 and 4), and set the M Scaler's output sample rate switch (OP SR) to white Head-Fi, Feb 2019Head-Fi, Sep 2018. Watts said in 2020 that only DAVE needs care over which cable goes where Head-Fi, Apr 2020.

Which cable goes where (DAVE)

• Which inputs. In February 2017, before the Hugo M Scaler was released, he wrote that "the M scaler connects to the dual BNC inputs (BNC 1 and 2 or BNC 3 and 4)" Head-Fi, Feb 2017. His February 2019 post on the Hugo M Scaler names the same pairs Head-Fi, Feb 2019.
• Left and right. In February 2019 he said the order matters for DAVE: "BNC2 or 4 is the right channel (on the top on Dave)" and connects to the M Scaler's right-hand output (Dual BNC out 2, looking from the front), while the left one (Dual BNC 1) goes to the bottom BNC 1 or 3 Head-Fi, Feb 2019.
• Only DAVE is fussy. In April 2020 he said the Hugo TT2 and all other DACs except DAVE auto-switch the data, because the data carries a left or right flag. For DAVE, looking from the front of the M Scaler, the left-hand BNC is left and goes to the bottom BNC pair you are using Head-Fi, Apr 2020. In January 2018 he explained that DAVE's code predates the AES3 dual data standard, so it uses a sync detector and "you had to connect each data line manually" Head-Fi, Jan 2018.
• Cables. In August 2018 he said an RF cable should be 75 ohms, the M Scaler's output impedance Head-Fi, Aug 2018. In February 2020 he said cable length matters for DAVE because it "requires sync", while other DACs use the AES3 dual data flag Head-Fi, Feb 2020. In August 2020 he added that differing lengths give different delays, which can show up on DAVE as drop-outs or "complete failure of 705/768 kHz dual data operation" Head-Fi, Aug 2020.

Why dual BNC needs white

• The rule. In September 2018 he wrote that "dual BNC operation is only for the full 1M taps at 705/768 kHz. Otherwise it is single data mode at max rate of 352/384 kHz" Head-Fi, Sep 2018. The next day he called this "correct behaviour" and said he would get the manual corrected Head-Fi, Sep 2018.
• The switch settings. The same month he listed the OP SR settings: red is pass through, green 176.4/192 (1/4 M taps), blue 352.8/384 (1/2 M taps) and white the full 705.6/768 with 1M taps. In an edit to that post he added that only white enables dual data mode, with the others single data mode "with one BNC active and the other disabled" Head-Fi, Sep 2018.
• What dual data means. In July 2018 he said each BNC in dual data mode is a mono SPDIF at 352.8/384 using the AES3 mono protocol, "so that the sample rate is doubled" Head-Fi, Jul 2018. In March 2019 he said "the only way of outputting 705k is via the dual BNC" Head-Fi, Mar 2019.

If you use a setting other than white

Below white only one BNC carries data Head-Fi, Sep 2018. In August 2019 he said that in pass-through "only one cable is active", so you would need to connect the right cable or select the appropriate coax input Head-Fi, Aug 2019. The post names neither the DAC nor which BNC is active.

Native DSD is a separate case: in October 2018 he said you must set the SR OP setting to white, "otherwise it will be silent as an OP" Head-Fi, Oct 2018.

If DAVE drops out or mutes

In November 2018 he said DAVE's dual data detector needs sync on both data lines, so DAVE can jump into single data mode, which forces a mute and makes the display flash. He said "Normally, using BNC 3 and 4 will fix this issue (using the supplied cables)", and to attach cables securely and check the center pin for dirt or bending. If the problem persists with the supplied cables, is not the source, and swapping BNC inputs does not help, DAVE needs to go back to Chord for service Head-Fi, Nov 2018.

Other Chord DACs

In December 2018 he said the original Hugo cannot accept dual data mode and treats the mono signal as stereo, so he advised its single BNC output, "which is stereo at 352/384 at white" Head-Fi, Dec 2018. In August 2017 he said the Hugo 2 can accept 768 kHz dual BNC, "so it is designed to work with an M scaler" Head-Fi, Aug 2017.

What he has not said. The relevant posts run from 2017 to August 2020, with no later restatement for the Hugo M Scaler and DAVE Head-Fi, Aug 2020. This archive does not show which BNC is active in each single-data setting Head-Fi, Sep 2018Head-Fi, Aug 2019.

Quotes

“only white enables the dual data mode, all the others are single data mode with one BNC active and the other disabled”2018-09-05 Head-Fi, Sep 2018

“dual BNC operation is only for the full 1M taps at 705/768 kHz. Otherwise it is single data mode at max rate of 352/384 kHz.”2018-09-22 Head-Fi, Sep 2018

“BNC2 or 4 is the right channel (on the top on Dave)”2019-02-26 Head-Fi, Feb 2019

“looking from the front of Hugo M scaler, the left hand BNC is left and that should go to the bottom BNC pair you are using with Dave”2020-04-26 Head-Fi, Apr 2020

“Dave uses sync detection, so the different delays can affect this”2020-08-15 Head-Fi, Aug 2020

Dated positions

Caveats and open points

• Most evidence is Head-Fi posts from 2017 to 2020; no later restatement was found.
• Head-Fi, Feb 2017 (Feb 2017) predates the Hugo M Scaler's release; the input pairs are confirmed for it in Head-Fi, Feb 2019 (Feb 2019).
• Head-Fi, Feb 2019 is truncated in the archive partway through a numbered list, so his full answer may not be shown.
• The statement that the M Scaler would ship with a 1 m 75 ohm BNC cable (Head-Fi, Aug 2018) was made in August 2018 and may not match what is in the box.
• The archive does not show which BNC is active in each single-data setting, and Head-Fi, Aug 2019 does not say which DAC was being discussed.

Topics: Dual BNC, dual data and DX links, M Scaler and Quartet settings and operation