Barebones explanation: On the FAC tuning used for the tuning, tune D4 and then set the SAT to A6. Play D4 and stop the lights using DOB (BlueShift+CentsUp). Use that DOB setting for tuning the treble (A4-C8). Make sure to remove the DOB when not tuning the treble.
A bit more explanation:
Once the FAC tuning is ready to be used for the tuning – after doing the other tweaks: making sure A4 is exactly correct, having already set the A Default Multiplier .8, lowering and correcting al the partial changes – it’s time to tweak the treble for a little more brightness and clarity and to give it that ‘personal’ touch.
Here’s how: Set the SAT to A6 on the FAC tuning and play D4. The lights will probably be rotating counter clockwise, indicating A6 is flat. Before making any adjustments, make note of the original setting for A6. Let’s say it’s @ 10.0 c.
Now, stop the lights using the cents buttons and see what the new cents reading is @ A6. Lets say the new setting is 14.0 c. This new setting for A6 is our target for where we want A6 to be on the FAC tuning.
Use the Double Octave Beat feature (DOB) to increase the stretch in the treble so that the FAC tuning’s setting for A6 will be 14.0 c. instead of 10 c. The button commands for DOB are BlueShift+CentsUp/Dn.
Each .1 DOB effects the setting @ A6 by .4 c. Since A4 needs to be raised by 4.0 c., 1.0 of DOB will need to be entered into the SAT and be used for tuning the treble. (10 x .4 = 4 c.)
(Press and hold the Blue Shift button and press the Cents up button to see the DOB amount in the SAT display. Put 1.0 DOB and release the buttons. The setting for A6 should now be 14.)
Use a DOB setting of 1.0 DOB to tune the treble (A4 – C8).
But JUST THE TREBLE (A4 – C8). Make sure you set the DOB back to 0.0 after tuning the treble. Just use this DOB setting for tuning A4-C8.
DOB gradually increases the stretch from A4 all the way to the top. This tweak uses A6 as the pass through ‘target’ for the tuning.
Since DOB @ A6 uses .4 c. increments, often getting the exact A6 setting is not possible. When that happens, always round up and use the higher one.
For example, if the FAC tuning put A6 @ 10, and stopping the lighs when playing D4 gave an A6 setting of 13.1 c., DOB will not put A6 @ exactly 13.0 – a .7 DOB would put it @ 12.8, and a .8 DOB would put it @ 13.2. Always use the higher DOB setting (of .8 in this example) and let A6 be just slightly sharper.
What is being done here is tweaking the FAC tuning so it tunes A6 as a double octave fifth above D4.
Aural checking is always good, so before tuning the whole treble using this DOB setting, start by tuning D6 and listen to the D6/A6 5th. If that 5th sounds nice, that’s probably all you need to do. But you can do more if you like. Tune A5 and E6, and listen to that octave (A5/A6) and the A5/E6 5th. Do as much and as many as you like. The more intervals you check the more of an ‘average’ you’ll need to determine. But before making any big adjustments to the DOB setting, check the tuning of the test notes. Also, when using notes below the B5/C6 partial change – if that partial change hasn’t been corrected (and hopefully lowered to G#5/A5) if there’s a bit of a hiccup at that partial change, it might be giving a false reading during aural checks.
But if you’ve done all the other FAC tweaks, tuning a few notes below A6 to confirm the goodness of the DOB setting to result in a D4/A6 double octave +5th, should yield reliable results given the tuning of all the test notes is accurate as well.
But really, after you’ve done this for a while, that first D6/A6 5th aural check will probably tell you all you need to know.
That’s all there is to it. Try this treble tweak on every piano you tune for a couple weeks and see if you like how it sounds.