What is de-emphasis in USB 3.1 standard

[engr_joni_ee]

I am looking at the USB 3.1 standard. I understand that CTLE is needed at the receiver but I don't get what is "de-emphasis" in the attachment. Is this pre-emphasis at the Tx with some +ve gain and then post-emphasis which some -ve gain. In other words is that gain or amplification at the Tx before actual transmission and then gain reduction at the receiver which is described as pre (+ve) and post (-ve) emphasis and de-emphasis respectively.

 

[FvM]

The table doesn't tell the whole story. Read the 3.1 physical layer specification completely, particularly 6.7 and 6.8. Did you realize that Gen 1 and Gen 2 are using adaptive equalization?

 

[D.A.(Tony)Stewart]

It is just like some radio & phonograph signals boost HF to normalize SNR when attenuated by the medium then restored to flat response in the Rx.
Except this is done for a few dB from 0.5 GHz to 5 GHz with the LPF pre-emphasis peaking at the 1.5 GHz pole using an underdamped up to + 3 dB peaking linear-phase filter.

This SNR boost improves the cable range and compensates for losses.

 

[engr_joni_ee]

Then it look like "pre-emphasis" at the Tx and "de-emphasis" at Rx mentioned in the figure in the original post, right ?

 

[FvM]

The term pre-emphasis isn't used in USB spec.

 

[D.A.(Tony)Stewart]

@FvM The USB3 specification refers to de-emphasis, not pre-emphasis.
They are effectively the same subject.
.
Pre- for before Tx and de- for after Rx.
For pre-emphasis, the peak the gain at max f or use a digital method of mixing the amplitude of the bits before and after with an over-sampled shift register then re-mixed to shape the bit. This compensates for the frequency attenuation of the cable at USB-3 's highest speed to improve the "eye-pattern"
For de-emphasis the amplitude refers to the amplitude of the bits before emphasis is removed.

I showed the analog filter method previously for various emphasis curves. Depending on the cable losses, auto-connect equalization optimizes the de-emphasis for phase margin or maximum eye-pattern or the centre of the error free zone comparing results with different clock phases.

The user specifies dB of emphasis in the driver.

Test methods https://cdn.teledynelecroy.com/files/manuals/qphy-usb3-tx-rx-operators-manual.pdf