Emphasis (telecommunications)

Removal of the distortion caused by pre-emphasis is called de-emphasis, making the output accurately reproduce the original input.

Emphasis is commonly used in many places ranging from FM broadcasting (preemphasis improvement) and vinyl (e.g. LP) records to PCI Express.

As CD players were originally implemented with affordable 14-bit converters, a specification for pre-emphasis was included to compensate for quantization noise.

After economies of scale eventually allowed full 16 bits, quantization noise became less of a concern, but emphasis remained an option.

[1] In serial data transmission, emphasis is used to improve signal quality at the output of a communication channel.

Both have the same net effect of producing a flatter system frequency response; de-emphasis is typically more convenient to do in real circuits since it only requires attenuation rather than amplification.

[2]: 9  Well-known serial data standards such as PCI Express, SATA and SAS require transmitted signals to use de-emphasis.

[4] Higher numbers of taps are possible but increase circuit complexity and tend to result in diminishing returns [2]: 14  so are not commonly used.

RIAA equalization curve for vinyl records
Baseline signal with no emphasis. Transition bits are clearly weaker than non-transition bits and the signal is touching the mask (fail).
Excessive pre-cursor emphasis. The eye is more closed than the baseline, indicating minimal pre-cursor ISI was present and the emphasis is doing more harm than good.
Well-tuned post-cursor emphasis. The eye is open and transition and non-transition bits are well matched in amplitude, indicating a correct level of equalization. The signal is passing the mask test.
Excessive post-cursor emphasis. The eye is starting to close and transition bits have significant overshoot, indicating excessive equalization. The "double banding" artifact visible in the eye indicates the presence of significant ISI [ 5 ] : 2 caused by the excessive emphasis.