High-Definition Multimedia Interface is a compact audio/video interface for transmitting uncompressed digital data.

Often multimedia PC's comes with a HDMI output.
Not everybody is enthusiastic about the sound quality.

Of the many problems associated with HDMI, the audio quality is totally handicapped for lack of -- a pin! They designed the connector before they finished designing the system. They didn't have enough pins to also have a master audio clock.

So with HDMI, the audio clock is derived from the video clock. For high-def TV, the video clock runs at either 74.25 MHz or 74.25 * (1000/1001) [thank you NTSC!!!]. The audio clock runs at multiples of 48 kHz. Of course, these are not related. So the receiver has a PLL to regenerate an audio clock based on instructions from the transmitter (source) telling it what to do.

The result is the worst jitter of any system yet invented. It truly sucks.

Much later, they added a thing called Audio Rate Control in HDMI 1.3a. This puts a buffer and the master audio clock in the receiver. Then commands are sent upstream on the CEC line telling the player to speed up and slow down as necessary to keep the buffer full.
Charles Hansen

In the Feb 2009 edition of the Hi-fi News magazine Paul Miller measured the following jitter results for a few A/V amplifiers:

Denon AVR-3803A
SPDIF: 560psec
HDMI: 3700psec

Onkyo TX-NR906
SPDIF: 470psec
HDMI: 3860psec

Pioneer SC-LX81
SPDIF: 37psec
HDMI: 50psec

Yamaha RX-V3900
SPDIF: 183psec
HDMI: 7660psec



Jitter level comparison (lower is better):

Jitter with SPDIF input:
10ps Classe SSP800
15ps DCS Scarlatti
37ps Pioneer SC-LX81
40ps Cambridge DACMagic
50ps Arcam AVR600
121ps Sony SCD-XA5400ES
183ps Yamaha RX-V3900
250ps Denon 2500/AVP-A1
430ps Onkyo PR-SC886
470ps Onkyo TX-NR906
485ps Audiolab 8000AP
560ps Denon 3808A

Jitter with HDMI input:
5ps Arcam AVR600
21ps Classe SSP800
50ps Pioneer SC-LX81
200ps Sony XA5400ES HATS on
2200ps Denon 2500/AVP-A1
3700ps Denon 3808A
3860ps Onkyo TX-NR906
3920ps Onkyo PR-SC886
7660ps Yamaha RX-V3900
8000ps Sony XA5400ES HATS off
8490ps Audiolab 8000AP

Source: AVS Forum.