HDMI Spec

Konwledging2026-01-06 8:29

Audio Clock Regeneration

Audio data being carried across the HDMI link, which is driven by a TMDS clock running at a rate corresponding to the video pixel rate, does not retain the original audio sample clock. The task of recreating this clock at the Sink is called Audio Clock Regeneration.
coherent clocks ：In many video source devices, the audio and video clocks are generated from a common clock
The Source shall determine the fractional relationship between the TMDS clock and an audio reference clock (128 *audio sample rate $fs$ ) and shall

pass the numerator and denominator of that fraction to the Sink across the HDMI link.
The Sink may then recreate the audio clock from the TMDS clock by using a clock divider and a clock multiplier.
- fs : audio sample rate
- N: 在video和audio用同一个clock的情况下, N的定义是在spec推荐的表中Recommended N and Expected CTS，不同一个clock的情况下需要计算(N = 128*fs / 1000Hz)
- CTS: 等于 (fTMDS_clock* N ) / (128 · fs)
Typically, this value N will be used in a clock divider to generate an intermediate clock that is slower than the 128·fs clock by the factor N.
The Source will typically determine the value of the denominator CTS (Cycle Time Stamp) by counting the number of TMDS clocks in each of the 128·fs/N clocks.