Understanding DisplayPort Maud and Naud Values

This article explains the function and significance of Maud and Naud values in DisplayPort audio transmission. In DisplayPort standards, these two parameters serve as the foundation for Audio Clock Regeneration (ACR), enabling receiving devices to accurately reconstruct the source audio clock from the video link clock. Understanding how Maud and Naud operate clarifies how audio stays synchronized and jitter-free without requiring a dedicated physical clock line.

The Role of Audio Clock Regeneration

DisplayPort transmits both video and audio data as serialized digital packets across high-speed differential pairs known as the Main Link. The Main Link operates at fixed symbol rates (such as 1.62 Gbps, 2.7 Gbps, 5.4 Gbps, or higher), which are asynchronous to standard audio sampling rates (such as 44.1 kHz, 48 kHz, 96 kHz, or 192 kHz).

Because the audio sample clock is not transmitted directly over a physical wire, the sink device (such as a monitor or AV receiver) must synthesize the original audio clock locally. This process is called Audio Clock Regeneration. To accomplish this, the source device transmits the fractional relationship between the link clock and the audio clock using the parameters Maud and Naud.

Definition of Maud and Naud

Within the DisplayPort Secondary Data Packets (specifically Audio Clock Regeneration or Audio Stream packets):

  • Maud (Audio M-value): Represents the numerator of the clock ratio. It is a dynamically measured or calculated value that reflects the exact frequency relationship of the audio sample clock relative to the link clock. Because physical clocks experience slight drift, Maud can vary dynamically to compensate for clock frequency variations over time.
  • Naud (Audio N-value): Represents the denominator of the clock ratio. Unlike Maud, Naud is typically a fixed parameter defined by the DisplayPort specification based on the link rate and audio format (commonly set to a power of two or a standard constant).

The Mathematical Relationship

The receiving sink uses a Phase-Locked Loop (PLL) or digital frequency synthesizer to derive the audio stream master clock (\(f_{audio}\)) from the link symbol clock (\(f_{LS\_CLK}\)) using the following equation:

\[f_{audio} = f_{LS\_CLK} \times \left(\frac{M_{aud}}{N_{aud}}\right)\]

The resulting generated frequency (\(f_{audio}\)) is typically a multiple of the audio sampling rate (often \(128 \times f_s\) or \(512 \times f_s\)), which the sink’s digital-to-analog converter (DAC) uses to process audio frames.

Why Maud and Naud Are Essential

  1. Jitter Reduction: Continuous transmission of Maud updates allows the sink's PLL to track and adapt to minor frequency drifts between the source and display, minimizing phase jitter.
  2. Buffer Management: Accurate clock regeneration ensures that the sink consumes audio data at precisely the rate it is sent, preventing audio buffer underflow (which causes dropouts) or overflow (which causes crackling and dropped samples).
  3. Hardware Efficiency: Transmitting virtual timing values eliminates the need for dedicated physical clock lines, reducing cable complexity and pin counts while supporting high-resolution multi-channel audio.