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Figure 132
illustrates the overall system architecture model used by HDMI devices for audio clock regeneration. The HDMI source
determines the fractional relationship between the video clock and an audio reference clock (128*fs) and passes the numerator and
denominator for 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. The exact relationship between the two clocks is shown in
The source determines the value of the numerator N as specified in the HDMI specification. Typically, this value N is used in a clock
divider to generate an intermediate clock that is slower than the 128*fs clock by the factor N. The source typically determines the value of
the denominator Cycle Time Stamp (CTS) by counting the number of TMDS clocks in each of the 128*fs/N clocks.
13.12.7 N Parameter
N is an integer number and is calculated using
approximately equals N for coherent audio and video clock sources.
coherent sources or sources where coherency is not known,
13.12.8 CTS Parameter
The CTS is an integer number that satisfies
13.12.9 Recommended N and Expected CTS Values
The recommended values of N for several standard pixel clocks are provided in
The ADV7850 has two modes for CTS generation.
Manual mode:
Manual mode is selected by setting
cts_manual[19:0]. Manual mode is suitable for coherent audio and video, where the audio and video clocks are generated from the same
crystal. Thus, CTS should be a fixed number.
Automatic mode:
Automatic mode is selected by setting
rates. The result can be read from cts_internal. Automatic mode is good for incoherent audio or video, where there is no simple integer
ratio between the audio and video clock.
The 20-bit N value used by the Tx core of the ADV7850 can be programmed in the field N.
cts_sel, Addr B8 (Main), Address 0x0A[7]
This control is used to select the CTS option; either internally generated CTS values or external user-defined CTS.
Rev. A May 2012
128
f
s
Equation 13: Relationship Between Audio Reference and TMDS Clocks
Equation 14
with the recommended optimal value shown in
Equation
128*fs/1500Hz ≤N ≤128*fs/300Hz
Equation 14: Restriction for N Value
Equation 15: Optimal N Value
Equation
16.
CTS
Average
Equation 16: Relationship Between N and CTS
cts_sel
to 1. In manual mode, the user can program the CTS number directly into the chip via
cts_sel
to 0. In automatic mode, the chip computes the CTS based on the actual audio and video
N
=
f
TMDS
_
CLK
CTS
Table 95
to
Table 97
14,
Equation
15, and
128*fs/1000Hz
f
N
=
TMDS
_
CLK
128
f
s
Table 95
371
Equation
13.
Equation 15
can be used to determine the value of N. For non
Equation 16
should be used.
to
Table
97.
ADV7850
which
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