GS4911B Datasheet, PDF (57/113 Pages) Gennum Corporation – HD/SD/Graphics Clock and Timing Generator with GENLOCK

GS4911B/GS4910B Data Sheet
To genlock the audio clock to the video clock, the user must lock the fundamental
sampling frequency (f_s) to the frequency of the output video clock (f_out). This is
accomplished by programming the set of integers (A_Feedback_Divide,
A_Reference_Divide) in the equation:
-^A----^_---^F----^e--^e---^d---^b---^a---^c---^k--^_---^D-----ⁱ-^v---ⁱ--^d---^e- = n × ---^f--^s--
A_Reference_Divide
f_out
where:
f_s = the fundamental audio sampling frequency
f_out = output video clock frequency
A_Feedback_Divide = numerator of the divide ratio (host register 3Bh-3Ch)
A_Reference_Divide = denominator of the divide ratio (host register 3Dh-3Eh)
n = an integer constant
The integer constant, n, will depend on the fundamental audio sampling frequency.
It will be one of the three values as defined in Table 3-5.
Table 3-5: Integer Constant Value
ASR_SEL[2:0]=100b
Enable_384fs = 0
Value of constant (n)
NO
X
3072
YES
YES
1024
YES
NO
1536
NOTES:
1. Enable_384fs corresponds to bit 5 of address 31h of the host interface. It is LOW by default.
2. ‘X’ signifies ‘don’t care.’ This bit will be ignored when determining n.
Before programming A_Feedback_Divide and A_Reference_Divide, the
numerator and denominator must be reduced to their lowest factors.
For example, to manually genlock a custom audio clock with a fundamental
sampling frequency of 42kHz to a 27MHz video clock, the following calculations are
necessary:
n = 1024
∴-^A----^_---^F----^e--^e---^d---^b---^a---^c---^k--^_---^D-----ⁱ-^v---ⁱ--^d---^e-= 1024 × -----4---2---0---0---0------ = ^{4---3----0--0---8--}= ^{1---7---9---2--}
A_Reference_Divide
27000000 27000 1125
Therefore, program A_Feedback_Divide = 1792 and A_Reference_Divide = 1125.
Note that n=1024 when programming a custom audio clock (see Section 3.9.2 on
page 73).
36655 - 2 April 2006
57 of 113