GS4911B Datasheet, PDF (73/119 Pages) Gennum Corporation – HD/SD/Graphics Clock and Timing Generator with GENLOCK

Before programming N_v and D_v, the numerator and denominator must be reduced to
their lowest factors. Two examples are given below:
Example 1: Programming an output video clock of 74.25MHz:
f_{---o---u----t = 7---4---.--2---5---M------H-----z-}
f_in
27MHz
∴^N-----^v- = ^{7----4--2---5--} = ^1---1--
D_v 2700 4
Therefore, program N_v = 11 and D_v = 4.
Example 2: Programming an output video clock of 74.175824MHz (74.25/1.001):
f_{---o---u----t = -71------4.---0---.---20------51-----}^M_------^H_----^z_--
f_in
27MHz
∴^N-----^v- = ^{7---4---2---5-----×}-----1---0---0---0- = ^2---5----0-
D_v 2700 × 1001 91
Therefore, program N_v = 250 and D_v = 91.
NOTE: The Nv and Dv values programmed in registers 20h-21h and 22h-23h are not
held until the custom video clock update bit (6) of register 16h is toggled.
3.9.2 Programming a Custom Audio Clock (GS4911B only)
The GS4911B’s audio clocks are derived from the fundamental audio sampling
frequency initially set by ASR_SEL[2:0]. At any time this fundamental sampling
frequency may be modified to create a custom output audio clock.
The user may generate any audio sampling frequency between 6.6kHz and 96kHz, and
therefore create a custom audio clock as high as 512*96kHz. When generating a custom
audio sampling frequency, ASR_SEL[2:0] must be set to 100b and bit 5 of register 31h
(enable_384fs) must be kept LOW.
The fundamental sampling frequency is determined using a ratio based on the 27MHz
reference. Therefore, to program a custom audio clock, the user must calculate and
program the set of integers (N_a, D_a) in the equation:
N_-----a-
D_a
=
1024 × --^f--^s--
f_in
where:
f_s = desired fundamental audio sampling frequency
f_in = 27MHz crystal reference
N_a = numerator of the ratio (host register 33h-34h)
D_a = denominator of the ratio (host register 35h-36h)
Before programming N_a and D_a, the numerator and denominator must be reduced to
their lowest factors.
GS4911B/GS4910B HD/SD/Graphics Clock and Timing
Generator with GENLOCK
Data Sheet
36655 - 5
June 2009
73 of 119