GS4910B GENNUM [Gennum Corporation], GS4910B Datasheet - Page 73

• Manufacturer Part Number: GS4910B
• Description: HD/SD/Graphics Clock and Timing Generator with GENLOCK
• Manufacturer: GENNUM [Gennum Corporation]

3.9.2 Programming a Custom Audio Clock (GS4911B only)
Example 2: Programming an output video clock of 74.175824MHz (74.25/1.001):
Therefore, program N
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.
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
where:
f
f
N
D
Before programming N
to their lowest factors.
For example, to program a fundamental audio sampling frequency of 42kHz:
Therefore, program N
update bit (6) of register 31h. Using registers 3Fh to 41h, the custom audio
sampling frequency generated may then be multiplied by a factor of 64, 128, 256,
or 512 before being presented to the ACLK pins.
NOTE: The AFS reset described in
active.
36655 - 2
∴
s
in
N
------
D
N
------
D
f
------- -
f
out
a
a
in
= desired fundamental audio sampling frequency
a
a
N
----- -
D
a
a
= 27MHz crystal reference
= numerator of the ratio (host register 33h-34h)
= denominator of the ratio (host register 35h-36h)
v
v
=
=
=
=
1024
1024
------------------------- -
74.25
------------ - MHz
1.001
7425 1000
----------------------------- -
2700 1001
27MHz
×
×
April 2006
×
×
----------------------- -
27000000
-----
f
f
in
s
42000
=
250
-------- -
91
v
=
a
a
= 250 and D
= 1792 and D
43008
-------------- -
27000
and D
=
a
, the numerator and denominator must be reduced
1792
----------- -
1125
v
Section 3.7.2 on page 63
a
= 91.
= 1125 and toggle the custom audio clock
GS4911B/GS4910B Data Sheet
a
, D
a
) in the equation:
will always remain
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