FAB2200 Datasheet, PDF (21/33 Pages) Fairchild Semiconductor – Audio Subsystem with Stereo Class-G Headphone Amplifier and 1.2W Mono Class-D Speaker Amplifier

Speaker Amplifier Gain Ramp and Zero-Crossing Detection (ZCD)
The SPRAMP, SPRAMPSPEED, and SPZCD bits
control the speed at which PRESENTGAIN is changed
when STARTGAIN is changed.
Table 16. Speaker Gain Change Behavior
SPRAMP SPZCD
Behavior When STARTGAIN Is Changed
0
0
STARTGAIN changes immediately.
0
1
Wait until a zero crossing occurs in the input before changing STARTGAIN. If a zero
crossing does not occur within SPRAMPSPEED, STARTGAIN is forced to the new setting.
1
0
STARTGAIN is ramped to the new setting at a rate of SPRAMPSPEED per step.
STARTGAIN is changed by one step when a zero crossing occurs. If a zero crossing does
1
1
not occur within SPRAMPSPEED, a step is forced. Only the first zero crossing within
SPRAMPSPEED triggers a gain change – gain does not change again until the next
SPRAMPSPEED.
Table 17. Speaker Gain Change Timing
SPRAMPSPEED[1:0]
00
01
10
11
Ramp and ZCD Time Between Steps (ms)
0.25
2.00
16.00
128.00
SPRAMP, SPRAMPSPEED, and SPZCD have no effect
on AGC and noise gate timing. AGC and noise gate
timing have no effect on speaker amplifier gain ramp
and zero-crossing detection. In the event of a conflict
between these systems, PRESENTGAIN chooses the
lowest gain setting. For example, SPRAMP is enabled
with a slow SPRAMPSPEED and a fast AGCATTACK.
The user changes STARTGAIN from 111111 to 000001.
As the ramp function begins to ramp PRESENTGAIN
down slowly (as defined by SPRAMPSPEED), a loud
sound surpasses the AGC threshold. This forces
PRESENTGAIN to react quickly (as defined by
AGCATTACK). If the sound’s amplitude falls below the
AGC threshold before PRESENTGAIN reaches 000001,
the quick gain reduction halts and the slow gain
reduction resumes.
Valid combinations of SPRAMP, SPZCD, SPNGZRA,
and SPNGZRR are shown in Table 18. Combinations
not listed may produce unpredictable results.
Table 18. Valid Speaker Amplifier Ramp / Zero Crossing / Noise Gate Combinations
SPRAMP
SPZCD
SPNGZRA
SPNGZRR
0
0
X
X
X
X
1
1
1
X
1
0
Note:
6. X = don’t care.
If the SPNGZRA bit is set to 0, the speaker noise gate
attack (mute) function occurs immediately rather than
waiting for zero-crossing detection or ramping. If the
SPNGZRA bit is set to 1, the speaker noise gate attack
function obeys speaker zero crossing detection and
ramp settings.
If the SPNGZRR bit is set to 0, the speaker noise gate
release (un-mute) function occurs immediately rather
than waiting for zero-crossing detection or ramping. If
the SPNGZRR bit is set to 1, the speaker noise gate
release (un-mute) function obeys speaker zero-crossing
detection and ramp settings.
© 2009 Fairchild Semiconductor Corporation
FAB2200 • Rev. 1.0.1
21
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