CEquel Digital Systems Overview
Page 14 of 16
Clip Eliminator
The clip eliminator is based completely on the clip signal from the amplifier. The DSP/uP has
no other information about output signal of the amplifier. Given the latency of the amplifier
and conversion stages, the fastest the output signals can be limited is about 15ms. So if a
heavily clipped signal comes out of the amplifier, gain reduction will not start until about 15ms
later. So….. the clip limiter is not perfect, in fact it is quite crude. Here is how the clip limiter
algorithm will adjust the limiter threshold based on clip events. The clip signal is detected by
an ISR(Interrupt Service Routine), which simply counts clip events (VERY FAST), almost
never misses. The clip limiter routine is called every 100us and adjusts the limiter threshold;
so the limiter threshold can be adjusted quite quickly. Also, the amplifier generates the clip
signal at every zero crossing. For example, a clipped 20k sine waveform(Period=50us), will
generate 4 clip events every 100us, one for each zero crossing. The clip eliminator routine
will adjust the limiter based on this number of clip events.
1
st
clip event Set limiter threshold to 0.0dB
2
nd
clip event Set limiter threshold to -0.2dB
3
rd
clip event Set limiter threshold to -0.3dB
4
th
clip event Set limiter threshold to -0.4dB
5
th
clip event Set limiter threshold to -0.2dB
6
th
clip event Set limiter threshold to -0.3dB
7
th
clip event Set limiter threshold to -0.4dB
8
th
clip event Set limiter threshold to -0.5dB
9
th
clip event Set limiter threshold to -0.6dB
10
th
clip event Set limiter threshold to -0.7dB
13
th
clip event Set limiter threshold to -1.0dB
:
:
23
rd
clip event Set limiter threshold to -2.0dB
43
rd
clip event Set limiter threshold to -3.0dB
83
rd
clip event Set limiter threshold to -4.0dB
:
:
243
rd
clip event Set limiter threshold to -24.0dB
Actually, the maximum gain reduction provide by the clip limiter is based on the amp model
.
The 1000 amplifiers will only provide -9.0dB, while the 2000 and 4000 will go all the way down
to -24dB.
The clip LED has also been somewhat desensitized. A single clip event will not fire the LED.
To turn the clip LED on, there must be more than 2 clip events occur in a 100us period.
If the amp is in bridge mode, only the ch1 clip eliminator routine is used. A clip event on either
ch1 or ch2 is used.
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