By Ray Rayburn
Signal processing in a free-wire DSP is like signal processing in an idealized analog system where every device has the same input and output clip levels. You can’t protect processing modules from over-driving using a limiter, because if the levels are high enough to distort the following modules they will already have distorted the input to the limiter. After all everything clips at the same level. If the input does not clip, how can anything following the input clip? Here are the main sources of internal clipping:
- Increasing the gain beyond unity in a processing block – yes it can be done, but why would you ever need to do so?
- Using EQ with boost at some frequencies – the safe way to fix this without needing a limiter is by dropping the level in an amount equal to the maximum boost, and doing so before the EQ.
- Combining multiple signal – again the fix is to reduce the input levels so the sum can never clip the output.
Now before you insert a limiter consider that a DSP limiter has the same input clip level as the modules that follow it. If the goal is clip prevention (as opposed to dynamic range limitation) then a limiter just can’t do the job. You must take one or more of the steps above to prevent clipping. The reasons limiters at the output of the DSP are of benefit in reducing the chances of power amplifier clipping are:
- In many DSP processors the clip point of the output D/A converters is a lower level than the clip point of the internal DSP signal processing chain.
- The power amplifier clip point may be lower than the D/A converter clip point thus allowing the limit threshold to be set even lower.
If the limit threshold is not set lower than the maximum internal digital level, the limiter block does nothing. The maximum amount of limiting possible is determined by how many dB the limit threshold is set below the maximum digital signal level.
Many modern DSP now use floating point processing rather than fixed point. This changes the above somewhat. While the A/D and D/A converters are still fixed point and have defined clip levels, the processing between the converters have if not infinite level capability at least a vastly wider dynamic range than the I/O converters. Therefore you can get away with sloppy gain structure inside the DSP without ever clipping an internal level. Limiters in such a modern floating point DSP are never needed for prevention of internal clipping because internal clipping is almost impossible to achieve. Limiters are needed at the outputs because you are going from the near unlimited internal dynamic range to the fixed maximum level of the D/A converts and the analog devices that follow them. Limiters may also be used profitably for dynamic range control.