No Amp Output After Reboot When Using AES Source With JBL Speaker Tunings
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Introduction:
When an amplifier is loaded with a speaker tuning that contains FIRs designed for a sample rate other than the one the amplifier is set to, the FIR will not work as intended, and it can cause the DSP to behave in unexpected ways.
Symptoms:
FIR clock speed mismatch issues have been seen to cause:
- Halted output and metering
- Incorrect EQ
- Phase issues between band passes that change tonality and sound incorrect
- Phase issues between band passes that change horizontal coverage
- LCD showing “Firmware N/A” after bootup has completed
- Incorrect load measurements from the amp
- Corrupted DSP requiring a system reset
The problems may occur only after a reboot. While this is not the only possible reason for these issues, the Processing Sample Rate (PSR) must be checked when there are issues when using FIR filters.
Details:
In Crown I-Tech HD amplifiers, the DSP is clocked to the Cobra Net clock. It has no relationship to the AES inputs, which are always Sample Rate Converted (SRC).
To get to the PSR/Cobranet setting, click on any input button on the 4x3500HD, IT12000HD, IT9000HD or IT5000HD device panel in Audio Architect or Performance Manager.
FIR filters are mathematical coefficients designed to change audio running at a specific sample rate. Speaker designers use them to adjust amplitude (level) at specific frequencies and also adjust phase (time) at specific frequencies. By applying FIRs, individual band passes can be optimized to perform at their best. Additionally, whole speakers can be optimized by optimizing frequency and phase overlaps between band passes. When optimizing phase overlaps between band passes, systems can manipulate the directionality of the overlapping frequencies. When done correctly, FIRs are a helpful and common tool used in speaker system design.
When FIR sample rates are mismatched with the DSP’s speed, the FIR will transpose the effects of the filter to the new speed. If the filter is designed for 96 k Hz, like most JBL presets, and is run at 48 k Hz, a filter designed to affect 2 k Hz, for example, will now affect 1 k Hz. Phase filters will also misbehave. This will cause the system to misbehave in tonality, introduce undesired phase issues between drivers, and may open the opportunity for catastrophic damage as high and low pass filters in the FIR will get transposed or may be removed entirely.
Inside the amplifier, the DSP will be running a filter that is now twice as long as intended. Depending on the FIR length and the number of FIRs affected, this may cause the DSP to be overloaded, causing it to overheat or crash. This will lead to the device no longer processing audio.
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