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62 lines
3.6 KiB
Markdown
62 lines
3.6 KiB
Markdown
## speakersafetyd - a software Smart Amp implementation
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speakersafetyd is a userspace daemon written in Rust that implements an
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analogue of the Texas Instruments Smart Amp speaker protection model.
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Apple Silicon Macs mostly use the Texas Instruments TAS2764 amp chip (codec
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in ALSA parlance), which provides sense lines for the voltage and current across
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the voice coil of the connected speaker. These codecs are designed to be used
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in embedded applications where device firmware takes this information and uses
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it to protect the speaker from damage. Apple instead implement this as machine-specific
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plugins to the userspace half of CoreAudio. An increasing number of other
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vendors in both the desktop and embedded/Android worlds are choosing to go down a similar
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route, folding this functionality into proprietary driver/userspace blobs that usually
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also bundle niceties like EQ (we have a solution for this too, see [asahi-audio](https://github.com/AsahiLinux/asahi-audio)).
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This puts users at serious risk of permanently destroying their expensive devices if they choose
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to run custom software, such as Asahi Linux or an Open Source Android ROM.
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speakersafetyd is the first (as far as we know) FOSS implementation of a speaker
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protection model. It solves the problem described above by allowing parties interested
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in compatible devices to quickly and easily implement a speaker protection model for those
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devices. Only Apple Silicon Macs under Linux are currently supported,
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however the model applies to all loudspeakers. The daemon itself should be easy enough to
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adapt for any device that provides V/ISENSE data in a manner similar to TAS2764.
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### Dependencies
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* Rust stable
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* alsa-lib
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* An Apple Silicon Mac running Asahi Linux
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### Some background on Smart Amps
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The cheap component speaker elements used in modern devices like
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Bluetooth speakers, TVs, laptops, etc. are very fragile. In order
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to eke the highest possible sound quality out of them, they need to be
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driven *hard*. This leaves us with a dilemma - how do we drive these
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speakers hard enoguh to get a loud, high-quality output but not hard
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enough to destroy them?
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A speaker's electromechanical characteristics can be modelled
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and boiled down to a set of parameters - the Thiele/Small Parameters.
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These can be used to predict what the speaker will do with certain
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inputs. When we add measured properties like the time constant of
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the speaker's voice coil's and magnet's temperature curve, we can
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accurately model a speaker's temperature for any given voltage/current
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across the coil. When the speaker is getting too hot, we just reduce the
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power going to it until it cools down.
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This lets us fearlessly drive the speakers as hard as they physically can
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be without being permanently damaged. This is extremely useful, as without it
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the output level on these devices would have to be hard limited to a very low
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level that is known to be safe for the worst possible input. Instead, we can
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simply duck the output in those cases and allow the speakers to oeprate at
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full power where possible.
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Many integrated amplifier chips implement this functionality in hardware, as well
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as additional advanced DSP features like compressors and limiters. Texas Instruments
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call their implementation "Smart Amp." Integrators need only communicate the parameter set
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to the chip for the speaker it is connected to, and it does the rest. Many do not however,
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and instead only provide facilities for measuring the voltage and current across the speaker's
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voice coil. It is up to the implementer to capture this data and do something with it.
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speakersafetyd is (as far as we know) the first and only FOSS implementation of the
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Smart Amp protection model.
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