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Fix audiomixer phase inversion and use DSP on Cortex-M33 - #11617
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On cores with DSP instructions, a level of 1.0 (1 << 15) shifted left by 16 overflowed into the sign bit, so the mixed sample came out negated: mono output was inverted and the right channel of a centered stereo voice was out of phase with the left. Shift by 15 instead. A centered pan now scales both channels by 1.0 instead of the left one by 32767/32768.
The SIMD helpers were limited to ARMv7E-M, so the Cortex-M33 in the RP2350 fell back to a float divide per sample. Check __ARM_FEATURE_DSP instead, and mix with integers where there is no DSP, which matches the DSP result bit for bit.
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On boards with DSP instructions (SAMD51, nRF52840, i.MX RT, STM32F4), a voice at level 1.0 came out negated. The level is 1 << 15, and the code shifted it left by 16, which overflowed into the sign bit. Mono output was inverted. With centered stereo the right channel was out of phase with the left, so it cancelled out when the two channels were summed to mono.
Speed: The DSP code ran only on ARMv7E-M. The RP2350 has the same DSP instructions, so it now uses them too. Chips without DSP, like the RP2040, now mix with integers instead of float. The output is the same on every chip.
Time to mix 4 voices, 1024 samples:
Flash is 1.1 KB smaller on RP2040 and 1.8 KB smaller on RP2350.
Testing: I read the mixer output with audiocore.get_buffer() on all four boards.
The code was written with AI help.