From d59476d0e102ba833eaa3a0d242d931a44fa245a Mon Sep 17 00:00:00 2001 From: Chris Beswick Date: Mon, 4 Aug 2025 15:43:44 +0100 Subject: [PATCH] [i2s_audio] Use high-pass filter for dc offset correction (#10005) --- .../microphone/i2s_audio_microphone.cpp | 64 +++++++++++++------ .../microphone/i2s_audio_microphone.h | 3 +- 2 files changed, 48 insertions(+), 19 deletions(-) diff --git a/esphome/components/i2s_audio/microphone/i2s_audio_microphone.cpp b/esphome/components/i2s_audio/microphone/i2s_audio_microphone.cpp index 5ca33b3493..cdebc214e2 100644 --- a/esphome/components/i2s_audio/microphone/i2s_audio_microphone.cpp +++ b/esphome/components/i2s_audio/microphone/i2s_audio_microphone.cpp @@ -24,9 +24,6 @@ static const uint32_t READ_DURATION_MS = 16; static const size_t TASK_STACK_SIZE = 4096; static const ssize_t TASK_PRIORITY = 23; -// Use an exponential moving average to correct a DC offset with weight factor 1/1000 -static const int32_t DC_OFFSET_MOVING_AVERAGE_COEFFICIENT_DENOMINATOR = 1000; - static const char *const TAG = "i2s_audio.microphone"; enum MicrophoneEventGroupBits : uint32_t { @@ -381,26 +378,57 @@ void I2SAudioMicrophone::mic_task(void *params) { } void I2SAudioMicrophone::fix_dc_offset_(std::vector &data) { + /** + * From https://www.musicdsp.org/en/latest/Filters/135-dc-filter.html: + * + * y(n) = x(n) - x(n-1) + R * y(n-1) + * R = 1 - (pi * 2 * frequency / samplerate) + * + * From https://en.wikipedia.org/wiki/Hearing_range: + * The human range is commonly given as 20Hz up. + * + * From https://en.wikipedia.org/wiki/High-resolution_audio: + * A reasonable upper bound for sample rate seems to be 96kHz. + * + * Calculate R value for 20Hz on a 96kHz sample rate: + * R = 1 - (pi * 2 * 20 / 96000) + * R = 0.9986910031 + * + * Transform floating point to bit-shifting approximation: + * output = input - prev_input + R * prev_output + * output = input - prev_input + (prev_output - (prev_output >> S)) + * + * Approximate bit-shift value S from R: + * R = 1 - (1 >> S) + * R = 1 - (1 / 2^S) + * R = 1 - 2^-S + * 0.9986910031 = 1 - 2^-S + * S = 9.57732 ~= 10 + * + * Actual R from S: + * R = 1 - 2^-10 = 0.9990234375 + * + * Confirm this has effect outside human hearing on 96000kHz sample: + * 0.9990234375 = 1 - (pi * 2 * f / 96000) + * f = 14.9208Hz + * + * Confirm this has effect outside human hearing on PDM 16kHz sample: + * 0.9990234375 = 1 - (pi * 2 * f / 16000) + * f = 2.4868Hz + * + */ + const uint8_t dc_filter_shift = 10; const size_t bytes_per_sample = this->audio_stream_info_.samples_to_bytes(1); const uint32_t total_samples = this->audio_stream_info_.bytes_to_samples(data.size()); - - if (total_samples == 0) { - return; - } - - int64_t offset_accumulator = 0; for (uint32_t sample_index = 0; sample_index < total_samples; ++sample_index) { const uint32_t byte_index = sample_index * bytes_per_sample; - int32_t sample = audio::unpack_audio_sample_to_q31(&data[byte_index], bytes_per_sample); - offset_accumulator += sample; - sample -= this->dc_offset_; - audio::pack_q31_as_audio_sample(sample, &data[byte_index], bytes_per_sample); + int32_t input = audio::unpack_audio_sample_to_q31(&data[byte_index], bytes_per_sample); + int32_t output = input - this->dc_offset_prev_input_ + + (this->dc_offset_prev_output_ - (this->dc_offset_prev_output_ >> dc_filter_shift)); + this->dc_offset_prev_input_ = input; + this->dc_offset_prev_output_ = output; + audio::pack_q31_as_audio_sample(output, &data[byte_index], bytes_per_sample); } - - const int32_t new_offset = offset_accumulator / total_samples; - this->dc_offset_ = new_offset / DC_OFFSET_MOVING_AVERAGE_COEFFICIENT_DENOMINATOR + - (DC_OFFSET_MOVING_AVERAGE_COEFFICIENT_DENOMINATOR - 1) * this->dc_offset_ / - DC_OFFSET_MOVING_AVERAGE_COEFFICIENT_DENOMINATOR; } size_t I2SAudioMicrophone::read_(uint8_t *buf, size_t len, TickType_t ticks_to_wait) { diff --git a/esphome/components/i2s_audio/microphone/i2s_audio_microphone.h b/esphome/components/i2s_audio/microphone/i2s_audio_microphone.h index 633bd0e7dd..de272ba23d 100644 --- a/esphome/components/i2s_audio/microphone/i2s_audio_microphone.h +++ b/esphome/components/i2s_audio/microphone/i2s_audio_microphone.h @@ -82,7 +82,8 @@ class I2SAudioMicrophone : public I2SAudioIn, public microphone::Microphone, pub bool correct_dc_offset_; bool locked_driver_{false}; - int32_t dc_offset_{0}; + int32_t dc_offset_prev_input_{0}; + int32_t dc_offset_prev_output_{0}; }; } // namespace i2s_audio