1 | /* |
2 | Copyright 2018 Google Inc. All Rights Reserved. |
3 | |
4 | Licensed under the Apache License, Version 2.0 (the "License"); |
5 | you may not use this file except in compliance with the License. |
6 | You may obtain a copy of the License at |
7 | |
8 | http://www.apache.org/licenses/LICENSE-2.0 |
9 | |
10 | Unless required by applicable law or agreed to in writing, software |
11 | distributed under the License is distributed on an "AS-IS" BASIS, |
12 | WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
13 | See the License for the specific language governing permissions and |
14 | limitations under the License. |
15 | */ |
16 | |
17 | #include "dsp/near_field_processor.h" |
18 | |
19 | #include "base/constants_and_types.h" |
20 | #include "dsp/filter_coefficient_generators.h" |
21 | #include "dsp/gain.h" |
22 | |
23 | namespace vraudio { |
24 | |
25 | namespace { |
26 | |
27 | // Cross-over frequency of the band-splitting filter. |
28 | const float kCrossOverFrequencyHz = 1000.0f; |
29 | |
30 | // +6dB bass boost factor converted to linear scale. |
31 | const float kBassBoost = 2.0f; |
32 | |
33 | // Average group delay of the HRTF filters in seconds. Please see |
34 | // [internal ref] |
35 | const float kMeanHrtfGroupDelaySeconds = 0.00066667f; |
36 | |
37 | // Average group delay of the shelf-filter in samples. |
38 | const size_t kMeanShelfFilterGroupDelaySamples = 1; |
39 | |
40 | } // namespace |
41 | |
42 | NearFieldProcessor::NearFieldProcessor(int sample_rate, |
43 | size_t frames_per_buffer) |
44 | : frames_per_buffer_(frames_per_buffer), |
45 | delay_compensation_(static_cast<size_t>(kMeanHrtfGroupDelaySeconds * |
46 | static_cast<float>(sample_rate)) - |
47 | kMeanShelfFilterGroupDelaySamples), |
48 | lo_pass_filter_(BiquadCoefficients(), frames_per_buffer_), |
49 | hi_pass_filter_(BiquadCoefficients(), frames_per_buffer_), |
50 | low_passed_buffer_(kNumMonoChannels, frames_per_buffer_), |
51 | delay_filter_(delay_compensation_, frames_per_buffer_) { |
52 | DCHECK_GT(sample_rate, 0); |
53 | DCHECK_GT(frames_per_buffer, 0); |
54 | DCHECK_LT(kCrossOverFrequencyHz, 0.5f * static_cast<float>(sample_rate)); |
55 | |
56 | // Generate biquad coefficients and construct low- and high-pass filter |
57 | // states. |
58 | BiquadCoefficients lo_pass_coefficients; |
59 | BiquadCoefficients hi_pass_coefficients; |
60 | ComputeDualBandBiquadCoefficients(sample_rate, crossover_frequency: kCrossOverFrequencyHz, |
61 | low_pass_coefficients: &lo_pass_coefficients, |
62 | high_pass_coefficients: &hi_pass_coefficients); |
63 | |
64 | // Create two biquad filters initialized with the above filter coefficients. |
65 | lo_pass_filter_.SetCoefficients(lo_pass_coefficients); |
66 | hi_pass_filter_.SetCoefficients(hi_pass_coefficients); |
67 | } |
68 | |
69 | void NearFieldProcessor::Process(const AudioBuffer::Channel& input, |
70 | AudioBuffer::Channel* output, |
71 | bool enable_hrtf) { |
72 | |
73 | DCHECK(output); |
74 | DCHECK_EQ(input.size(), frames_per_buffer_); |
75 | DCHECK_EQ(output->size(), frames_per_buffer_); |
76 | |
77 | // Low-pass filter the input and put it in the temporary low-passed buffer. |
78 | auto* low_passed_channel = &low_passed_buffer_[0]; |
79 | lo_pass_filter_.Filter(input_channel: input, output_channel: low_passed_channel); |
80 | |
81 | // High-pass filter the input and put it in the output channel (unmodified). |
82 | hi_pass_filter_.Filter(input_channel: input, output_channel: output); |
83 | // Iterate through all the samples in the |low_passed_buffer_| and apply |
84 | // the bass boost. Then, combine with the high-passed part in order to form |
85 | // the shelf-filtered output. Note: phase flip of the low-passed signal is |
86 | // required to form the correct filtered output. |
87 | ConstantGain(/*offset_index=*/0, gain: -kBassBoost, input_samples: *low_passed_channel, output_samples: output, |
88 | /*accumulate_output=*/true); |
89 | |
90 | if (enable_hrtf) { |
91 | // Delay the output to compensate for the average HRTF group delay. |
92 | delay_filter_.InsertData(input: *output); |
93 | delay_filter_.GetDelayedData(delay_samples: delay_compensation_, buffer: output); |
94 | } |
95 | } |
96 | |
97 | } // namespace vraudio |
98 | |