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Improvements to high order low- and high-pass filters.
This changes the Q value of the chain of second order low-pass filters making up the high order filters. Previously the Q value was always set to 0.707107 for the entire chain, but this is incorrect for Butterworth filters when the order is greater than 2. This should make a small improvement to the quality of the linear resampler.
This commit is contained in:
David Reid
+43
-30
@@ -1072,13 +1072,13 @@ Low-Pass Filtering
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------------------
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Low-pass filter is achieved with the following APIs:
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|---------|-----------------------------------------|
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| API | Description |
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|---------|-----------------------------------------|
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| ma_lpf1 | First order low-pass filter |
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| ma_lpf2 | Second order low-pass filter |
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| ma_lpf | Low-pass filter with configurable order |
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|---------|-----------------------------------------|
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|---------|------------------------------------------|
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| API | Description |
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|---------|------------------------------------------|
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| ma_lpf1 | First order low-pass filter |
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| ma_lpf2 | Second order low-pass filter |
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| ma_lpf | High order low-pass filter (Butterworth) |
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|---------|------------------------------------------|
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Low-pass filter example:
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@@ -1103,11 +1103,11 @@ Filtering can be applied in-place by passing in the same pointer for both the in
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ma_lpf_process_pcm_frames(&lpf, pMyData, pMyData, frameCount);
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```
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The maximum filter order is limited to MA_MAX_FILTER_ORDER which is set to 8. If you need more, you can chain filters together.
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The maximum filter order is limited to MA_MAX_FILTER_ORDER which is set to 8. If you need more, you can chain first and second order filters together.
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```c
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for (iFilter = 0; iFilter < filterCount; iFilter += 1) {
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ma_lpf_process_pcm_frames(&lpf[iFilter], pMyData, pMyData, frameCount);
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ma_lpf2_process_pcm_frames(&lpf2[iFilter], pMyData, pMyData, frameCount);
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}
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```
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@@ -1126,13 +1126,13 @@ High-Pass Filtering
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-------------------
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High-pass filtering is achieved with the following APIs:
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|---------|------------------------------------------|
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| API | Description |
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|---------|------------------------------------------|
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| ma_hpf1 | First order high-pass filter |
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| ma_hpf2 | Second order high-pass filter |
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| ma_hpf | High-pass filter with configurable order |
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|---------|------------------------------------------|
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|---------|-------------------------------------------|
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| API | Description |
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|---------|-------------------------------------------|
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| ma_hpf1 | First order high-pass filter |
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| ma_hpf2 | Second order high-pass filter |
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| ma_hpf | High order high-pass filter (Butterworth) |
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|---------|-------------------------------------------|
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High-pass filters work exactly the same as low-pass filters, only the APIs are called `ma_hpf1`, `ma_hpf2` and `ma_hpf`. See example code for low-pass filters
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for example usage.
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@@ -1142,12 +1142,12 @@ Band-Pass Filtering
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-------------------
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Band-pass filtering is achieved with the following APIs:
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|---------|------------------------------------------|
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| API | Description |
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|---------|------------------------------------------|
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| ma_bpf2 | Second order band-pass filter |
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| ma_bpf | Band-pass filter with configurable order |
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|---------|------------------------------------------|
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|---------|-------------------------------|
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| API | Description |
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|---------|-------------------------------|
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| ma_bpf2 | Second order band-pass filter |
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| ma_bpf | High order band-pass filter |
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|---------|-------------------------------|
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Band-pass filters work exactly the same as low-pass filters, only the APIs are called `ma_bpf2` and `ma_hpf`. See example code for low-pass filters for example
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usage. Note that the order for band-pass filters must be an even number which means there is no first order band-pass filter, unlike low-pass and high-pass
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@@ -30262,10 +30262,16 @@ static ma_result ma_lpf_reinit__internal(const ma_lpf_config* pConfig, ma_lpf* p
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for (ilpf2 = 0; ilpf2 < lpf2Count; ilpf2 += 1) {
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ma_lpf2_config lpf2Config;
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double q;
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double a;
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/* Tempting to use 0.707107, but won't result in a Butterworth filter if the order is > 2. */
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if (lpf1Count == 1) {
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a = (1 + ilpf2*1) * (MA_PI_D/(pConfig->order*1)); /* Odd order. */
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} else {
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a = (1 + ilpf2*2) * (MA_PI_D/(pConfig->order*2)); /* Even order. */
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}
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q = 1 / (2*ma_cos(a));
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/* TODO: Calculate Q. */
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q = 0;
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lpf2Config = ma_lpf2_config_init(pConfig->format, pConfig->channels, pConfig->sampleRate, pConfig->cutoffFrequency, q);
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if (isNew) {
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@@ -30758,9 +30764,15 @@ static ma_result ma_hpf_reinit__internal(const ma_hpf_config* pConfig, ma_hpf* p
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for (ihpf2 = 0; ihpf2 < hpf2Count; ihpf2 += 1) {
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ma_hpf2_config hpf2Config;
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double q;
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double a;
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/* TODO: Calculate Q. */
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q = 0;
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/* Tempting to use 0.707107, but won't result in a Butterworth filter if the order is > 2. */
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if (hpf1Count == 1) {
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a = (1 + ihpf2*1) * (MA_PI_D/(pConfig->order*1)); /* Odd order. */
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} else {
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a = (1 + ihpf2*2) * (MA_PI_D/(pConfig->order*2)); /* Even order. */
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}
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q = 1 / (2*ma_cos(a));
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hpf2Config = ma_hpf2_config_init(pConfig->format, pConfig->channels, pConfig->sampleRate, pConfig->cutoffFrequency, q);
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@@ -31078,8 +31090,8 @@ static ma_result ma_bpf_reinit__internal(const ma_bpf_config* pConfig, ma_bpf* p
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ma_bpf2_config bpf2Config;
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double q;
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/* TODO: Calculate Q. */
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q = 0;
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/* TODO: Calculate Q to make this a proper Butterworth filter. */
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q = 0.707107;
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bpf2Config = ma_bpf2_config_init(pConfig->format, pConfig->channels, pConfig->sampleRate, pConfig->cutoffFrequency, q);
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@@ -38318,11 +38330,12 @@ ma_decoder_config ma_decoder_config_init(ma_format outputFormat, ma_uint32 outpu
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config.format = outputFormat;
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config.channels = outputChannels;
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config.sampleRate = outputSampleRate;
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ma_get_standard_channel_map(ma_standard_channel_map_default, config.channels, config.channelMap);
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config.resampling.algorithm = ma_resample_algorithm_linear;
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config.resampling.linear.lpfOrder = ma_min(MA_DEFAULT_RESAMPLER_LPF_ORDER, MA_MAX_FILTER_ORDER);
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config.resampling.speex.quality = 3;
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/* Note that we are intentionally leaving the channel map empty here which will cause the default channel map to be used. */
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return config;
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}
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