diff --git a/firmware/code/run.c b/firmware/code/run.c
index 53e7120..14333a0 100644
--- a/firmware/code/run.c
+++ b/firmware/code/run.c
@@ -106,7 +106,7 @@ static void _as_audio_packet(struct usb_endpoint *ep) {
     multicore_fifo_push_blocking(CORE0_READY);
     multicore_fifo_push_blocking(samples);
 
-    for (int j = 0; j < FILTER_STAGES; j++) {
+    for (int j = 0; j < filter_stages; j++) {
         // Left channel filter
         for (int i = 0; i < samples; i += 2) {
             fix16_t x_f16 = fix16_from_int((int16_t) out[i]);
@@ -128,6 +128,23 @@ static void _as_audio_packet(struct usb_endpoint *ep) {
     usb_packet_done(ep);
 }
 
+static void update_volume()
+{
+    if (audio_state._volume != audio_state._target_volume) {
+        // PCM3060 volume attenuation:
+        //  0: 0db (default)
+        //  55: -100db
+        //  56..: Mute
+        uint8_t buf[3];
+        buf[0] = 65;    // register addr
+        buf[1] = 255 + (audio_state.target_volume[0] / 128); // data left
+        buf[2] = 255 + (audio_state.target_volume[1] / 128); // data right
+        i2c_write_blocking(i2c0, PCM_I2C_ADDR, buf, 3, false);
+
+        audio_state._volume = audio_state._target_volume;
+    }
+}
+
 void core1_entry() {
     uint8_t *userbuf = (uint8_t *) multicore_fifo_pop_blocking();
     int32_t *out = (int32_t *) userbuf;
@@ -142,7 +159,7 @@ void core1_entry() {
         
         uint32_t limit = multicore_fifo_pop_blocking();
 
-        for (int j = 0; j < FILTER_STAGES; j++) {
+        for (int j = 0; j < filter_stages; j++) {
             for (int i = 1; i < limit; i += 2) {
                 fix16_t x_f16 = fix16_from_int((int16_t) out[i]);
 
@@ -159,26 +176,13 @@ void core1_entry() {
         // Update the volume if required. We do this from core1 as
         // core0 is more heavily loaded, doing this from core0 can
         // lead to audio crackling.
-        if (audio_state._volume != audio_state._target_volume) {
-            // PCM3060 volume attenuation:
-            //  0: 0db (default)
-            //  55: -100db
-            //  56..: Mute
-            uint8_t buf[3];
-            buf[0] = 65;    // register addr
-            buf[1] = 255 + (audio_state.target_volume[0] / 128); // data left
-            buf[2] = 255 + (audio_state.target_volume[1] / 128); // data right
-            i2c_write_blocking(i2c0, PCM_I2C_ADDR, buf, 3, false);
-
-            audio_state._volume = audio_state._target_volume;
-        }
+        update_volume();
     }
 }
 
 void setup() {
     set_sys_clock_khz(SYSTEM_FREQ / 1000, true);
     sleep_ms(100);
-    // stdio_init_all();
 
     userbuf = malloc(sizeof(uint8_t) * RINGBUF_LEN_IN_BYTES);
     
@@ -532,8 +536,7 @@ static bool do_get_current(struct usb_setup_packet *setup) {
                 else if (cn == AUDIO_CHANNEL_RIGHT_FRONT) {
                     usb_start_tiny_control_in_transfer(audio_state.target_volume[1], 2);
                 }
-                else
-                {
+                else {
                     return false;
                 }
                 return true;
diff --git a/firmware/code/user.c b/firmware/code/user.c
index f50bc8d..ac76599 100644
--- a/firmware/code/user.c
+++ b/firmware/code/user.c
@@ -19,7 +19,7 @@
 #include "bqf.h"
 #include "run.h"
 
-int FILTER_STAGES = 0;
+int filter_stages = 0;
 
 /*****************************************************************************
  * Here is where your digital signal processing journey begins. Follow this
@@ -33,32 +33,32 @@ int FILTER_STAGES = 0;
 
 void define_filters() {
     // First filter.
-    bqf_memreset(&bqf_filters_mem_left[FILTER_STAGES]);
-    bqf_memreset(&bqf_filters_mem_right[FILTER_STAGES]);
-    bqf_peaking_config(SAMPLING_FREQ, 38.0, -19.0, 0.9, &bqf_filters_left[FILTER_STAGES]);
-    bqf_peaking_config(SAMPLING_FREQ, 38.0, -19.0, 0.9, &bqf_filters_right[FILTER_STAGES++]);
+    bqf_memreset(&bqf_filters_mem_left[filter_stages]);
+    bqf_memreset(&bqf_filters_mem_right[filter_stages]);
+    bqf_peaking_config(SAMPLING_FREQ, 38.0, -19.0, 0.9, &bqf_filters_left[filter_stages]);
+    bqf_peaking_config(SAMPLING_FREQ, 38.0, -19.0, 0.9, &bqf_filters_right[filter_stages++]);
     
     // Second filter.
-    bqf_memreset(&bqf_filters_mem_left[FILTER_STAGES]);
-    bqf_memreset(&bqf_filters_mem_right[FILTER_STAGES]);
-    bqf_lowshelf_config(SAMPLING_FREQ, 2900.0, 3.0, 4.0, &bqf_filters_left[FILTER_STAGES]);
-    bqf_lowshelf_config(SAMPLING_FREQ, 2900.0, 3.0, 4.0, &bqf_filters_right[FILTER_STAGES++]);
+    bqf_memreset(&bqf_filters_mem_left[filter_stages]);
+    bqf_memreset(&bqf_filters_mem_right[filter_stages]);
+    bqf_lowshelf_config(SAMPLING_FREQ, 2900.0, 3.0, 4.0, &bqf_filters_left[filter_stages]);
+    bqf_lowshelf_config(SAMPLING_FREQ, 2900.0, 3.0, 4.0, &bqf_filters_right[filter_stages++]);
 
     // Third filter.
-    bqf_memreset(&bqf_filters_mem_left[FILTER_STAGES]);
-    bqf_memreset(&bqf_filters_mem_right[FILTER_STAGES]);
-    bqf_peaking_config(SAMPLING_FREQ, 430.0, 6.0, 3.5, &bqf_filters_left[FILTER_STAGES]);
-    bqf_peaking_config(SAMPLING_FREQ, 430.0, 6.0, 3.5, &bqf_filters_right[FILTER_STAGES++]);
+    bqf_memreset(&bqf_filters_mem_left[filter_stages]);
+    bqf_memreset(&bqf_filters_mem_right[filter_stages]);
+    bqf_peaking_config(SAMPLING_FREQ, 430.0, 6.0, 3.5, &bqf_filters_left[filter_stages]);
+    bqf_peaking_config(SAMPLING_FREQ, 430.0, 6.0, 3.5, &bqf_filters_right[filter_stages++]);
 
     // Fourth filter.
-    bqf_memreset(&bqf_filters_mem_left[FILTER_STAGES]);
-    bqf_memreset(&bqf_filters_mem_right[FILTER_STAGES]);
-    bqf_highshelf_config(SAMPLING_FREQ, 8400.0, 3.0, 4.0, &bqf_filters_left[FILTER_STAGES]);
-    bqf_highshelf_config(SAMPLING_FREQ, 8400.0, 3.0, 4.0, &bqf_filters_right[FILTER_STAGES++]);
+    bqf_memreset(&bqf_filters_mem_left[filter_stages]);
+    bqf_memreset(&bqf_filters_mem_right[filter_stages]);
+    bqf_highshelf_config(SAMPLING_FREQ, 8400.0, 3.0, 4.0, &bqf_filters_left[filter_stages]);
+    bqf_highshelf_config(SAMPLING_FREQ, 8400.0, 3.0, 4.0, &bqf_filters_right[filter_stages++]);
 
     // Fifth filter.
-    bqf_memreset(&bqf_filters_mem_left[FILTER_STAGES]);
-    bqf_memreset(&bqf_filters_mem_right[FILTER_STAGES]);
-    bqf_peaking_config(SAMPLING_FREQ, 4800.0, 6.0, 5.0, &bqf_filters_left[FILTER_STAGES]);
-    bqf_peaking_config(SAMPLING_FREQ, 4800.0, 6.0, 5.0, &bqf_filters_right[FILTER_STAGES++]);
+    bqf_memreset(&bqf_filters_mem_left[filter_stages]);
+    bqf_memreset(&bqf_filters_mem_right[filter_stages]);
+    bqf_peaking_config(SAMPLING_FREQ, 4800.0, 6.0, 5.0, &bqf_filters_left[filter_stages]);
+    bqf_peaking_config(SAMPLING_FREQ, 4800.0, 6.0, 5.0, &bqf_filters_right[filter_stages++]);
 }
diff --git a/firmware/code/user.h b/firmware/code/user.h
index c70da24..290aff2 100644
--- a/firmware/code/user.h
+++ b/firmware/code/user.h
@@ -20,8 +20,10 @@
 
 #include "bqf.h"
 
+// In reality we do not have enough CPU resource to run 8 filtering
+// stages without some optimisation.
 #define MAX_FILTER_STAGES 8
-extern int FILTER_STAGES;
+extern int filter_stages;
 
 extern bqf_coeff_t bqf_filters_left[MAX_FILTER_STAGES];
 extern bqf_coeff_t bqf_filters_right[MAX_FILTER_STAGES];
diff --git a/firmware/tools/filter_test.c b/firmware/tools/filter_test.c
index afe066b..045ee11 100644
--- a/firmware/tools/filter_test.c
+++ b/firmware/tools/filter_test.c
@@ -59,7 +59,7 @@ int main(int argc, char* argv[])
         out[i] = in[i];
     }
 
-    for (int j = 0; j < FILTER_STAGES; j++)
+    for (int j = 0; j < filter_stages; j++)
     {
         for (int i = 0; i < samples; i ++)
         {