Issue with usb sound card at higher sample rates

[alessandromrc]

Hi, I've tried to implement 96KHz support but I get weird pitch and distorted audio apparently because there's some sample rate mismatch inside the audio system that creates this issue and I would like to know how to make it work with higher frequencies than 48k/16 bits.

Here's the code I changed:

/*
 * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include <stdio.h>

#include "pico/stdlib.h"
#include "pico/usb_device.h"
#include "pico/audio.h"
#include "pico/audio_i2s.h"
#include "pico/multicore.h"
#include "lufa/AudioClassCommon.h"

// todo forget why this is using core 1 for sound: presumably not necessary
// todo noop when muted

CU_REGISTER_DEBUG_PINS(audio_timing)

// ---- select at most one ---
//CU_SELECT_DEBUG_PINS(audio_timing)

// todo make descriptor strings should probably belong to the configs
static char *descriptor_strings[] =
        {
                "PiAudio",
                "Test",
                "13124324"
        };

// todo fix these
#define VENDOR_ID   0x520au
#define PRODUCT_ID  0xf2ddu

#define AUDIO_OUT_ENDPOINT  0x01U
#define AUDIO_IN_ENDPOINT   0x82U

#undef AUDIO_SAMPLE_FREQ
#define AUDIO_SAMPLE_FREQ(frq) (uint8_t)(frq), (uint8_t)((frq >> 8)), (uint8_t)((frq >> 16))

#define AUDIO_MAX_PACKET_SIZE(freq) (uint8_t)(((freq + 999) / 1000) * 4)
#define FEATURE_MUTE_CONTROL 1u
#define FEATURE_VOLUME_CONTROL 2u

#define ENDPOINT_FREQ_CONTROL 1u

struct audio_device_config {
    struct usb_configuration_descriptor descriptor;
    struct usb_interface_descriptor ac_interface;
    struct __packed {
        USB_Audio_StdDescriptor_Interface_AC_t core;
        USB_Audio_StdDescriptor_InputTerminal_t input_terminal;
        USB_Audio_StdDescriptor_FeatureUnit_t feature_unit;
        USB_Audio_StdDescriptor_OutputTerminal_t output_terminal;
    } ac_audio;
    struct usb_interface_descriptor as_zero_interface;
    struct usb_interface_descriptor as_op_interface;
    struct __packed {
        USB_Audio_StdDescriptor_Interface_AS_t streaming;
        struct __packed {
            USB_Audio_StdDescriptor_Format_t core;
            USB_Audio_SampleFreq_t freqs[3];
        } format;
    } as_audio;
    struct __packed {
        struct usb_endpoint_descriptor_long core;
        USB_Audio_StdDescriptor_StreamEndpoint_Spc_t audio;
    } ep1;
    struct usb_endpoint_descriptor_long ep2;
};

static const struct audio_device_config audio_device_config = {
        .descriptor = {
                .bLength             = sizeof(audio_device_config.descriptor),
                .bDescriptorType     = DTYPE_Configuration,
                .wTotalLength        = sizeof(audio_device_config),
                .bNumInterfaces      = 2,
                .bConfigurationValue = 0x01,
                .iConfiguration      = 0x00,
                .bmAttributes        = 0x80,
                .bMaxPower           = 0x32,
        },
        .ac_interface = {
                .bLength            = sizeof(audio_device_config.ac_interface),
                .bDescriptorType    = DTYPE_Interface,
                .bInterfaceNumber   = 0x00,
                .bAlternateSetting  = 0x00,
                .bNumEndpoints      = 0x00,
                .bInterfaceClass    = AUDIO_CSCP_AudioClass,
                .bInterfaceSubClass = AUDIO_CSCP_ControlSubclass,
                .bInterfaceProtocol = AUDIO_CSCP_ControlProtocol,
                .iInterface         = 0x00,
        },
        .ac_audio = {
                .core = {
                        .bLength = sizeof(audio_device_config.ac_audio.core),
                        .bDescriptorType = AUDIO_DTYPE_CSInterface,
                        .bDescriptorSubtype = AUDIO_DSUBTYPE_CSInterface_Header,
                        .bcdADC = VERSION_BCD(1, 0, 0),
                        .wTotalLength = sizeof(audio_device_config.ac_audio),
                        .bInCollection = 1,
                        .bInterfaceNumbers = 1,
                },
                .input_terminal = {
                        .bLength = sizeof(audio_device_config.ac_audio.input_terminal),
                        .bDescriptorType = AUDIO_DTYPE_CSInterface,
                        .bDescriptorSubtype = AUDIO_DSUBTYPE_CSInterface_InputTerminal,
                        .bTerminalID = 1,
                        .wTerminalType = AUDIO_TERMINAL_STREAMING,
                        .bAssocTerminal = 0,
                        .bNrChannels = 2,
                        .wChannelConfig = AUDIO_CHANNEL_LEFT_FRONT | AUDIO_CHANNEL_RIGHT_FRONT,
                        .iChannelNames = 0,
                        .iTerminal = 0,
                },
                .feature_unit = {
                        .bLength = sizeof(audio_device_config.ac_audio.feature_unit),
                        .bDescriptorType = AUDIO_DTYPE_CSInterface,
                        .bDescriptorSubtype = AUDIO_DSUBTYPE_CSInterface_Feature,
                        .bUnitID = 2,
                        .bSourceID = 1,
                        .bControlSize = 1,
                        .bmaControls = {AUDIO_FEATURE_MUTE | AUDIO_FEATURE_VOLUME, 0, 0},
                        .iFeature = 0,
                },
                .output_terminal = {
                        .bLength = sizeof(audio_device_config.ac_audio.output_terminal),
                        .bDescriptorType = AUDIO_DTYPE_CSInterface,
                        .bDescriptorSubtype = AUDIO_DSUBTYPE_CSInterface_OutputTerminal,
                        .bTerminalID = 3,
                        .wTerminalType = AUDIO_TERMINAL_OUT_SPEAKER,
                        .bAssocTerminal = 0,
                        .bSourceID = 2,
                        .iTerminal = 0,
                },
        },
        .as_zero_interface = {
                .bLength            = sizeof(audio_device_config.as_zero_interface),
                .bDescriptorType    = DTYPE_Interface,
                .bInterfaceNumber   = 0x01,
                .bAlternateSetting  = 0x00,
                .bNumEndpoints      = 0x00,
                .bInterfaceClass    = AUDIO_CSCP_AudioClass,
                .bInterfaceSubClass = AUDIO_CSCP_AudioStreamingSubclass,
                .bInterfaceProtocol = AUDIO_CSCP_ControlProtocol,
                .iInterface         = 0x00,
        },
        .as_op_interface = {
                .bLength            = sizeof(audio_device_config.as_op_interface),
                .bDescriptorType    = DTYPE_Interface,
                .bInterfaceNumber   = 0x01,
                .bAlternateSetting  = 0x01,
                .bNumEndpoints      = 0x02,
                .bInterfaceClass    = AUDIO_CSCP_AudioClass,
                .bInterfaceSubClass = AUDIO_CSCP_AudioStreamingSubclass,
                .bInterfaceProtocol = AUDIO_CSCP_ControlProtocol,
                .iInterface         = 0x00,
        },
        .as_audio = {
                .streaming = {
                        .bLength = sizeof(audio_device_config.as_audio.streaming),
                        .bDescriptorType = AUDIO_DTYPE_CSInterface,
                        .bDescriptorSubtype = AUDIO_DSUBTYPE_CSInterface_General,
                        .bTerminalLink = 1,
                        .bDelay = 1,
                        .wFormatTag = 1, // PCM
                },
                .format = {
                        .core = {
                                .bLength = sizeof(audio_device_config.as_audio.format),
                                .bDescriptorType = AUDIO_DTYPE_CSInterface,
                                .bDescriptorSubtype = AUDIO_DSUBTYPE_CSInterface_FormatType,
                                .bFormatType = 1,
                                .bNrChannels = 2,
                                .bSubFrameSize = 2,
                                .bBitResolution = 16,
                                .bSampleFrequencyType = count_of(audio_device_config.as_audio.format.freqs),
                        },
                        .freqs = {
                                AUDIO_SAMPLE_FREQ(44100),
                                AUDIO_SAMPLE_FREQ(48000),
                                AUDIO_SAMPLE_FREQ(96000)
                        },
                },
        },
        .ep1 = {
                .core = {
                        .bLength          = sizeof(audio_device_config.ep1.core),
                        .bDescriptorType  = DTYPE_Endpoint,
                        .bEndpointAddress = AUDIO_OUT_ENDPOINT,
                        .bmAttributes     = 5,
                        .wMaxPacketSize   = AUDIO_MAX_PACKET_SIZE(AUDIO_FREQ_MAX),
                        .bInterval        = 1,
                        .bRefresh         = 0,
                        .bSyncAddr        = AUDIO_IN_ENDPOINT,
                },
                .audio = {
                        .bLength = sizeof(audio_device_config.ep1.audio),
                        .bDescriptorType = AUDIO_DTYPE_CSEndpoint,
                        .bDescriptorSubtype = AUDIO_DSUBTYPE_CSEndpoint_General,
                        .bmAttributes = 1,
                        .bLockDelayUnits = 0,
                        .wLockDelay = 0,
                }
        },
        .ep2 = {
                .bLength          = sizeof(audio_device_config.ep2),
                .bDescriptorType  = 0x05,
                .bEndpointAddress = AUDIO_IN_ENDPOINT,
                .bmAttributes     = 0x11,
                .wMaxPacketSize   = 3,
                .bInterval        = 0x01,
                .bRefresh         = 2,
                .bSyncAddr        = 0,
        },
};

static struct usb_interface ac_interface;
static struct usb_interface as_op_interface;
static struct usb_endpoint ep_op_out, ep_op_sync;

static const struct usb_device_descriptor boot_device_descriptor = {
        .bLength            = 18,
        .bDescriptorType    = 0x01,
        .bcdUSB             = 0x0110,
        .bDeviceClass       = 0x00,
        .bDeviceSubClass    = 0x00,
        .bDeviceProtocol    = 0x00,
        .bMaxPacketSize0    = 0x40,
        .idVendor           = VENDOR_ID,
        .idProduct          = PRODUCT_ID,
        .bcdDevice          = 0x0200,
        .iManufacturer      = 0x01,
        .iProduct           = 0x02,
        .iSerialNumber      = 0x03,
        .bNumConfigurations = 0x01,
};

const char *_get_descriptor_string(uint index) {
    if (index <= count_of(descriptor_strings)) {
        return descriptor_strings[index - 1];
    } else {
        return "";
    }
}

static struct {
    uint32_t freq;
    int16_t volume;
    int16_t vol_mul;
    bool mute;
} audio_state = {
        .freq = 96000,
};

static struct audio_buffer_pool *producer_pool;

static void _as_audio_packet(struct usb_endpoint *ep) {
    assert(ep->current_transfer);
    struct usb_buffer *usb_buffer = usb_current_out_packet_buffer(ep);
    DEBUG_PINS_SET(audio_timing, 1);
    // todo deal with blocking correctly
    struct audio_buffer *audio_buffer = take_audio_buffer(producer_pool, true);
    DEBUG_PINS_CLR(audio_timing, 1);
    assert(!(usb_buffer->data_len & 3u));
    audio_buffer->sample_count = usb_buffer->data_len / 4;
    assert(audio_buffer->sample_count);
    assert(audio_buffer->max_sample_count >= audio_buffer->sample_count);
    uint16_t vol_mul = audio_state.vol_mul;
    uint16_t *out = (uint16_t *) audio_buffer->buffer->bytes;
    uint16_t *in = (uint16_t *) usb_buffer->data;
    for (unsigned int i = 0; i < audio_buffer->sample_count * 2; i++) {
        out[i] = (uint16_t) ((in[i] * vol_mul) >> 15u);
    }

    give_audio_buffer(producer_pool, audio_buffer);
    // keep on truckin'
    usb_grow_transfer(ep->current_transfer, 1);
    usb_packet_done(ep);
}

static void _as_sync_packet(struct usb_endpoint *ep) {
    assert(ep->current_transfer);
    DEBUG_PINS_SET(audio_timing, 2);
    DEBUG_PINS_CLR(audio_timing, 2);
    struct usb_buffer *buffer = usb_current_in_packet_buffer(ep);
    assert(buffer->data_max >= 3);
    buffer->data_len = 3;

    // todo lie thru our teeth for now
    uint feedback = (audio_state.freq << 14u) / 1000u;

    buffer->data[0] = feedback;
    buffer->data[1] = feedback >> 8u;
    buffer->data[2] = feedback >> 16u;

    // keep on truckin'
    usb_grow_transfer(ep->current_transfer, 1);
    usb_packet_done(ep);
}

static const struct usb_transfer_type as_transfer_type = {
        .on_packet = _as_audio_packet,
        .initial_packet_count = 1,
};

static const struct usb_transfer_type as_sync_transfer_type = {
        .on_packet = _as_sync_packet,
        .initial_packet_count = 1,
};

static struct usb_transfer as_transfer;
static struct usb_transfer as_sync_transfer;

static bool do_get_current(struct usb_setup_packet *setup) {
    usb_debug("AUDIO_REQ_GET_CUR\n");

    if ((setup->bmRequestType & USB_REQ_TYPE_RECIPIENT_MASK) == USB_REQ_TYPE_RECIPIENT_INTERFACE) {
        switch (setup->wValue >> 8u) {
            case FEATURE_MUTE_CONTROL: {
                usb_start_tiny_control_in_transfer(audio_state.mute, 1);
                return true;
            }
            case FEATURE_VOLUME_CONTROL: {
                /* Current volume. See UAC Spec 1.0 p.77 */
                usb_start_tiny_control_in_transfer(audio_state.volume, 2);
                return true;
            }
        }
    } else if ((setup->bmRequestType & USB_REQ_TYPE_RECIPIENT_MASK) == USB_REQ_TYPE_RECIPIENT_ENDPOINT) {
        if ((setup->wValue >> 8u) == ENDPOINT_FREQ_CONTROL) {
            /* Current frequency */
            usb_start_tiny_control_in_transfer(audio_state.freq, 3);
            return true;
        }
    }
    return false;
}

// todo this seemed like aood guess, but is not correct
uint16_t db_to_vol[91] = {
        0x0001, 0x0001, 0x0001, 0x0001, 0x0001, 0x0001, 0x0002, 0x0002,
        0x0002, 0x0002, 0x0003, 0x0003, 0x0004, 0x0004, 0x0005, 0x0005,
        0x0006, 0x0007, 0x0008, 0x0009, 0x000a, 0x000b, 0x000d, 0x000e,
        0x0010, 0x0012, 0x0014, 0x0017, 0x001a, 0x001d, 0x0020, 0x0024,
        0x0029, 0x002e, 0x0033, 0x003a, 0x0041, 0x0049, 0x0052, 0x005c,
        0x0067, 0x0074, 0x0082, 0x0092, 0x00a4, 0x00b8, 0x00ce, 0x00e7,
        0x0104, 0x0124, 0x0147, 0x016f, 0x019c, 0x01ce, 0x0207, 0x0246,
        0x028d, 0x02dd, 0x0337, 0x039b, 0x040c, 0x048a, 0x0518, 0x05b7,
        0x066a, 0x0732, 0x0813, 0x090f, 0x0a2a, 0x0b68, 0x0ccc, 0x0e5c,
        0x101d, 0x1214, 0x1449, 0x16c3, 0x198a, 0x1ca7, 0x2026, 0x2413,
        0x287a, 0x2d6a, 0x32f5, 0x392c, 0x4026, 0x47fa, 0x50c3, 0x5a9d,
        0x65ac, 0x7214, 0x7fff
};

// actually windows doesn't seem to like this in the middle, so set top range to 0db
#define CENTER_VOLUME_INDEX 91

#define ENCODE_DB(x) ((uint16_t)(int16_t)((x)*256))

#define MIN_VOLUME           ENCODE_DB(-CENTER_VOLUME_INDEX)
#define DEFAULT_VOLUME       ENCODE_DB(0)
#define MAX_VOLUME           ENCODE_DB(count_of(db_to_vol)-CENTER_VOLUME_INDEX)
#define VOLUME_RESOLUTION    ENCODE_DB(1)

static bool do_get_minimum(struct usb_setup_packet *setup) {
    usb_debug("AUDIO_REQ_GET_MIN\n");
    if ((setup->bmRequestType & USB_REQ_TYPE_RECIPIENT_MASK) == USB_REQ_TYPE_RECIPIENT_INTERFACE) {
        switch (setup->wValue >> 8u) {
            case FEATURE_VOLUME_CONTROL: {
                usb_start_tiny_control_in_transfer(MIN_VOLUME, 2);
                return true;
            }
        }
    }
    return false;
}

static bool do_get_maximum(struct usb_setup_packet *setup) {
    usb_debug("AUDIO_REQ_GET_MAX\n");
    if ((setup->bmRequestType & USB_REQ_TYPE_RECIPIENT_MASK) == USB_REQ_TYPE_RECIPIENT_INTERFACE) {
        switch (setup->wValue >> 8u) {
            case FEATURE_VOLUME_CONTROL: {
                usb_start_tiny_control_in_transfer(MAX_VOLUME, 2);
                return true;
            }
        }
    }
    return false;
}

static bool do_get_resolution(struct usb_setup_packet *setup) {
    usb_debug("AUDIO_REQ_GET_RES\n");
    if ((setup->bmRequestType & USB_REQ_TYPE_RECIPIENT_MASK) == USB_REQ_TYPE_RECIPIENT_INTERFACE) {
        switch (setup->wValue >> 8u) {
            case FEATURE_VOLUME_CONTROL: {
                usb_start_tiny_control_in_transfer(VOLUME_RESOLUTION, 2);
                return true;
            }
        }
    }
    return false;
}

static struct audio_control_cmd {
    uint8_t cmd;
    uint8_t type;
    uint8_t cs;
    uint8_t cn;
    uint8_t unit;
    uint8_t len;
} audio_control_cmd_t;

static void _audio_reconfigure() {
    switch (audio_state.freq) {
        case 44100:
        case 48000:
        case 96000:
            break;
        default:
            audio_state.freq = 44100;
    }
    // todo hack overwriting const
    ((struct audio_format *) producer_pool->format)->sample_freq = audio_state.freq;
}

static void audio_set_volume(int16_t volume) {
    audio_state.volume = volume;
    // todo interpolate
    volume += CENTER_VOLUME_INDEX * 256;
    if (volume < 0) volume = 0;
    if (volume >= count_of(db_to_vol) * 256) volume = count_of(db_to_vol) * 256 - 1;
    audio_state.vol_mul = db_to_vol[((uint16_t)volume) >> 8u];
//    printf("VOL MUL %04x\n", audio_state.vol_mul);
}

static void audio_cmd_packet(struct usb_endpoint *ep) {
    assert(audio_control_cmd_t.cmd == AUDIO_REQ_SetCurrent);
    struct usb_buffer *buffer = usb_current_out_packet_buffer(ep);
    audio_control_cmd_t.cmd = 0;
    if (buffer->data_len >= audio_control_cmd_t.len) {
        if (audio_control_cmd_t.type == USB_REQ_TYPE_RECIPIENT_INTERFACE) {
            switch (audio_control_cmd_t.cs) {
                case FEATURE_MUTE_CONTROL: {
                    audio_state.mute = buffer->data[0];
                    usb_warn("Set Mute %d\n", buffer->data[0]);
                    break;
                }
                case FEATURE_VOLUME_CONTROL: {
                    audio_set_volume(*(int16_t *) buffer->data);
                    break;
                }
            }

        } else if (audio_control_cmd_t.type == USB_REQ_TYPE_RECIPIENT_ENDPOINT) {
            if (audio_control_cmd_t.cs == ENDPOINT_FREQ_CONTROL) {
                uint32_t new_freq = (*(uint32_t *) buffer->data) & 0x00ffffffu;
                usb_warn("Set freq %d\n", new_freq == 0xffffffu ? -1 : (int) new_freq);

                if (audio_state.freq != new_freq) {
                    audio_state.freq = new_freq;
                    _audio_reconfigure();
                }
            }
        }
    }
    usb_start_empty_control_in_transfer_null_completion();
    // todo is there error handling?
}


static const struct usb_transfer_type _audio_cmd_transfer_type = {
        .on_packet = audio_cmd_packet,
        .initial_packet_count = 1,
};

static bool as_set_alternate(struct usb_interface *interface, uint alt) {
    assert(interface == &as_op_interface);
    usb_warn("SET ALTERNATE %d\n", alt);
    return alt < 2;
}

static bool do_set_current(struct usb_setup_packet *setup) {
#ifndef NDEBUG
    usb_warn("AUDIO_REQ_SET_CUR\n");
#endif

    if (setup->wLength && setup->wLength < 64) {
        audio_control_cmd_t.cmd = AUDIO_REQ_SetCurrent;
        audio_control_cmd_t.type = setup->bmRequestType & USB_REQ_TYPE_RECIPIENT_MASK;
        audio_control_cmd_t.len = (uint8_t) setup->wLength;
        audio_control_cmd_t.unit = setup->wIndex >> 8u;
        audio_control_cmd_t.cs = setup->wValue >> 8u;
        audio_control_cmd_t.cn = (uint8_t) setup->wValue;
        usb_start_control_out_transfer(&_audio_cmd_transfer_type);
        return true;
    }
    return false;
}

static bool ac_setup_request_handler(__unused struct usb_interface *interface, struct usb_setup_packet *setup) {
    setup = __builtin_assume_aligned(setup, 4);
    if (USB_REQ_TYPE_TYPE_CLASS == (setup->bmRequestType & USB_REQ_TYPE_TYPE_MASK)) {
        switch (setup->bRequest) {
            case AUDIO_REQ_SetCurrent:
                return do_set_current(setup);

            case AUDIO_REQ_GetCurrent:
                return do_get_current(setup);

            case AUDIO_REQ_GetMinimum:
                return do_get_minimum(setup);

            case AUDIO_REQ_GetMaximum:
                return do_get_maximum(setup);

            case AUDIO_REQ_GetResolution:
                return do_get_resolution(setup);

            default:
                break;
        }
    }
    return false;
}

bool _as_setup_request_handler(__unused struct usb_endpoint *ep, struct usb_setup_packet *setup) {
    setup = __builtin_assume_aligned(setup, 4);
    if (USB_REQ_TYPE_TYPE_CLASS == (setup->bmRequestType & USB_REQ_TYPE_TYPE_MASK)) {
        switch (setup->bRequest) {
            case AUDIO_REQ_SetCurrent:
                return do_set_current(setup);

            case AUDIO_REQ_GetCurrent:
                return do_get_current(setup);

            case AUDIO_REQ_GetMinimum:
                return do_get_minimum(setup);

            case AUDIO_REQ_GetMaximum:
                return do_get_maximum(setup);

            case AUDIO_REQ_GetResolution:
                return do_get_resolution(setup);

            default:
                break;
        }
    }
    return false;
}

void usb_sound_card_init() {
    //msd_interface.setup_request_handler = msd_setup_request_handler;
    usb_interface_init(&ac_interface, &audio_device_config.ac_interface, NULL, 0, true);
    ac_interface.setup_request_handler = ac_setup_request_handler;

    static struct usb_endpoint *const op_endpoints[] = {
            &ep_op_out, &ep_op_sync
    };
    usb_interface_init(&as_op_interface, &audio_device_config.as_op_interface, op_endpoints, count_of(op_endpoints),
                       true);
    as_op_interface.set_alternate_handler = as_set_alternate;
    ep_op_out.setup_request_handler = _as_setup_request_handler;
    as_transfer.type = &as_transfer_type;
    usb_set_default_transfer(&ep_op_out, &as_transfer);
    as_sync_transfer.type = &as_sync_transfer_type;
    usb_set_default_transfer(&ep_op_sync, &as_sync_transfer);

    static struct usb_interface *const boot_device_interfaces[] = {
            &ac_interface,
            &as_op_interface,
    };
    __unused struct usb_device *device = usb_device_init(&boot_device_descriptor, &audio_device_config.descriptor,
                                                         boot_device_interfaces, count_of(boot_device_interfaces),
                                                         _get_descriptor_string);
    assert(device);
    audio_set_volume(DEFAULT_VOLUME);
    _audio_reconfigure();
//    device->on_configure = _on_configure;
    usb_device_start();
}

static void core1_worker() {
    audio_i2s_set_enabled(true);
}

int main(void) {

    stdout_uart_init();

    //gpio_debug_pins_init();
    puts("USB SOUND CARD");

#ifndef NDEBUG
    for(uint i=0;i<count_of(audio_device_config.as_audio.format.freqs);i++) {
        uint freq = audio_device_config.as_audio.format.freqs[i].Byte1 |
                (audio_device_config.as_audio.format.freqs[i].Byte2 << 8u) |
                (audio_device_config.as_audio.format.freqs[i].Byte3 << 16u);
        assert(freq <= AUDIO_FREQ_MAX);
    }
#endif
    // initialize for 48k we allow changing later
    struct audio_format audio_format_48k = {
            .format = AUDIO_BUFFER_FORMAT_PCM_U16,
            .sample_freq = 96000,
            .channel_count = 2,
    };

    struct audio_buffer_format producer_format = {
            .format = &audio_format_48k,
            .sample_stride = 4
    };

    producer_pool = audio_new_producer_pool(&producer_format, 8, 96); // todo correct size
    bool __unused ok;
    struct audio_i2s_config config = {
            .data_pin = PICO_AUDIO_I2S_DATA_PIN,
            .clock_pin_base = PICO_AUDIO_I2S_CLOCK_PIN_BASE,
            .dma_channel = 0,
            .pio_sm = 0,
    };

    const struct audio_format *output_format;
    output_format = audio_i2s_setup(&audio_format_48k, &config);
    if (!output_format) {
        panic("PicoAudio: Unable to open audio device.\n");
    }

    ok = audio_i2s_connect_extra(producer_pool, false, 2, 96, NULL);
    assert(ok);
    usb_sound_card_init();

    multicore_launch_core1(core1_worker);
    printf("HAHA %04x %04x %04x %04x\n", MIN_VOLUME, DEFAULT_VOLUME, MAX_VOLUME, VOLUME_RESOLUTION);
    // MSD is irq driven
    while (1) __wfi();
}

In the CMake I did this change here:

AUDIO_FREQ_MAX=96000

Hope to get a response back as this project seems dead. I was thinking if it would also make more sense to recreate the entire soundcard using TinyUSB at this point...

[alessandromrc]

Apparently the issue is related to USB buffers which I'm actually looking into right now and I've got 96kHz 16 bit working without any issue... will look forward at adding more sample rates and maybe higher up the bitrate to 24