static void midi_transfer_cb(usb_transfer_t *transfer)
{
	if (Device_Handle == transfer->device_handle) {
		int in_xfer = transfer->bEndpointAddress & USB_B_ENDPOINT_ADDRESS_EP_DIR_MASK;
		if ((transfer->status == 0) && in_xfer) {
			uint8_t *const p = transfer->data_buffer;
			for (int i = 0; i < transfer->actual_num_bytes; i += 4) {
				if ((p[i] + p[i+1] + p[i+2] + p[i+3]) == 0) break;

				switch ( p[i+1] & 0xF0 ) {
					case 0x90: midiNoteOn (p[i+1] & 0x0F, p[i+2], p[i+3]); break;
					case 0x80: midiNoteOff(p[i+1] & 0x0F, p[i+2], p[i+3]); break;
					default:                                               break;
				}
			}
			esp_err_t err = usb_host_transfer_submit(transfer);
		}
	}
}