Design Considerations

Depending on the audio data format, that may be lossless, lossy or compressed, e.g. WAV, MP3 or FLAC and the quality expressed in sampling rate and bitrate, the project will require different resources: memory, storage space, input / output throughput and the processing power. The resources will also depend on the project type and features discussed in Project Design.

This section describes capacity and performance of ESP32 system resources that should be considered when designing an audio project to meet required data format, audio quality and functionality.

Memory

The spare internal Data-RAM is about 290kB with “hello_world” example. For audio system this may be insufficient, and therefore the ESP32 incorporates the ability to use up to 4MB of external SPI RAM (i.e. PSRAM) memory. The external memory is incorporated in the memory map and is, within certain restrictions, usable in the same way internal Data-RAM is.

Refer to External SPI-connected RAM section in IDF documenation for details, especially pay attention to its Restrictions section which is very important.

To be able to use the PSRAM, if installed on your board, it should be enabled in menucofig under Component config > ESP32-specific > SPI RAM config. The option CONFIG_SPIRAM_CACHE_WORKAROUND, set by default in the same menu, should be kept enabled.

Note

Bluetooth and Wi-Fi can not coexist without PSRAM because it will not leave enough memory for an audio application.

Optimization of Internal RAM and Use of PSRAM

Internal RAM is more valuable asset since there are some restrictions on PSRAM. Here are some tips for optimizing internal RAM.

  • If PSRAM is in use, set all the static buffer to minimum value in Component config > Wi-Fi; if PSRAM is not used then dynamic buffer should be selected to save memory. Refer to Wi-Fi Buffer Usage section in IDF documentation for details.
  • If PSRAM and BT are used, then CONFIG_BT_ALLOCATION_FROM_SPIRAM_FIRST and CONFIG_BT_BLE_DYNAMIC_ENV_MEMORY should be set as “yes” under Component config > Bluetooth > Bluedroid Enable, to allocate more of 40kB memory to PSRAM
  • If PSRAM and Wi-Fi are used, then CONFIG_WIFI_LWIP_ALLOCATION_FROM_SPIRAM_FIRST should be set as “yes” under Component config > ESP32-specific > SPI RAM config, to allocate some memory to PSRAM
  • Set CONFIG_WL_SECTOR_SIZE as 512 in Component config > Wear Levelling

Note

The smaller the size of sector be, the slower the Write / Read speed will be, and vice versa, but only 512 and 4096 are supported.

  • Call char *buf = heap_caps_malloc(1024 * 10, MALLOC_CAP_SPIRAM | MALLOC_CAP_8BIT) instead of malloc(1024 * 10) to use PSRAM, and call char *buf = heap_caps_malloc(512, MALLOC_CAP_INTERNAL | MALLOC_CAP_8BIT) to use internal RAM.
  • Not relying on malloc() to automatically allocate PSRAM allows to make a full control of the memory. By avoiding the use of the internal RAM by other malloc() calls, you can reserve more memory for high-efficiency usage and task stack since PSRAM cannot be used as task stack memory.
  • The task stack will always be allocated at internal RAM. On the other hand you can use of the xTaskCreateStatic() function that allows to create tasks with stack on PSRAM (see options in PSRAM and FreeRTOS menuconfig), but pay attention to its help information.

Important

Don’t use ROM code in xTaskCreateStatic task: The ROM code itself is linked in components/esp32/ld/esp32.rom.ld. However, you also need to consider other pieces of code that call ROM functions, as well as the code that is not recompiled against the CONFIG_SPIRAM_CACHE_WORKAROUND patch, like the Wi-Fi and Bluetooth libraries. In general, we advise using this only in threads that do not call any IDF libraries (including libc), doing only calculations and using FreeRTOS primitives to talk to other threads.

Memory Usage by Component Overview

Below is a table that contains ESP-ADF components and their memory usage. Choose the components needed and find out how much internal RAM is left. The table is divided into two parts, when PSRAM is used or not. If PSRAM (external RAM) is in use, then some of the memory will be allocated at PSRAM automatically.

The initial spare internal RAM is 290kB.

Component Internal RAM Required
PSRAM not used With PSRAM
Wi-Fi 1 50kB+ 50kB+
Bluetooth 140kB (50kB if only BLE needed) 95kB (50kB if only BLE needed)
Flash Card 2 12kB+ 12kB+
I2S 3 Configurable, 8kB for reference Configurable, 8kB for reference
RingBuffer 4 Configurable, 30kB for reference 0kB, all moved into PSRAM

Notes to the table above

  1. According to the Wi-Fi menuconfig each Tx and Rx buffer occupies 1.6kB internal RAM. The value of 50kB RAM is assuming use of 5 Rx static buffers and 6 Tx static buffers. If PSRAM is not in use, then the “Type of WiFi Tx Buffer” option should be set as DYNAMIC in order to save RAM, in this case, the RAM usage will be far less than 50kB, but programmer should keep at least 50kB available for the Wi-Fi to be able to transmit the data. [Internal RAM only]
  2. Depending on value of SD_CARD_OPEN_FILE_NUM_MAX in audio_hal/board/board.h, that is then used in sd_card_mount() function, the RAM needed will increase with a greater number of maximum open files. 12kB is the RAM needed with 5 max files and 512 bytes CONFIG_WL_SECTOR_SIZE. [Internal RAM only]
  3. Depending on configuration settings of the I2S stream, refer to audio_stream/include/i2s_stream.h and audio_stream/i2s_stream.c. [Internal RAM only]
  4. Depending on configuration setting of the Ringbuffer, refer to DEFAULT_PIPELINE_RINGBUF_SIZE in audio_pipeline/include/audio_pipeline.h or user setting, if the buffer is created with e.g. rb_create().

System Settings

The following settings are recommended to achieve a high Wi-Fi performance in an audio project.

Note

Use ESP32 modules and boards from reputable vendors that put attention to product design, component selection and product testing. This is to have confidence of receiving well designed boards with calibrated RF.

  • Set these following options in menuconfig.

    • Flash SPI mode as QIO
    • Flash SPI speed as 80MHz
    • CPU frequency as 240MHz
    • Set Default receive window size as 5 times greater than Maximum Segment Size in Component config > LWIP > TCP
  • If external antenna is used, then set PHY_RF_CAL_PARTIAL as PHY_RF_CAL_FULL in ‘’esp-idf/components/esp32/phy_init.c’’