LD3320 Chinese Speech Recognition and MP3 Player Module

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UPDATED (March 2018): A better English version of the datasheet can be found here.


After my previous success in getting the SYN6288, a Chinese text-to-speech IC, to produce satisfactory Chinese speech and pronouncing synthetic English characters, I purchased the LD3320, another Chinese voice module providing speech recognition as well as MP3 playback capabilities.

The module’s Chinese voice recognition mechanism can be initialized with the Pinyin transliterations of the Chinese text to be recognized. The module will then listen to the audio sent to its input channel (either from a microphone or from the line-in input) to identify any voice that resembles the programmed list of Chinese words sent during initialization. Audio during MP3 playback is sent via the headphone/lineout (stereo) and speaker (mono) pins. Data communication with the module is done using either a proprietary parallel protocol or SPI.

The board I purchased comes with a condenser microphone and 2.54mm connection headers for easy prototyping:

Board Schematics

The detailed schematics of the board is below:

The connection headers on the breakout board expose several useful pins, namely VDD, GND, parallel/SPI communication lines and audio input/output pins. The detailed pin description can be found below, where ^ denotes an active low signal:

VDD          3.3V Supply
GND          Ground
RST^         Reset Signal
MD           Low for parallel mode, high for serial mode.
INTB^        Interrupt output signal
A0           Address or data selection for parallel mode. If high, P0-P7 indicates address, low for data.
CLK          Clock input for LD3320 (2-34 MHz).
RDB^         Read control signal for parallel input mode
CSB^/SCS^    Chip select signal (parallel mode) / SPI chip select signal (serial mode).
WRB^/SPIS^   Write Enable (parallel input mode) / Connect to GND in serial mode
P0           Data bit 0 for parallel input mode / SDI pin in serial mode
P1           Data bit 1 for parallel input mode / SDO pin in serial mode
P2           Data bit 2 for parallel input mode / SDCK pin in serial mode
P3           Data bit 3 for parallel input mode
P4           Data bit 4 for parallel input mode
P5           Data bit 5 for parallel input mode
P6           Data bit 6 for parallel input mode
P7           Data bit 7 for parallel input mode
MBS          Microphone Bias
MONO         Mono Line In 
LINL/LINR    Stereo Line In (Left/Right)
HPOL/HPOR    Headphone Output (Left/Right)
LOUL/LOUTR   Line Out (Left/Right)
MICP/MICN    Microphone Input (Pos/Neg)
SPOP/SPON    Speaker Ouput (Pos/Neg)

The LD3320 requires an external clock to be fed to pin CLK, which is already provided by the breakout board via a 22.1184 MHz crystal. No external components are needed, even for the audio input/output lines, as the breakout board already contains all the required parts.

To use SPI for communication, connect MD to VDD, WRB^/SPIS^ to GND and use pins P0, P1 and P2 for SDI, SDO and SDCK respectively. For simplicity, the rest of this article will use SPI to communicate with this module.

Official documentation (in Chinese only) can be found on icroute’s website. The Chinese datasheet can be downloaded here. With the help of onlinedoctranslator, I made an English translation, which can be downloaded here.

Breakout board issues

Before you proceed to explore the LD3320, please be aware of possible PCB issues causing wrong signals to be fed to the IC and resulting in precious time wasted debugging the circuit. In my case, after getting the sample program to compile and run on my PIC microcontroller only to find out that it did not work, I spent almost a day checking various connections and initialization codes to no avail. I could easily have debugged till the end of time and still could not get it to work if I hadn’t noticed by chance a 22.1184 MHz sine wave on the pin marked as WRB, raising suspicion that the PCB trace may have issues.

I decided to use a multimeter and cross-checked the connections between the labelled pins on the connection headers and the actual pins on the IC while referring to the LD3320 pin configuration described in the datasheet:

This is the pin description printed on the connection header at the back of the board:

To my surprise, apart from the GND/VDD pins which are fortunately correctly labelled (otherwise I could have damaged the module by applying power in reverse polarity), the rest of the pin labels on the left and right columns of the left connection header are swapped! For example, RSTB should be INTB, CLK should be WRB and vice versa. This explained why I got a clock signal on the WRB pin as their labels are swapped! The correct labelling for these pins should be:

For the right and bottom connection headers, the labelling is correct. However, further tests showed that the condenser microphone is connected in reverse polarity and that there are several other connection issues between the microphone and the LD3320. The connections on the PCB did not seem to match the board schematics, which could indicate a faulty PCB or a mismatched schematics. Either way, the microphone input still could not work even with the ECM replaced, and I could only get it to work using the line-in input (more on that later) after removing the ECM from the board. The presence of the microphone, even if unused, will disturb the line-in input channel and prevent the module from working.

Therefore, before you apply power to the board, check to make sure that the pin labelling is correct – or at least check that the VDD and GND pins are correctly labelled.  Also, your board may not have any issue or have a different issue than those described above.

Speech recognition

The only few examples I found for this IC are from coocox’s LD3320 driver and some 8051 codes downloadable from here. By comparing the codes with the initialization protocol provided in the datasheet, the steps to use this module can be summarized below:

1. Reset the module by pulling the RST pin low, and then high for a short while.
2. Initialize the module for ASR (Automatic Speech Recognition) mode. In particular, set the input channel to be used for speech recognition.
3. Initialize the list of Chinese words to be recognized. For each Chinese word, send the Pinyin transliteration of the word (without tone marks) in ASCII (e.g. bei jing for 北京) and an associated code (a number between 1 and 255) to identify this word. The codes for the words in the list need not be continuous and multiple words can have the same identification code.
4. Look for an interrupt on the INTB pin, which will trigger when a voice has been detected on the input channel.
5. When the interrupt happens, instruct the LD3320 to perform speech recognition, which will analyse the detected voice for any patterns similar to the list of Chinese words programmed in step 3. If a match is found, the chip will return the identification code associated with the word.
6. After a speech recognition task is completed, go back to step 1 to be ready for another recognition task.

To specify which input channel will be used for speech recognition, use register 0x1C (ADC Switch Control). Write 0x0B for microphone input (MICP/MIN pins), 0x07 for stereo input (LINL/LINR pins) and 0x23 for mono input (MONO pins).

In my tests, as the microphone input channel cannot be used due to the PCB issues mentioned above, I used the stereo input channels with an ECM and a preamplifier circuit based on a single NPN transistor. The output of this circuit is then connected to the LINL/LINR audio input pins of the LD3320. Below is the diagram of the preamplifier:

To achieve the highest recognition quality possible, several registers of the LD3320 are used to adjust the sensitivity and selectivity of the recognition process:

  • Register 0x32 (ADC Gain) can be set to values between 00 and 7Fh. The greater the value, the greater the input audio gain and the more sensitive the recognition. However, higher values may result in increased noises and mistaken identifications. Set to 10H-2FH for noisy environment. In other circumstances, set to between 40H-55H.
  • Register 0xB3 (ASR Voice Activity Detection). If set to 0 (disable), all sounds detected on the input channel will be taken as voice and trigger the INTB interrupt. Otherwise, INTB will only be triggered when a voice is detected on the audio input channel whereas other static noises will be ignored. Set to a value between 1 and 80 to control the sensitivity of this detection – the lower the value, the higher the sensitivity. In general, the higher the SNR (signal-to-noise) ratio in the working environment, the higher the recommended value of this register. Default is 0x12.
  • Register 0xB4 (ASR VAD Start) defines how long a continuous speech should be detected before it is recognized as voice. Set to value between 1 and 80 (10 to 800 milliseconds). Default is 0x0F (150ms).
  • Register 0xB5 (ASR VAD Silence End) defines how long a silence period should be detected at the end of a speech segment before the speech is considered to have ended. Set to 20-200 (200-2000 ms). Default is 60 (600 ms).
  • Register 0xB6 (ASR VAD Voice Max Length) defines the longest possible duration of a detected speech segment. Set to 5-200 (500ms-20sec). Default is 60 (6 seconds)

After initializing the LD3320 according to the datasheet and tweaking the speech recognition setup registers, I could get the LD3320 to recognize Chinese proper names such as bei jing (北京) and other words like a li ba ba. The quality of the recognition is satisfactory.

MP3 playback

The LD3320 also supports playback of MP3 data received via SPI. Playback is done using the following steps:

1. Reset and initialize the LD3320 in MP3 mode.
2. Set the correct audio output channel for audio playback.
3. Send the first segment of the MP3 data to be played.
4. Check if the MP3 has finished playing. If so, stop playback.
5. If not, continue to send more MP3 data and go back to step 4.

Three types of audio output are supported: headphone (stereo), line out (stereo), or speaker (mono). The headphone and line out channels are always enabled whereas the speaker channel must be enabled independently. Line out and headphone output volume can be adjusted by writing a value to bits 5-1 of registers 0x81 and 0x83 respectively, with 0x00 indicating maximum volume. Speaker output volume can be changed by writing to bits 5-2 of register 0x83, with 0x00 indicating maximum volume.

According to the datasheet, the speaker output line can support an 8-ohm speaker. However, in my tests, connecting an 8-ohm speaker to the speaker output will cause the module to stop playback unexpectedly, presumably due to high power consumption, although the sound quality through the speaker remains clear. The headphone and line out channels seem to be stable and deliver good quality audio.

I also tried to connect a PAM8403 audio amplifier to the line-out channel to achieve a stereo output using two 8-ohm speakers. At first, with the PAM8403 sharing the same power and ground lines with the LD3320, the same issue of unexpected playback termination persisted, even with the usage of decoupling capacitors. Suspecting the issue may be due to disturbance caused by the 8-ohm speaker sharing the same power lines, I used a different power supply for the PAM8403 and the LD3320 managed to play MP3 audio smoothly with no other issues.

Demo video

I made a video showing the module working with a PIC microcontroller and an ST7735 128×160 16-bit color LCD to display the speech recognition results. It shows the results of the module trying to recognize proper names in Chinese (bei jing 北京, shang hai 上海, hong kong 香港, chong qing 重庆, tian an men 天安门) and other words such as a li ba ba. A single beep means that the speech is recognized while a double beep indicates unrecognized speech. Although the speech recognition quality highly depends on the input audio, volume level and other environmental conditions, overall the detection sensitivity and selectivity seems satisfactory as can be seen from the video.

The end of the video shows the stereo playback of an MP3 song stored on the SD card – using a PAM8403 amplifier whose output is fed into two 8-ohm speakers. Notwithstanding the background noises presumably due to effects of breadboard stray capacitance at high frequency (22.1184 MHz for this module), MP3 playback quality seems reasonably good and comparable to the VS1053 module.

The entire MPLAB X demo project for this module can be downloaded here.

See also

SYN6288 Chinese Speech Synthesis Module
Interfacing VS1053 audio encoder/decoder module with PIC using SPI 

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A tough developer who likes to work on just about anything, from software development to electronics, and share his knowledge with the rest of the world.

17 thoughts on “LD3320 Chinese Speech Recognition and MP3 Player Module

  • December 5, 2014 at 5:23 pm

    Thank you very much for posting this information. I purchased the board for a simple two command (up/down) voice recognition device to operate a switch. It appears you used SPI via the PIC controller.

  • December 5, 2014 at 6:01 pm

    Hope you'll enjoy the LD3320 module :) I bought mine out of curiosity – just want to see how good the recognition is. I preferred SPI over parallel protocol – it is just as efficient and uses less communication pins, leaving free pins for other purposes (LCD, SD card, UART, etc.).

    Feel free to leave a comment if you have any questions.

  • December 5, 2014 at 7:32 pm

    Will it be possible to recognise the english words……….

  • December 5, 2014 at 9:24 pm

    Unfortunately the LD3320 is made for Chinese speech recognition so it is not suitable for English. I could get it to recognize simple non-Chinese words such as alibaba but that's about it. For English speech recognition I think Microchip provides some great working sample code for the PIC32 platform which I am going to try when my free time allows. :) When that is done I will post another article on my blog :)

  • December 11, 2014 at 4:24 pm

    I lost my last response. I am trying two commands only, UP, Down. I gues I will be using Xiang Shang, Xiang Xia. I looked at PICs awhile back but the circuits were over load. I just need to drive a servo and recognize these two commands. Thanks very much.

  • December 11, 2014 at 11:59 pm

    Hope you'll get it to recognize these 2 words – that'll be fun! In my experience the module doesn't do really well with words that sound similar, so maybe you can try with words that sound different first. Once you get it working, the recognition parameters can be tweaked to make it more optimal to distinguish between similar words.

  • April 7, 2015 at 8:18 am

    In which PIC you programmed this?

  • April 7, 2015 at 10:34 am

    I used a device in the PIC24 family. The exact model is not important – the code can be ported to other PIC devices with just minor changes

  • April 7, 2015 at 10:42 am

    I know, I just wanted to see which specific PIC just to understand the code… Im working with MSP430G2553..

  • April 7, 2015 at 10:45 am


    I just checked the code again. If you open main.c, at the first line there is an include to the header "p24fj64ga002.h". So the code was using PIC24FJ64GA002.

    Let me know if you have any other questions.

  • April 7, 2015 at 10:53 am

    You're right,thanks!

  • April 10, 2015 at 1:24 am

    hello is it possible to interface with arduino? help out

  • June 19, 2016 at 7:45 pm

    I used arduino to operate the LD3320, i can’t read the correct values of some registers, like 0x06,0x35 and 0xb3, their values are 8f,81 and ff. What’s wrong?

  • June 20, 2016 at 12:29 pm


    It is hard to say what was wrong without taking a look at your connectionz and your code. However, in general, check if all connections have been made correctly and that the SPI routines in your code can work properly with other SPI peripherals. One particular test you can try is to write a value to a register and read it back. If you can get back the same value – that means the SPI routines and the connections are most likely OK, if not, go back and check your code and your connections.

    If you have an oscilloscope you can also check if there is a clock signal on the CLK pin, and verify if data is being sent out on the DATA pin.

    Sorry I can’t be more specific. Let me know if you can get it to work.

  • June 21, 2016 at 6:12 pm

    Thanks for the reply. I didn’t get an oscilloscope, so it is rather difficult to debug the clock input and other things.
    I am using 5v arduino uno to run it, I have checked the connections several times already, but I am using a ttl converter to convert the signals from 5v to3.3v. (5v and 3.3v are both from the arduino uno)

    I have tried to write an register, I wrote 0xaa, 0x33 and 0x08, receiving 0xff, 0x36 and 0x08 respectively.

    For the SPI, I used div128, MSB and mode2. But I have tried different combinations and still can’t get it working.

  • ToughDev
    March 29, 2018 at 6:22 pm

    Thanks for helping with the translation, the article has been updated.

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