[0:00] Hey Everyone, we’re back looking at some more options for getting audio data into the ESP32.
[0:09] In the previous video, we looked at a couple of microphone boards (the MAX4466 and the MAX9814)
[0:18] Both these boards use Electret Condensor Microphones which are essentially a parallel plate capacitor
[0:25] with the distance between the capacitor plates varying depending on the sounds waves hitting the diaphragm.
[0:32] Both the MAX4466 and the MAX9814 integrate both the microphone and the audio amplifier.
[0:40] They output an analogue signal which then needs to be fed into the ESP32’s analogue to digital converter.
[0:48] In this video, we’re going to be looking at a couple of boards with MEMS microphones.
[0:54] These microphones are constructed using a Micro-Electro-Mechanical System component.
[1:00] The two boards I’m looking at are the SPH0645 and the INMP441.
[1:08] What’s interesting with these boards is that they integrate the audio amplifier, ADC and
[1:15] the I2S interface.
[1:17] So the output from the boards can be fed directly into the ESP32 without using the ADC.
[1:24] The actual microphones on these boards are packaged up into a small surface mounted device
[1:30] which will either have a hole in the top or the bottom for the sound to enter.
[1:35] Both the boards I am looking at are bottom ported so there is a hole in the PCB to let
[1:39] the sound enter.
[1:40] The principle is similar to the Electret microphone with a capacitor that changes value due to
[1:47] air pressure.
[1:50] You may recall from the previous video that we had some problems with noise on the 3v3
[1:55] power line from the ESP32 dev board.
[1:59] I solved that using an RC filter followed by a Low Dropout Voltage regulator.
[2:05] For this follow-up video, I’ve gone even further.
[2:07] I have an LC filter followed by a capacitor multiplier.
[2:11] This is followed by our LDO regulator, which then has another LC filter on its output.
[2:18] I may have gone too far…
[2:22] I’ve had this circuit made up into a PCB and had it fabricated by JLPCB.
[2:29] The board includes headers for plugging in the MAX9814 and the two I2S boards I’m testing
[2:35] in this video.
[2:37] I’ve wired up the board so that the SPH0645 is on the left channel and the INMP441 is
[2:45] on the right channel.
[2:46] Ideally, I want to be able to record from all three microphone boards simultaneously
[2:53] to compare the outputs.
[2:57] Let’s have a look at how the I2S interface is wired up.
[3:01] There are at least three required lines:
[3:04] We have a serial clock - this is used to clock data to or from the peripheral.
[3:10] We have a word select (also called the left-right clock or LRCLK) - this selects the channel
[3:17] that you want to send or receive data for.
[3:19] And finally, we have the data line.
[3:22] When the Word Select is low the right device will tri-state its output
[3:28] and the left channel will send data.
[3:31] when the word select is high the left device will tri-state its output
[3:35] and the right device will send data.
[3:38] I’ve plugged in the SPH0645 in and left the INMP441 unpopulated.
[3:46] The I2S peripheral is set up to record audio from both left and right channels.
[3:54] When we look at what has been recorded from this configuration
[3:57] we can see there is a small problem.
[4:00] All our data should be coming out in the left channel,
[4:02] but we have some data also coming out on the right channel.
[4:06] This matches with some of the comments I’ve seen in the forums about the SPH0645 and the ESP32.
[4:14] The output from the board doesn’t seem to match up with the I2S standards.
[4:20] After a lot of googling, I came across some forum posts with a potential solution.
[4:26] These settings should fix the timing issues.
[4:28] After some testing, I don’t think they interfere with the INMP441 so we can capture some audio.
[4:37] As I said earlier, my initial plan was to capture from all three boards are once,
[4:42] but I’ve noticed quite a lot of interference from the I2S signals with the MAX9814 output.
[4:48] So I’ve captured the ADC signal separately from the two I2S signals.
[4:55] Here’s the audio captured via the built-in ADC.
[5:03] I’ve set the gain on the MAX9814 to its minimum value and that does seem to have improved
[5:10] the amount of noise coming through.
[5:12] Here’s the audio captured directly from the INMP441
[5:22] And finally, the audio captured from the SPH0645
[5:33] Another question I wanted to answer is how good these I2S boards are
[5:39] at dealing with the noisy power supply.
[5:41] Do we need my power supply filter?
[5:45] Here’s the audio with the INMP441 connected directly to the dev board without any filtering.
[6:02] There’s no sign of the noise that we saw with the two analogue boards.
[6:06] in the previous video.
[6:08] So, what have we learnt?
[6:10] I think the INMP441 board wins this competition hands down.
[6:16] It seems to be standards compliant
[6:18] It produces a good signal with no DC offset
[6:23] And it seems to handle the noisy power supply without any problems
[6:28] It’s much less noisy than the MAX9814
[6:33] The only downside I can find at the moment is the lack of a built-in AGC
[6:38] you will need to do something in software if you want that kind of functionality.
[6:42] So, that’s it for this video - please hit the subscribe button if you found it useful
[6:48] and check out the previous video where we looked at the other two microphone boards.
[6:52] The code I used is all in the GitHub repo linked in the description.
[6:57] In the next video, we’ll look at getting audio out of the ESP32,
[7:01] another small step to finishing my next project.
[7:04] Thanks for watching!
[7:06] I’ll see you in the next video!