VS1053B intercom, white noise amplification solution!

Designing hardware that uses VLSI Solution's devices as slave codecs such as an external MP3 decoder chip for a host microcontroller.
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Luc Landry
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VS1053B intercom, white noise amplification solution!

Post by Luc Landry »

Hello,

We are designing a second 4 layer prototype board for an intercom using the VS1063A.

On our first prototype pcb we had the following hardware:
- a VS1053B
- an electret microphone used in the differetial mode (MICP/LIN1 and MICN) as an audio input source.
- a class D amplifier (PAM8403) with speakers.
- a W5500 ethernet interface chip.
- a microcontroller (STM32F103RCT6) that managed the SPI communications with the W5500 and the VS1053B.
- a talk/listen button to switch between talking and listening when we are communicating with another intercom station. After establishing a communication with another intercom station we need to press the button to talk to the other station and when we release the button we can hear the audio from the other station. The microcontroller send the audio data in a one way communication via SPI.

This first prototype worked quite well on the SPI communication side. We decided to make the following modification on the second board:
- Migrate to the VS1063A. After looking over the forum, we decided to go with the VS1063A for the increase speed and the ability of outputing the data stream from LINE IN directly to the UART output which will be received by the microcontroller. This might be good if we ever decide to go with a two way realtime communication intercom system.
- Preamplification of the microphone signal. We noticed that there was a lot of white noise at the speakers when we amplified the microphone signal using the VS1053B. When a person is far away from the microphone or speaks silently we need to amplify the signal and the ambiant noise is also amplified. We are thinking of using the using the SSM2167, which is a Low Voltage Microphone Preamplifier with Variable Compression and Noise Gating. Do you think that this would be a good preamplifier for the microphone signal without the white noise?
- We also plan to use the AOM-5024L-HD-R electret microphone from PUI Audio. They say that their HD Series microphones use premium-grade FETs and diaphragms for high sensitivity and superior signal-to-noise ratio. It has a -24dB sensitivity and a 80dB signal to noise ratio.
- Use only LINE1 instead of the differential MICP/LINE1 and MICN. The SSM2167 can only take one audio input for amplification. The preamplified audio input signal from the SSM2167 will be sent to the MICP/LINE1 of the VS1063A while the MICN and LINE2 will be left unconnected.
- Added a MicroSD card. We plan to use the SPI3 from the microcontroller to access a microSD card to play mp3 music files.
- We did some changes on the ground and power plane layers to try to improve the analog and digital separation.

I was wondering if a VLSI expert would be kind enough to give us some feedback on what we are planning to do. We also would like to post our schematics and board layout so that someone could have deeper look for any possible problems but for the moment I don't know how to post a file.

Kind Regards,
Luc
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pasi
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Re: VS1053B intercom, white noise amplification solution!

Post by pasi »

I know little about improving the analog parts, so my contribution is limited to digital.

It looks like you correctly used the mic amplifier, so this doesn't apply to you. However, for general caution:

Sometimes people connect a microphone, but still enable the line input in SCI_MODE and use automatic gain control (AGC). Not using the mic amplifier produces a very low digital signal and the AGC will then produce a very high noise floor.

Limiting the AGC gain helps control the amount of noise at the expense of keeping the amplification of the signal itself limited. More directional microphone pickup pattern could be a solution for some applications.
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Luc Landry
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Re: VS1053B intercom, white noise amplification solution!

Post by Luc Landry »

Pasi, thanks for replying.

Is Panu or someone else available to check the analog design?

Here are the audio schematic and the pcb:
Intercom audio schematic.pdf
Schematic
(160.99 KiB) Downloaded 19 times
Intercom PCB.pdf
PCB
(408.78 KiB) Downloaded 15 times

We would really like to reduce the white noise to the minimum when the sound comes out of the speakers. This is why we included the SSM2167 (Low Voltage Microphone Preamplifier with Variable Compression and Noise Gating) and the AOM-5024L-HD-R (-24dB sensitivity and a 80dB signal to noise ratio) in our second design.

Thanks again,
Luc
Hannu
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Re: VS1053B intercom, white noise amplification solution!

Post by Hannu »

Hello and welcome to the forum.

I was so delighted when you had written the instruction to connect AGND and GND under the chip.

With a quick look I didn't find usual silly mistakes. And the analog side looked nice and compact.
One thing I would do is to have some pads just to test if connecting the amplifier ground and AGND separation is too much and connecting them makes things better. I just kept asking myself how the audio signal returns from the amplifier.

With good noisegate, compressor and pickup direction pattern handling you should be able to get small hiss noise floor and wide dynamics.

Few ideas what to debug next:
  1. Mic noise. Have you tried short the microphone in your current prototype? When mic is shorted, no signal is sent to chip.
  2. Amplifier noise. VS10x3 in reset, apmlifier on.
  3. DAC noise. decode mp3 or piece of wav. Any way so that the VS10x3 has outputted some audio and the audio drivers are left on. If it generates noise the problem is too high low pass filter or ground between amplifier and the chip. SDM switching frequency aliases to audible frequencies.
  4. AGC noise. As Pasi alreaydy mentioned, AGC can increase noise when singal level is low. The external amplifier with dynamics control can help with this.
Luc Landry
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Re: VS1053B intercom, white noise amplification solution!

Post by Luc Landry »

Thanks Hannu for the recommendations.

We will try them out.

Kind regards,
Luc
Luc Landry
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Re: VS1053B intercom, white noise amplification solution!

Post by Luc Landry »

Hello Hannu,

We did the suggested tests on our current prototype in which we installed a VS1063A chip instead of the original VS1053B:

1) Mic noise.
- Microphone shorted connection: It gaved some white noise. As we increased the gain in the VS1063A the white noise also increased.
- Microphone opened connection: Here there results were equivalent as when we shorted the microphone connection. There was some white noise and it increased when we increased the gain in the VS1063A.

2) Amplifier noise. VS1063A in reset and the amplifier ON.
- The white noise was very low, almost quiet. Very acceptable.

3) DAC noise.
- We played an mp3 audio file. After the file was finished playing, we left the VS1063A audio drivers ON. There was no noticeable difference in the white noise when the audio file was playing and after it finished.
- We played an mp3 audio file in which we included a silent pause af several seconds in the middle of the file. There was no noticeable difference in the white noise between the audio and the silence pause when we played the file.

4) AGC noise.
- We did not do this test at this time because we already came to that conclusion when we did our previous tests.

Kind regards,
Luc
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