Clicking noise issue
Hi,
I'm developing digital avionics intercom system using AD1938 CODEC and ADAU1446 DSP.
Analog audio signals are digitized by AD1938 and processed by ADAU1446 and converted to analog by AD1938.
The AD1938 CODEC is used with stand alone (No SPI control) mode and audio interface is I2S which has 48kHz sampling rate.
The DSP also has 48kHz sampling rate and schematic is as below.
Whenever the temperature of DSP package becomes more than 75 Celsius degrees, clicking noise comes out to headset.
I already check the analog part of my intercom system and the clicking noise didn't come out due to analog circuit.
The clicking noise has following symptoms.
- At the high temperature, the noise comes out.
- As the temperature increases, the noise comes out more frequently.
- If there is no audio input (ex, 1kHz sine wave), no noise comes out.
I have modified and checked following items, but the clicking noise still comes out.
- I added pull-up and pull-down at the clocks between the CODEC and the DSP to prevent distortion of clock due to high temperature.
- I modified external filter connected with LF pin of the DSP and checked the noise.
- Master clock status of the DSP and CODEC. (The DSP and CODEC are using same master clock.)
So, I wonder that what point I can check more? Please let me know.
Audio Signal Processors
adau1446
ad1938
Integrated/Special Purpose A/D Converters
SigmaDSP Audio Processors
Audio CODECs
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Hello Freewill,
There could be several reasons for this with several causes. I will go with my first guess. This sounds like it is a signal integrity issue. It is possible that the timing is on the edge but when you raise the temperature that changes the timing and drive strength just enough to cause an error periodically. Then there are power and grounding details that also could cause this. If there are issues with power and grounding the noise could be on the edge of acceptable but when temperature rises so does the current/power requirements of the parts. The drive strength increases and so the noise on the power and ground will increase and this could theoretically rise to the point where noise spikes can cause false timing to occur.
So let's start looking at these causes to rule out possible problems. I really could use a more complete schematic and a few more details. If you do not want to post on a public forum I can set up a private message for you to post the files only to a select group of people.
I also need to know the following:
Where the MCLK is coming from?
Who is master for BCLK and LRCLK?
It is probably a good idea to know your register settings for the DSP.
Thanks,
Dave T
Hellow Mr. Thib,
Thank you for your immediate response.
Actually, I resolved this issue at last night (in Korean time).
The issue was occured by clock timing mismatch as your guessing.
The CODEC uses LRCK from another CODEC, but the DSP generates the LRCK by itself.
So, I change the DSP start pulse resistor (0xE220) from "00000" to "00101" (the DSP is using LRCK and BCLK pair 2), and then perform high temp. testing.
After change 0xE220 as above, the noise is disappeared.
I think that the syncronization between DSP start pulse and CODEC LRCK is very important.
Thank you.
S. J. Lee
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