Glitches with SAI/SPDIF. Different behaviour on L/R channels

[jmf1]

Posted on November 20, 2016 at 17:52

Dear all,

I have a issue with SPDIF_TX on a nucleo F746ZG. Working on this since some days and can't identify the problem. I have some hearable ''glitches'' in an otherway working operation. A frequency sweep signal allows to reproduce those glitches on a regular basis. I recorded the output of the DAC/Amp and you can see in the attachment the visualization of the glitches: 4 ms amplitude ''dents'' in the signal. The 3 files attached allow to precisely visualize the issue. Please have a look. What is odd is that there is still the expected signal oscillation, but the amplitude is reduced for about 4ms or what looks like of multiple of 4 ms.

The upper part of the dataflow that prepares the data for the SAI has been well tested on a stm32F4 Discovery with I2S and onboard DAC without showing such a behaviour.

I prepare a buffer that is read by DMA (circular mode) to feed the SAI configured for SPDIF protocol. As SPDIF data are fixed in Stm32 to 24 bits for SPDIF, I prepare a buffer of int32 that I fill with int16 shifted by 8 bits (<<8). I alternate Left and Right music samples.

I noticed that I don't have the same result in the 3 below cases:

- 0 for Left channel and music for Right: music and glitches in Right channel, silence on left channel

- music (same data) in Left and Right channels: glitches in Left and right channels,

- strange: - Music for Left channel and 0 for Right: music and NO glitches in Left channel, silence on right channel !!!!

So feeding the SAI with comparable data does not give the same result:  0=>L/Music=>R different from  0=>R/Music=>L

I have tried to play with:

- length of the buffer: no effect,

- amplitude of the signal: no effect,

- frequency: the pure frequency signals that I tried doesn't seems to exhibit the glitches...

- and the signal that continues to oscillate during the glitch, with a suddenly wrong amplitude.

Problem of DMA configuration?

Problem of SAI configuration?

Note: I have to set the 

hsai_BlockB1.

Init

.

AudioFrequency

= SAI_AUDIO_FREQUENCY_96K to efectively have 48k recognized by my DAC. With this setting, I can play music at the right pace. 

DMA is set to work with WORDS (I believe) as: 

hdma_sai1_a.Instance = DMA2_Stream1;

hdma_sai1_a.Init.Channel = DMA_CHANNEL_0;

hdma_sai1_a.Init.Direction = DMA_MEMORY_TO_PERIPH;

hdma_sai1_a.Init.PeriphInc = DMA_PINC_DISABLE; hdma_sai1_a.Init.MemInc = DMA_MINC_ENABLE;

hdma_sai1_a.Init.PeriphDataAlignment = DMA_PDATAALIGN_WORD;

hdma_sai1_a.Init.MemDataAlignment = DMA_MDATAALIGN_WORD;

hdma_sai1_a.Init.Mode = DMA_CIRCULAR;

hdma_sai1_a.Init.Priority = DMA_PRIORITY_HIGH;

hdma_sai1_a.Init.FIFOMode = DMA_FIFOMODE_ENABLE;

hdma_sai1_a.Init.FIFOThreshold = DMA_FIFO_THRESHOLD_FULL;

hdma_sai1_a.Init.MemBurst = DMA_MBURST_SINGLE;

hdma_sai1_a.Init.PeriphBurst = DMA_PBURST_SINGLE;

SAI is configured as:

hsai_BlockB1.

Instance

= SAI1_Block_B;

hsai_BlockB1.

Init

.

Protocol

= SAI_SPDIF_PROTOCOL;

hsai_BlockB1.

Init

.

AudioMode

= SAI_MODEMASTER_TX;

hsai_BlockB1.

Init

.

Synchro

= SAI_ASYNCHRONOUS;

hsai_BlockB1.

Init

.

OutputDrive

= SAI_OUTPUTDRIVE_DISABLE;

hsai_BlockB1.

Init

.

FIFOThreshold

= SAI_FIFOTHRESHOLD_EMPTY;

hsai_BlockB1.

Init

.

AudioFrequency

= SAI_AUDIO_FREQUENCY_96K;

hsai_BlockB1.

Init

.

SynchroExt

= SAI_SYNCEXT_DISABLE;

hsai_BlockB1.

Init

.

MonoStereoMode

= SAI_STEREOMODE;

hsai_BlockB1.

Init

.

CompandingMode

= SAI_NOCOMPANDING;

Start of the SAI is done with:

HAL_SAI_Transmit_DMA(&hsai_BlockA1, (

uint8_t

*)Audio_output_bufferA, AUDIO_OUTPUT_BUF_SIZE*2);

Full code is here: 

https://github.com/jmf13/F7USBAudio/tree/DebugGlitch

#spdif #sai

[jmf1]

Posted on November 21, 2016 at 21:18

You can also see in the big zoom that the signal is perfectly continuous. In case of buffer underrun/overrun, you expect some discontinuity in the signal...

Looking at the specs of the SPDIF interface, I wonder if it could be related to some protection scheme of the consumer type SPDIF interface. I currently don' t manage the Channel Status bits. Could it trigger some faults in my SPDIF receiver like the ones in the above attachments? JMF

[jmf1]

Posted on November 22, 2016 at 19:55

Solved !!!!

You can have a look at

http://www.eevblog.com/forum/microcontrollers/stm32-sai-spdif_tx-issue/

or

http://www.diyaudio.com/forums/digital-line-level/293603-can-low-jitter-achieved-stm32-microcontroller-25.html#post4894411

I was writing by error randomly in the SPDIF Channel Status bits and tickling the ''pre-emphasis'' bit, having the de-emphasis filter switching on and off. I now have a USB Asynch => DSP => 4 digital channels is perfectly reproducing the music on a 20$ stm32 Nucleo board. JMF

[waclawek.jan]

Posted on November 22, 2016 at 21:01

Thanks for coming back with the solution.

JW