RTOS/SIMPLELINK-CC2640R2-SDK: I2S Frame Size

Trevor Hsu

Part Number: SIMPLELINK-CC2640R2-SDK

Tool/software: TI-RTOS

We refer to TI's github:

https://github.com/ti-simplelink/ble_examples/tree/master/examples/rtos/CC26X2R1_LAUNCHXL/ble5apps/central_bidirectional_audio

Example	Streaming method	Input method	Output method	S/W Compression Supported	Sample Rate	Frame Size	Notifications per Frame
Bidirectional Audio	bidirectional	Analog mic + external codec	external codec + headphones	mSBC, ADPCM	16kHz	100 Octets	1
HID Advanced Remote, SensorTag Voice	unidirectional	Digit mic + pulse density modulation (PDM)	None	ADPCM	16kHz	100 Octets	5

And also refer to TI's voice over BLE application note,

software-dl.ti.com/.../voice_platform.html

Data	Calculation	rate
Raw	(16000 S / s) * (16 bits / S)	256kbps
Compressed	256kbps / 4	64kbps
Metadata	4 B / 12ms	2.67kbps
Compressed frame with metadata	64kbps + 2.667kbps	66.67kpbs

The data flow is : I2S capture raw data -> ADPCM encoder -> BLE transmission

Regarding the BLE package size / encoded audio data size is configured to 100 (96 adpcm data + 4 byte header), so the I2S frame size should be set to 192 (96 *2 )

But we found from source code, the I2S frame size is configured to 96

#define AUDIO_DUPLEX_BUFSIZE_ADPCM 96

streamVariables.samplesPerFrame = AUDIO_DUPLEX_BUFSIZE_ADPCM

i2sParams.blockSize = streamVariables.samplesPerFrame;

In order to save bandwidth, so we need to encode the raw data,

Why the I2S frame size equal to the encoding size????

over 7 years ago

0 Trevor Hsu over 7 years ago

Genius 4605 points

Regarding 4:1 IMA-ADPCM, it compresses the 16-bit raw data into 4-bit encoded data.

16 bit -> 4 bit

So, for demo code, it sets the encoding buffer size to 96 Bytes

96 * 8 / 4 = 192, so the PCM size should be 192 instead of 96.

Can you help check this out?

Thanks.

Trevor

0 IUsedToBeARealPerson (M-W) over 7 years ago in reply to Trevor Hsu

TI__Guru 58425 points

Hi Trevor,

I will look into this and get back to you with an answer as soon as possible.

0 Trevor Hsu over 7 years ago in reply to IUsedToBeARealPerson (M-W)

Genius 4605 points

Thanks, waiting for your feedback

0 Trevor Hsu over 7 years ago in reply to Trevor Hsu

Genius 4605 points

Do you have any update????

BLE5 minimum connection interval is 7.5ms.

Regarding the I2S frame size should be greater than 7.5ms

If the I2S Frame Size is set to 192, the frame interval is ~12ms, it can transmit audio packet.

but, if you set the I2S Frame Size to 96, the frame interval is ~6ms, it may lose some audio packets.

0 Sean2 over 7 years ago in reply to Trevor Hsu

TI__Mastermind 33460 points

Hey Trevor,

This is a bug. Thanks for finding it.

ADPCM converts 4:1 from unint16_t to uint8_t.

So if you want a resultant frame size of 96 bytes (uint8_t) of compressed data then you need a framesize of 384.

However, the I2S module only supports an 8-bit frame size (i.e. max = 255).

You can adjust the examples and defines accordingly.

0 Trevor Hsu over 7 years ago in reply to Sean2

Genius 4605 points

ADPCM converts 4:1 from unint16_t to uint8_t. >>>>>>>

I don't think so, regarding 4:1 , it should be 1 * unint16_t to 4bit.

so 2 * unint16_t to 1* uint8_t, 4 * unint16_t to 2* uint8_t,

so I think the correct frame size should be set to 192. (12ms)

could you help me to double check????

0 IUsedToBeARealPerson (M-W) over 7 years ago in reply to Trevor Hsu

TI__Guru 58425 points

Hi Trevor,

You are correct, 192 should be the frame size. We have noted down this, these audio examples are deprecated and will be replaced by new versions in upcoming SDKs so we will make sure to bring this (the correct frame size, not the wrong one) with us to those examples.

0 Trevor Hsu over 7 years ago in reply to IUsedToBeARealPerson (M-W)

Genius 4605 points

Looking forward to your new SDK.

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RTOS/SIMPLELINK-CC2640R2-SDK: I2S Frame Size