nrf5340 stereo output using external codec on SDK 2.6.1

[EchoCharlieWhiskey]

I am trying to make my gateway send stereo data to the headset where it will be played on an external codec. I want this to be a single stereo stream.

I had this working in SDK 2.5.1 but can't seem to get it working with SDK 2.6.1.

The gateway is an unmodified nrf5340 audio DK using the standard board files.

The headset is also an nrf5430 audio DK with an external codec wired up. It uses a Zephyr board file to keep the codec interface line high (codec LED is orange). The headset code does not use the cirrus_cs47l63 driver (#def'd out with CONFIG_HW_CODEC_CIRRUS_LOGIC), my external codec is being configured externally.

I've added STEREO_CIS_HEADSET to my KConfig and set it in prj.conf along with:

CONFIG_STEREO_CIS_HEADSET=y

CONFIG_STREAM_BIDIRECTIONAL=y

CONFIG_TRANSPORT_CIS=y

CONFIG_AUDIO_SOURCE_I2S=y

In unicast_server\Kconfig.defaults, I modified LC3_DEC_CHAN_MAX:

config LC3_DEC_CHAN_MAX

    default 2 if STEREO_CIS_HEADSET

    default 1

In audio_system.c, audio_headset_configure, I added

#endif /* (CONFIG_STREAM_BIDIRECTIONAL) */

	if (IS_ENABLED(CONFIG_STEREO_CIS_HEADSET)) {
		sw_codec_cfg.decoder.num_ch = 2;
		sw_codec_cfg.decoder.channel_mode = SW_CODEC_STEREO;
	} else {
		sw_codec_cfg.decoder.num_ch = 1;
		sw_codec_cfg.decoder.channel_mode = SW_CODEC_MONO;
	}

In bt_le_audio_tx.c, bt_le_audio_tx_send, I modified

	if (streams_to_tx > SRC_STREAM_COUNT) {
		return -ENOMEM;
	}
	if (IS_ENABLED(CONFIG_STEREO_CIS_HEADSET)) {
		data_size_pr_stream = enc_audio.size;
	} else {
		if ((enc_audio.num_ch == 1) || (enc_audio.num_ch == streams_to_tx)) {
			data_size_pr_stream = enc_audio.size / enc_audio.num_ch;
		} else {
			LOG_ERR("Num encoded channels must be 1 or equal to num streams");
			return -EINVAL;
		}
	}

which led to runtime errors in the gateway about "The encoded data size does not match the SDU size"
where data_size_pr_stream was 120, LE_AUDIO_SDU_SIZE_OCTETS(bitrate) was 60, and bitrate was 48000.

I commended out the error, further down in the same function:

//        if (data_size_pr_stream != LE_AUDIO_SDU_SIZE_OCTETS(bitrate)) {
//            LOG_ERR("The encoded data size does not match the SDU size");
//            return -EINVAL;
//        }

In unicast_client.c, at the top, I added

static struct bt_audio_codec_qos lc3_preset_sink_stereo_qos = BT_AUDIO_CODEC_LC3_QOS_10_UNFRAMED(
	LE_AUDIO_SDU_SIZE_OCTETS(CONFIG_BT_AUDIO_BITRATE_UNICAST_SINK) * 2,
	CONFIG_BT_AUDIO_RETRANSMITS, CONFIG_BT_AUDIO_MAX_TRANSPORT_LATENCY_MS,
	CONFIG_BT_AUDIO_PRESENTATION_DELAY_US);

and then later in the same file, in unicast_client_enable I added the pointer to lc3_preset_sink_stereo_qos:

		if ((i % 2) == 0) {
			if (IS_ENABLED(CONFIG_STEREO_CIS_HEADSET)) {
				group_stream_params[i].qos = &lc3_preset_sink_stereo_qos;
			} else {
				group_stream_params[i].qos = &lc3_preset_sink.qos;
			}
			group_stream_params[i].stream = &headsets[headset_iterator].sink_stream;

In unicast_server.c, near the top, I added the two channels support:

static struct bt_audio_codec_cap lc3_codec_sink = BT_AUDIO_CODEC_CAP_LC3(
	BT_AUDIO_CODEC_CAPABILIY_FREQ,
	(BT_AUDIO_CODEC_LC3_DURATION_10 | BT_AUDIO_CODEC_LC3_DURATION_PREFER_10),
#if CONFIG_STEREO_CIS_HEADSET
	BT_AUDIO_CODEC_LC3_CHAN_COUNT_SUPPORT(2),
#else
	BT_AUDIO_CODEC_LC3_CHAN_COUNT_SUPPORT(1), 
#endif /* CONFIG_STEREO_CIS_HEADSET */
	LE_AUDIO_SDU_SIZE_OCTETS(CONFIG_LC3_BITRATE_MIN),
	LE_AUDIO_SDU_SIZE_OCTETS(CONFIG_LC3_BITRATE_MAX), 1u, AVAILABLE_SINK_CONTEXT);

I made no changes to the child image. 

Using a logic analyzer on my headset-to-codec I2S connection I can see that only one channel of data is being sent from the gateway tones mode. (Once tones works, I will hook the gateway up to USB and get stereo audio from my computer.)

As I said, I had this working in 2.5.1 based on reading past forum posts but can't get it in 2.6.1. Advice would be greatly appreciated.

Thank you very much,
ECW

[dejans]

Hi,

Are all mentioned configuration and code changes specified for NCS v2.6.1?

Best regards,
Dejan

[EchoCharlieWhiskey]

Yes.

[dejans]

Hi,

Could you specify all changes which you made when you used NCS v2.5.1 and when everything was working as expected?

When providing code changes you mentioned unicast_server and unicast_client. Did you modify these samples on the headset and gateway side?

Best regards,
Dejan

[EchoCharlieWhiskey]

My code aside, how should a CIS stereo stream from the gateway to the headset (with an external codec) work for SDK 2.6.1? Note that I also want a mono stream from the headset to the gateway and I have only one headset.

(And yes, my modifications above were for the unicast_server and unicast_client example code in 2.6.1, recompiling both gateway and headset.)

Also, from the forums here, having stereo in unicast mode is a common application. Does a white paper exist? 

[dejans]

Hi,

I had a discussion with our developers. Below are some relevant points.

EchoCharlieWhiskey said:

how should a CIS stereo stream from the gateway to the headset (with an external codec) work for SDK 2.6.1?

At the moment our application supports only mono bidirectional. However, you should be able to do either stereo in one BIS to the headset or 2 mono BIS to the headset, but this would require some code changes.

EchoCharlieWhiskey said:

having stereo in unicast mode is a common application. Does a white paper exist?

There is currently no such white paper. We are aiming at implementing full support for this.

The error that you see is what is expected. You could try to print sizes and compare values. You would need to split the data yourself and decode it. Please note that the tone module is mono by design. Even if this approach works, you would still see mono data on I2S. 
The suggestion is to first start with mono stream and duplicate the data after it has been received. Audio part of it can then be modified so that one gets 2 mono outputs, Then, changes can happen on the BLE and sending side.

Best regards,
Dejan